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Since the mid-nineteenth century the Earth's surface has warmed, and models indicate that human activities have caused part of the warming by altering the radiative balance of the atmosphere. Simple theories suggest that global warming will reduce the strength of the mean tropical atmospheric circulation. An important aspect of this tropical circulation is a large-scale zonal (east-west) overturning of air across the equatorial Pacific Ocean--driven by convection to the west and subsidence to the east--known as the Walker circulation. Here we explore changes in tropical Pacific circulation since the mid-nineteenth century using observations and a suite of global climate model experiments. Observed Indo-Pacific sea level pressure reveals a weakening of the Walker circulation. The size of this trend is consistent with theoretical predictions, is accurately reproduced by climate model simulations and, within the climate models, is largely due to anthropogenic forcing. The climate model indicates that the weakened surface winds have altered the thermal structure and circulation of the tropical Pacific Ocean. These results support model projections of further weakening of tropical atmospheric circulation during the twenty-first century.
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© 2006 Nature Publishing Group
© 2006 Nature Publishing Group
Weakening of tropical Pacific atmospheric
circulation due to anthropogenic forcing
Gabriel A. Vecchi
1
, Brian J. Soden
2
, Andrew T. Wittenberg
1
, Isaac M. Held
1
, Ants Leetmaa
1
& Matthew J. Harrison
1
Since the mid-nineteenth century the Earths surface has
warmed
1–3
, and models indicate that human activities have caused
part of the warming by altering the radiative balance of the
atmosphere
1,3
. Simple theories suggest that global warming will
reduce the strength of the mean tropical atmospheric circula-
tion
4,5
. An important aspect of this tropical circulation is a large-
scale zonal (east–west) overturning of air across the equatorial
Pacific Ocean
driven by convection to the west and subsidence to
the east
known as the Walker circulation
6
. Here we explore
changes in tropical Pacific circulation since the mid-nineteenth
century using observations and a suite of global climate model
experiments. Observed Indo-Pacific sea level pressure reveals a
weakening of the Walker circulation. The size of this trend is
consistent with theoretical predictions, is accurately reproduced
by climate model simulations and, within the climate models, is
largely due to anthropogenic forcing. The climate model indicates
that the weakened surface winds have altered the thermal struc-
ture and circulation of the tropical Pacific Ocean. These results
support model projections of further weakening of tropical
atmospheric circulation during the twenty-first century
4,5,7
.
The Walker circulation is fundamental to climate throughout the
globe: its variations are closely linked to those of the El Nin
˜
o/
Southern Oscillation
6
and monsoonal circulations over adjacent
continents
8
, and variations in its intensity and structure affect climate
across the planet
8,9
. The strength of equatorial Pacific zonal wind-
stress, associated with the Walker circulation, is critical to equatorial
Pacific Ocean circulation
10
and to biogeochemical processes
11
.
Observational and modelling evidence indicates that since the
mid-nineteenth century tropical sea surface temperatures (SSTs)
have warmed by 0.5–0.6 8C (refs 1–3). Climate models predict a
weakening of the atmospheric convective overturning in response to
surface warming driven by increases in greenhouse gases
4,5,7
. One
expects this weakening to be manifest, in part, as a reduction in the
zonal overturning of tropical air
4
, a large component of which is the
Pacific Walker circulation
6
.
The weakening of tropical atmospheric overturning circulations in
response to warming can be understood in terms of the energy and
mass balance of the ascending branch of these circulations. As the
surface warms, the water vapour concentration in the lower tropo-
sphere increases by roughly 7% per 8C of surface warming
12,13
,
consistent with the Clausius–Clapeyron equation and fixed relative
humidity. However, the rate of precipitation (which on long time-
scales is limited by the rate of change of the radiative cooling of the
troposphere) increases more slowly
approximately 2% per 8C
warming
1,14,15
. The global-mean rate of precipitation must be
balanced by the moisture transport from the atmospheric boundary
layer to the free troposphere, which is a product of the boundary layer
water vapour concentration and the exchange of air between the
boundary layer and free troposphere. Thus, the differential rate of
response to surface warming of water vapour and precipitation
LETTERS
Figure 1 | Spatial pattern of observed and modelled sea level pressure linear
trends.
Linear trend of sea level pressure (SLP) from: a, Kaplan SLP
reconstruction
29
(1861–1992), and ensemble-mean of GCM experiments
(1861–1992) as follows; b, all-forcing (five-member mean), c, natural forcing
(three-member mean) and d, anthropogenic forcing (three-member mean).
The trend averaged over the domain 158 S–158 N, 08–3608 is removed
from each panel. Dashed rectangles indicate the regions used to define the
large-scale Indo-Pacific SLP gradient index (DSLP).
1
NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08540-6649, USA.
2
Rosenstiel School for Marine and Atmospheric Sciences, University of Miami,
Miami, Florida 33149-1098, USA.
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73
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© 2006 Nature Publishing Group
implies a weakening of the boundary layer/troposphere mass
exchange of ,5% per 8C warming
4
. Closely related arguments have
been provided for a weakening of tropical overturning circulations
based on the energy balance of the subsiding branch of these
circulations
5
. On the basis of observed tropical warming since the
mid-nineteenth century (0.5–0.6 8C), these theoretical arguments
predict a 2.5–3% reduction in the strength of tropical atmospheric
overturning circulation. Is there evidence of such a slowdown in the
observational record?
We use historical observations of sea level pressure (SLP) to assess
observed changes in the Walker circulation over the tropical Pacific;
an index of the large-scale tropical Indo-Pacific SLP gradient (DSLP)
serves as a proxy for the mean intensity of the Pacific Walker
circulation (see Methods). Ensembles of global climate model
(GCM) experiments
with different radiative forcings
serve to
explore the origin of the observed circulation changes, and allow
for the estimation of the statistical significance of the observed
changes. The model used here is the US National Oceanic and
Atmospheric Administration (NOAA) Geophysical Fluid Dynamics
Laboratory (GFDL) CM2.1 GCM
16–18
, with three historical inte-
gration sets over the period 1861–2000: (1) a five-member ensemble
including estimates of natural (solar variations, volcanoes) and
anthropogenic (well-mixed greenhouse gases, ozone, direct aerosol
forcing and land use) sources of climate change; (2) a three-member
ensemble applying only natural forcing; and (3) a three-member
ensemble applying only anthropogenic forcing (see Methods and
Supplementary Information).
There have been spatially coherent patterns in observed trends of
tropical SLP since the mid-nineteenth century (Fig. 1a), and similar
patterns are evident in the five-member ensemble-mean of the
historically forced GCM integrations (Fig. 1b). These trends indicate
a reduced zonal SLP gradient, and thus a weakened zonal circulation,
because the climatological pattern in the tropical Indo-Pacific has
SLP larger in the east than in the west. The naturally forced GCM
experiments are unable to recover these observed patterns in the SLP
trends (Fig. 1c); however, the GCM recovers many of the principal
observed SLP trend patterns using only anthropogenic forcing
(Fig. 1d).
Trends computed from observed DSLP are inconsistent with
those expected from the variability in the pre-industrial control
simulation, and they are inconsistent with the trends from the
‘natural-forcing’ GCM ensemble experiment (Fig. 2). However,
the trends in observed DSLP fall within the range of trends from
the ‘all-forcing’ GCM ensemble, and within that of the ‘anthropo-
genic-forcing’ GCM ensemble (Fig. 2). Thus, within the framework
of this GCM, a significant part of the observed reduction of
DSLP since the mid-nineteenth century resulted from anthropogenic
forcing. The trend computed from observed DSLP is also signifi-
cantly distinct from that expected from the pre-industrial control
experiments of all other models in the Intergovernmental Panel on
Climate Change 4th Assessment Report (IPCC/AR4) archive (Sup-
plementary Fig. 3). Most other models in the IPCC/AR4 archive also
show a weakening of the equatorial Pacific pressure gradient,
although CM2.1 shows the largest reduction (Supplementary Fig. 4.)
Though we use linear trends to summarize the changes in tropical
SLP since the mid-nineteenth century, these changes have not
appeared as a smooth reduction (Fig. 3). Both the observational
record and individual GCM ensemble members exhibit substantial
decadal variability (Supplementary Fig. 1), arising from processes
internal to the coupled system. The GCM decadal variability in DSLP
is comparable to that in observations, even though the GCM
interannual variability is overly energetic owing to a simulated
El Nin
˜
o variability that has too large an amplitude
17
. The strong
decadal variability complicates the detection of a relatively small
forced change even in multi-decadal records of DSLP; on the basis
of the 2,000-year control integration, a record shorter than
100–120 years is insufficient to detect the forced linear trend in
DSLP from the GCM at P ¼ 0.05 (Supplementary Fig. 2).
Figure 2 | Summary of the linear trends in SLP gradient across the
Indo-Pacific (DSLP) from observations and the various GCM historical
radiative forcing experiments.
Circles indicate the trend value from each
observational data set: K, Kaplan (1854–1992)
29
; H, Hadley Centre
(1871–1998)
28
; and B, a blend of Hadley
28
and Kaplan
29
, extended into 2005
using the NCEP gridded ship data (1854–2005)
27
. Model trends are
computed over the period 1861–2000. Confidence intervals are computed
from a 2,000-year control experiment, at the two-sided P ¼ 0.05 level.
Figure 3 | Observed and modelled evolution of DSLP since the nineteenth
century.
Five-year running-mean DSLP from: a, observations (black from
Kaplan
29
, and blue from Hadley Centre
28
; the record is extended through to
2005 with NCEP ship-based observations
27
(dashed line)); and b, GCM
historical integrations (five-member ensemble mean in black, an illustrative
ensemble member (number 3) in blue, dashed lines indicate linear trend of
ensemble members 1, 2, 4 and 5). In both panels, the linear trends in DSLP
are shown as thick lines, with shadings corresponding to each time-series.
LETTERS NATURE|Vol 441|4 May 2006
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© 2006 Nature Publishing Group
In the 1970s there was a rapid reduction in observed DSLP; such
rapid changes are also evident in other decades and in individual
ensemble members. Fifty-year DSLP trends ending in the 1990s are
significantly larger than those computed over the entire record,
suggesting an amplifying trend in DSLP. However, in recent
years the statistical significance of the amplification of the trend
disappears; though nominally larger than the long-term trend, the
1956–2005 trend is not significantly different from the long-term
trend at P ¼ 0.05. Further work is required to determine if the larger
recent trend since the 1950s is forced or a result of internal variability.
We estimate zonal wind stress across the equatorial Pacific using
DSLP, following the method of ref. 19 (Fig. 4). Linear relationships
with D SLP capture most of the interannual and longer timescale
variability in equatorial Pacific zonal-mean easterlies (,t
x
.; zonal
wind-stress averaged 1208 E–708 W, 58 S–58 N; easterlies are winds
from the east). DSLP-reconstructed ,t
x
. recovers the principal
extremes and transitions of the observed and modelled ,t
x
.,
including the long-term weakening of ,t
x
. in the model, and the
various El Nin
˜
o and La Nin
˜
a events observed in the recent decades
(Fig. 4, upper panels). The trend of the linear fit of ,t
x
. to observed
DSLP represents a ,t
x
. reduction of ,7% since 1860; as surface
stress is roughly proportional to the square of wind speed, this
suggests a mean reduction in equatorial Pacific zonal wind of ,3.5%,
roughly consistent with the theoretical prediction.
Equatorial Pacific ,t
x
. is of critical importance to the large-scale
oceanic circulation in the equatorial Pacific
9
. The GCM experiments
indicate that since the mid-nineteenth century, the weakening of
equatorial ,t
x
. has resulted in a weakening of surface equatorial
currents, a vertical shift in sub-surface currents, and a reduction in
the intensity and depth of equatorial upwelling (Supplementary
Fig. 5). By bringing nutrient-rich waters close to the surface,
equatorial upwelling exerts a strong control on biological activity
in the tropical Pacific
10
; its weakening and shoaling suggest a possible
reduction of biological productivity under global warming.
The GCM experiments indicate a substantial shoaling of the
western equatorial Pacific thermocline depth (Z
tc
) since the mid-
nineteenth century (Fig. 4, lower panels); the thermocline is the zone
of rapid temperature change in a typical vertical oceanic temperature
profile between the warm well-mixed surface layer and cold abyssal
waters. Reduction of ,t
x
. affects both the east–west tilt of the
equatorial Pacific thermocline and its mean depth
20
(Supplementary
Fig. 6); these changes could affect the character of El Nin
˜
o varia-
bility
21
. On seasonal to interannual timescales (timescales too short
for the equatorial thermocline to come to equilibrium with the
winds), a reduction in ,t
x
. has a strong impact on both western
and eastern equatorial Pacific Z
tc
. However, on timescales longer
than that of equatorial adjustment, the impact of reductions in
,t
x
. is felt almost entirely by the western equatorial Pacific Z
tc
(Fig. 4). Because Z
tc
and SST are tightly coupled only in the eastern
equatorial Pacific
22
, these long-term thermocline depth changes in
the GCM are unlikely to affect SST directly. The observational record
of equatorial Pacific thermocline slope and depth over the past
50 years is consistent with the recent reduction in the strength of
the easterlies and the low-frequency relationship between Z
tc
and
,t
x
. in the GCM.
Atmosphere–ocean conditions in the equatorial Pacific have
changed since the mid-nineteenth century: there has been a signifi-
cant slowdown of atmospheric circulation, which models indicate
has driven a response in ocean circulation. The observed trend in the
Pacific surface zonal SLP gradient is unlikely to be due to natural
variability. However, much of the long-term trend is reproduced in
model simulations that account for human impacts on the radiative
budget of the planet, and is consistent with the changes expected
from simple thermodynamic arguments
4
. The agreement between
Figure 4 | Observed and modelled equatorial Pacific zonal-mean zonal
wind-stress anomaly, <t
x
>, and equatorial thermocline depth anomaly,
Z
tc
. Upper panels: model/observed ,t
x
. and reconstruction using linear
relation to DSLP; dashed line shows (1854–2005) trend in ,t
x
.
reconstructed using blended Kaplan
29
/Hadley
28
/NCEP
27
DSLP. Lower
panels: Z
tc
in the western (black line, 28 S–28 N, 1408 E–1808 E) and eastern
(blue line, 28 S–28 N, 1308 W–908 W) equatorial Pacific. Left panels:
ensemble-mean all-forcing CM2.1 GCM experiment, showing five-year
running mean. Right panels: five-year running mean (thick lines) and
annual-mean (thin lines) observational estimates. Observed stress is from
European Centre for Medium Range Weather Forecasting Reanalysis 40,
observed Z
tc
is from GFDL ocean data assimilation
30
. Z
tc
is the location of
the maximum vertical temperature gradient. Note different scales in each
panel.
NATURE|Vol 441|4 May 2006 LETTERS
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the theoretical, observed and model-produced changes in strength
of atmospheric circulation suggests increased confidence in the
model-projected reduction in the strength of tropical circulation
during the twenty-first century
4,5,7
; on the basis of climate model
simulations, this weakening may be of the order of 10% by the end of
the twenty-first century
4,7
.
METHODS
SLP data sets and calculations. Direct assessment of long-term changes in the
strength of tropical circulation is problematic, because there have been changes
in the methods used to make wind measurements at sea
23,24
. Trends in tropical
winds over recent decades are ambiguous, with some studies showing a
strengthening
25
and others a weakening
26
. SLP offers a proxy to recover changes
in wind velocity, because of the dynamical connection between large-scale zonal
gradients of SLP and zonal-mean zonal wind-stress
19
. Further, SLP measure-
ments at sea have always been made using instruments, in contrast with
measurements of surface wind, which have been instrument-based only in
recent decades
23,24
. Equatorial Pacific wind-stress estimated from the zonal
gradients of SLP has shown a reduction over the period 1920–90
19
. Ship-based
SLP measurements
27
have been used to build global gridded data sets of monthly
SLP
28,29
using knowledge of its global co-variability, and allow the exploration of
changes from the mid-nineteenth century.
In this study two global reconstruction data sets of monthly SLP are used: (1)
the Kaplan SLP data set
29
version 1 spanning the period 1854–1992, with data
only over the ocean, and (2) the Hadley Centre SLP reconstruction
28
version 1
spanning the period 1871–1998, with data over both land and ocean. These data
sets are extended into recent years using the gridded monthly ship-based
SLP observations available from NOAAs National Center for Environmental
Prediction (NCEP)
27
. For the analyses presented here, the monthly long-term
climatology of each data set is removed to compute SLP anomalies.
Using the GCM and observed SLP data, a large-scale tropical Indo-Pacific SLP
gradient (DSLP) is computed from the difference in SLP averaged over the
central/east Pacific (1608 W–808 W, 5 8 S–58 N) and over the Indian Ocean/west
Pacific (808 E–1608 E, 58 S–58 N). The index is computed with SLP anomalies
from monthly climatology; positive values indicate a strengthened Indo-Pacific
SLP gradient. A least-squares linear trend in DSLP is used as a concise metric for
the long-term changes in the strength of zonal circulation. As shown in ref. 19 a
large-scale SLP gradient, like DSLP, provides a useful proxy for the mean
intensity of Pacific zonal surface winds. For the observational record of surface
winds available to us (1957–2003) and for the entire climate model record
(1861–2000), DSLP provides a useful proxy for the strength of the mean zonal
circulation over the equatorial Pacific Ocean (Fig. 4).
Climate models. Global climate models (GCMs) simulate the variations of, and
interactions between, various elements of the climate system (ocean, atmos-
phere, cryosphere and land) forced by radiatively active naturally occurring and
anthropogenic gases and aerosols. Internal climate variability can be isolated
from that forced by changes to atmospheric composition through ensemble
experiments, in which the same model physics and forcing fields are applied to
different initial conditions. The model used here is the NOAA GFDL CM2.1
GCM
16–18
, which uses estimated radiative forcings over the period 1861–2000.
The principal historical integration set is a five-member ensemble including
estimates of natural (solar variations, volcanoes), and anthropogenic (well-
mixed greenhouse gases, ozone, direct aerosol forcing and land use) sources of
climate change. Two additional sets of experiments isolate the effects of each set
of forcing elements, by applying only natural or anthropogenic forcing; each
consists of a three-member ensemble. Statistical significance estimates are
computed from a 2,000-year control integration with invariant radiative
conditions from the 1860s (see Supplementary Information).
Received 27 October 2005; accepted 22 March 2006.
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Acknowledgements G.A.V. was supported by the Visiting Scientist Program at
the NOAA/GFDL administered by UCAR. We are grateful to the model
development teams at GFDL, and thank A. E. Johansson, M. P. Vecchi,
T. Knutson, T. Delworth and J. Russell for comments and suggestions.
Author Information Reprints and permissions information is available at
npg.nature.com/reprintsandpermissions. The authors declare no competing
financial interests. Correspondence and requests for materials should be
addressed to G.A.V. (Gabriel.A.Vecchi@noaa.gov).
LETTERS NATURE|Vol 441|4 May 2006
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... Moreover, global climate models are not typically run at scales needed to resolve changes in TC characteristics (particularly intensity) at island-scale, substantially limiting our ability to understand the effects of climate change on TCs over small island countries Fig. 5 Schematic representation of the linkages between natural climate variability, human-induced global warming and tropical cyclones. Note that this diagram is not exclusive and does not quantify the changes, but instead demonstrates how different climatic factors may interact to affect TC characteristics and associated impacts over the SWP region ( 1 Vecchi et al. 2006;2 Yeh et al. 2009;3 Power and Kociuba 2011;4 Kim and Yu 2012;5 Sugi et al. 2012;6 Sugi and Yoshimura 2012;7 Tokinaga et al. 2012;8 Church et al. 2013;9 Hartmann et al. 2013;10 Tory et al. 2013;11 Woodruff et al. 2013;12 Cai et al. 2014;13 Kossin et al. 2014;14 Lucas et al. 2014;15 Walsh et al. 2016;16 Chand et al. 2017;17 Taupo and Noy 2017;18 Chand 2018;19 Kossin 2018;20 Sharmila and Walsh 2018;21 Andrew et al. 2019;22 Chand et al. 2020) ▸ Content courtesy of Springer Nature, terms of use apply. Rights reserved. ...
... in the SWP region. Nevertheless, from past literature (Vecchi et al. 2006;Power and Kociuba 2011;Tokinaga et al. 2012;Tory et al. 2013;Woodruff et al. 2013;Kossin et al. 2014;Lucas et al. 2014;Walsh et al. 2016;Chand et al. 2017 Lee et al. 2021), trends in observation records, climate model simulations and theoretical understanding, we have a good understanding of how various climatic conditions may interact to affect TCs and associated impacts in the SWP region as depicted in Fig. 5. It is important to note here that this schematic does not make any attempt to quantify the changes, but instead demonstrates how different climatic factors may interact to affect TC characteristics and associated impacts in the SWP region. ...
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Tropical cyclones (TCs) are amongst the costliest natural hazards for southwest Pacific (SWP) Island nations. Extreme winds coupled with heavy rainfall and related coastal hazards, such as large waves and high seas, can have devastating consequences for life and property. Effects of anthropogenic climate change are likely to make TCs even more destructive in the SWP (as that observed particularly over Fiji) and elsewhere around the globe, yet TCs may occur less often. However, the underpinning science of quantifying future TC projections amid multiple uncertainties can be complex. The challenge for scientists is how to turn such technical knowledge framed around uncertainties into tangible products to inform decision-making in the disaster risk management (DRM) and disaster risk reduction (DRR) sector. Drawing on experiences from past TC events as analogies to what may happen in a warming climate can be useful. The role of science-based climate services tailored to the needs of the DRM and DRR sector is critical in this context. In the first part of this paper, we examine cases of historically severe TCs in the SWP and quantify their socio-economic impacts. The second part of this paper discusses a decision-support framework developed in collaboration with a number of agencies in the SWP, featuring science-based climate services that inform different stages of planning in national-level risk management strategies.
... The Pacific Walker circulation (PWC) is an important component of the global climate system; it features low-level winds blowing from east to west across the central Pacific, a rising motion over the Maritime Continent and the warm western Pacific, returning flow from west to east in the upper troposphere, and a sinking motion over the cold water of the eastern Pacific [1][2][3][4][5][6]. The PWC regulates the global exchange of heat energy, momentum, and water vapor within the tropics through substantial overturning motions. ...
... This recent PWC intensification and westward shift contribute greatly to the observed moistening over the Indo-Pacific warm pool and drying (cooling) over the central (eastern) tropical Pacific. This differs from the findings of [4,6,28,47,48], which reported an observed and simulated twentieth-century weakening of PWC because of global warming. , and (c) the monthly variation in PWC with ZMS for the ERA5 and the NCEP2 derived by applying a 3-year running mean to the annual anomalies. ...
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The Pacific Walker circulation (PWC) is one of the most important components of large-scale tropical atmospheric circulations. The PWC and its influences have been studied extensively by numerical models and reanalysis. The newly released ERA5 and NCEP2 are the most widely used reanalysis datasets and serve as benchmarks for evaluation of model simulations. If the results of these datasets differ significantly, this could lead to a bias in projected long-term climate knowledge. For better understanding of future climate change, it is necessary to evaluate PWC rea-nalysis productions. As a result, we compared the PWC structures between the ERA5 and NCEP2 datasets from month to seasonal time scales. We used the zonal mass streamfunction (ZMS) over the equatorial Pacific to indicate the strength of the PWC. The PWC's average monthly or seasonal cycle peaks around July. From February to June, the NCEP2 shows a higher PWC intensity, whereas the ERA5 shows greater intensity from July to December. The circulation center in the NCEP2 is generally stronger and wider than in the ERA5. The ERA5, however, revealed that the PWC's west edge (zero line of ZMS over the western Pacific) had moved 10 degrees westward in comparison to the NCEP2. In addition, we compared the PWC mean state in the reanalysis and CMIP6 models; the mean state vertical structures of the tropical PWC in the CMIP6 multi-model ensemble (MME) are similar to those of the reanalyses in structure but weaker and wider than in the two reanalysis datasets. The PWC is broader in CMIP6, and the western boundary is 7 and 17 degrees farther west than in the ERA5 and NCEP2, respectively. This study suggests that, when using reanalysis datasets to evaluate PWC structural changes in intensity and western edge, extreme caution should be exercised .
... It displays El Niño-like SST warming, Southern Hemisphere warming, and Northern Hemisphere cooling. The reduced zonal SST gradient over the tropical Pacific could lead to large-scale weakening and an eastward shift of the Walker circulation ( Figure 3b; Tanaka et al., 2004;Vecchi et al., 2006;Vecchi & Soden, 2007). Moreover, anomalous downward motions over the Indian Ocean occur in the RDN period, together with weakened Hadley circulation during the boreal summer (Figures 3b and 3c; Ma & Xie, 2013). ...
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Understanding precipitation changes over the Northern Hemisphere land monsoon (NHLM) region, where nearly 60% of the world's population resides, is fundamental for hydrological projections and adaptations against climate change. There are many studies on the hydrological cycle under various climate change scenarios. However, there is still a lack of research on the hydrological responses to CO2 removal as a global warming mitigation measure from a global perspective. This study demonstrates the distinguished hysteresis responses of mean NHLM precipitation based on idealized CO2 ramp‐up and ramp‐down experiments using the Community Earth System Model|Community Earth System model. The Indian and North African monsoons have time asymmetry in the mean precipitation changes under the CO2 increase and decrease pathways, while the North American monsoon does not. The zonal contrasting hysteresis is attributed to longitudinally contrasting changes in the intertropical convergence zone position driven by the inter‐hemispheric and land–sea thermal contrast. On the contrary, changes in extreme precipitation exhibit little temporal asymmetries over any of the NHLM domains. These results provide new insights into climate hysteresis of the hydrological cycle from regional and global perspectives.
... The Pacific trends also share many similarities with salinity anomalies observed during El Niño events (e.g., freshening western Pacific and salinifying southeast Pacific; Figure 6 and Figure S10 in Supporting Information S1). These similarities could provide evidence of a long-term weakening in the Walker Circulation (e.g., Power & Kociuba, 2011;Power & Smith, 2007;Vecchi et al., 2006;Zhang & Song, 2006) (contours in Figure S10 of Supporting Information S1). This dynamical response is difficult to disentangle from the thermodynamical component; however, locations that subvert the "fresh gets fresher, salty gets saltier" paradigm can provide evidence of such dynamical changes. ...
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Climate observations in much of the tropical oceans are scarce during most of the 20th century, so paleoclimate proxies are needed to understand the full range of natural climate variability. Past proxy studies have focused primarily on sea surface temperatures, but there are comparatively few salinity reconstructions. Such reconstructions can extend our understanding of hydroclimate across the tropical oceans, including variability in precipitation, evaporation, and ocean circulation. Here we compile a network of salinity‐sensitive coral δ¹⁸O records, then apply a reduced‐space method based on empirical orthogonal function analysis to reconstruct annual tropical salinity anomalies over the 20th century. A comparison of surface salinity data sets, including reanalyzes (SODA2/3, Ocean ReAnalysis System 5 (ORAS5), Global Ocean Data Assimilation System) and objective analyses (Institute of Atmospheric Physics (IAP), EN4, Delcroix), show large discrepancies in the spatial structure, temporal evolution, and importance of the leading modes of variability. Two salinity data sets, IAP and ORAS5, are retained for climate reconstruction. Our coral‐based salinity reconstructions reveal significant long‐term trends over the 20th century, which are likely associated with hydrological cycle intensification and possibly a weakening of the Walker Circulation. These reconstructions also capture the spatial and temporal patterns of salinity anomalies associated with the El Niño‐Southern Oscillation, Interdecadal Pacific Oscillation, and Atlantic Multidecadal Oscillation. Ultimately, this approach can enhance our understanding of tropical hydroclimate prior to the observational era.
... Near the equator where the Coriolis effect vanishes, the upper ocean currents are sensitive to wind stress changes (32,33). In a warmer climate, the Walker circulation slows down (34,35), with easterly and westerly wind anomalies over the equatorial Indian and Pacific, respectively ( fig. S11B). ...
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How the ocean circulation changes in a warming climate is an important but poorly understood problem. Using a global ocean model, we decompose the problem into distinct responses to changes in sea surface temperature, salinity, and wind. Our results show that the surface warming effect, a robust feature of anthropogenic climate change, dominates and accelerates the upper ocean currents in 77% of the global ocean. Specifically, the increased vertical stratification intensifies the upper subtropical gyres and equatorial currents by shoaling these systems, while the differential warming between the Southern Ocean upwelling zone and the region to the north accelerates surface zonal currents in the Southern Ocean. In comparison, the wind stress and surface salinity changes affect regional current systems. Our study points a way forward for investigating ocean circulation change and evaluating the uncertainty.
... There have been several theories outlined to explain such mean state changes. One theory involves a weakening of the hydrological cycle in response to ACC would slow the Walker circulation, warming eastern Pacific SSTs [14][15][16] . Other theories call for a stronger evaporative damping of SST changes in the warm pool than in the cold tongue 17,18 , or different cloud radiative feedbacks in the western and eastern tropical Pacific [19][20][21] . ...
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Future changes in the seasonal evolution of the El Niño—Southern Oscillation (ENSO) during its onset and decay phases have received little attention by the research community. This work investigates the projected changes in the spatio-temporal evolution of El Niño events in the 21 st Century (21 C), using a multi-model ensemble of coupled general circulation models subjected to anthropogenic forcing. Here we show that El Niño is projected to (1) grow at a faster rate, (2) persist longer over the eastern and far eastern Pacific, and (3) have stronger and distinct remote impacts via teleconnections. These changes are attributable to significant changes in the tropical Pacific mean state, dominant ENSO feedback processes, and an increase in stochastic westerly wind burst forcing in the western equatorial Pacific, and may lead to more significant and persistent global impacts of El Niño in the future.
... In addition, annual cycles are well known to the region's many indigenous communities, who associate the rainy season with greater fruit abundance, though timing of peak fruiting varies across the island (Hosen et al., 2020). Thus, wildlife habitat use likely reflects fruit scarcity both outside masting years (multi-year cycle) and during annual dry seasons (annual cycle), and these dynamics are likely to become more erratic with global climate change (Harrison, 2000;Laurance et al., 2012;Vecchi et al., 2006). ...
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Community‐managed forests (CF) bordering protected areas are critical to conservation in Borneo. Iban‐managed CF retain tree species characteristic of primary forests within pulau, remnant old growth forests conserved for harvesting forest products. However, the selective felling of large trees, and proximity to surrounding mixed‐use mosaic habitat, likely influence pulau structure and composition. Tropical Asian forests exhibit supra‐annual mast fruiting (3–7 years) and extended periods of fruit scarcity, but fruit trees encouraged and planted by communities, in mixed‐use mosaic bordering pulau, may benefit wildlife during periods of fruit scarcity. We investigated seasonal availability of foods important to wildlife within protected primary forest and pulau at the boundary of the Lanjak Entimau Wildlife Sanctuary (LEWS), in Sarawak, Malaysia. We tracked the presence of fruits and seeds, comparing relative composition and fruiting activity between forests, using bi‐monthly phenology surveys across 50 locations (October 2016–2019). We also compared fruit presence along walking transects within LEWS’s primary forest and the mixed‐use mosaic in Iban territories. We found forests within and adjacent to LEWS shared similar composition, synchrony, and extended periods of fruit scarcity. Mixed‐use mosaic bordering pulau provided more consistent fruits, however, due largely to an invasive tree (Bellucia pentamera). Our study suggests pulau retain diversity and synchrony of fruit resources comparable to primary forest, sustaining valuable habitat for wildlife within the greater mixed‐use mosaic of traditional Iban land management practices. These findings are important for understanding resources available to wildlife outside of protected areas, and how CF contribute to conserving biodiversity. Findings of our 3‐year study suggest that community‐managed forests (pulau) bordering Lanjak Entimau Wildlife Sanctuary (LEWS) in Malaysian Borneo retained comparable diversity and synchrony of fruiting within mature primary forest, sustaining valuable habitat for wildlife. In addition, we observed increased presence of more consistently reproductively active trees, likely facilitated by human activities, which may benefit wildlife during periods of fruit scarcity, a phenomenon that is common in SE Asian forests. We also discuss implications of the presence of an early successional invasive tree (Bellucia pentamera), a potential threat to forest regeneration following swidden clearings and neighboring logging concessions.
... moisture content arising from oceanic and land surface evaporation and enhances moisture transport, contributing to P-E increases over wet regions and decreases over dry regions, which is often termed as the thermodynamic response (Chou et al., 2009;. However, deviations from this hypothesized thermodynamic response occur over most land regions (Greve et al., 2014), partly due to anomalous circulation change associated with warming of sea surface temperature (SST) (Lau & Kim, 2015;Vecchi et al., 2006). As SST evolves over a range of relatively slow timescales (Zappa et al., 2020), the thermodynamic and dynamic hydrological responses caused by SST warming are recognized as a slow hydrological response to the CO2 radiative forcing (Allan et al., 2020;He & Soden, 2017;Monerie et al., 2021). ...
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Increasing atmospheric CO2 and associated global warming are expected to alter the global hydrological cycle, thereby posing widespread threats to freshwater availability. However, future hydrological projections differ greatly between models, particularly over the tropical regions. The large difference between model projections directly limits policy planning efforts, and the responsible modelling processes remain unclear. Here, we identify the primary processes accounting for model differences in tropical hydrological changes using multiple CO2 sensitivity experiments in the Coupled Model Intercomparison Project. We show that differences in projected changes to tropical evapotranspiration, precipitation, and surface water availability mainly arise from model representations of vegetation cover and stomatal conductance responses to elevated CO2 and associated changes in atmospheric moisture and circulation. Atmospheric responses to sea surface warming contribute additionally to the divergence in hydrological projections. Given the importance of vegetation responses to elevated CO2 and associated atmosphere feedbacks, our results underscore the need to improve representations of the vegetation physiological response to rising CO2 and its coupling to the atmosphere, to provide reliable tropical hydrological projections.
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Tree restoration is an effective way to store atmospheric carbon and mitigate climate change. However, large-scale tree-cover expansion has long been known to increase evaporation, leading to reduced local water availability and streamflow. More recent studies suggest that increased precipitation, through enhanced atmospheric moisture recycling, can offset this effect. Here we calculate how 900 million hectares of global tree restoration would impact evaporation and precipitation using an ensemble of data-driven Budyko models and the UTrack moisture recycling dataset. We show that the combined effects of directly enhanced evaporation and indirectly enhanced precipitation create complex patterns of shifting water availability. Large-scale tree-cover expansion can increase water availability by up to 6% in some regions, while decreasing it by up to 38% in others. There is a divergent impact on large river basins: some rivers could lose 6% of their streamflow due to enhanced evaporation, while for other rivers, the greater evaporation is counterbalanced by more moisture recycling. Several so-called hot spots for forest restoration could lose water, including regions that are already facing water scarcity today. Tree restoration significantly shifts terrestrial water fluxes, and we emphasize that future tree-restoration strategies should consider these hydrological effects.
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