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Jet streams, the meandering bands of fast winds located near the tropopause, are driving factors for weather in the midlatitudes. This is the first study to analyze historical trends of jet stream properties based on the ERA-40 and the NCEP/NCAR reanalysis datasets for the period 1979 to 2001. We defined jet stream properties based on mass and mass-flux weighted averages. We found that, in general, the jet streams have risen in altitude and moved poleward in both hemispheres. In the northern hemisphere, the jet stream weakened. In the southern hemisphere, the sub-tropical jet weakened, whereas the polar jet strengthened. Exceptions to this general behavior were found locally and seasonally. Further observations and analysis are needed to confidently attribute the causes of these changes to anthropogenic climate change, natural variability, or some combination of the two.
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Historical trends in the jet streams
Cristina L. Archer
and Ken Caldeira
Received 12 February 2008; revised 10 March 2008; accepted 14 March 2008; published 18 April 2008.
[1] Jet streams, the meandering bands of fast winds located
near the tropopause, are driving factors for weather in the
midlatitudes. This is the first study to analyze historical
trends of jet stream properties based on the ERA-40 and the
NCEP/NCAR reanalysis datasets for the period 1979 to
2001. We defined jet stream properties based on mass and
mass-flux weighted averages. We found that, in general, the
jet streams have risen in altitude and moved poleward in
both hemispheres. In the northern hemisphere, the jet stream
weakened. In the southern hemisphere, the sub-tropical jet
weakened, whereas the polar jet strengthened. Exceptions to
this general behavior were found locally and seasonally.
Further observations and analysis are needed to confidently
attribute the causes of these changes to anthropogenic
climate change, natural variability, or some combination of
the two.
Citation: Archer, C. L., and K. Caldeira (2008),
Historical trends in the jet streams, Geophys. Res. Lett., 35,
L08803, doi:10.1029/2008GL033614.
1. Introduction
[2] Jet streams are narrow bands of fast, meandering air
currents that flow around the globe near the tropopause
level in both hemispheres. They are often classified in two
categories: sub-tropical jets, found at the poleward margin
of the upper branch of the Hadley circulation, and polar jets,
located above the polar-frontal zone, a region of sharp
thermal contrast between cold polar air and warm tropical
air [Holton, 1992; Bluestein, 1993].
3] Jet streams are important bec ause synoptic scale
disturbances tend to form in the regions of maximum jet
stream wind speed and to propagate downstream along
storm t racks that follow the jet axes [Holton, 1992].
Changes in jet stream location, intensity, or altitude can
therefore cause variations in frequency and intensity of
storms. Also, jet streams inhibit formation and development
of hurricanes, which preferentially develop in low-shear
regions of the atmosphere [Gray, 1968; Vecchi and Soden,
2007]. They affect air transport not only because of their
high winds, but also because of the clear-air turbulence
associated with jet cores [Bluestein , 1993].
2. Data
[4] To study if and how the jet streams have changed in
the past few decades, we used two reanalyses of historical
weather data: ERA-40 [Uppala et al., 20 05], from the
European Centre for Medium-Ra nge Weather Forecasts,
which covers 44 years from 1958 to 2001, and NCEP/
NCAR [Kalnay et al., 1996; Kistler et al., 1999], from the
National Centers for Environmental Protection and the Na-
tional Center for Atmospheric Research, covering the years
from 1948 to 2006. Monthly averages of zonal and merid-
ional (u and v respectively) wind velocity components were
available at 2.5 degrees hori zontal resolution and with
6 vertical levels between 400 and 100 hPa. Whereas
conventional observations (e.g., upper-air winds, tempera-
ture, and humidity from radiosondes; surface data from
various land and buoy networks; ocean wave heights) were
assimilated in both datasets throughout the entire periods,
satellite-borne observations (e.g., infrared and microwave
radiances; total and column ozone; surface-pressure and
winds over ocean) were only assimilated from 1979 on.
For this reason, this study will focus only on the period
1979-2001. Despite limita tions [Pawson and Fiorino ,
1999], the ERA-40 and NCEP/NCAR datasets are the best
sets of reanalyzed weather data available [Uppala et al.,
3. Methods
[5] In bot h hemispheres, the jet streams are located
between the 400 and the 100 hPa levels. The Northern
Hemisphere (NH) jet has a single-band spiral-like structure,
generally beginning south of the Canary Islands and ending
one eastward circumnaviga tion later over England. The
Southern Hemisphere (SH) jet has a more c oncentric
structure, with a persistent ring around Antarctica, hereafter
referred t o as Southern Hemisphere Polar (SHP) jet, and a
seasonally varying second ring at about 30 S, hereafter
referred to as the Southern Hemisphere sub-Tropical (SHT)
jet [Koch et al., 2006].
6] Jet streams are not continuous, but rather fragmented,
meandering, and with notable wind speed and elevation
variations. As such, the task of clearly identifying jet stream
boundaries at a given time can be difficult and ambiguous
[Koch et al., 2006]. To overcome this problem, we define jet
stream properties via integrated quantities, which are more
numerically stable and less grid-dependent than are simple
maxima and minima.
7] First, for each horizontal grid point in the reanalyses,
we define the mass weighted average wind speed between
400 and 100 hPa (WS) as:
i; j
i; j; k
þ v
i; j; k
; ð1Þ
where u
and v
are the monthly-average horizontal
wind components at grid point (i,j,k), and m
is the mass at
level k. Figure 1a shows the 23-year average of WS,
GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L08803, doi:10.1029/2008GL033614, 2008
Department of Global Ecology, Carnegie Institution of Washington,
Stanford, California, USA.
Copyright 2008 by the American Geophysical Union.
L08803 1of6
hereafter referre d to as the jet stream wind speed,
obtained from the ERA-40 dataset for all grid points
(the pattern of WS from NCEP/NCAR is almost identical
and is shown in the auxiliary material Figure S1).
1a shows wind maxima to the East of the continents in
the Northern Hemisphere, and to the East of Australia
and to the South of Africa in the Southern Hemisphere,
as observed [Koch et al., 2006]. Seasonal differences
between the two hemispheres were also found (Figures 1c
and 1e). The NH jet forms a nearly continuous band
between northern Africa and Hawaii in DJF (Figure 1),
but it shifts northward, fragments, and weakens in JJA. In
the SH, the Polar jet is omnipresent in all seasons, with
wind speed maxima south of Africa; the SH sub-Tropical
jet is only present in JJA as a nearly continuous band
between Australia and South America.
8] Given mass and wind speed at each pressure level z
between 400 and 100 hPa, for each horizontal grid cell, the
mass-flux weighted pressure P is defined as:
i; j
i; j; k
þ v
i; j; k
i; j; k
þ v
i; j; k
; ð2Þ
where p
is the pressure at level k. P represents the average
pressure of flows near the tropopause, and therefore the
average altitude of these flows. Because jet streams are
found near the tropopause, we can use P to characterize the
height of both the jet streams and the tropopause. In both
hemispheres, the jet streams are lower (and closer to the
poles) in the summer than they are in the winter, but the NH
jets are generally lower than the SH jets (Figures 2a, 2c, and
2e for the ERA-40 and auxiliary material Figure S2 for the
NCEP/NCAR). In the NH, the jets are lowest downwind of
their wind speed maxima, whereas in the SH the jets are
lowest where they are fastest.
9] Given the total mass-flux between 400 and 100 hPa,
we calculate the mass-flux weighted latitude in the NH for
each longitude i in the gridded reanalysis fields as follows:
i; j; k
þ v
i; j; k
i; j
i; j; k
þ v
i; j; k
where f
i, j
is the grid cell latitude. We use this integrated
value L
to characterize the latitude of the NH jet stream at
each longitude i. The following latitude bands will be used
in equation (3), and in the rest of the paper, to define the
three jet streams: 15N-70N for the NH, 40S-15S for the
SHT, and 70S-40S for the SHP jets. These intervals were
chosen based on zonal mean wind speeds (not shown).
10] Because these jet stream properties are, by design,
weighted averages over large volumes (i.e., all grid cells
between the 400 and 100 hPa levels within a given latitude
band worldwide), it is possible that trends occurring in a
sub-volume are partially masked by the lack of trends in other
sub-volumes. Trends in jet stream properties calculated in
Auxiliary materials are available in the HTML. doi:10.1029/
Figure 1. Jet stream wind speed from the ERA-40 reanalyses in 1979 2001: (a, c, e) annual, DJF, and JJA averages (m/s);
(b, d, f ) linear regression trends (m/s/decade), hatched where statistically significant (P-value <0.15), taking auto-correlation
into account.
this study are therefore generally expected to be conservative
lower-bound estimates.
4. Annual and Global Trends
[11] Using these jet stream properties, simple scalar
metrics for the entire globe (or for regions of interest) can
be obtained. We calculated global-average latitude, wind
speed, and altitude for the three jet streams (NH, SHT, and
SHP) for the period 1979-2001 from both the ERA-40 and
the NCEP/NCAR datasets. Since the temporal evolution of
the annual and seasonal averages from the ERA-40 and the
NCEP/NCAR reanalyses were nearly identical, only the
former are shown in Figure 3, whereas the latter are
available in the auxiliary material Figure S3. Parameters
of the linear regression against a linear trend from both
reanalyses are listed in Table 1. The auto-correlation of the
time series, when larger than 0.15, was taken into account to
correct the P-value of the linear regression.
12] We found that all three jet streams moved poleward
during the period 19792001, at rates varying from 0.06
0.11 degrees/decade in the SHT, to 0.070.10 degrees/
decade in the SHP, and to 0.170.19 degrees/decade in
the NH jet. The SHP jet, however, shifted equatorward in
the austral winter, but not enough to compensate for the
poleward shift in the austral summer (Figure 3, left panels).
A poleward shift of the jet streams i s consistent with
numerous other signals of global warming found in previ-
ous studies, such as the expansion of the Hadley cell, the
poleward shift of the storm tracks, the widening of the
tropical belt, and the cooling of the stratosphere. However,
this is the first study to examine jet stream latitude trends in
the reanalyses.
13] Lorenz and DeWeaver [2007] looked at climate
projections from 15 models under the Intergovernmental
Panel for Climate Change (IPCC) A2 scenario (‘‘business as
usual’ in the 21st century) and found qualitatively consis-
tent jet shifts poleward in both hemispheres. Kushner et al.
[2001] found that strong anthropogenic greenhouse gas
emissions can cause a similar poleward shift of the SHP
jet (by 0.08 degrees/decade) in a climate model. The
connection of poleward expansion of the Hadley cell with
global warming was identified by Frierson et al. [2007], in
idealized simulations with increased mean temperature, and
by Lu et al. [2007], who used climate models in a strong
greenhouse gas emission scenario. Both studies found
similar expansion rates, between 0.2 and 0.6 degrees/K
(degrees of latitude per degree K of warming), equivalent
to a poleward shift of the jet streams by 0.020.06 degrees/
decade (given a 0.5 K of warming in 19792005), smaller
than the values found in this study. On the other hand, other
studies have reported larger poleward shifts of parameters
related to the jets. Fu et al. [2006] and Hu and Fu [2007]
estimated a widening of the Hadley cell of 24.5 degrees in
19792005, corresponding to a shift of the jets by 0.37
0.86 degrees/decade. Also, a poleward (and upward) shift in
the storm tracks in both hemispheres ( 0.6 degrees/decade
from the plots) was found by Yin [2005] in a 15-member
ensemble of 21st century climate models in a moderate
green house gas emission scenario. Seidel et al. [2008]
reported several estimates of the widening of the Tropics
varying between 1 and 8 d egrees during 1979 200 5,
corresponding to poleward shifts of 0.21.6 degrees/decade
for the jets. Finally, Williams [2006] and Haigh et al. [2005]
showed that stratospheric warming would cause a lowering
Figure 2. Jet stream pressure (as a measure for altitude) from the ERA-40 reanalyses in 1979 2001: (a, c, e)
annual, DJF, and JJA averages (hPa); (b, d, f ) linear regression trends (hPa/decade), hatched where statistically significant
(P-value <0.15), taking auto-correlation into account. At an altitude of 10 km, the pressure is approximately 265 hPa in
the standard atmosphere and a pressure change of 1 hPa represents an altitude change of about 26 m.
of the tropopause and a shift of the jets equatorward,
therefore confirming that the strato spheric cooling expected
from anthropogenic emissions would cause a poleward (and
upward) shift of the jets.
14] An increase in jet stream altitude implies a negative
change in pressure and is reflected in a negative pressure
trend in Table 1. All jet streams had negative pressure trends
(except for a non-significant positive trend in the SHT jet
Figure 3. Annual, DJF, and JJA: anomalies from the 23-year (19792001) average latitude, average pressure (hPa, as a
measure of altitude), and average wind speed (m/s) of the Northern Hemisphere (NH), Southern Hemisphere sub-Tropical
(SHT), and Southern Hemisphere Polar (SHP) jet streams from the ERA-40 reanalyses. Positive latitude trends in the
northern hemisphere indicate a poleward shift in the jet stream, as do negative trends in the southern hemisphere.
Table 1. Statistical Regression Analysis of Annual Averages of Latitude, Altitude, and Wind Speed for Each Jet Stream From Both the
ERA-40 and the NCEP/NCAR Datasets in 1979 2001
Parameter Jet Stream Dataset Slope Per Decade Correlation Coefficient P-value
Latitude, deg NH ERA-40 0.165 0.363 0.096
NCEP 0.185 0.399 0.064
SHT ERA-40 0.063 0.268 0.215
NCEP 0.111 0.445 0.033
SHP ERA-40 0.073 0.222 0.308
NCEP 0.101 0.279 0.195
Pressure, hPa NH ERA-40 0.419 0.545 0.009
NCEP 0.036 0.066 0.765
SHT ERA-40 0.412 0.458 0.027
NCEP 0.017 0.022 0.920
SHP ERA-40 0.832 0.746 <0.001
NCEP 0.410 0.506 0.013
Wind speed, m/s NH ERA-40 0.156 0.287 0.183
NCEP 0.182 0.337 0.115
SHT ERA-40 0.365 0.381 0.214
NCEP 0.422 0.429 0.153
SHP ERA-40 0.237 0.251 0.336
NCEP 0.404 0.429 0.045
NH = Northern Hemisphere, SHT = Southern Hemisphere sub-Tropical, and SHP = Southern Hemisphere Polar. Bold values indicate P-values that have
been corrected to take into account the auto-correlation of the series, if greater than 0.15.
for the NCEP/NCAR reanalyses). Statistically significant
pressure decreases occurred in the SHP jet (0.41 to
0.83 hPa/decade, corresponding to +11 to +22 m/decade
in the standard atmosphere with 26.3 m/hPa altitude
decrease with pressure). The NH jet showed a decrease in
pressure in both datasets ( 0.04 to 0.42 hPa/decade or
about +2 to +11 m/decade). Because of their highly aver-
aged nature, these estimated trends in jet stream pressure are
lower than those found in previous studies of tropopause
height from climate models or observations. The increase in
altitude of the jets occurred mostly in the boreal winter
(Figure 3, center panels).
15] Santer et al. [2003] reported an average decrease in
tropopause pressure of 1.45 hPa/decade (+60 m/decade)
for the 1979 1999 period. In a climate model under a
‘business as usual’ CO
emission scenario, Kushner et al.
[2001] found a rising of the SHP by up to 3 hPa/decade
(+79 m/decade in standard atmosphere), and Lorenz
and DeWeaver [2007] an average tropopause rising of
40 m/decade (1.52 hPa/decade in standard atmosphere ).
Seidel and Randel [2006] used radiosonde data to estimate
that the tropopause rose by 1.6 hPa/decade (+64 m/decade)
during 1980-2004.
16] The strength, or wind speed, decreased in the NH
(0.16 to 0.18 m/s/decade) and in the SHT jet (0.37 to
0.42 m/s/decade), and increased in the SHP jet ( +0.25 to
+0.42 m/s/decade). Whereas no significant seasonal differ-
ences were found for the wind speed decrease in the SHT
jet, the strengthening of the SHP jet occurred mostly in DJF,
as did the weakening of the NH jet (Figure 3, right panels).
17] The increasing trend in the SHP jet winds is consis-
tent with findings by Russell et al. [2006], who reported an
increase in the westerlies over the Southern Ocean of 20%
in the past 20 years, by Kushner et al. [2001], who found
smal l increases (+0.08 m/s/decade) due to green house
gases alone, and by Thompson and Salomon [2002], who
related wind speed increases to photochemical ozone losses.
5. Seasonal and Spatial Trends
[18] More details about changes in jet stream properties
can be obtained by looking at two-dimensional and seasonal
maps of trends from the ERA-40 dataset (the corresponding
trends from the NCEP/NCAR dataset are available in the
auxiliary material, Figures S1 and S2, right panels). In
general, the SH trends are more horizontally homogeneous
than in the NH, due to the greater ocean extent.
19] The jet stream shift towards the poles can be detected
in Figure 1, where the areas of high average wind speed (left
panels) are generally bounded by areas with statistically-
significant positive trends on their poleward sides, and by
areas of statistically-significant negative trends on their
equatorward sides (right panel s). In the NH in D JF
(Figure 1d), the jet wind speed has been increasing over a
band covering northern Europe, central Asia, and the
northern Pacific, to the north of the jet core (Figure 1c),
accompanied by a second band with negative trends over
southern Asia and the Pacific, to the south of the jet core.
This pattern in the NH is also evident in the boreal summer
(JJA, Figure 1f ). In the SH during the austral summer
(DJF), strong wind speed increases were found all around
Antarctica and strong wind speed decreases were found
further north along a band centered at about 40S (Figure 1d),
again consistent with a poleward shift of the jets. In the SH
during the austral winter (JJA), however, the SHP jet tended
to shift equatorward (Figure 1f ), but not enough to com-
pensate for the poleward shift observed in the SH austral
summer. This seasonal difference, with a strong poleward
shift in the SH summer, was found in a climate model by
Kushner et al. [2001], who related it to anthropogenic
20] Figure 2b further confirms that jet stream pressure
trends are overall negative on average, to indicate that they
are rising in altitude. In both hemispheres, the jets have
risen more in the summer than in the winter. For example, in
DJF (Figure 2d), statistically-significant negative pressure
trends in the SH are larger and more wide spread than in
JJA (Figure 2f ). In both hemispheres during their respective
winters (DJF for NH and JJA for SH), there is a strong
correspondence between wind speed and pressure trends
(Figures 1d and 2d for the NH, Figures 1f and 2f for the
SH). This suggests that, in both hemispheres in their
respective winters only, jet stream wind speed e ither
increases as the jet lowers (e.g., over central Asia), or
decreases as the jet rises (e.g., over the Southern Ocean
between Africa and Australia).
6. Conclusions
[21] Global warming is expected to affect the distribution
of mass (and thus pressure) in the atmosphere and therefore
affect the strength and location of the jet streams. Because
of the complex nature of jet streams, which are discontin-
uous in time and space, meandering, and with notable wind
speed and elevat ion vari ations, it i s important to use
objective and numerically stable metrics to define their
properties. We introduced mass and mass-flux weighted
averages of wind speed, pressure, and latitude of the jets
in both hemispheres and used these quantities to study jet
stream trends from a subset (1979 2001) of the ERA-40
and the NCEP/NCAR reanalyses. We found that, on aver-
age, the jets are generally moving poleward in both hemi-
spheres. The northern hemisphere jet is weakening. In the
southern hemisphere, the sub-tropical jet is also weakening,
whereas the polar jet is strengthening. However, seasonal
and local trends may differ from this general behavior. For
example, the northern hemisphere jet strengthened while
decreasing in altitude in the boreal winter over Asia.
22] In general, trends of jet stream properties found in
this study are consistent in sign, but smaller in magnitude,
with those found in previous studies. This suggests that the
weighted averages over large volumes used to characterize
the jet streams in this study correctly capture the jet
properties, but they must be considered conservative
lower-bound estimates for larger trends that may manifest
themselves in subsets of these volumes.
23] These changes in jet stream latitude, altitude, and
strength have likely affected, and perhaps will continue to
affect, the formation and evolution of storms in the mid-
latitudes and of hurricanes in the sub-tropical regions.
Further observations and analyses are needed to confidently
attribute the causes of these changes to anthropog enic
climate change, natural variability, or some combination of
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C. L. Archer and K. Caldeira, Department of Global Ecology, Carnegie
Institution of Washington, Stanford, CA 94305, USA. (
... При различных температурных тенденциях в слоях тропосферы и стратосферы возможны различные тенденции для режимов СТ [Мохов, 2022]. Проявляются также различия режимов СТ и их изменений в Северном и Южном полушариях [Archer and Caldeira, 2008;Pena-Ortiz et al, 2013]. В связи с отмеченными особенностями СТ и их изменчивости необходимо наряду с локальными режимами анализировать интегральные характеристики атмосферной динамики, связанные с СТ. ...
... Кинетическая энергия слоя атмосферы СП рассчитывается как в [Archer and Caldeira, 2008] ...
... Since model circulation responses are weak, present-day jet behaviour might be expected to be dominated by natural variability 18 , with the exception of the strong southern hemisphere summertime jet shift associated with stratospheric ozone loss 19 . Some studies have detected weak signals associated with poleward jet shifts in observed data 20 but the patterns of change have so far appeared complex and regional 21 . In this paper, we revisit the observed jet trends using the latest reanalyses and attempt to reconcile these with the apparently weak warming of the tropical upper troposphere. ...
... Also apparent is an upwards extension of the jets associated with the lifting of the tropopause. The structure of these trends is clearer in contrast to previous assessments using shorter time periods or more regional methods, especially in the northern hemisphere 20,21 . There is Fig. 1 Linear trends in DJF zonal mean zonal wind and temperature as a function of latitude and pressure from ERA5 and the CMIP6 historical simulations. ...
Full-text available
Climate models predict a weak poleward shift of the jets in response to continuing climate change. Here we revisit observed jet trends using 40 years of satellite-era reanalysis products and find evidence that general poleward shifts are emerging. The significance of these trends is often low and varies between datasets, but the similarity across different seasons and hemispheres is notable. While much recent work has focused on the jet response to amplified Arctic warming, the observed trends are more consistent with the known sensitivity of the circulation to tropical warming. The circulation trends are within the range of historical model simulations but are relatively large compared to the models when the accompanying trends in upper tropospheric temperature gradients are considered. The balance between tropical warming and jet shifts should therefore be closely monitored in the near future. We hypothesise that the sensitivity of the circulation to tropical heating may be one factor affecting this balance.
... The PFJ shows a poleward shift and an ascent into the upper levels of the atmosphere in SA, which is robust to results previously obtained for other reanalyses (Pena-Ortiz et al. 2013;Manney and Hegglin 2018;WMO 2018). Climate warming favors an expansion of the tropical circulation (Lucas et al. 2014) and an increase in the altitude of the tropopause (Santer 2003;Lorenz and DeWeaver 2007;Seidel and Randel 2007;Archer and Caldeira 2008;Xian and Homeyer 2019). The introduction of newly defined parameters allows us to make significant additional observations on the changes in the PFJ over the last decades. ...
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The upper-level jet stream is a critical element of atmospheric circulation, driving synoptic systems and extreme weather events. This study analyzes the impact of upper-level jets on South American (SA) summer temperature and precipitation under different El Niño-Southern Oscillation (ENSO) phases. Using the ERA5 reanalysis dataset from 1979 to 2022, we perform a daily multiparametric characterization of the jet stream, considering its spatial and temporal discontinuities. Besides latitude and intensity, we find that the departure and number of branches of the subtropical jet (STJ) and the longitudinal extent of the Pacific branch of the polar front jet (PFJ) are needed for their description. An additional parameter is required to characterize the STJ due to its absence on around 40% of summer days over SA. Moreover, we observe distinct long-term changes in PFJ parameters across different ocean basins. Three synoptic weather types (WTs) of the upper-level zonal wind are identified: normal conditions, a prominent STJ pattern, and a PFJ-only pattern. The latter pattern is associated with anticyclonic anomalies at 500hPa in the South Atlantic Ocean and an active SA Convergence Zone, which favors clear skies and warm (wet and cold) conditions in southern SA (Brazil). Consistently, the probability of experiencing warm spells in central Argentina is increased more than twofold. Finally, we detect that the temperature anomalies associated with the WTs are independent of the ENSO phase. However, ENSO modulates the frequency of the WTs: during La Niña (El Niño), the PFJ-only (prominent STJ) pattern is more common.
... They play an essential role in transporting heat and moisture, and in balancing the energy distribution between low-and high-latitude regions [2][3][4][5] . The westerlies have shifted poleward since the 1960s in response to global warming, which caused more severe midlatitude weather events, such as extreme storms and precipitation [6][7][8] . The increasing effects of such extreme events make it necessary to understand how the westerlies responded to different climate states. ...
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The mid-latitude westerly winds are a major component of the global atmospheric circulation and a dominant factor in mid-latitude climate change. Understanding their behaviour and the controls on their variations under different climate background states is essential for assessing climate system feedback. Here we present a midlatitude North Pacific Ocean aeolian dust record from core NP02 through the last glacial cycle, during which extreme and abrupt climatic oscillations occurred. We find low dust contents during Heinrich stadials 2, 4, 5, and 5a that we attribute using proxy-model comparison to westerly transportation path changes associated with Atlantic meridional overturning circulation (AMOC) reductions, which caused North Atlantic cooling and modified the westerly wave train pattern, particularly over the Tibetan Plateau. The finding that AMOC variations had significant impacts on the westerlies half-way around the world, through ocean-atmosphere interactions, improves understanding of large-scale westerly sensitivity to different climate states.
... It is interesting to note that historical records disagree on changes in Northern Hemisphere jet stream speeds [54][55][56] . Shifts in position are more robust. ...
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The trigger, pace, and nature of the intensification of Northern Hemisphere Glaciation (iNHG) are uncertain, but can be probed by study of ODP Site 1208 North Pacific marine sediments. Herein, we present magnetic proxy data that indicate a 4-fold increase of dust between ~ 2.73 and ~ 2.72 Ma, with subsequent increases at the start of glacials thereafter, indicating a strengthening of the mid-latitude westerlies. Moreover, a permanent shift in dust composition after 2.72 Ma is observed, consistent with drier conditions in the source region and/or the incorporation of material which could not have been transported via the weaker Pliocene winds. The sudden increase in our dust proxy data, a coeval rapid rise in dust recorded by proxy dust data in the North Atlantic (Site U1313), and the Site 1208 shift in dust composition, suggest that the iNHG represents a permanent crossing of a climate threshold toward global cooling and ice sheet growth, ultimately driven by lower atmospheric CO2.
... The interannual variations in turbulence are noticeably greater in the North Atlantic than the USA, possibly because of the influence of the North Atlantic Oscillation (NAO); see Kim et al. (2016Kim et al. ( , 2020. Note that, over the 42-year period, the average latitude of the subpolar jet may have shifted, but this shift is negligible compared to the latitudinal extent of these boxes (Archer & Caldeira, 2008;Simmons, 2022). Figure 3a by decomposing it into the 21 constituent CAT diagnostics. ...
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Plain Language Summary Turbulence is unpleasant to fly through in an aircraft. Strong turbulence can even injure air passengers and flight attendants. An invisible form called clear‐air turbulence (CAT) is predicted to become more frequent because of climate change. Here we analyze modern atmospheric data based on four decades of observations (1979–2020) to investigate whether CAT has already started to increase. We use 21 different turbulence calculations to ensure our results are as reliable as possible. We find clear evidence of large CAT increases in various places around the world at aircraft cruising altitudes since satellites began observing the atmosphere. For example, at a typical point over the North Atlantic, the upward trend is such that the strongest category of CAT was 55% more frequent in 2020 than 1979. Our study represents the best evidence yet that CAT has increased over the past four decades, consistent with the expected effects of climate change.
... Large-scale extratropical atmospheric circulation is shifting to the poles amid global warming. Including poleward shifts of westerly winds [4], jet streams [5], storm tracks [6] and the pattern of precipitation [7]. At the same time, the ocean fronts related to the atmosphere also move in the same trend. ...
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Using high-resolution NOAA Optimum Interpolation sea surface temperature (OISST) data, the temporal and spatial characteristics of typical oceanic fronts in the past 39 years, including Oyashio front, Gulf Stream front, Peru current front and southern Indian Ocean polar front, are analyzed. The results show that the intensities of the four typical fronts have all strengthened, among which the Peru current is the most significant and the South Indian Ocean polar front is the weakest. The movement of the fronts are quite different: the Oyashio front has a significant poleward movement, and the Gulf Stream front extending into the open sea part has a marginal poleward movement; while the south Indian Ocean polar front and Peru current front have no obvious movements.
... The PSJ maximum is located in the upper troposphere between 400 and 100 hPa. Previous studies define the jet stream core position using a mass-weighted mean wind speed from multiple levels (Archer and Caldeira 2008), or wind speed in a single level (Rikus 2018). We simplify consider the PSJ configurations and metrics derived from the 250-hPa zonal wind (U250), consistent with that for the midlatitude jet stream in many studies (e.g., Teng et al. 2014;Coumou et al. 2018;Belmecheri et al. 2017). ...
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The westerly wind on the poleward side of the summer polar jet stream (PJS) over the Western Hemisphere has significantly weakened since the 1980s. A weak summer PJS causes warming surface temperatures and deficient precipitation over Alaska and western North America, favoring extreme wildfire events. This study investigates influences of Arctic sea ice loss on the summer PJS variability over the Western Hemisphere. Regression analysis first provides observational evidence that Arctic sea ice reduction is related to a weakening summer Western Hemisphere PJS at interannual time scales. Atmospheric model ensemble simulations are then used to demonstrate that Arctic sea ice loss significantly contributes to observed Western Hemisphere Arctic warming and reduced meridional temperature gradient between midlatitudes and the pole in the lower and middle troposphere, acting to weaken the troposphere zonal wind and vertical wind shear from 55° to 75°N, and about 20–30% of observed weakened summer PJS trend during 1979–2014. Observational analysis and the model-based results also indicate that a significant portion of the observed trends of the PJS and vertical wind shear during 1979–2014 might be attributed to the decadal variability of the summer North Atlantic Oscillation (NAO). In the future climate, as more and more ice melts in the summer, the weakening effect of sea ice on the PJS will continue and will be superimposed onto the natural decadal variability of the PJS.
... I n recent decades the global atmospheric circulation has been subject to robust trends 1 . Some of these trends, such as tropical expansion and poleward shifts of the tropospheric extratropical jets, have been observed in both the Northern and Southern Hemispheres (NH and SH) [2][3][4][5][6][7] . Apart from these coherent trends, there are changes that are unique for specific regions, e.g., an increase in the mean sea-level pressure (MSLP) over subtropical regions and reduction in precipitation in Mediterranean-type climates, except in North America [8][9][10] , an amplification of the temperature trend over the Arctic and a reduction in the annual mean Arctic sea ice [11][12][13] , a weakening of the stratospheric polar vortex in the NH with an increased frequency of sudden stratospheric warming events [14][15][16] , and a decrease in the lower stratospheric geopotential heights over Antarctica in spring and summer [17][18][19] . ...
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The circulation of the atmosphere is subject to natural and anthropogenic forcings that alter the energy balance of the climate system. In each hemisphere the zonally averaged atmospheric circulation can be represented by a single overturning cell if viewed in isentropic coordinates, highlighting the connections between tropics and extratropics. Here we present clusters of the meridional atmospheric circulation based on reanalysis data. Our results reveal preferred global circulation regimes with two clusters in each solstice season. These clusters show strong trends in their occurrence in the last two decades of the 20th century coincident with the depletion of the low-stratospheric ozone over Antarctica. We hypothesize that a change in the occurrence of short-term circulation regimes may lead to some long-term atmospheric trends. Finally, we show a strong coupling between the atmospheric circulation in boreal and austral winters and propose a mechanism linking anomalies in both seasons to the stratospheric ozone that requires confirmation with modelling experiments.
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Previous studies proposed convective limits on Northern Hemisphere 500-hPa temperature from a maximum of ~−3 °C in the tropics to a minimum of ~−42 °C in the Arctic. Here, we further explore this topic using three current generation reanalyses. All three reanalyses indicate that there has been statistically significant trends in the yearly maximums in the coldest temperatures in the Arctic at 500 hPa (from 0.40 to 0.66 °C decade⁻¹), while two have statistically significant trends in the yearly minimums in the warmest 500-hPa temperatures in the Northern Hemisphere (0.13 and 0.19 °C decade⁻¹). As upper-level tropospheric winds are related to the meridional temperature gradient in the Northern Hemisphere through the thermal wind balance, we also analyze the trends in maximum zonal wind speed. There are very small trends in the yearly maximum in the highest 200-hPa zonal wind speeds in the Northern Hemisphere and a slight poleward movement in the latitude of the highest winds in the reanalyses. This does not point to the jet stream becoming wavier as was hypothesized by others. The reanalysis climatology is then used to evaluate four current generation Earth system models. These models driven by observed sea surface temperature and sea ice generally produce larger trends than represented by the reanalyses. They are all too cold when the warmest tropical temperatures are at their lowest in the mean annual cycle. Only one model produces the poleward movement of the latitude of highest winds. The reanalysis trends presented here can be used to assess which of the CMIP models are more reliable in the historic period and hence may provide more trustworthy future projections.
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Upper-level jet streams are among the most salient atmospheric flow features and are often associated with major tropopause breaks. Climatological analysis can help to better describe their structure, geographical distribution, seasonal variation and dynamical characteristics. Climatological mean zonal wind distributions exhibit a single maximum, the so-called subtropical jet, associated with the corresponding space-time average climatological tropopause break. In contrast, daily upper-air charts may reveal more intricate multiple-jet structures, including extratropical jets associated with in-situ transient synoptic-scale weather systems. In this study, a novel type of event-based jet stream climatology is presented for both hemispheres using ECMWF reanalysis data for the years 1979-1993 (ERA15). It involves the analysis of transient jet streams based upon an objective criterion for defining jets in the upper troposphere/lower stratosphere region. The knowledge of the space-time distribution of these jets provides a rich dataset for climatological studies. In addition to traditional monthly and seasonal jet climatologies, a particular attention is given to the baroclinic characteristics of jets, their classification and the relative frequency of single and double jet configurations.
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.
Climate variability in the high-latitude Southern Hemisphere (SH) is dominated by the SH annular mode, a large-scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex. We present evidence that recent trends in the SH tropospheric circulation can be interpreted as a bias toward the high-index polarity of this pattern, with stronger westerly flow encircling the polar cap. It is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex, which are due largely to photochemical ozone losses. During the summer-fall season, the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the Antarctic Peninsula and Patagonia and to the cooling over eastern Antarctica and the Antarctic plateau.
ERA-40 is a re-analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) in collaboration with many institutions. The observing system changed considerably over this re-analysis period, with assimilable data provided by a succession of satellite-borne instruments from the 1970s onwards, supplemented by increasing numbers of observations from aircraft, ocean-buoys and other surface platforms, but with a declining number of radiosonde ascents since the late 1980s. The observations used in ERA-40 were accumulated from many sources. The first part of this paper describes the data acquisition and the principal changes in data type and coverage over the period. It also describes the data assimilation system used for ERA-40. This benefited from many of the changes introduced into operational forecasting since the mid-1990s, when the systems used for the 15-year ECMWF re-analysis (ERA-15) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis were implemented. Several of the improvements are discussed. General aspects of the production of the analyses are also summarized.