ArticlePDF Available

The expanding tropics: A critical assessment of the observational and modeling studies

January 2014
Wiley interdisciplinary reviews: Climate Change 5(1)

DOI:10.1002/wcc.251

Authors:

Chris Lucas

Bureau of Meteorology

Bertrand Timbal

Bureau of Meteorology

Hanh Nguyen

Bureau of Meteorology

This review provides comprehensive coverage of the tropical expansion literature to date. The primary focus is on the annual- and zonal-mean behavior of the phenomenon. An idealized model that identifies the mean meridional circulation as a hemisphere-wide structure with significant tropical–extratropical interaction is introduced as background for the understanding of the expansion and the methodologies used for detection. A variety of metrics from different data sources have been used to identify an expansion of the global tropics since 1979 by 1°–3° latitude in each hemisphere, an average trend of approximately 0.5°–1.0° decade−1. The symmetry of this expansion—whether Northern and Southern hemispheres are expanding at the same rate—is unclear. Limitations of observational datasets, including reanalyses, prevent a more precise determination at this time. General circulation models are able to qualitatively reproduce this expansion, but generally underestimate its magnitude. Multiple factors have been identified as potential drivers of the expansion, including increasing greenhouses gases, stratospheric ozone depletion, and anthropogenic aerosols. No single factor by itself appears to explain the full expansion, perhaps a shortcoming of the models or experiment design. It may be that some combination of these forcings is producing the change, but the relative contribution of each forcing to the expansion is currently unknown. The key issues remaining to be resolved are briefly summarized at the end. WIREs Clim Change 2014, 5:89–112. doi: 10.1002/wcc.251 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website.

Acronyms and References for the Reanalysis Products

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Summary of Studies Using the Tropopause-Based Methodologies

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Summary of Studies of Tropical Expansion Using Satellite-Based Methodologies

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Summary of Studies Using Streamfunction () Methods

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Summary of result of observational tropical expansion studies, broken down by categories. Pluses (+) represent SH values, crosses (×) NH values of tropical expansion trend. Symbols in the corresponding hemisphere indicates expansion, symbols in the ‘opposite’ hemisphere indicate contraction. Within a particular methodology, the vertical position of each symbol is randomized to improve clarity of individual symbols; no other meaning is implied. Colors of symbols refer to source and/or methodology, with legends for each category in boxes to side of plot, where numbers represent the cited reference.

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Content may be subject to copyright.

Advanced Review

The expanding tropics: a critical

assessment of the observational

and modeling studies

Christopher Lucas,∗Bertrand Timbal and Hanh Nguyen

This review provides comprehensive coverage of the tropical expansion literature

to date. The primary focus is on the annual- and zonal-mean behavior of the

phenomenon. An idealized model that identiﬁes the mean meridional circulation

as a hemisphere-wide structure with signiﬁcant tropical–extratropical interaction

is introduced as background for the understanding of the expansion and the

methodologies used for detection. A variety of metrics from different data sources

have been used to identify an expansion of the global tropics since 1979 by 1◦–3◦

latitude in each hemisphere, an average trend of approximately 0.5◦–1.0◦decade−1.

The symmetry of this expansion—whether Northern and Southern hemispheres

are expanding at the same rate—is unclear. Limitations of observational datasets,

including reanalyses, prevent a more precise determination at this time. General

circulation models are able to qualitatively reproduce this expansion, but generally

underestimate its magnitude. Multiple factors have been identiﬁed as potential

drivers of the expansion, including increasing greenhouses gases, stratospheric

ozone depletion, and anthropogenic aerosols. No single factor by itself appears to

explain the full expansion, perhaps a shortcoming of the models or experiment

design. It may be that some combination of these forcings is producing the change,

but the relative contribution of each forcing to the expansion is currently unknown.

John Wiley & Sons, Ltd.

How to cite this article:

WIREs Clim Change 2013. doi: 10.1002/wcc.251

SUBTROPICAL DRYING AND

TROPICAL EXPANSION

In recent decades, observations suggest that the

hydroclimate in many regions has been chang-

ing. Reduced precipitation and more frequent

droughts have been observed in southern portions

of Australia,1,2 much of the Mediterranean region,3,4

portions of North America, particularly the south-

western United States,5the Altiplano in South

America,6and northern China.7Similar tendencies

have also been noted in Africa and southeast Asia.8,9

∗Correspondence to: c.lucas@bom.gov.au

Centre for Australian Weather and Climate Research/Bureau of

Meteorology, Melbourne, Vic, Australia

Conﬂict of interest: The authors have declared no conﬂicts of

interest for this article.

While some places may only show seasonal rain-

fall declines, these deﬁcits can drive extreme heat in

later seasons.10 Droughts and precipitation have many

natural inﬂuences across multiple timescales; telecon-

nections from changes in sea surface temperature

(SST) driven by phenomena like the El Ni ˜

no-Southern

Oscillation (ENSO) or other decadal and centennial

variability8,11,12 have a particularly noteworthy effect.

However, some portion of the apparent trend may be

linked to changes in the broader circulation patterns of

the globe caused by anthropogenic global warming,8

trends that are projected to continue through the 21st

century.9

A focus of these drying trends is found in the

subtropics, among the driest regions on Earth. The

subtropics are broadly associated with the downward

branch of the Hadley circulation (HC), a planetary-

scale tropical overturning circulation spanning half the

Advanced Review wires.wiley.com/climatechange

globe. Conceptually, this part of the circulation can

be considered the ‘edge of the tropics’. One projected

consequence of anthropogenic climate change is that

this edge is expected to expand toward the poles,

shifting precipitation patterns, and broadening the

area covered by frequent drought.13,14 During the last

decade, numerous studies using a variety of metrics to

identify the tropical edge have emerged, suggesting

just such a shift referred to as an expansion of

the tropics. Circulation patterns outside the tropics

have also been related to the expected hydroclimatic

changes.1–4These extratropical changes may, in fact,

be closely related to tropical expansion.

Earlier reviews13,14 summarized the basic

understanding of tropical expansion. However, much

remains unclear, including the exact amount of

tropical expansion and the physical forcings driving it.

A signiﬁcant amount of new research has subsequently

improved the understanding. Herein, we provide a

comprehensive review of the state of knowledge in

relation to tropical expansion.

Understanding tropical expansion necessitates

knowledge of the components of the mean meridional

circulation (MMC) and the interactions between

them. A key factor lies in deﬁning the ‘edge of the trop-

ics’, a nebulous concept that is somewhat resistant to

precise deﬁnition. Part of this nebulosity arises because

the edge is not ﬁxed, varying with both the change of

season and geographic location. We will focus primar-

ily on the zonally and annually averaged circulation

in this review, ignoring some of the ﬁner detail that

ultimately may be important to understanding the

issue at hand. To provide a background, an idealized

model of the MMC is described. This qualitative

model presents the MMC as a pair of hemisphere-wide

structures in which the extratropics play a crucial role.

The observational studies of tropical expansion are

then summarized and organized around the features

of the MMC that are used to identify the tropical

edge. The various deﬁnitions of the tropical edge that

have been used are given. A discussion of the sources

of observational uncertainty in the different estimates

follows. Many results are derived from reanalysis-

based data, which may contain inhomogeneities and

other artifacts that could affect the understanding of

the long-term behavior of the climate. Potential uncer-

tainty introduced by the individual methodologies of

the studies is also described. The mechanisms and

climate forcings behind tropical expansion are then

explored, largely described from modeling studies.

The forcings are generally anthropogenic in origin,

although natural factors do inﬂuence the tropical

edge. A synthesis of the observations and modeling

results is then presented, where model-predicted

impacts are compared to the observed results to pro-

vide evidence for the relative roles of the prospective

drivers. The ﬁnal section addresses some remaining

questions and directions for future research into

this topic.

AN IDEALIZED MODEL OF THE MMC

Figure 1 presents a schematic diagram of the idealized

MMC, depicting the zonally and annually averaged

POLAR FRONT

FIGURE 1|Schematic diagram detailing the components and physical processes of the idealized mean meridional circulation (MMC). Diagram

represents the hemispheric annual and zonal-mean ﬂow; a mirror image about the equator would represent the global circulation. Labels in all

capitals represent the main features of the MMC. Bold labels refer to subcomponents of the circulation. Text in italics identify important physical

processes. See text for further discussion.

WIREs Climate Change The expanding tropics

circulation. Only one hemisphere is shown; the full

global MMC would be represented by a ‘mirror image’

in the other hemisphere. This view of the ﬂow is devel-

oped from ‘classical’ meteorological concepts and

studies using the (dry) isentropic streamfunction15,16

to diagnose the ﬂow. Paluis et al.17,18 have developed

a conceptually similar model based on a moist

isentropic view of the circulation. These models differ

from the more conventional ‘three-cell’ model based

on the isobaric streamfunction, while still sharing

many characteristics. The most notable differences

are the enhanced extratropical circulation and the

idea of a complex hemisphere-wide overturning

circulation. This diagram and its discussion provide

a qualitative framework for the methodologies and

results discussed in this article.

In the tropics, many similarities to the three-cell

model are observed. The ﬂow is characterized by a

deep meridional overturning circulation known as

the Hadley circulation (HC). Mean ascent, driven by

tropical mesoscale convective systems,19 is observed

near the equator in the Intertropical Convergence

Zone (ITCZ) with a broad compensating subsidence

near 30◦latitude. Radiative cooling drives much

of the descent. In the lower-troposphere, the trade

winds move air toward the equator; in the upper-

troposphere, the ﬂow moves poleward and terminates

in the subtropical jet (STJ). Webster20 provides a

thorough elementary review of the characteristics

and physics of the HC. The circulation varies with

season and with the rising branch generally located

off the equator by 5◦latitude or more in the summer

hemisphere. The descending branch also migrates

with the season. The circulation in the winter

hemisphere is signiﬁcantly stronger than that in the

summer hemisphere.21

Idealized axisymmetric models of the HC22,23

suggest that the HC width depends on the size and

rotation rate of the Earth, the tropical tropopause

height, and the pole-to-equator temperature gradient.

In idealized aquaplanet simulations,24 the width of the

HC increases with increasing mean temperature and

temperature gradient. Other experiments25 indicate

more complex relationships, with feedbacks between

many different processes. No single parameter solely

determines the scale of the HC. One important factor,

often neglected in simpler models, is the effect of

extratropical baroclinic eddies.26 A signiﬁcant role for

these eddies has been suggested since the middle of

the 20th century; Palm´

en and Newton27 summarize

this earlier work. Allowing for baroclinic instability

suggests a dependence of the HC width on the gross

static stability,28 a relationship broadly supported in

both idealized and realistic general circulation model

(GCM) simulations.24,29,30 This relationship has been

found to apply in all seasons.31

In Figure 1, eddies dominate the circulation in

the ‘polar front’, a symbiosis of the eddy-driven polar

front jet (PFJ) and baroclinic waves. The polar front

marks the position of the ‘storm track’, whose location

and variability is related to the ‘annular modes’.32

When the storm track is active, an overturning

circulation similar to the Ferrel circulation is observed

in association with the baroclinic waves and transient

weather systems.33 On average, the ﬂow in the

extratropics is quasi-isentropic. Moist air ascends in

the polar front, resulting in enhanced precipitation,

and returns equatorward, its moisture largely

removed.34 This circulation also varies seasonally,

generally moving poleward and strengthening in the

winter hemisphere, a response to greater baroclinicity.

Signiﬁcant longitudinal variability in wave activity

arises from surface topographic features like mountain

ranges and land-ocean boundaries.27

The exact nature of the relationship between the

extratropical eddies and the HC remains unclear and

may also depend on season. Bordoni and Schneider35

suggest that the impact of eddies on the HC is variable

during the year. In their study of the Asian monsoon,

they ﬁnd that direct thermal driving becomes more

important than eddies with the onset of the monsoon.

Kang and Polvani36 found a strong interannual

relationship of the HC edge and the position of

the eddy-driven jet in the Southern Hemisphere (SH)

during summer, with the HC edge moving 1◦latitude

for a 2◦shift in the position of the westerly jet. Ding

et al.37 show a linkage between tropical SST and the

SH annular mode (SAM) through tropically forced

Rossby wave trains.

In the idealized MMC model (Figure 1),

tropical–extratropical interactions have the greatest

impact in the subtropics, loosely deﬁned as the region

between the HC and the polar front. In general, the

region is dominated by subsidence and the subtropical

ridge (STR). In a relative sense, the air is dry and

precipitation is low. Much of the variability here,

on both seasonal and interannual time scales, is due

to the continual evolution of tropical–extratropical

interactions and their response to different forcings.

For example, deep tropical convection—modulated

by the Madden Julian Oscillation—has been shown

to produce Rossby wave trains which extend well into

the higher latitudes.38

The stratosphere appears to play a signiﬁcant

role in the MMC. Much of its effect is related to

ozone, and its observed depletion in recent decades.39

The Antarctic ozone hole which forms every spring

in particular has a signiﬁcant impact on the MMC.

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Increasing wind shear in the lower stratosphere has

an effect on baroclinic waves, increasing their phase

speed.40 Stratospheric water vapor concentrations are

known to affect the decadal rate of warming of the

surface climate41 and these changing concentrations

could drive changes in the tropospheric MMC.42 This

is consistent with evidence that suggests that the

stratospheric circulation changes precede anomalous

tropospheric weather regimes. These stratospheric

changes are translated to the troposphere through

the annular modes.43

The model represents the ‘typical’ structure of

the MMC. At any given time or place, the represented

features may be more or less prominent and/or

in a different position. As noted earlier, seasonal

variation is particularly important. Also, the seasonal

evolution of the MMC is quite different in the two

hemispheres; the variability of intensity and extent

of the circulation is much larger in the Northern

Hemisphere (NH) compared with the SH.21 The

distribution of land, more prevalent in the NH, is an

important factor in this different evolution.44 Quasi-

stationary waves, associated with mountain ranges,

also play a role.45 Rind and Perlwitz46 suggest that

topography is a factor in HC width based on a

set of paleoclimate simulations. These factors also

contribute to signiﬁcant longitudinal variability in the

behavior of the MMC, particularly in the extratropics.

OBSERVATIONS OF TROPICAL

EXPANSION

A variety of datasets and/or metrics have been

used in tropical expansion studies. From a practical

standpoint, the deﬁnition of the ‘edge of the tropics’ is

critical. With most metrics the point of transition from

tropics to extratropics is neither intuitively obvious

nor unambiguous, but rather is a transition occurring

over a range of latitudes (i.e., the subtropics);

deﬁning ‘the edge’ is then a matter of choosing some

point within this transition zone that is (hopefully)

representative of the whole. The methods for making

this choice can be subjective (but rationally decided)

or objective (e.g., chosen through an algorithm), but

ultimately the distinction is arbitrary. The subtleties of

a complex region of the atmosphere are reduced to a

single number that may or may not be representative of

the behavior of the whole MMC. There is no universal

deﬁnition of the tropical edge that is applicable across

all datasets; each sees the transition zone in a different

way and as such, the responses of a given metric

show different sensitivities. Adding further complexity

to these metrics are the effects of seasonal and/or

longitudinal variability of the MMC; the zonal and

annual averaging used in many studies may introduce

uncertainty or mask important processes in its own

right. These aspects, along with the different time

periods covered by different studies, confound the

interpretation and comparison of the trends.

This section is organized around the different

features of the MMC and/or datasets that have been

used to identify the tropical edge. For each, a brief

overview of the methodology is given and the results

brieﬂy discussed. A table is produced that summarizes

the information from each study, including the data

sources, the exact metric used, and the annual

observed rate of expansion, given by hemisphere when

available. As many studies here use the various global

reanalysis products, references to those products and

their acronyms are shown in Table 1.

Tropopause Methods

The tropopause height frequency (THF) method-

ology55 is frequently used in tropical expansion

studies. Outside of the subtropics, annual histograms

of THF have a single peak. This peak is found at

15–17 km in the tropics and at 11–13 km in the

extratropics. Within the subtropics, the THF is

bimodal, an indicator used to deﬁne the tropical edge.

The number of tropical tropopause days (TTD) is

deﬁned as the number of days that the tropopause

exceeds a speciﬁed height threshold representative of

the tropics. An array of TTD in time-latitude space is

created and given values contoured. The slope of the

contours provides an estimate of the long-term trend.

Lu et al.56 also applied this basic methodology.

The deﬁnition of the various thresholds is

crucial. Generally, tropopause height thresholds from

TABLE 1 Acronyms and References for the Reanalysis Products

Discussed in the Article

Acronym Expansion and References

NCEP National Center for Environmental Prediction

(NCEP)/National Center for Atmospheric

Research (NCAR) Reanalysis47

ERA40 European Centre for Medium-Range Weather

Forecasting (ECMWF) 40-year Reanalysis48

NCEP2 NCEP/Department of Energy (DOE) Reanalysis49

JRA25 Japanese 25-year Reanalysis50

ERA-I ECMWF Interim Reanalysis51

CFSR NCEP Climate Forecast System Reanalysis52

20CR NOAA-CIRES 20th Century Reanalysis53

MERRA National Aeronautics and Space Administration

(NASA) Modern Era Reanalysis for Research

and Applications54

WIREs Climate Change The expanding tropics

14.5 to ∼16 km have been chosen, although there is

some sensitivity to the choice of tropical tropopause

threshold and the deﬁnition of the tropical edge.57

This sensitivity arises, at least in part, from the

observed global rise in tropopause heights58 of

∼40–80 m decade−1,59–61 and the use of a ﬁxed

height threshold could act to confound the tropical

expansion signal. Some studies57,58 deﬁne relative

tropical tropopause thresholds to try and remove the

effect of the ‘background’ tropopause trends. These

methodologies reduce the magnitude of the tropical

expansion trends. However, the amount of trend

reduction has little apparent relationship with the

time tendencies in the tropopause thresholds and the

results are not robust.

A comparison of rates of THF-based tropical

expansion in the SH computed directly from historical

radiosonde data and from four reanalyses62 suggests

that much of the sensitivity is related to issues with the

underlying data quality; reanalysis-derived tropopause

heights63 are subject to strong biases in the subtropics.

Global tropopause trends were estimated to have a

small effect on the estimated expansion trends (less

than 0.1◦decade−1).62

A blended tropopause height dataset61 derived

from the two ECMWF reanalyses combines thermal

and dynamic deﬁnitions of the tropopause is also used

to examine expansion rates. This dataset reduces the

bias in the subtropics. The THF methodology is not

used in this study, but expansion rates are consistent

with those studies. A non-THF tropopause-based

methodology—the potential temperature difference

between the tropopause and the surface (θ ), a dry

bulk stability parameter—has also been used to iden-

tify the edges of the tropics and its long-term trend.64

Table 2 summarizes the results from studies

using a tropopause-based metric. For the most part,

these indicate an expansion of less than 1◦decade−1

in each hemisphere. Many THF-based studies indicate

that the SH has expanded more rapidly than the NH.

Results with relative thresholds57,58 generally show

smaller trends and even contraction in some cases.

The results are dependent on the reanalysis chosen.

In several studies, signiﬁcant interannual variability

is identiﬁed,56,58,62 with effects due to ENSO and

large volcanic eruptions noted. Seasonal differences

in tropopause-based trends may also be present,

generally suggesting a weak tendency toward greater

TABLE 2 Summary of Studies Using the Tropopause-Based Methodologies

Investigators Data Sources Thresholds Time Period

Expansion

(◦Latitude Decade−1)

Siedel and Randel55 NCEP, ERA40

radiosondes

15.5 km/TTD =300, 200, or

100

1979–2005 Both: 1.7–3.1

Equal between

hemispheres

Lu et al.56 NCEP and ERA40 120 hPa/200 days 1958–1999 SH: 0.5–0.7

NH: 0.05–0.2

Birner57 NCEP, NCEP2, ERA40,

and JRA25

Variable/equivalent latitude 1979–2008 SH: −0.4 to 0.7

NH: −0.2 to 0.0

NCEP, NCEP2, ERA40,

and JRA25

Variable 2/equivalent

latitude

1979–2008 SH: 0.0–0.6

NH: −0.2 to 0.2

Wilcox et al.61 S ERA-I, ERA40 blended

tropopause

Average of 15.0, 15.5, and

16.0 km/NA

ERA40: 1958–2001

ERA-I: 1989–2007

ERA40 Both: 0.7

ERA-I Both: 0.9

(60–80% in SH)

Davis and Rosenlof58 S NCEP, CFSR, JRA25,

and MERRA

15.0 km/equivalent latitude 1979–2009 SH: 0.2–0.8

NH: −0.1 to 0.6

S NCEP, CFSR, JRA25,

and MERRA

Variable, 1.5 km below

tropical mean/equivalent

latitude

1979–2009 SH: 0.1–0.4

NH: −0.3 to 0.3

Lucas et al.62 Radiosonde 14.5 km/200 days 1979–2010 SH: 0.4, regionally varies

0.2–0.6

Davis and Birner64 MERRA, ERA-I, and

NCEP

Subtropical stability maxima 1979–2010 Both: −0.5 to 0.9

Positive values equal expansion regardless of hemisphere. Thresholds refer to tropical tropopause height threshold and the number of TTD used to deﬁne the

edge. An ‘s’ after the investigators indicates the study examined the seasonality of the expansion.

NH, Northern Hemisphere; SH, Southern Hemisphere.

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expansion in the summer and autumn months of each

hemisphere58,61 although these trends are not always

statistically signiﬁcant.58 Longitudinal variability in

the amount of tropical expansion was also noted with

this methodology.62

Satellite Methods

Varied metrics derived from satellite-based platforms

have been widely used to investigate tropical

expansion. One common method is analogous to

the THF methodology; zonal averages of the metric

are arranged in a time-latitude array, a deﬁnition

of the tropical edge chosen, and trends estimated

from the slope of chosen contours. As with the THF

methodology, background trends in the quantity of

interest may mask the true rate of tropical expansion.

A common metric in satellite studies is outgoing

longwave radiation (OLR).58,65,66 Since 1979, broad

trends have been noted in many OLR datasets, on

the order of 1–3 W m−2decade−1, depending on the

dataset and the area examined.58,67–69 Several recent

studies70,71 have examined cloudiness measurements

from the International Satellite Cloud Climatology

Project (ISCCP)72 and the Pathﬁnder Atmosphere

(PATMOS)73 datasets, deﬁning the position of the

edge from either a ﬁxed threshold or the minimum

in subtropical cloudiness. One study74 examined the

long-term changes in different NH ozone regimes

derived from satellite data, whose boundaries are

closely linked with upper-tropospheric fronts.75 A

later study76 used this approach to extend these results

into the SH. Trends in upper-troposphere and lower-

stratosphere temperature data retrieved from the

(Advanced) Microwave Sounding Unit (MSU/AMSU)

have also been used to infer tropical expansion,

either through analysis of global temperature trends77

or changes to the idealized temperature structure

associated with the STJ.78

Table 3 summarizes the satellite-based studies.

Large differences exist in many estimates using the

same or similar data sources. Four OLR datasets each

shows a general widening trend in both hemispheres,

greater in the NH.65,66 However, differences between

the datasets are large, in some cases statistically

signiﬁcant.58 Using a relative threshold reduces the

TABLE 3 Summary of Studies of Tropical Expansion Using Satellite-Based Methodologies

Investigators Data Sources Metric Time Period

Expansion

(◦Latitude Decade−1)

Hudson et al.75 Satellite-derived ozone Total ozone 1979–2003

1979–1991

NH: about 1◦

NH: 1.1

Fu et al.77 MSU Global temperature trends 1979–2005 Estimated total shift of

1◦per hemisphere

Hu and Fu65 S HIRS, ISSCP, and GEWEX Monthly OLR, 250 W m−21984–2004

1980–2003 (HIRS)

SH: 0.3–0.9

NH: 0.6–1.1

Johanson and Fu79 HIRS, ISSCP, and GEWEX Multidataset OLR,

250 W m−2

1984–2004

1979–2003 (HIRS)

Both: 3.0

Hu et al.66 S NOAA OLR, 250 W m−21979–2009 SH: 0.3

NH: 0.9

Fu and Lin78 3 MSU datasets Lower stratospheric

temperature

1979–2009 SH: 0.2

NH: 0.3

Zhou et al.70 S ISSCP cloudiness Subtropical 60% coverage 1984–2006 SH: 0.7–1.3

NH: 1.3–1.6

S ICCSP cloudiness Subtropical cloud cover

minimum

1984–2006 SH: −0.1 to −0.6

NH: 0.3–1.6

Davis and Rosenlof58 S NOAA, HIRS, GEWEX,

and ISSCP

OLR/variable: 20 W m−2

below subtropical

maximum

1979–2005,

1979–2009 for

NOAA

SH: −0.3 to 0.4

NH: 0.1–0.5

Allen et al.71 ISSCP, AVHRR cloud

climatology

Subtropical cloud cover

minimum

1983–2008 SH: 0.36

NH: 0.39

Hudson76 Satellite-derived ozone Total ozone 1979–2010 SH: 2.0

NH: 1.2

Positive values equal expansion regardless of hemisphere. An ‘S’ after the investigators indicates the study examined the seasonality of the expansion.

NH, Northern Hemisphere; SH, Southern Hemisphere.

WIREs Climate Change The expanding tropics

magnitude of the trends, such that two OLR datasets

suggest contracting trends in the SH.58 The studies

using minimum cloudiness metrics show considerably

different results,70,71 with differing signs in the SH.

Global temperature trends suggested a total shift in

the jet stream of about 1◦in each hemisphere,77 later

reﬁned to identify total expansions of 1◦in the NH and

0.6◦in the SH.78 Studies using ozone data74,76 indicate

a broadening of about 1◦decade−1in the NH, partic-

ularly between 1979 and 1991. In the SH, expansion

using this methodology has averaged 2.0◦decade−1

between 1979 and 2010. The OLR-based studies show

only weak signals of seasonal variability.58,65,66 Inter-

annual variability was not discussed.

Streamfunction Methods

One common metric is the edge of the HC as derived

from calculations of the (isobaric) mass streamfunc-

tion ().21,58,64,65,71,79,80The mass streamfunction is a

vertical integral of the meridional wind and produces

the familiar three-cell model of the MMC. The edge

of the tropics is generally taken as the latitude of

the subtropical zero isopleth in the mid-troposphere.

The monthly position of the HC is determined, which

can be averaged to the desired resolution and tracked

in time to estimate the expansion. This metric,

annually averaged, for eight reanalysis products is

shown in Figure 2. Visual examination of the time

FIGURE 2|Time series of the position of the subtropical edge of

the Hadley cell as deﬁned from the streamfunction in the eight modern

reanalyses for the Northern (top) and Southern (bottom) Hemisphere.

Meteorological Society). Time series from ERA40 (post 2002) and 20CR

(post 2008) are partially extrapolated from statistical relations with the

other reanalyses.

series shows an expansionary trend is apparent in

each hemisphere—particularly the NH—although

considerable scatter exists in the estimates.

Table 4 summarizes the -based results. Over-

all, expansion rates are below 0.8◦decade−1, with

little difference between the hemispheres.21,58,65 Stud-

ies that examine both hemispheres combined64,79,80

suggest expansion rates of 0.4 and 3.2◦decade−1.

No studies using this methodology indicate con-

traction. Seasonally, expansion is more rapid in

the respective summer and autumn seasons of each

hemisphere,21,58,65 when the HC is extended furthest

poleward and its intensity is lowest. Despite the

large trends, the NH expansion in many reanalyses

is not statistically signiﬁcant due to high variability

within the time series.21 The SH expansion is more

robust across the different products, with a shift in

the location indicated across all reanalyses in the late

1990s (Figure 2).

Jet Stream Methods

Another metric used to examine tropical expansion is

the change in position of the various jet streams,

which can be deﬁned in different ways. Several

studies58,81 identify jet streams in several reanalyses

from the mass-weighted wind between 400 and

100 hPa. Jet streams have also been deﬁned from

the zonal-mean zonal wind at 850 hPa and from the

location of the speed maxima of the jet streams as

deﬁned above.58,64 The NH 850 to 300 hPa maximum

wind derived from radiosonde data has also been

investigated.71 While not explicitly considered in this

section, the satellite temperature study noted earlier

interpret their data based on an idealized model

of the STJ, although no wind data are actually

used there.78

Table 5 shows the trends based on these metrics.

For the jet streams deﬁned from the mean wind, trends

of less than 0.2◦decade−1are generally observed.

No consistent difference is noted between upper and

lower jet streams in these metrics.58,81 When using

the latitude of the maximum mean wind, the time

series are much more variable and hence have greater

uncertainty in the trend values.58 This is particularly

notable for the 850 hPa zonal winds in the SH, where

some individual reanalyses identify contractive trends

on an annual basis.58 The seasonality of the trends

is inconsistent across the metrics. For zonal-mean

wind metrics, the seasonality tends to be weaker. For

those based on maxima, the seasonality is larger with

changes of sign and strength throughout the year.

For the 850 hPa maximum wind deﬁned edge in the

SH, expansion is noted in summer and autumn while

strong contraction is noted in winter and spring.58

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TABLE 4 Summary of Studies Using Streamfunction () Methods

Investigators Data Sources Metric Time Period

Expansion

(◦Latitude Decade−1)

Hu and Fu65 S NCEP, ERA40, and NCEP2 Isobaric mass streamfunction 1979–2005 SH: 0.4–1.2

NH: 1.0–2.3

Johanson and Fu79 NCEP, ERA40, and NCEP2 Multidataset average isobaric

mass streamfunction

1979–2004 Both: 3.2

Stachnik and Schumacher80 S NCEP, ERA40, NCEP2,

ERA-I, MERRA, CSFR,

JRA25, and 20CR

Isobaric mass streamfunction 1979–2008 Both: 1.10 (ensemble)

Range: 0.29–1.48

Davis and Rosenlof58 S NCEP, JRA25, CFSR, and

MERRA

Isobaric mass streamfunction 1979–2009 SH: 0.1–0.8

NH: 0.1–0.8

Allen et al.71 NCEP, NCEP2, ERA40,

MERRA, and CFSR

Multidataset average isobaric

mass streamfunction

1979–1999 SH: 0.36 ±0.3

NH: 0.42 ±0.2

Nguyen et al.21 S NCEP, ERA40, NCEP2,

ERA-I, MERRA, CFSR,

JRA25, and 20CR

Isobaric mass streamfunction 1979–2009 SH: 0.1–0.8

NH: 0.2–1.0

Davis and Birner64 MERRA, ERA-I, and NCEP Isobaric mass streamfunction 1979–2010 Both: 0.4–2.0

Positive values equal expansion regardless of hemisphere. An ‘S’ after the investigators indicates the study examined the seasonality of the expansion.

NH, Northern Hemisphere; SH, Southern Hemisphere.

TABLE 5 Summary of Studies Using Jet Stream Methods

Investigators Data Sources Metric Time Period

Expansion

(◦Latitude Decade−1)

Archer and Caldeira81 NCEP and ERA40 Mean latitude of mean 400

to 100 hPa wind

1979–2001 SH: 0.06–0.12

NH: 0.17–0.19

Davis and Rosenlof58 S NCEP, JRA25, CFSR, and

MERRA

Mean latitude of mean 400

to 100 hPa wind

1979–2009 SH: 0.0–0.1

NH: 0.0–0.1

S Mean latitude of mean

850 hPa wind

1979–2009 SH: 0.0–0.2

NH: 0.1–0.3

S Latitude of maximum 400

to 100 hPa wind

1979–2009 SH: 0.1–0.3

NH: 0.0–0.2

S Latitude of maximum

850 hPa wind

1979–2009 SH: −0.4 to 0.2

NH: 0.0–0.2

Allen et al.71 Radiosonde Latitude of 850 to 300 hPa

wind

1979–2009 SH: 0.20

NH: 0.08

Davis and Birner64 MERRA, ERA-I, and NCEP Latitude of maximum wind 1979–2010 Both: 0.3–0.6

Positive values equal expansion regardless of hemisphere. An ‘S’ after the investigators indicates the study examined the seasonality of the expansion.

NH, Northern Hemisphere; SH, Southern Hemisphere.

Surface-Based Methods

Several studies rely upon surface-based variables to

investigate tropical expansion. Several studies66,70,71

utilize the Global Precipitation Climatology Project

(GPCP) monthly dataset82,83 to examine shifts in the

locations/boundaries of the subtropical dry zones, as

deﬁned by the location of the precipitation minimum

or the 2.4 mm day−1contour (Ref 70 only). Several

studies have examined metrics based on P-E (precip-

itation minus evaporation) from several sources; an

observationally based composite P-E dataset derived

from GPCP and Woods Hole Oceanographic Institute

oceanic evaporation data71 and the output from

several reanalysis products.58 The global position of

the STR, derived from sea level pressure data from

three reanalyses and the HadSLP2 dataset,84 has also

been used to estimate tropical expansion.66

Table 6 summarizes these results of the surface-

based studies. The multiple studies using GPCP data

to examine subtropical precipitation minimum have

conﬂicting results. One study ﬁnds that the tropical

edge is moving poleward around 0.5–0.7◦decade−1in

WIREs Climate Change The expanding tropics

TABLE 6 Summary of Surface-Based Methodologies

Investigators Data Sources Metric Time Period

Expansion

(◦Latitude Decade−1)

Hu et al.66 S GPCP Subtropical precipitation

minimum

1979–2009 SH: 0.52

NH: 0.71

S NCEP, ERA40, NCEP2,

and HadSLP2

Subtropical ridge location 1979–2009 SH: 0.18

NH: 0.22

Davis and Rosenlof58 S NCEP, JRA25, CFSR, and

MERRA

Subtropical P-E =0 1979–2009 SH:−0.7 to 2.0

NH: 0.0–0.5

Zhou et al.70 S GPCP Subtropical 2.4 mm day−1

isopleth

1979–2007 SH: 0.4

NH: 0.5

S GPCP Subtropical minimum

precipitation

1979–2007 SH: 0.1

NH: 0.0

Allen et al.71 GPCP and WHOI

evaporation

Subtropical P-E =0 1979–2009 SH: 0.31

NH: 0.52

GPCP Subtropical precipitation

minimum

1979–1999 SH: 0.42

NH: 0.31

Positive values equal expansion regardless of hemisphere. An ‘S’ after the investigators indicates the study examined the seasonality of the expansion.

NH, Northern Hemisphere; SH, Southern Hemisphere.

the SH, slightly more in the NH.66 Another indicates

a slower rate of expansion, with slightly more seen

in the SH.71 The third reports little expansion in

either hemisphere using the annual metric, but show

considerable expansion in the NH summer; this study

reports similar results from estimates using the edge

the subtropical dry zone.70 The STR metrics suggest

a poleward trend of around 0.2◦decade−1in each

hemisphere, although there is considerable difference

between datasets.66 Expansion estimates using P-E as

a metric show considerable scatter, particularly in the

SH.58,71 Little consensus exists among these metrics

regarding the strength or timing of the seasonality of

the expansion.

Intensity Changes

Changes to the intensity of large-scale tropical

circulations—both the HC and the zonal Walker

circulation (WC)—have also been reported by several

studies. These changes, while not the main focus of

the article, are useful in interpreting the expansion

results. Table 7 brieﬂy summarizes these ﬁndings.

For the HC, the intensity is often estimated using

the peak value of within the cell. Figure 3 shows

the time series of this metric for eight reanalyses.

In the NH, a signiﬁcant strengthening of the HC

is indicated in ﬁve of eight reanalyses. In the SH,

the results are mixed, with both strengthening and

weakening being identiﬁed in different products; only

four of eight deﬁne a signiﬁcant trend in either

direction. Seasonally the most robust intensiﬁcation

in these data is observed in NH winter and spring

(i.e., DJF and MAM), with the SH and other seasons

producing weaker trends, often of a different sign.21

Other studies85,89 using different approaches generally

support these ﬁndings, particularly the changes to the

strength of the NH winter (DJF) cell. An even stronger

trend in the intensity of the DJF winter cell was noted

in the ERA40 reanalysis.86 A general intensiﬁcation in

the annual average HC intensities in both hemispheres

is suggested by a reanalysis ensemble average; two

exceptions were noted in the SH.80 The interannual

variability of the HC using global radiosonde data

shows no obvious trend in the data.86,87 Satellite

data, in particular OLR and reﬂected shortwave,

were used to infer an intensiﬁcation of the HC.66

The interpretation of these results was questioned,90

and a subsequent investigation91 signiﬁcantly lowered

the estimates of radiative imbalance, a result of

adjustments for satellite altitude errors and instrument

drift. Satellite-derived estimates of water vapor

transport indicate an intensiﬁcation of the HC during

DJF, also suggested by reanalysis data.88

Studies of the trends in WC are contradictory;

decadal variability appears to play a signiﬁcant

role. Weakening trends have been noted in the

NCEP reanalysis85 and an independent historical

sea level pressure dataset.92 However, satellite water

vapor transport observations suggest that the WC is

strengthening since 1979.88 On an interannual basis,

radiosonde estimates87 suggest that the strength of the

HC and WC varied in opposition to one another, an

observation also supported by satellite data.88

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TABLE 7 Summary of the Observed Changes to the Intensity of the Tropical Circulations

Investigator Subject Data Source Comments

Chen et al.67 Broad tropics Satellite shortwave and

longwave ﬂuxes

Intensity shift not quantiﬁed; subtle changes to satellite

radiances suggestive of intensiﬁcation of the HC

and/or WC

Tanaka et al.85 HC NCEP velocity potential DJF winter cell is intensifying. No signiﬁcant trend in JJA

WC WC weakening since (at least) 1979. A strong link the

SOI

Mitas and Clement86 DJF winter cell NCEP/ERA40/NCEP2,

isobaric streamfunction

Trends in peak : 7.5/21.2/1.2 ×109kg s−1decade−1

for NCEP/ERA40/NCEP2

Radiosonde [extended Oort

and Yienger (1996)

series] Ref 87

Trend in peak :0.3×109kg s−1decade−1

GCM multimodel ensemble Modeled trends over the 20th century mostly range −10

to +4×109kg s−1decade−1, suggest weakening

Stachnik and

Schumacher80

HC Eight reanalysis ensemble NH: 4.0 ×109kg s−1decade−1

Ranges from 0.3 to 1.43 ×109kg s−1decade−1

SH: 0.7 ×109kg s−1decade−1

Ranges from −13.9 to +5.4 ×109kg s−1decade−1

Sohn and Park88 DJF HC NCEP, ERA40, and SSM/I Intensiﬁcation of NH winter cell of ∼15–25% since 1979

DJF WC Intensiﬁcation of Paciﬁc WC of ∼25% since

1979/signiﬁcant decadal variability, with decrease

from mid-20th century to 1979

Nguyen et al.21

(see also Figure 3)

HC Eight reanalyses NH: −0.5 to 12.7 ×109kg s−1decade−1

SH: −3.2to9.0×109kg s−1decade−1

Positive values equal expansion regardless of hemisphere. An ‘S’ after the investigators indicates the study examined the seasonality of the expansion.

HC, Hadley circulation; NH, Northern Hemisphere; SH, Southern Hemisphere; WC, Walker circulation.

FIGURE 3|Time series of the peak intensity (vertically-averaged

maximum ) of the Hadley cell in the eight reanalyses for the Northern

(top) and Southern (bottom) Hemispheres. (Reprinted with permission

Time series from ERA40 (post 2002) and 20CR (post 2008) are partially

extrapolated from statistical relations with the other reanalyses .

SOURCES OF OBSERVATIONAL

UNCERTAINTY

As seen in the previous section, considerable

uncertainty exists in the observations. While the

broad consensus of studies suggests that the tropics

are expanding, results from different datasets and

methodologies do not necessarily agree. In this section,

possible sources of this uncertainty are discussed. In

particular, the potential shortcomings of the reanalysis

products are addressed, followed by a description of

some of the limitations associated with the individual

metrics and methodologies.

Reanalysis Considerations

The majority of observational studies of tropical

expansion rely on one or more of the eight available

reanalysis data products, and this potentially has a

wide-ranging effect on the results. These products are a

hybrid of numerical weather prediction model output

and assimilated data from various observational

platforms. They are very useful for climate analysis as

they incorporate historical observations in a physically

WIREs Climate Change The expanding tropics

consistent manner.93 Generally, reanalyses adequately

reproduce many mean ‘dynamic’ quantities like wind

and temperature, albeit with some systematic error.94

Perhaps most importantly, their globally complete

spatial and temporal coverage provides information

in otherwise data-sparse regions.

However, potential shortcomings remain. A

lack of observations may not fully constrain the

reanalysis, and allow possibly unrealistic model

variability to dominate.95 The observational network,

particularly the satellite-based components, has

evolved considerably over the previous decades,

which may introduce inhomogeneities into the

reanalyses.90,96,97 Comparisons of radiosonde and

reanalysis-based estimates of tropical expansion using

the THF methodology62 indicate that inhomogeneities

in reanalysis products may be behind the differences

in estimates of tropical expansion.

Further, differences in the physics of the

underlying models used within the various reanalyses

can result in different representations of the

circulation. The strong HC intensiﬁcation in the

ERA40 was demonstrated to be a result of

a (unrealistic) tropical mid-tropospheric cooling

trend.98 The NH winter cells in the NCEP and

ERA40 reanalyses show signiﬁcant differences, driven

by the differing cloud amounts and radiation budgets

predicted by the two products.99 Newer reanalyses

are not free from these shortcomings; considerable

variability is present in the strength and width of the

HC within the different products.21,80

These issues conceivably account for the

differences observed in tropical expansion trend

estimates across reanalysis products. The differences

in performance are readily apparent in Figures 2 and 3,

where considerable scatter is noted in the position and

strength of the NH and SH Hadley cells. Simpler

metrics, like changes to the mean jet stream position,

are better-captured by the reanalyses and have less

scatter in the estimates. More complex metrics, like

tropopause height, or P-E, are more subject to the

vagaries of the reanalysis models and cover a broader

range, even within a given metric. The scatter in the

estimates is often larger in the SH, where fewer in situ

observations are available and the reanalyses are more

reliant on their internal dynamics and assimilated

satellite data.100 The available evidence, while not

conclusive at this time, indicates that these effects are

potentially quite signiﬁcant and cannot be ignored.

Methodological Concerns

In addition to underlying problems with the

reanalyses, differences in the expansion rates between

the different methodologies and datasets are, at

least to some degree, a manifestation of the

differing physics represented by the different metrics.58

Further, individual datasets and/or methodologies

may introduce their own unique shortcomings. These

are described below.

Tropopause-based methods are the most widely

used and the most widely critiqued methodology

employed to date.57,58 Results using this technique

are broadly consistent across the range of studies,

the initial effort of Seidel and Randel55 being the

exception. The interpretation of the results from this

methodology is relatively straightforward, without

relying on too many assumptions, although seasonal

variability could confound the results. Identifying the

tropopause in the subtropics can be problematic in

the reanalyses, but the use of blended tropopause data

is promising.61 The reanalyses do broadly capture

the interannual variability of TTD, although the

possibility of inhomogeneities and/or other reanalysis

issues cannot be discounted.62 The use of appropriate

relative thresholds57,58 is likely important, but needs

further work to enhance their robustness. The use of a

ﬁxed threshold may account for up to 20% of the total

trend due to global background tropopause trends.62

Satellite-based data are subject to many

uncertainties. In a study of the shifts of the midlatitude

storm track using ISCCP cloud cover data, correction

of data problems signiﬁcantly reduced or even

eliminated the trends in the data.101 Similar issues

may affect estimates of tropical expansion that use

satellite-based metrics and potentially account for the

differences in cloudiness-based estimates of tropical

expansion, where both corrected71 and uncorrected70

data are used.

Satellite estimates using broadband OLR are

a particular concern. The observational data have a

strong trend, while modeling studies102,103 suggest

that net changes to broadband OLR should be

relatively small, on the order of 0.5 K or less. One

potential factor responsible for the trend—overlooked

in tropical expansion studies to date—is a change

in the equatorial crossing time (ECT) of the polar-

orbiting satellites.104 This alters the time of day

when the satellite views a scene and has been

shown to produce signiﬁcant biases, particularly over

land.105–107 Figure 4 explores the possible impact on

tropical expansion calculations. The left panel shows

the uncorrected NOAA OLR data108; expansionary

trends of the 250 W m−2contours of 0.82 and −0.32◦

decade−1are seen in the NH and SH, respectively.

These values are similar to those previously reported

using the same dataset and analysis protocol.66 Using a

dataset that has a statistically based ECT correction107

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FIGURE 4|Time-latitude proﬁles of annually averaged NOAA AVHRR OLR for uncorrected (left) and the equatorial crossing time (ECT)-corrected

data (Ref 107) for the period 1979–2008. Vertical dashed lines indicate the changes in the satellite. Contours are every 10 Wm−2, with the

250 W m−2contour colored in red. The trend along this contour is also indicated with a dashed line.

applied effectively removes any expansionary trends

in the data. While it cannot be absolutely stated

that the statistical correction algorithm has not

removed a valid long-term climate signal, these

results raise questions about the true magnitude of

OLR-based estimates of tropical expansion. Clearly,

more investigation into this ECT bias and its effects

on tropical expansion calculations is warranted.

Further, a physically based model for the

interpretation of the OLR in terms of the ‘edge

of the tropics’ is required, as opposed to a simple

climatological deﬁnition. The expected behavior of

OLR in response to a changing climate is complex.

While changes to overall broadband OLR were small,

changes in speciﬁc spectral regions could be quite

large.103 Changes to the surface, tropospheric and

stratospheric temperatures all play a role, as does the

variability in water vapor, clouds, and aerosol. These

effects vary in different portions of the spectrum,

with sources often making opposite contributions.

Consideration of these effects and the examination of

speciﬁc spectral regions could lead to more reliable

estimates of tropical expansion, and explain any

differences in trends between hemispheres.

At ﬁrst glance, the use of to identify the HC

and the edge of the tropics would appear ideal. It

provides an unambiguous, quantitatively deﬁned deﬁ-

nition of the tropical edge. In practice, these deﬁnitions

are sensitive to the differences in the reanalyses, which

show signiﬁcant variations in the strength, width, and

position of the HC (Figures 2 and 3). Further, the

HC shows a larger degree of seasonality in the trends

than other metrics, particularly in the NH. The HC

during summer and early autumn is generally weak.21

The meridional gradient about the zero-contour is

‘ﬂat’ and as a consequence the identiﬁed position of

the tropical edge is sensitive to small perturbations.

These seasons are when the tropical expansion trends

with this metric are the largest.21,58,65 Conceivably,

even small errors/uncertainties in the reanalyses could

overwhelm the comparatively weak signals at this

time of year and result in an artiﬁcial trend. Temporal

inhomogeneities may also be important here, as

signiﬁcant changes were made to the global observing

system in the early 21st century,51,54 particularly the

addition of new satellite data sources. Conceivably,

these could be behind the SH shift noted in Figure 2.

Further, the seasonal cycle of width deﬁned from is

different than those deﬁned from the jet streams and

θ stability parameter, with different amplitude and

timing.64 More investigation of these effects and their

impact on the results is required.

The jet stream methods from reanalysis data58,81

and radiosonde data71 appear to produce reasonably

robust results. These results indicate very small (and

often not statistically signiﬁcant) amounts of expan-

sion. These studies tend to be based on the annual or

seasonal mean winds; care must be taken to identify

the SH STJ during austral summer.64 Studies based on

maximum winds produce noisier results.58 Jet streams

objectively identiﬁed on daily time scales suggest

a more complex behavior of these phenomena,109

with more than one jet often apparent and a distinct

annual cycle in the position and strength of the jet in

both hemispheres. How well the mean quantities used

in tropical expansion studies represent this complex

behavior and what its impact may be are unknown.

The suitability of the other methodologies that

have been used to assess the rate of tropical expansion

is unclear. In many cases, there are simply not enough

studies to support (or refute) their robustness. The

source of the disparity between results using the

WIREs Climate Change The expanding tropics

GPCP subtropical minimum precipitation is unclear,

and may reﬂect the use of different versions of

the GPCP dataset (version 2.1 vs 2.2). If this is

the case, it suggests a strong sensitivity to the

input data. Further, the use of absolute thresholds70

with this data needs to be carefully considered;

as with the OLR and THF methods, the chosen

threshold should be physically linked with the tropical

edge, beyond just a climatological coincidence. Other

metrics derived from reanalyses (like P-E) are subject

to reanalysis uncertainties, particularly in the data-

poor SH and should be treated with caution until

better understood. The effects of seasonal variability

also need consideration with these methods.

MECHANISMS AND FORCINGS

BEHIND TROPICAL EXPANSION

In this section, the inﬂuences on the position of

the tropical edge are examined. Tropical expansion

and changes to the HC have been examined in

numerous GCMs using paleoclimate, 20th century,

and climate change scenarios. From these simulations,

physical mechanisms have been proposed. Simulations

from simpliﬁed GCMs with idealized forcing are

instructive. Natural climate forcing factors, as seen

through observational studies, can have a signiﬁcant

effect. Modeling also suggests a signiﬁcant role for

anthropogenic inﬂuences. Experiments examining the

role of individual forcings are described; these climate

forcings include greenhouse gases, stratospheric ozone

depletion, and aerosols.

Tropical Expansion in CMIP3 and Other

Simulations

The GCMs used in the World Climate Research

Programme (WCRP) Coupled Model Intercompar-

ison Project phase 3 (CMIP3) multimodel dataset

project110 show signiﬁcant changes to the tropical cir-

culation during the 21st century as a result of increas-

ing greenhouse gas concentrations (generally either the

A1B or A2 scenarios), a poleward expansion of the HC

of 1o–2oover the 21st century along with a decrease

in the intensity of the HC and WC.30,67,111,112 Closely

associated to this are a shift in the subtropical dry zone

and changes to the hydrologic cycle, with wet regions

becoming wetter and dry regions drier.113 Much of

this change is hypothesized to be driven by the ther-

modynamic effects of increasing speciﬁc humidity,

although dynamic changes also allow the poleward

ﬂanks of the HC to dry.114 A recent study115 suggest

that the poleward shifts of the MMC are more respon-

sible for subtropical drying than the thermodynamic

effect. On an annual basis, these projected changes are

largely symmetric about the equator.56

While CMIP3 simulations generally show an

expansion of the tropics, uncertainties remain. The

strength of the modeled HC across the suite of

models varies by a factor of 2.112 Other GCMs

outside of the CMIP3 framework are more equivocal

in their response. Several studies46,116 investigated

the HC in simulations of paleo, present day, and

future climate simulations, covering a wide ‘parameter

space’ in overall temperature, surface characteristics,

topography, and solar forcing. Despite the range,

changes in the characteristics of the HC were noted

between these different eras were generally small.

These results highlight the model dependency in

the results, owing (at least in part) to the various

formulations, parameterizations, and assumptions

made within models. One of the studies116 reports

that the formulation of the ocean model plays a strong

role in determining the ﬁnal characteristics of the HC.

The climate sensitivity of a given model, related to the

strength of the feedbacks within the model, can also

have an impact on future trends in the circulation.117

As noted in the idealized model of the MMC, a

strong interaction exists between the tropics and extra-

tropics. Hence, it seems plausible that variability in the

extratropical circulation has an effect on the tropical

edge (or vice versa). This is consistent with results36

that identify an interannual relationship between

the HC edge and the latitude of the eddy-driven

jet in the SH during summer. Similar relationships

between the HC extent and the annular modes in both

hemispheres have been identiﬁed in most reanalysis

products, although again not in JJA.21 The CMIP3

simulations suggest that tropical expansion is to some

degree a consequence of circulation changes in the

extratropical portion of the MMC, rather than a

driver of those changes. Up to half of the projected

change to the subtropical dry zones can be associated

with changes to the annular mode.118 Midlatitude

dynamics are likely important to the expansion.119

Hadley cell scaling arguments indicate that the extra-

tropical tropopause heights are a better predictor of

modeled tropical expansion than tropical tropopause

heights.56 Overall, the role of tropical–extratropical

interactions in tropical expansion remains uncertain,

but it appears to be signiﬁcant.

Proposed Mechanisms of Tropical

Expansion

Two potential dynamical mechanisms for tropical

expansion have been proposed.29 Both mechanisms

require tropical–extratropical interactions and invoke

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a poleward shift in the position of extratropical jet

stream. The ﬁrst mechanism involves a reduction

in baroclinicity in the subtropics, which dampens

eddy activity there and extends the HC further

poleward. This reduction is a result of an increase

in the (dry) static stability, related to a quasi-moist

adiabatic adjustment to increasing speciﬁc humidity.

A recent study120 suggests that changing wind shear

characteristics may be more important than static

stability changes.

The second mechanism relates to an increase

in the phase speed of upper-tropospheric baroclinic

waves. The faster moving waves are unable to

penetrate as far equatorward, producing a poleward

shift in eddy momentum ﬂux convergence and an

associated shift in the position of the eddy-driven

jet.121,122 This may be related to a rising tropopause

in the subtropics.123 An examination124 of this

mechanism in greater detail suggests that a transition

in the wave type (equatorward- vs poleward-breaking

waves40) may also play a role. Changes to the eddy

length scale may be a factor,125 perhaps in response

to an increase in upper-tropospheric baroclinicity in

the midlatitudes.126

Idealized Simulations

Experiments with simpliﬁed GCMs and idealized

heating proﬁles have been used to explore tropical

expansion and the shift in the eddy-driven jet stream.

Using idealized heating proﬁles reminiscent of those

expected from global warming scenarios, Butler

et al.127 found that both tropical upper-tropospheric

heating and stratospheric cooling resulted in a

poleward shift in midlatitude jet and an expansion of

the HC. Although qualitatively similar in terms of the

jet response, the resulting circulations due to forcings

from the two regions were different. Allen et al.128

performed a suite of simulations using a range of

idealized heating proﬁles, both stratospheric cooling

and tropospheric heating. In their experiments, heat-

ing in the deep tropics resulted in little change in jet

stream and HC, while midlatitude heating produced a

poleward jet shift and expansion of the HC. Tandon

et al.120 examined the circulation response to a series

of tropospheric heating proﬁles of varying widths.

Heating near the equator produced a contraction

of the tropics and poleward shift in the jet stream.

Subtropical heating produced the opposite responses.

While the model simulations provide a degree

of insight into the dynamical mechanisms of tropical

expansion, some uncertainty remains. In the idealized

runs, the results are sensitive to the choice of

initial conditions and the details of the forcing used.

However, the meridional extent of the forcing appears

to be particularly important factor in determining the

response of the HC in these idealized experiments.

Natural Variability

The position of the tropical edge varies on interannual

(and longer) time scales. A primary source of climate

variability is ENSO. Both the width and intensity of

the HC varies with the phase of ENSO.86 During

the warm El Ni ˜

no phase, the HC is narrower

and more intense; during La Ni ˜

na, the opposite is

observed. This general tendency is also suggested in

GCM simulations,29 which also show an equatorial

shift of the STJ during El Ni ˜

no. The various

reanalysis products indicate an intensiﬁcation80 and

contraction21 of the HC during El Ni ˜

no compared

with Neutral and La Ni ˜

na phases in reanalyses.

Using THF metrics, the expansion and variability

were more pronounced in the extratropical portions

of the subtropics, less so toward the deep tropics, with

changes in width from La Ni ˜

na to El Ni ˜

no in the order

of 1◦–2◦latitude.62 The Paciﬁc Decadal Oscillation

(PDO) has also been hypothesized to impact the width

of the tropics. During cool phases of the PDO, the

HC is ∼1◦latitude further poleward compared with

its warm phase position. This is primarily observed

during the equinox seasons.129

Globally signiﬁcant volcanic eruptions can also

have an impact on the tropics. Eruptions can increase

stratospheric aerosol, warming the lower stratosphere

and lowering tropopause height.130 Tropical precip-

itation can be lowered by ∼5%131 and particularly

large eruptions may act as a trigger for El Ni ˜

no.132

A signal of tropical contraction following large

eruptions is apparent, particularly in studies using

tropopause-based methodologies. The magnitude of

this effect is estimated to be ∼1◦latitude.62

Greenhouse Gases and SST

The relative roles of radiative forcing and SST

changes on tropical expansion from 1958 to 2000

have been examined using the THF methodology.56

Expansion trends were accurately replicated by the

model, with more pronounced trends in the SH. The

expansion was attributed to changes in atmospheric

radiative forcing; indirect SST changes had little effect.

Stratospheric cooling was found to be particularly

important in producing the observed trends, although

no distinction between the effects of greenhouse

gases and stratospheric ozone depletion could be

made due to the design of the experiment. Seasonal

changes to SST do not appear to account for tropical

expansion in CMIP3 simulations, suggesting that

WIREs Climate Change The expanding tropics

extratropical eddies have induced the expansion.119

Deser and Phillips133 found that the direct effects of

atmospheric radiative forcing led to a strengthening

and intensiﬁcation of midlatitude westerly winds in

the SH over the latter half of the 20th century. In

the NH, SST changes, particularly over the tropics,

were responsible for the intensiﬁcation of the Aleutian

low and a weakened WC. While the HC was not

speciﬁcally examined, an expansionary trend in the

SH is implied by the poleward shift of the westerlies.

Other studies indicate that as a whole the indirect SST

change is the main driver of circulation change from

both the preindustrial (1870) to the present period

and extending into the future, although factors like

ozone depletion may have a signiﬁcant, but temporary

impact on the hemispheric circulation.134 Hu et al.66

also emphasized that historical SST changes played

a signiﬁcant, but incomplete role in reproducing the

observed expansion.

Stratospheric Ozone Depletion

Numerous studies have suggested that stratospheric

ozone depletion plays a role in tropical expansion

of the SH. For the 20th century, the GCMs in

CMIP3 with ozone depletion consistently showed

greater tropical expansion through a variety of metrics

than those without.135 In future scenarios, GCMs

with ozone recovery showed on average near zero

trends in the tropopause, jet location, HC, and SAM

index, in contrast to runs without ozone recovery.

These results were only qualitative, as details of

the ozone forcing were unavailable. Using seven

reanalyses and a multimodel GCM dataset, optimal

ﬁngerprinting techniques identiﬁed a separable signal

of stratospheric ozone depletion in SH tropical

expansion during DJF, but not in other seasons.136

A multimodel ensemble of 17 climate-chemistry

models (CCM) has been used to understand the

response of the SH circulation to the effects of

explicitly simulated stratospheric ozone depletion.137

This study found that ozone depletion in the late-

spring led to a poleward shift and intensiﬁcation of

the SH tropospheric jet and an expansion of the

HC. These changes to the HC were primarily found

during the SH summer. Similar results were obtained

using a coupled CCM,124 suggesting that the observed

changes in the 20th century were better explained by

stratospheric ozone changes compared to changing

GHG concentrations.

Model simulations using separate ozone and

GHG forcings have also suggested that stratospheric

ozone depletion was the main driver of 20th century

SH climate change. In particular, ozone depletion

resulted in a widening of the HC and an expansion

of the SH subtropical dry zones.138 In the ﬁrst half of

the 21st century, ozone recovery may approximately

cancel the effects of GHG on the MMC, resulting

in a reduced rate of expansion.139 Other models

have suggested that the distinct effects of polar

stratospheric ozone depletion could extend well into

the SH subtropics, resulting in a precipitation increase

at those latitudes.140

Aerosol

Aerosol forcing of the climate is complex, but quite

signiﬁcant. They can directly affect the radiative

ﬂux through scattering (primarily sulfate aerosol)

or absorption (e.g., black carbon), contributing to

surface cooling and/or atmospheric solar heating.141

Semi-direct and indirect aerosol effects may also

impact the climate.142 The semi-direct effect arises

when the heating associated with absorbing aerosols

changes relative humidity, impacting the lifetime

of clouds. The indirect effects of aerosol involve

their interactions with cloud properties, including

increasing their albedo and lifetime and alteration of

their precipitation characteristics. Modeling studies143

suggest that these indirect effects can be approximately

of equal magnitude (but oppositely signed) to the

forcing by GHG. Further complicating the role of

aerosol is their observed regional distribution and

relatively short lifetime.144

Earlier studies have examined aerosol effects

on aspects of the tropical circulation, particularly on

regional scales. The direct effects of increasing black

carbon may intensify the Asian summer monsoon,

while increases of only scattering aerosol weakens the

circulation.145 The indirect aerosol effect may drive

a southward shift in the ITCZ, potentially associated

with past Sahelian drought.146 While aerosol loadings

are regional in scope, they can have circulation impacts

at remote regions around the globe.147,148

Aerosol impacts on the broader MMC have

also been reported. The direct effects of black carbon

included a northward displacement of the ITCZ

and a strengthened (weakened) HC in the Northern

(Southern) Hemisphere.148 Anthropogenic aerosols

(largely absorbing) weaken the HC and shift the peak

streamfunction northward. Natural aerosols (mostly

reﬂecting) had the opposite effects. Opposing shifts

in the STJ were also noted, with anthropogenic

aerosols shifting the jet stream poleward in both

hemispheres.149 The observed shift in the NH tropics

has been attributed to midlatitude heating by black

carbon aerosol and tropospheric ozone. The modeled

response was notably less than that observed in

reality.71 Model simulations that included smoke

Advanced Review wires.wiley.com/climatechange

aerosol from landscape ﬁres showed a widening and

weakening of the HC.150

The response of the tropical circulation to

both direct and indirect aerosol effects has been

modeled.151 Radiative cooling from aerosol is most

prominent in the NH, creating an interhemispheric

radiative imbalance. The atmosphere attempts to

moderate this asymmetry by altering the zonal-mean

circulation in the tropics, resulting in a weaker

(stronger) HC in the Northern (Southern) Hemi-

sphere. The asymmetrical portions of the circulation

(i.e., the WC) strengthen in both hemispheres. In

the zonal-mean, the subtropics (deﬁned here as

20◦–30◦latitude) of both hemisphere become wetter.

However, the authors conclude that the atmospheric

responses may be overestimated because of a lack of

an interactive ocean model in these simulations.

SYNTHESIS

A variety of methodologies have been used to examine

tropical expansion. Most studies begin at the start of

the satellite era (1979) and extend into the ﬁrst decade

of the 21st century. Figure 5 summarizes the rates

of expansion estimated by these studies, stratiﬁed

into the methodological categories discussed earlier.

The numerous studies indicate a robust poleward

expansion of the tropical edges of both hemispheres

over the last ∼30 years. Almost all studies suggest that

the rate of expansion in each hemisphere has been less

than 1◦decade−1, with many suggesting rates around

half of that value. Many of these widening trends

may not be statistically signiﬁcant at the 95% level.58

Tropopause-based studies generally suggest a greater

widening in the SH; satellite-based studies indicate

the opposite. Studies using suggest a greater

seasonality and an (highly uncertain) intensiﬁcation

of the HC. As a whole, jet stream metrics suggest a

generally smaller rate of expansion. Taken together, a

total expansion of 1◦–3◦latitude in each hemisphere

is implied since 1979.

The seasonality of the changes, if any, is unclear.

Some metrics, notably the -based ﬁndings, show

strong seasonality with the largest changes in each

hemisphere’s respective summer and autumn.21,65

Jet stream metrics based on the position of the

peak zonal-mean 850 hPa zonal wind show notable

seasonal differences in the SH.58 Seasonal variability

is suggested in other metrics,58,70 but many of these

differences are not statistically signiﬁcant.58 As noted

earlier, some of the metrics, particularly during the

warm season, may not produce robust results.

Modeling studies suggest that multiple climate

forcings can result in an expansion of the

FIGURE 5|Summary of result of observational tropical expansion

studies, broken down by categories. Pluses (+) represent SH values,

crosses (×) NH values of tropical expansion trend. Symbols in the

corresponding hemisphere indicates expansion, symbols in the

‘opposite’ hemisphere indicate contraction. Within a particular

methodology, the vertical position of each symbol is randomized to

improve clarity of individual symbols; no other meaning is implied.

Colors of symbols refer to source and/or methodology, with legends for

each category in boxes to side of plot, where numbers represent the

cited reference.

tropics, including increasing GHG concentrations and

associated changes to the SST, stratospheric ozone

depletion (particularly in the SH) and the effects

of aerosols, both direct and indirect. Each of these

forcings results in subtly different impacts on the

tropical edge, as summarized in Table 8. A comparison

of these projections with the relevant climatological

variables can provide a qualitative assessment of the

relative importance of these factors.

Tropical expansion is noted in both the NH

and the SH, suggesting some degree of symmetry in

the response. This suggests that the observed increase

in speciﬁc humidity (water vapor),152–154 associated

with the increase in GHG and global temperature, is

playing a role in tropical expansion. However, the

observations of HC intensity, while highly uncertain,

suggest no trend or a slight intensiﬁcation, which

is inconsistent with this response. Some studies111,151

suggest that this thermodynamic effect may impact the

zonal WC more than the HC. Most studies, but not all,

report that the WC has weakened in recent decades,

although internal climate variability may account for

a signiﬁcant fraction of the observed weakening.155

During the 20th century, zonal-mean precip-

itation has increased in the SH tropics and sub-

tropics, while decreasing in the NH.156 Coupled

climate models indicate that these changes are a result

of anthropogenic radiative forcing, with GHG and

sulfate aerosol forcing speciﬁcally noted. Kang et al.140

also report a similar tendency in the subtropical

WIREs Climate Change The expanding tropics

TABLE 8 Summary of Tropical Expansion Projections Following Different Forcings

Forcing Citations Projected Effects

A2, A1B (many contain ozone, but

generally O3 recovery when

evaluated)

Lu et al.,56 Seager et al.114 Expansion rate: 0.1–0.2◦decade−1

Symmetric about equator

Weakening and expansion of HC

Reduction of precipitation in subtropics

Enhancement of rain in ITCZ/monsoon

Poleward shift in jet

Stratospheric ozone depletion Son et al.,135 Son et al.,137 Polvani

et al.,139 Kang et al.,140 Staten et al.,134

Min and Son136

Greater impact in SH spring and summer

SH subtropical precipitation increase

HC widening and poleward shift in SH

Direct aerosol (sulfate) Wang,148 Allen et al.71 Strengthened HC

Equatorward shift in jet stream

Direct aerosol (black carbon) Wang,148 Allen et al.71 Weakening of HC

Northward shift in ITCZ

Poleward shift of jet stream

Indirect aerosol Rostayn and Lohmann,146 Rotstayn

et al.,147 Ming and Ramaswamy151

Asymmetric response—intensiﬁcation of SH

HC, weakening in NH

Precipitation increase in subtropics

Southward shift in ITCZ

Sea surface temperature Hu et al., 2011,65 Deser and Phillips,133

Staten et al.134

Poleward shift in jet stream

precipitation in the SH, but attribute the increase

to stratospheric ozone depletion. As noted in earlier, a

considerable amount of modeling evidence exists for a

signiﬁcant role of stratospheric ozone depletion in pro-

ducing SH tropical expansion, although these changes

are largely conﬁned to the SH spring and summer

seasons. Some degree of SH asymmetry in expansion

rates is observed in tropopause-based metrics, but not

in others (Figure 5).

Aerosols, particularly from volcanoes, clearly

play a role in tropical expansion on interannual

time scales, but longer term effects are unlikely.

The asymmetric expansion (in some results) and

precipitation response is reminiscent of the indirect

aerosol effect,143 a forcing that has not been included

in many climate simulations to date, including the

CMIP3 archive. However, the evidence for this factor

is unclear; little change has been observed in either a

proxy for the global ITCZ21 or in the satellite-detected

eastern Paciﬁc ITCZ157 and the apparent HC

intensiﬁcation has occurred in both hemispheres, more

in the NH. The role of direct aerosol effects is also

incomplete; while Allen et al.71 explicitly attribute NH

expansion to black carbon aerosol and tropospheric

ozone, the projected response is considerably less that

the observed increase.

The dynamical mechanisms behind tropical

expansion are unclear, but many hypotheses proposed

to date29,122,124 indicate that the extratropics and

baroclinic eddies play a signiﬁcant role. One

manifestation of this is a poleward shift in the

storm track, closely associated with the position of

the eddy-driven jet stream. Yin158 notes this as a

response to climate change in simulations of the 21st

century. Bender et al.101 identiﬁed such a shift in

recent decades, albeit with some reservations due

to data issues. Overall, a shift in the storm track

results in less precipitation over subtropical regions,

with this mechanism possibly more important than

the thermodynamic effect.115 Other modeling studies

have indicated a close linkage of the jet position

and the HC.135,137 Relationships between the HC

edge (identiﬁed using the methodology) and the jet

stream position have been identiﬁed for some seasons,

particularly in the SH.21,36 A poleward shift in the

jet stream is also closely related to poleward shift in

the annular mode. Up to half of the projected change

to the subtropical dry zones may be associated with

changes to the annular mode.118

To summarize, no clear consensus exists

regarding the primary forcing mechanism behind

tropical expansion. Any singular forcing generally

results in an under-prediction of the observed

amount of tropical expansion.79 This may reﬂect

a shortcoming of the modeling approaches used

to date; for example, the models simply are not

correct or sensitive enough to reproduce the observed

changes. Certainly, given the varied response of the

Advanced Review wires.wiley.com/climatechange

different models in play, this suggestion cannot be

dismissed out-of-hand. However, it may be that

tropical expansion simply does not have a single cause.

Rather, it may arise from a variety of causes that have

acted in concert to produce the observed response,

with different factors acting in the NH and SH. Other

factors not thoroughly considered to date by modeling

may also play a role. The atmosphere is complex, and

an attempt to attribute the whole change to a single

forcing may not be appropriate.

OUTSTANDING ISSUES

As summarized in this review, considerable efforts

have been made within the climate community

to understand the causes and effects of tropical

expansion. From this research a coherent picture is

emerging, but many gaps remain in our knowledge.

These are brieﬂy summarized below.

A key issue is the determination of the true rate

of tropical expansion. A broad consensus from current

studies suggests the value is below 1◦decade−1, but

which hemisphere is expanding at a faster rate (if

either) remains unclear. Also unclear is the seasonality

of the expansion, with signiﬁcant differences identiﬁed

between the various methodologies. Developing a

clearer picture requires improving our understanding

and interpretation of current methodologies used to

estimate tropical expansion, as well as the formulation

of new ones. Clarifying the linkages between different

metrics is crucial. For example, do THF methods tell

us the same thing as methods? Addressing data

quality issues through homogenized datasets and/or

reanalysis products is paramount.

One particular shortcoming in our understand-

ing is the role of tropical–extratropical interaction

in the MMC. To what degree does tropical expan-

sion arise from changes in the extratropical circu-

lation? Several studies discussed herein suggest that

the changes to the extratropics may be the source

of the tropical expansion. Traditionally in the atmo-

spheric sciences, the tropics, and extratropics have

been treated as separate entities. To overcome this,

a new conceptual framework of understanding for

the hemispheric (and global) MMC is required. The

idealized model shown earlier provides a qualitative

description of such a framework, a useful starting

point.

Another signiﬁcant issue is the understanding of

the climate factors responsible for tropical expansion.

This includes not only identifying the most signiﬁcant

climate forcings, but also the dynamical mechanisms

by which they act. This requires the ongoing efforts

of model development and evaluation, as well as

observational studies to propose new hypotheses and

verify the results. A sharper focus on the seasonality

and/or longitudinal variability of the expansion

could improve the understanding of the hypothesized

forcings. The current understanding of many potential

forcings (e.g., the role of aerosol in the climate)

remains uncertain. It is helpful to understand the trop-

ical expansion observed to date, which can then lead

to a better interpretation of projected climate futures.

Finally, a better understanding of the impacts

of tropical expansion on the climate and society is

required. Much of the research to date has focused

on the global or zonal-mean responses to expansion,

e.g., the subtropical dry zone will expand. Some

studies have identiﬁed this as a factor in recently

observed SH rainfall declines.159 However, questions

remain about what this means at a given location

in the subtropics. For instance, GCM simulations

often indicate a signiﬁcant regional variability in the

response. The reality of the MMC is more complicated

than that presented in a simple zonal-mean picture.

For purposes of planning and climate adaptation, a

better understanding of regional responses to tropical

expansion is required.

ACKNOWLEDGMENTS

This research was funded as part of the South Eastern Australia Climate initiative Phase 2 (SEACI 2).

Interpolated OLR data were provided by the NOAA/OAR/ESRL PSD, Boulder, CO, USA, from their Web site

http://www.esrl.noaa.gov/psd/. The ECT-corrected version of the same dataset was provided by Matt Sapiano.

Tim Cowan, Pandora Hope, and two anonymous reviewers provided helpful comments on the manuscript.

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Environmental Sustainability and Climate Change Changing Patterns of Long-term Climatic Elements and Efficiency Levels of Adaptation Strategies Adopted by smallholder Farmers in Edo North, Nigeria

Article

Full-text available

Jan 2022

Understanding the long term patterns and trends in climatic variables in relations to their effects on farming operations and community-based adaptation techniques employed by smallholder farmers in minimizing the negative effects of climatic variability, is a perquisite towards achieving food security and key SDGs. Thus this study aimed to examine the long-term trends and patterns of agro-climatic variables in Edo north and efficient of existing climate change adaptation strategies employed by mainly smallholder farmers. Datasets used include minimum temperature (TMin), maximum temperature (TMax), rainfall, soil moisture (SM) and potential evapotranspiration (PET). The dataset were assembled on monthly basis and spanned across 119 years starting from SM. These dataset were sourced from the University of East Anglia-Climatic Research Unit, the National Aeronautical and Space Administration Goddard Space Flight Center along with the University of Maryland. Primary data was collected from field survey through questionnaire and oral interviews. Results of long-term annual distribution of climatic variables revealed marked variations in TMin, TMax, and PET over time, while rainfall and SM showed no statistical significant changes. Of all the climatic variables investigated, only rainfall exhibited a rising trend. The study found that the three most adaptation strategies deployed by farmers were the use improved crop varieties (WMS = 4.51), application of early maturing plants (WMS = 4.49) and the use of intensive fertilizer and/or manure application for crop production (WMS = 4.48). The fact that other adaptation strategies are not widely employed in the study area, maybe attributed to low level of rural infrastructures, high poverty level and illiteracy etc. There therefore need for the formulation of climate change adaptation workable policy, programme development/implementation that is geared towards massive rural infrastructure transformations and access to extension services.

Estado del conocimiento de los efectos del cambio climático en el Océano Atlántico Sudoccidental sobre los recursos pesqueros y sus implicancias para el manejo sostenible

Technical Report

Full-text available

Dec 2022

Este documento es una novedosa recopilación de información científica sobre el impacto del cambio climático en el Mar Argentino. Es el primer informe nacional de estas características y un insumo fundamental para la formulación de políticas públicas y metas y objetivos de gestión

Estado del conocimiento de los efectos del cambio climático en el Océano Atlántico Sudoccidental sobre los recursos pesqueros y sus implicancias para el manejo sostenible

Technical Report

Full-text available

Dec 2022

El cambio climático es un fenómeno a escala global, con efectos evidentes que repercuten sobre los diversos cuerpos de agua provocando migraciones de especies, incremento en el nivel del mar, y aumento de la frecuencia e intensidad de condiciones climáticas extremas. En línea con lo expresado por la FAO en múltiples ámbitos y publicaciones, es posible afirmar que estos cambios generan impactos continuos a nivel ambiental, social y económico. La pesca y la acuicultura en general, y particularmente la pesca y la acuicultura de pequeña escala son actividades particularmente vulnerables a los efectos del cambio climático. Por ello, es necesario desarrollar políticas y acciones que contribuyan a mitigar los cambios y a acelerar la adaptación del sector a una realidad desafiante. En este sentido, la carencia periódica de agua y la alteración de sus parámetros de calidad, las sequías y las variaciones en los patrones de las temperaturas guardan correlación con el cambio climático, y son parte de un abanico mucho más amplio de situaciones que afectan a los sistemas productivos, y de cuya evolución depende el equilibrio natural en los lagos, ríos y mares. Por otra parte, la confluencia de factores naturales y antrópicos en los fenómenos que se están sucediendo, indican que resulta esencial tener presente que los impactos asociados a la variabilidad natural del clima pueden verse intensificados y su frecuencia incrementada debido al cambio climático. El escenario planteado hace necesaria una profunda innovación y adaptación de las políticas públicas, las legislaciones, y sus instrumentos de gestión asociados, para dar lugar a acciones capaces de mitigar eficazmente los efectos del cambio climático. Este proceso deberá originarse en la recolección, sistematización y análisis de los datos que puedan resultar relevantes, y deberá orientarse hacia la concientización sobre la naturaleza y profundidad de la problemática que se aborda. Asimismo, es imperioso consolidar un sistema de monitoreo que permita detectar con la mayor antelación posible los cambios ambientales y las variaciones en las poblaciones de recursos acuáticos. En línea con lo expuesto, el presente trabajo se centra en la investigación y recopilación de información científica sobre el cambio climático y sus consecuencias sobre la actividad pesquera. El mismo fue realizado por un grupo interdisciplinario de expertos, y constituye el primer avance para aportar información de calidad a los tomadores de decisión, fundamentalmente a quienes tienen la responsabilidad de estructurar e implementar políticas públicas. De hecho, resulta impensable la posibilidad de una planificación estratégica del sector pesquero y acuícola que no contemple la variabilidad ambiental, así como su incidencia y consecuencias. Por ello, este trabajo resulta un aporte fundamental para la formulación de metas y objetivos de gestión, tomando en cuenta la dificultad propia de las determinaciones relacionadas con los recursos acuáticos, así como el desafío que implica asegurar la sustentabilidad del sector, incluyendo en el análisis situacional el impacto de la reciente pandemia, que modificó la actividad normal de las flotas y la explotación de los distintos recursos a lo largo del litoral argentino. En un escenario marcado por la incertidumbre, una planificación certera aunada al compromiso y el esfuerzo mancomunado del sector público y privado, ha demostrado ser una senda virtuosa que se traduce en una productividad que ya supera los niveles anteriores a la pandemia. Sin dudas, esta capacidad de adaptación y cooperación debe dar lugar al sano orgullo por el desenvolvimiento alcanzado. Finalmente, este primer informe nacional sobre las implicancias del cambio climático en las pesquerías argentinas constituye la primera evaluación integral del conocimiento sobre cambio climático y pesquerías en el mar argentino. Su génesis y desarrollo implica la necesidad de una actualización permanente. Esto nos obliga a mantener un vínculo estrecho de colaboración e x intercambio con los distintos investigadores e instituciones públicas y privadas, cuya participación y compromiso desinteresado los hacen merecedores de un especial reconocimiento.

Simulation and Projection of Tropical Cyclone Activities over the Western North Pacific by CMIP6 HighResMIP

Article

Full-text available

Nov 2022
J CLIMATE

This study assesses the relative impacts of model resolutions, tropical cyclone (TC) trackers, and ocean coupling on simulating TC climatology over the western North Pacific (WNP) based on six Coupled Model Intercomparison Project phase 6 (CMIP6) High-Resolution Model Intercomparison Project (HighResMIP) models from 1979 to 2014. The HighResMIP multimodel ensemble (MME) analysis shows that the high resolution has a higher Taylor skill score II (S 2) in both temporal and spatial patterns of TC genesis frequency and accumulated cyclone energy (ACE) than the low resolution. In contrast, the TempestExtremes tracker (coupled run) results in a higher S 2 in temporal patterns but a lower S 2 in spatial patterns than the TRACK tracker (uncoupled run). Among the three factors, increased resolution leads to the greatest improvement in S 2 in both temporal and spatial patterns. Furthermore, this study investigates the projections of future TC activity over the WNP by HighResMIP under the SSP5-8.5 scenario. Overall, HighResMIP MMEs project a decrease in the genesis frequency, track density, and ACE of all TCs, with the high-resolution, TRACK tracker, and un-coupled run showing greater magnitude. The high-resolution MMEs, using both trackers, project an increase in the genesis frequency and ACE of intense TCs in the coupled run. Moreover, TC track density and ACE show a larger poleward migration in the coupled run than in the uncoupled run, consistent with the significant surface warming in the northern WNP.

ATAScongress2020

Conference Paper

Full-text available

Sep 2020

Alice Soccodato

Sustainable Management of biodiversity and Fisheries Based on EO Data

Strengthening of the maritime influence on the Balkans in summer as a result of changes in atmospheric circulation

Article

Full-text available

Sep 2022
CLIM DYNAM

Peter Nojarov

This study focuses on the strengthening of the maritime influence on the Balkan Peninsula in summer. The study uses mainly statistical methods such as trend analysis, multiple linear regression models, etc. The research period is 1979–2020. The results show that in the beginning of the twenty-first century in southeastern Europe there is a tendency of faster increase in the average August air temperatures compared to the increase in average July air temperatures. Thus, the temperature in August is already higher than that in July. The causes for these changes are changes in atmospheric circulation in summer. Over the last two decades, the transport of air masses from east and southeast has been strengthening, which for the Balkan Peninsula means a transport from water basins toward land. The intra-annual course of air temperature above a water basin, which has a maximum in August, is now becoming typical over land. This feature is not yet present in the entire studied region, but the trends from the measurements in meteorological stations show that very soon the temperatures in August could be higher than that in July over the entire territory of the Balkan Peninsula. On a more global scale, it is still unclear which mechanisms of atmospheric circulation influence the processes in southeastern Europe. Changes in ocean temperatures and atmospheric circulation in the North Atlantic are likely to have an impact. However, other factors (e.g. the increase in global temperatures, the expansion of Hadley cell) may also have an impact.

Opposing Shifts of the Hadley Cell Edge and Eddy-Driven Jet Latitude in the Last Glacial Maximum: A Parameter Sweep Study Using a Dynamical Core GCM

Article

Feb 2023

A coherent poleward displacement of the Hadley cell (HC) edge and eddy-driven jet latitude has been well documented in the Southern Hemisphere (SH) under the present and future climate changes. However, a recent study showed that during the Last Glacial Maximum (LGM) winter, an equatorward shift of the HC edge but a poleward shift of the jet latitude are found in the SH. These opposing circulation changes are investigated in this study by conducting the parameter sweep experiments using a dynamical core general circulation model (GCM). By systematically varying the amplitude of tropical upper-tropospheric and polar surface cooling, mimicking the LGM-like climate state, an opposing shift of circulation is reproduced when polar cooling is much stronger than tropical cooling. This is due to the higher sensitivity of the jet-latitude change compared to the HC-edge change in response to polar cooling. Eddy cospectra analysis reveals that the poleward jet shift is dominated by fast waves as the baroclinic zone expands poleward with increasing polar surface cooling. Instead, the HC-edge change is largely attributed to the activity of slow waves and the axisymmetric circulation change. They lead to the HC edge being weakly influenced by extratropical baroclinicity, resulting in an equatorward shift of the HC edge under a global cooling–like condition. Similar circulation changes but with an opposite sign are also found in global warming–like experiments. This result suggests that a poleward HC shift together with an equatorward jet shift can occur even in a future climate if Arctic amplification is accelerated relative to tropical warming.

A rare episode of minor circulation embedded in the northern hemispheric zonal mean hadley cell

Article

Feb 2023
J ATMOS SOL-TERR PHY

Can southern Australian rainfall decline be explained? A review of possible drivers

Article

Full-text available

Jan 2023

of observed seasonal trends in southern Australia rainfall and associated climate drivers during 1979‐2019, based on a literature survey. Inner rings indicate decline (orange) or increase (green) in rainfall (R), including rainfall from thunderstorms (TS), and from cutoff lows and fronts (CFL). Outer rings indicate a positive (red) or negative (blue) trend in indices of the large‐scale atmospheric circulation: Hadley Cell extent (HCE); subtropical ridge intensity (STRI); subtropical jet (STJ), or large‐scale climate drivers: Southern Annular Mode (SAM); Indian Ocean Dipole (IOD); El Niño Southern Oscillation (ENSO). Hatching indicates trend confidence and neutral colours are used when no trend is found. A consistent rainfall change story is revealed when a change in rainfall aligns with the wetting (W) or drying (D) typically favoured by the changes the large‐scale circulation and drivers in the outer rings. In this way, we see that winter rainfall decline in southern Australia is consistent with several changing climatic factors, including decreases in rainfall from weather systems, a strengthening of the subtropical ridge, a poleward shift in the Hadley Cell extent, and the positive SAM and IOD trends in that season. Conversely, rainfall changes in other seasons are less robust. In particular, southern Australian summer rainfall has no clear trend, despite wetting favoured by changes in some of the large‐scale circulation features and drivers. In each season, inconsistencies persist between changes in southern Australian rainfall, weather systems, large‐scale circulation, and large‐scale drivers, highlighting gaps in our understanding of atmospheric dynamics and climate change processes. To better understand how rainfall has changed, and may continue to change, future research should focus on reconciling these inconsistencies. image

Dipole response of early‐summer rainfall in eastern China to 1.5°C and 2.0°C global warming

Article

Sep 2022

The 2015 Paris Agreement set the goal of pursuing efforts to keep global warming to 1.5°C above the pre‐industrial level. It remains unclear how the early‐summer rainfall in China responds to moderate warming. Here, we investigate the changes in June rainfall in eastern China by 1.5°C and 2.0°C warming using the ensemble projection from Half a degree additional warming, prognosis and projected impacts (HAPPI) dataset. Results reveal a dipole response in the June rainfall with the significant increase in southeastern China but a slight decrease in northeastern China at 1.5°C and 2.0°C warming compared to the present day. The moisture budget analysis indicates that the dipole rainfall response is caused predominantly by the dynamic response in the anomalous circulation and in turn moisture transport. Compared to the present day, the upper‐tropospheric temperature is significantly enhanced in the tropics but decreased in middle latitudes at the 1.5°C and 2.0°C warming. The enhanced poleward temperature gradient strengthens zonal wind on the south of the East Asian westerly jet, leading to a southward displacement of jet. As a result, an anomalous upward motion is reinforced over the north of 30°N with an anomalous downward motion over the south of 30°N, leading to a dipole rainfall anomaly in eastern China. In addition, the warming also induces a thermodynamic response with increased moisture and in turn rainfall across eastern China, further enhancing rainfall in southeastern China, but weakening the decreased rainfall in northeastern China. This article is protected by copyright. All rights reserved.

A High-quality Historical Humidity Database for Australia

Technical Report

Full-text available

Jul 2010

Chris Lucas

methodology and evaluation of a homogenized dewpoint database for Australia from 1957 through 2003

Measurements of the movement of the jet streams at mid-latitudes, in the Northern and Southern Hemispheres, 1979 to 2010

Article

Full-text available

Aug 2012

R. D. Hudson

Previous studies have shown that the mean latitude of the sub-tropical jet streams in both hemispheres have shifted toward the poles over the last few decades. This paper presents a study of the movement of both the subtropical and Polar fronts, the location of the respective jet streams, between 1979 and 2010 at mid-latitudes, using total ozone measurements to identify the sharp horizontal boundary that occurs at the position of the fronts. Previous studies have shown that the two fronts are the boundaries of three distinct regimes in the stratosphere, corresponding to the Hadley, Ferrel, and polar meridionally overturning circulation cells in the troposphere. Over the period of study the horizontal area of the Hadley cell has increased at latitudes between 20 and 60 degrees while the area of the Polar cell has decreased. A linear regression analysis was performed to identify the major factors associated with the movement of the subtropical jet streams. These were: (1) changes in the Tropical land plus ocean temperature, (2) direct radiative forcing from greenhouse gases in the troposphere, (3) changes in the temperature of the lower tropical stratosphere, (4) the Quasi-Biennial Oscillation, and (5) volcanic eruptions. The dominant mechanism was the direct radiative forcing from greenhouse gases. Between 1979 and 2010 the poleward movement of the subtropical jet streams was 3.7 ± 0.3 degrees in the Northern Hemisphere and 6.5 ± 0.2 degrees in the Southern Hemisphere. Previous studies have shown that weather systems tend to follow the jet streams. The observed poleward movement in both hemispheres over the past thirty years represents a significant change in the position of the sub-tropical jet streams, which should lead to significant latitudinal shifts in the global weather patterns and the hydrologic cycle.

Global impact of smoke aerosols from landscape fires on climate and the Hadley circulation

Article

Full-text available

May 2013

Each year landscape fires across the globe emit black and organic carbon smoke particles that can last in the atmosphere for days to weeks. We characterized the climate response to these aerosols using an Earth system model. We used remote sensing observations of aerosol optical depth (AOD) and simulations from the Community Atmosphere Model, version 5 (CAM5) to optimize satellite-derived smoke emissions for high biomass burning regions. Subsequent global simulations using the adjusted fire emissions produced AODs that were in closer agreement with surface and space-based measurements. We then used CAM5, which included radiative aerosol effects, to evaluate the climate response to the fire-aerosol forcing. We conducted two 52 yr simulations, one with four sets of monthly cycling 1997–2009 fire emissions and one without. Fire emissions increased global mean annual AOD by 10% (+0.02) and decreased net all-sky surface radiation by 1% (1.3 W m−2). Elevated AODs reduced global surface temperatures by 0.13 &pm; 0.01 °C. Though global precipitation declined only slightly, patterns of precipitation changed, with large reductions near the Equator offset by smaller increases north and south of the intertropical convergence zone (ITCZ). A combination of increased tropospheric heating and reduced surface temperatures increased equatorial subsidence and weakened the Hadley circulation. As a consequence, precipitation decreased over tropical forests in South America, Africa and equatorial Asia. These results are consistent with the observed correlation between global temperatures and the strength of the Hadley circulation and studies linking tropospheric heating from black carbon aerosols with tropical expansion.

Climate of the Past Precipitation changes in the South American Altiplano since 1300 AD reconstructed by tree-rings

Article

Full-text available

Mar 2012
CLIM PAST

Throughout the second half of the 20th century, the Central Andes has experienced significant climatic and environmental changes characterized by a persistent warming trend, an increase in elevation of the 0 degrees C isotherm, and sustained glacier shrinkage. These changes have occurred in conjunction with a steadily growing demand for water resources. Given the short span of instrumental hydroclimatic records in this region, longer time span records are needed to understand the nature of climate variability and to improve the predictability of precipitation, a key factor modulating the socio-economic development in the South American Altiplano and adjacent arid lowlands. In this study we present the first quasi-millennial, tree-ring based precipitation reconstruction for the South American Altiplano. This annual (November-October) precipitation reconstruction is based on the Polylepis tarapacana tree-ring width series and represents the closest dendroclimatological record to the Equator in South America. This high-resolution reconstruction covers the past 707 yr and provides a unique record characterizing the occurrence of extreme events and consistent oscillations in precipitation. It also allows an assessment of the spatial and temporal stabilities of the teleconnections between rainfall in the Altiplano and hemispheric forcings such as El Nino-Southern Oscillation. Since the 1930s to present, a persistent negative trend in precipitation has been recorded in the reconstruction, with the three driest years since 1300 AD occurring in the last 70 yr. Throughout the 707 yr, the reconstruction contains a clear ENSO-like pattern at interannual to multidecadal time scales, which determines inter-hemispheric linkages between our reconstruction and other precipitation sensitive records modulated by ENSO in North America. Our reconstruction points out that century-scale dry periods are a recurrent feature in the Altiplano climate, and that the future potential coupling of natural and anthropogenic-induced droughts may have a severe impact on socio-economic activities in the region. Water resource managers must anticipate these changes in order to adapt to future climate change, reduce vulnerability and provide water equitably to all users.

Climate Change: Observed Impacts on Planet Earth: Second Edition

Book

Jan 2015

Trevor Melvyn Letcher

Earth's climate is always changing. As the debate over the Earth's climate has grown, the term "climate change" has come to refer primarily to changes we've seen over recent years and those that are predicted to be coming, mainly as a result of human behavior. Climate Change: Observed Impacts on Planet Earth, Second Edition, serves as a broad, accessible guide to the science behind this often political and heated debate by providing scientific detail and evidence in language that is clear to both the climatologist and the non-specialist. The book contains 35 chapters on all scientific aspects of climate change, written by the world's authority of each particular subject. It collects the latest information on all of these topics in one volume. In this way, readers can make connections between the various topics covered in the book, leading to new ways of solving problems and looking at related issues. The book also contains major references and details for further research and understanding on all issues related to climate change, giving a clear indication of a looming crisis in global warming and climate change. Provides an up-to-date account of the current understanding of climate change and global warming. Includes 23 updated chapters and 12 new chapters. Includes coverage on modeling climate change, geological history of climate change, and on engineering aspects of climate change. Written by the world's leading experts on the issues related to climate change.

The NCEP/NCAR reanalysis 40-year project

Article

Jan 1996
B AM METEOROL SOC

Removing satellite equatorial crossing time biases from the OLR and HRC datasets

Article

Sep 1997

The objective of this study is to examine the impacts from satellite equatorial crossing time (ECT) changes on the outgoing longwave radiation (OLR) and highly reflective cloud (HRC) datasets and to design appropriate and robust methods to remove these satellite-dependent biases. The OLR record covers the period from June 1974 to July 1996 and is on a 2.5° grid extending from 30°S to 30°N over the global Tropics. The HRC record covers the period from January 1971 to December 1987 and is on a 2° grid extending from 25°S to 25°N over the global Tropics. Rotated empirical orthogonal function analysis (REOF) is performed on both the monthly OLR and HRC anomalies to help distinguish between artificial modes of variability and those associated with real variability. Results from the analysis show that significant errors are introduced by changes in the satellite ECT, and they appear differently in the two datasets. The primary satellite-related bias in the OLR appears as the fourth REOF mode, which accounts for 4.4% of the OLR anomaly variance. Its spatial pattern exhibits a strong surface signature over land, with the opposite sign over many of the deep convective regions of the ocean. During some periods, these biases result in widespread errors of over 10 W m-2, which are sustained for several months to over a year. In other cases, the transition between satellites induces abrupt, artificial changes in the OLR as high as 16 W m-2. In the HRC, the satellite-related bias appears as the leading two REOF modes, which account for 13.1% of the HRC anomaly variance. The spatial patterns of the HRC biases are indicative of an overall change in the mean climatological convection pattern. The above results can be understood by considering the sampling and radiometric characteristics of the OLR and HRC datasets. To remove the satellite ECT biases, the REOF time series of the satellite-related modes are modified by using the detailed knowledge of the satellite ECTs so that only artificial variability related to the satellite changes is captured and the natural variability is excluded. These modified time series are used in conjunction with their associated spatial patterns to compute the satellite-related artificial variability, which is then removed from the two datasets. These datasets provide an improved resource to study intraseasonal and longer timescale regional climate variations, large-scale interannual variability, and global-scale climate trends. Analyses of the long-term trends in both datasets show that the satellite biases induce artificial trends in the data and that these artificial trends are reduced in the corrected datasets. Further, each of the corrected datasets exhibits a trend in the tropical western-central Pacific that appears spatially independent of the satellite biases and agrees with results of previous studies that indicate an increase in precipitation has occurred in this region over the period encompassed by these datasets.

The WCRP CMIP3 multi-model dataset: A new era in climate change research

Article

Jan 2007
B AM METEOROL SOC

Open access to an unprecedented, comprehensive coordinated set of global coupled climate model experiments for twentieth and twenty-first century climate and other experiments is changing the way researchers and students analyze and learn about climate. The history of climate change modeling was first characterized in the 1980s by a number of distinct groups developing, running, and analyzing model output from their own models with little opportunity for anyone outside of those groups to have access to the model data. This was partly a consequence of relatively primitive computer networking and data transfer capabilities, along with the daunting task of collecting and storing such large amounts

Western Australia's Weather and Climate: A Synthesis of Indian Ocean Climate Initiative Stage 3 Research

Article

Jun 2013

Lachie Mccaw

A Consistent Poleward Shift of the Storm Tracks in Simulations of 21st Century Climate

Article

Sep 2005
GEOPHYS RES LETT

Jeffrey H. Yin

A consistent poleward and upward shift and intensification of the storm tracks is found in an ensemble of 21st century climate simulations performed by 15 coupled climate models. The shift of the storm tracks is accompanied by a poleward shift and upward expansion of the midlatitude baroclinic regions associated with enhanced warming in the tropical upper troposphere and increased tropopause height. The poleward shift in baroclinicity is augmented in the Southern Hemisphere and partially offset in the Northern Hemisphere by changes in the surface meridional temperature gradient. The poleward shift of the storm tracks also tends to be accompanied by poleward shifts in surface wind stress and precipitation, and a shift towards the high index state of the annular modes. These results highlight the integral role that the storm tracks play in the climate system, and the importance of understanding how and why they will change in the future.

The expanding tropics: A critical assessment of the observational and modeling studies

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