1. Introduction
Earth’s troposphere features strong westerly jet streams in the subtropics and midlatitudes. These westerly jets arise from momentum convergence of both eddies and large-scale overturning circulation and are modulated by effects of topography and land–sea distribution. They play a vital role in shaping the regional climate and extreme weather. For example, the East Asian subtropical jet closely regulates the variation of the East Asian summer monsoon (e.g., Zhou et al. 2019; Kong and Chiang 2020; Chiang et al. 2020) while weather events near the west coasts of the United States and Europe are highly sensitive to the extension and location of the upstream westerly jets (Neelin et al. 2013; Simpson et al. 2019; Zhou et al. 2020). It is of great importance to understand the responses of the westerly jets to global warming.
Previous studies have identified a poleward shift in the zonal or annual mean westerly jet under global warming (e.g., Fyfe and Saenko 2006; Meehl et al. 2007; Barnes and Polvani 2013). The poleward jet shift, in concert with changes in the Hadley cell (e.g., Lu et al. 2007) and storm tracks (e.g., Yin 2005; Chang et al. 2012), has important consequences for future changes in regional climate and weather. The causes of the poleward jet shift have been extensively studied from various perspectives, including dynamics (e.g., Lorenz and DeWeaver 2007; Chen et al. 2008; Lu et al. 2008; Butler et al. 2010; Kidston et al. 2011; Wu et al. 2013), thermodynamics (e.g., Shaw and Voigt 2016), and poleward energy transport (e.g., Held 2015). The dynamic perspective, in particular, argues that global warming can affect the propagation, growth, and dissipation of midlatitudinal transient eddies, leading to a poleward shift in the eddy momentum convergence and consequently the westerly jet.
The eddy-driven zonal or annual mean poleward jet shift, however, does not necessarily hold at regional and seasonal scales. In the Northern Hemisphere (NH), the projected changes of the westerly jets vary substantially across regions and seasons (Simpson et al. 2014; Wills et al. 2019). In particular, the westerly jets can shift equatorward in the early-summer Asia–Pacific region, in the late-winter America–Atlantic region, and in the winter to spring seasons over the eastern Pacific. These equatorward jet shifts imply distinct climate impacts from those of the poleward shifts, yet they receive much less attention, and their causes are not well understood.
In this study, we investigate the causes of these equatorward jet shifts based on a hierarchy of coupled, prescribed SST, and aquaplanet models. We highlight an equatorward jet-shift mechanism from the enhanced tropical upper-level warming (ETUW), which under favorable seasonal/regional climatology overcomes the eddy-driven poleward jet shift. In section 2, the datasets and methods are described. In section 3, we describe in detail the projected seasonally and regionally dependent jet shifts under global warming. In section 4, we zoom into the pressure–latitude cross section of the distinct seasonal/regional jet shifts and illustrate the climatological-state-dependent competition between the poleward and equatorward jet shift mechanisms. In sections 5 and 6, we examine the relative contributions to the equatorward jet shifts of the global-scale uniform warming and the decomposed features of the surface warming pattern. In section 7, we further demonstrate the climatological-state tendency of the jet response through idealized aquaplanet simulations. Section 8 provides a summary with discussion.
2. Datasets and methods
a. Observations and reanalysis datasets
The responses of the atmospheric temperature, large-scale circulation, and eddy momentum flux convergence to El Niño are estimated from regression to the Niño-3.4 index. The atmospheric variables are obtained from the ERA5 reanalysis dataset (Hersbach et al. 2020). The Niño-3.4 index (i.e., the SST anomalies averaged across 5°N–5°S and 170°–120°W) is computed based on the HadISST dataset (Rayner et al. 2003).
b. Models
A hierarchy of coupled, prescribed-SST, and aquaplanet simulations is used for understanding the seasonally and regionally dependent jet shift. Figure 1 provides a summary of these simulations.
List of simulations used in this study as described in section 2b.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
1) Coupled simulations
The responses to global warming are projected by 20 global climate models (listed in Fig. 1) from phase 6 of the Coupled Model Intercomparison Project (CMIP6) (Eyring et al. 2016). The current and future climates are represented respectively by the 1980–2005 period in the historical simulation and the 2080–99 period under the SSP585 scenario. To maintain an equal weight for each model, we only use the first realization (r1i1p1), which is available for all models. Monthly outputs are used to illustrate the mean circulation changes while daily outputs are used to conduct the eddy momentum flux analysis. To facilitate comparison with the prescribed-SST simulations, the projected changes in the coupled models are normalized by the global-mean SST warming and then multiplied by 4 K.
2) Prescribed-SST simulations
The effects of uniform warming and surface warming pattern on the jet changes are examined based on a series of prescribed-SST simulations.
First, we quantify the effects of global-scale uniform warming and surface warming pattern using the AMIP, AMIP-p4K, AMIP-future4K, and AMIP-4CO2 simulations provided by AMIP6. Outputs are currently available from eight models (listed in Fig. 1). The AMIP simulations are forced with observed SST, radiative gases, and aerosols over the 1979–2008 period. In the AMIP-4CO2 experiment, the CO2 concentration is quadrupled with respect to AMIP. In the AMIP-p4K experiment, a uniform 4-K warming is superimposed on the prescribed observed SST. In the AMIP-future4K experiment, a composite SST warming pattern (derived from projections of coupled models and scaled to a global mean of 4 K) is superimposed [see details in Webb et al. (2017)]. The difference between AMIP-4CO2 and AMIP measures the direct effect of the increased CO2 without changes in SST; the difference between AMIP-p4K and AMIP measures the effect of uniform surface warming; and the difference between AMIP-future4K and AMIP-p4K measures the effect of surface warming pattern. The responses to global warming projected in coupled models consist of both the direct effect of the CO2 increase and the effect of the resultant SST warming (often referred to as the indirect effect). The direct and indirect effects can drive opposite responses of the westerly jets and what matters is the residual. Here, the effect of uniform warming is quantified as the sum of (AMIP-p4K minus AMIP) and 0.7 × (AMIP-4CO2 minus AMIP). The factor of 0.7 is introduced to approximately account for the mismatch between the 4-K SST warming and the quadrupled CO2 increase. As the mean climate sensitivity under CO2 doubling is roughly 3 K, a quadrupled CO2 would correspond to an SST warming of 6 K instead of 4 K. The conclusions of this study are not sensitive to the exact value of this factor (varied from 0.5 to 1).
Second, we isolate the effects of the enhanced equatorial warming (EEW) and zonally inhomogeneous warming through a series of idealized warming experiments using the CAM6 model (Danabasoglu et al. 2020). The control simulation is forced by the observed historical SST and radiative forcing. In the Full warming experiment, the projected SST warming pattern and CO2 increase from historical to future climate are superimposed. In the Full-noEEW experiment, the EEW signal (enhanced SST warming over the central to eastern equatorial Pacific Ocean and the equatorial Atlantic Ocean) is removed from the imposed SST warming pattern. The difference between Full and Full-noEEW then measures the effect of the EEW. In the Full-ZEEW warming experiment, a zonally averaged EEW is imposed by averaging the warming pattern within 15°S and 15°N. The difference between Full-ZEEW and Full-noEEW illustrates the effect of the zonally averaged EEW. In the Full-0.5NAWH experiment, the magnitude of the North Atlantic subpolar warming hole (NAWH) is halved. The NAWH is defined as where the warming in the subpolar North Atlantic is smaller than the zonal mean SST warming. In the Full-0.5NPEW experiment, the magnitude of the North Pacific subpolar enhanced warming (NPEW) is halved. The NPEW is defined as where the warming in the subpolar North Pacific is larger than the zonal mean SST warming. All the simulations are 45 years long, and the last 30 years are used for analysis.
3) Aquaplanet simulations with designed control climatology
Idealized aquaplanet models are employed to further illustrate the climatological-state-dependent jet responses to uniform warming and enhanced equatorial warming. By removing the influences of topography and land–sea distribution, aquaplanet models provide a simple and clearer setup for illustrating the dependency of the jet responses on the climatological state. The aquaplanet model is constructed by coupling the GFDL-AM2 (Anderson et al. 2004) atmosphere model to a saturated ocean surface with time-invariant and zonally homogeneous SST. Sea ice is not considered. The atmospheric model has 24 vertical levels and a horizontal resolution of C48, ∼1.875°. By manipulating the prescribed SST in the control simulations, we conduct two sets of experiments with distinct control climatology. In the first experiment, the control climatology is constructed to be favorable for the ETUW effect; that is, the climatological jet is neither too strong nor too poleward. To achieve this, we start from the observed MJJ Asia–Pacific SST and slightly adjust it to make the resultant jet climatology closer to the observed climatological jet in the MJJ Asia–Pacific region. The adjustment is guided by our observation that an increased meridional SST gradient will enhance the zonal wind on its poleward side. In the second experiment, we construct an unfavorable control climatology for the ETUW effect. The observed zonal-mean autumn SST is used so that the simulated westerly jet mimics that observed in the autumn zonal mean, strong and away from the tropics. In these two control simulations, the CO2 concentration is set at 360 ppm and the solar insolation is set constant at the value of the middle of the season.
Based on these two Aqua-Cntl simulations, we then examine the jet responses to uniform warming and enhanced equatorial warming. In the Aqua-p4KpCO2 simulation, the SST is increased uniformly by 4 K and the CO2 concentration is tripled. In the Aqua-p4KpCO2pEEW simulation, a zonally uniform enhanced equatorial warming (with a peak magnitude of 0.4 K) is further imposed. All the simulations are 15 years long and the last 10 years are used for analysis.
c. Spectral analysis of the eddy momentum flux convergence
The latitude–phase speed spectrum of eddy momentum flux convergence is analyzed at 300 hPa to illustrate the eddy feedbacks on the changes of the zonal wind (Randel and Held 1991; Chen et al. 2007). First, the eddy momentum flux convergence is decomposed as a function of zonal wavenumber and frequency using the mixed space–time cross-spectral analysis (Hayashi 1982). Then, the cospectrum is transformed as a function of zonal wavenumber and angular phase speed. Finally, the momentum flux spectrum at each latitude is summed over wavenumbers, resulting in a spectrum density as a function of latitude and angular phase speed. The ENSO regression is based on the ERA5 reanalysis dataset and the response to global warming is based on the projections of GFDL-CM4, MIROC6, and CESM2. Note that only the spectral analysis is applicable to the zonal mean, not the regional analysis.
3. The seasonally and regionally dependent shifts of the westerly jets under global warming
Figure 2 shows the spatial patterns of the projected changes in the NH westerly jets under global warming in the late winter [January–March (JFM)], early summer [May–July (MJJ)], and autumn [September–November (SON)] seasons at the upper (250 hPa), middle (400–700 hPa), and lower (850 hPa) troposphere. The projection is based on the ensemble mean of 20 coupled models in CMIP6 [section 2b(1); Fig. 2]. Climatologically, there are two dominant regional jets in the NH, one over the Asia–Pacific region and the other over the America–Atlantic region. The projected future jet changes vary substantially across seasons and regions. In JFM, the Asia–Pacific jet shifts poleward while the America–Atlantic jet shifts equatorward (Figs. 2a–c). In MJJ, the Asia–Pacific jet shifts equatorward while the America–Atlantic jet shifts poleward (Figs. 2d–f). It is only in autumn that a zonally uniform poleward jet shift is projected (Figs. 2g–i). The equatorward shifts are most notable in the upper and middle levels while the poleward jet shifts are significant throughout the troposphere.
Spatial pattern of future changes in the zonal wind at the upper (250 hPa), middle (400–700 hPa), and lower (850 hPa) troposphere in JFM, MJJ, and SON based on the ensemble-mean projection of the CMIP6 models. The current seasonal climatology is shown in the black contours. The white dots indicate that at least 75% of the models agree on the sign.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
Figure 3 shows the monthly series of the projected zonal wind changes and meridional jet shifts in the middle troposphere (400–700 hPa) for the Asia–Pacific region (125°E–180°), the America–Atlantic region (90°–50°W), and the zonal mean. Amid the general poleward jet shift, the equatorward jet shifts in the early-summer Asia–Pacific and winter America–Atlantic regions are notable (Figs. 3a,b). Based on the shift in the latitude of the peak middle tropospheric zonal wind, the ensemble-model mean projects an ∼0.6° equatorward shift for the MJJ Asia–Pacific jet (Fig. 3d) and a ∼1.5° equatorward shift for the JFM America–Atlantic jet (Fig. 3e). In the zonal mean, the westerly jet is projected to shift poleward throughout the year (Figs. 3c,f), but it should be remembered that the zonal mean change averages out the opposite changes across different longitudinal sectors.
(a)–(c) Projected future changes in the Asia–Pacific jet (125°–180°E), in the America–Atlantic jet (90°–50°W), and in the zonal-mean NH jet, respectively, as a function of month. The current monthly climatology is shown in black contours. The white dots indicate that at least 75% of the models agree on the sign. (d)–(f) Projected meridional shifts in the Asia–Pacific jet, in the America–Atlantic jet, and in the zonal-mean NH jet, respectively, as a function of month. The bars indicate the median of the ensemble projections, and the whiskers indicate the 25th and 75th percentile range.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
The seasonal/regional equatorward jet shifts under global warming are reminiscent of the observed equatorward jet shifts during El Niño. During El Niño, equatorial warming (Fig. 4a) leads to enhanced tropical upper-level warming (ETUW) (Fig. 4b), which steepens the subtropical meridional temperature gradient (Fig. 4c) and consequently enhances the westerlies equatorward of the climatological jet (Fig. 4d). The climatology and future changes of the meridional temperature gradient (measured using the 300–500-hPa mean) are indicated by the dashed lines and the orange shading in Fig. 4c. This thermally driven effect is assisted by positive eddy feedbacks, as enhanced zonal winds allow transient eddies to dissipate and deposit momentum convergence farther equatorward (Seager et al. 2005; Lu et al. 2008) (Fig. 4e). Under global warming, the thermal response of the troposphere also features an ETUW (Fig. 4g), which arises from both surface warming pattern that features El Niño–like enhanced warming near the equator (Fig. 4f) and tropical moist adiabat physics that amplifies the upper-level warming. While the zonal-mean response of the westerly jet to global warming features a poleward shift (Fig. 4i) driven by the midlatitudinal eddy changes (Fig. 4j), enhanced subtropical zonal winds are observed in the upper troposphere (Fig. 4i), consistent with an increased subtropical temperature gradient from ETUW (Fig. 4h). The zonal-mean picture reflects an average of the competition between the equatorward-shift mechanism from the ETUW and the poleward-shift mechanism from midlatitudinal eddies; it is possible that over certain regions and seasons the ETUW effect dominates and leads to equatorward jet shifts.
(a),(b) Spatial pattern of temperature anomalies at the ocean surface and 300–500 hPa during El Niño (regressed onto the Niño-3.4 index). (c) Zonal-mean temperature anomaly during El Niño as a function of latitude and pressure. The 240-K isotherm is shown in the solid lines, and the meridional gradient of the 300–500-hPa mean temperature is shown in the dashed lines (black for La Niña and red for El Niño). The orange shading shows the increase in the meridional temperature gradient from La Niña to El Niño (the decrease is not shown). (d) Zonal-mean zonal wind anomaly during El Niño, with the climatology shown in black contours. (e) Eddy momentum flux convergence anomaly during El Niño, with the climatology shown in black contours. The zonal-mean zonal wind is shown in the solid lines (black for La Niña and red for El Niño). (f)–(j) As in (a)–(e), but for the response to global warming, comparing the future (red) and current (black) climate. The isotherm is plotted at 240 K for current climate and 245 K for future climate. The ENSO regression in (a)–(e) is based on ERA5 and the future projection in (f)–(j) is based on climate models.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
4. Equatorward jet shifts as a climatological-state dependent response to the ETUW
Figure 5 shows the pressure–height cross section of the projected changes in the temperature and zonal wind averaged over the Asia–Pacific (125°–180°E) and America–Atlantic (90°–50°W) regions in the JFM, MJJ, and SON seasons. The speed and latitude of the climatological midlevel jet and the degrees of future jet shifts at the upper, middle, and lower troposphere are noted. In all regions and seasons, the ETUW is consistently present. In the JFM America–Atlantic and the MJJ Asia–Pacific regions, the enhanced subtropical zonal wind from the ETUW extends close to the lower troposphere and the jet changes feature an equatorward shift. In other seasons and regions, the enhanced subtropical zonal wind from the ETUW is confined to the upper level and the jet changes are dominated by a poleward shift.
(a) Longitudinal-mean changes in the (top) temperature and (bottom) zonal wind as a function of latitude and pressure for the Asia–Pacific region in (a) JFM, (b) MJJ, and (c) SON based on the ensemble-mean projection of CMIP6 models. For temperature, the climatological isotherms (240 K for current climate and 245 K for future climate) are shown in the solid lines and the meridional gradients of the 300–500-hPa mean temperature are shown in the dashed lines (black for the current climate and red for the future climate). The orange shading shows the increases in the meridional temperature gradient from current to future climate (the decrease is not shown). For zonal wind, the climatology in current climate is shown in contours. The latitude and speed of the climatological midlevel (400–700 hPa) jet and the degrees of the jet shifts at the upper, middle, and lower levels are noted (blue for poleward shifts and red for equatorward shifts). (d)–(f) As in (a)–(c), but for the America–Atlantic region.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
This raises the question of why the effect of the ETUW dominates in certain regions and seasons but not others. Intuitively, one would expect the ETUW to be most effective to affect the temperature gradient and drive the equatorward jet shift when the climatological meridional temperature gradient (or equivalently the climatological jet) is weak and peaks close to the tropics. Such intuition is consistent with the changes in the meridional temperature gradient and zonal wind shown in Fig. 5. In the JFM Asia–Pacific, the climatological meridional temperature gradient is so steep that the enhanced subtropical temperature gradient from the ETUW does not notably affect the climatological temperature gradient (Fig. 5a). In the JFM America–Atlantic, in contrast, the meridional temperature gradient is weaker and extends rather equatorward (Fig. 5d). The ETUW, by steepening the subtropical temperature gradient, extends the zone of the strong temperature gradient equatorward and shifts the jet equatorward. In MJJ (Figs. 5b,e), the climatological temperature gradients are weaker but peak more poleward compared to those in JFM (Figs. 5a,d). In the MJJ America–Atlantic (Fig. 5e), the climatological temperature gradient peaks poleward of 40°N, which is far away from the reach of the ETUW. As a result, while the ETUW steepens the meridional temperature gradient in the subtropics and enhances the subtropical zonal wind at the upper level, the midlatitudinal peak of the climatological temperature gradient is not affected. In the MJJ Asia–Pacific (Fig. 5b), the climatological temperature gradient peaks less poleward so the ETUW extends the zone of the strong temperature gradient equatorward and drives the westerly jet equatorward. In SON (Figs. 5c,f), the climatological temperature gradients are stronger and peak even farther poleward compared to those in MJJ. In both the America–Atlantic and Asia–Pacific regions, the ETUW does not notably affect the midlatitudinal peak of the climatological temperature gradient and the jet changes are dominated by poleward jet shifts.
The climatological-state dependency of the ETUW effect is further illustrated in Fig. 6a, which relates future jet shifts to current jet climatology for each month in different regions. The months are labeled by the numbers 1–12 and equatorward jet shifts are circled. Besides the America–Atlantic jet and the Asia–Pacific jet, we have also included the results of the zonal-mean SH jet and the NH east Pacific (150°–125°W) region. The zonal-mean SH jet features a poleward jet shift that is consistently seen at the middle and lower levels across seasons, and notable in the upper level in the austral summer when the climatological jet is strong (Kushner et al. 2001). The NH east Pacific does not have a strong westerly jet in climatology and the projected zonal wind change features an equatorward shift in the winter and spring seasons. With a large variation in the jet latitude and strength among different regions and seasons, the MJJ Asia–Pacific, the JFM America–Atlantic, and the winter/spring east Pacific are found to be in a special regime; that is, their climatological jets are neither too strong nor too poleward (Fig. 6a). This provides a favorable condition for the equatorward jet-shift effect of the ETUW to dominate the poleward jet-shift effect of the midlatitudinal eddies (schematically shown in Fig. 6b). In contrast, in other regions and seasons, the jets are either too strong or too poleward (Fig. 6a), leaving the ETUW effect dominated by the eddy-driven poleward jet-shift mechanism.
(a) Dependency of the projected jet shift (equatorward shifts are circled) on the climatological jet latitude (x axis) and speed (y axis) for the Asia–Pacific jet (red), the America–Atlantic jet (blue), the east Pacific region (gray), and the zonal-mean SH jet (yellow). The numbers indicate the month (e.g., 1 is January and 6 is June). (b) Schematic for the competing jet-shift mechanisms between the enhanced tropical upper-level warming (ETUW) and the midlatitudinal transient eddies.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
5. The contributions of global-scale uniform warming and surface warming pattern
To further understand the projection of coupled models, we decompose the jet responses into contributions of global-scale uniform warming and surface warming pattern, based on the prescribed-SST simulations of AMIP6 models [section 2b(2); Fig. 1].
Figures 7a–e show the effect of global-scale uniform warming, measured as the combined effect of a 4-K surface warming and a 4 × CO2 increase [AMIP-p4K − AMIP + 0.7 × (AMIP-4CO2 − AMIP); see section 2b(2) for details]. In the absence of a surface warming pattern, the ETUW arises only from the moist-adiabat physics and is thus weaker than that in coupled projection (Figs. 7d,e vs Figs. 5b,d). Nevertheless, the favorable climatology in the JFM America–Atlantic and in the MJJ Asia–Pacific are not changed, allowing the ETUW to effectively enhance the subtropical zonal wind. With a weaker magnitude, however, the effect of the ETUW no longer overcomes the poleward shift mechanisms. Instead, the westerly jet changes feature a broadening with enhanced zonal winds on both the equatorward and poleward flanks of the climatological jet, suggesting competing effects between the ETUW effect and the eddy-driven poleward jet shift. In unfavorable seasons/regions such as the JFM Asia–Pacific, the MJJ America–Atlantic, and SON, the jets shift poleward under uniform warming, largely reproducing what is seen in the coupled model projections.
(a)–(c) Spatial pattern of the responses of the midlevel (400–700 hPa) zonal wind to uniform warming in JFM, MJJ, and SON, respectively, based on the AMIP6 experiments. The climatological zonal wind is shown in the black contours. (d),(e) Longitudinal-mean responses of the temperature (in upper subplots) and zonal wind (in lower subplots) to uniform warming as a function of latitude and pressure for the JFM America–Atlantic and the MJJ Asia–Pacific. For temperature, the climatological isotherms are shown in solid lines and the meridional temperature gradients are shown in dashed lines, black for AMIP and red for AMIP-4K + 0.7 × (AMIP-4CO2 − AMIP). The orange shading shows the increases in the meridional temperature gradient (the decrease is not shown). The color map corresponds to the right set of values in the color bar. For zonal wind, the climatology in current climate is shown in contours. (f)–(j) As in (a)–(e), but for the responses to surface warming pattern. In the upper panels of (i) and (j), the climatological isotherms and meridional temperature gradients are shown in black for AMIP and red for AMIP+ (AMIP-future4K − AMIP-4K), and the color map corresponds to the left set of values in the color bar.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
Figures 7f–j show the effect of surface warming pattern, measured as the difference between the patterned and uniform warming experiments (AMIP-future4K − AMIP-p4K). The surface warming pattern includes many notable features such as the enhanced warming near the equator, the warming hole in the North Atlantic subpolar region (NAWH), and the enhanced warming in the North Pacific subpolar region (NPEW). In particular, the enhanced equatorial warming (EEW) is similar to the equatorial warming observed during El Niño (Figs. 4a,f); it would induce ETUW and drive equatorward jet shifts. Indeed, with favorable climatology in the JFM America–Atlantic and the MJJ Asia–Pacific, the ETUW from surface warming pattern substantially enhances the subtropical zonal wind and leads to equatorward jet shifts (Figs. 7f–j). In other seasons and regions with unfavorable climatological states, such as the JFM Asia–Pacific, the MJJ America–Atlantic, and SON, equatorward jet shifts are not notable (Figs. 7f–h).
The contributions of uniform warming and surface warming pattern are further illustrated from future shifts in the monthly Asia–Pacific and America–Atlantic jets (Fig. 8). The climatological-state dependency of the ETUW effect (from either moist adiabat or warming pattern) is clearly seen. When the climatology is favorable in the summer Asia–Pacific and the winter America–Atlantic, subtropical zonal winds are substantially enhanced by ETUW, so the general poleward jet shifts under global warming become insignificant (Figs. 8a,b, and orange bars in Figs. 8e,f) and the equatorial jet shifts under surface warming pattern become most notable (Figs. 8c,d, and green bars in Figs. 8e,f). Together, the effects of uniform warming and surface warming pattern accomplish the final equatorward jet shifts projected in coupled models.
(a),(b) Responses of the midlevel (400–700 hPa) Asia–Pacific jet and the America–Atlantic jet to uniform warming as a function of month in the AMIP6 experiments. The monthly climatology is shown in black contours. The white dots indicate that at least 75% of models agree on the sign. (c),(d) As in (a) and (b), but for the responses to the warming pattern. (e),(f) Meridional shifts in the midlevel Asia–Pacific jet and the America–Atlantic jet as a function of month in response to uniform warming (orange) and warming pattern (green). The bars indicate the median of the ensemble projection, and the whiskers indicate the 25th and 75th percentile range.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
6. The effects of the decomposed features of surface warming pattern
As mentioned above, the surface warming pattern includes many features such as EEW, NAWH, and NPEW. Their individual effects are examined here through idealized warming experiments using the CAM6 model [see details in section 2b(3)].
With the full warming pattern (Full), the CAM6 reproduces the seasonally and regionally dependent jet shifts seen in coupled projections (Fig. 9a). When the EEW is artificially removed from the full warming pattern (Full-noEEW), the equatorward shift in the JFM America–Atlantic jet becomes insignificant and the equatorward shift in the MJJ Asia–Pacific jet is substantially weakened (Fig. 9b). According to the absolute zonal-wind changes, the EEW (Full − Full-noEEW) is the dominating driver of the equatorward shift in the JFM America–Atlantic jet and contributes to about half of the equatorward shift in the MJJ Asia–Pacific jet (Fig. 9c). In projections of coupled models, the EEW is most significant in the central to eastern equatorial Pacific Ocean and in the equatorial Atlantic Ocean (Fig. 9a). To examine how important this zonal inhomogeneity of the EEW is, we have conducted an idealized experiment with the EEW signal zonally averaged (Full-ZEEW). Compared to those forced by the original El Niño–like EEW (Fig. 9c), the equatorward jet shifts under zonally averaged EEW become less pronounced in the JFM America–Atlantic region and more pronounced over the MJJ Asia–Pacific region (Fig. 9d). This is consistent with the fact that zonally averaging the EEW leads to a decrease in the equatorial warming in the central to eastern Pacific and an increase elsewhere.
(a) The Full warming pattern and the corresponding responses of the midlevel zonal wind in JFM (shading) and MJJ (contours). (b) As in (a), but for the warming pattern with EEW removed (Full-noEEW). (c) As in (a), but for the effect of the projected El Niño–like EEW, measured as Full − Full-noEEW. (d) As in (a), but for the effect of a zonally averaged EEW, measured as Full-ZEEW − Full-noEEW. (e) As in (a), but for the effect of the North Atlantic subpolar warming hole (NAWH), measured as Full − Full-0.5NAWH. (f) As in (a), but for the effect of the North Pacific subpolar enhanced warming (NPEW), measured as Full − Full0.5NPEW. Results are based on the CAM6 experiments with different SST warming patterns.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
The NAWH and the NPEW are two major features of the extratropical SST warming pattern. Their effects on regional jet changes are examined by halving their magnitudes in the full warming pattern (Figs. 9e,f). Overall, neither the NAWH nor the NPEW presents a strong influence on the equatorward jet shifts. The NAWH induces a general poleward jet shift and an eastward extension of the Atlantic jet in MJJ (Fig. 9e) whereas the NPEW tends to weaken the strength of the Asia–Pacific jet (Fig. 9f).
7. Jet shifts in idealized aquaplanet simulation with designed control climatology
The climatological-state dependency of the jet responses to uniform warming and EEW is further investigated in idealized aquaplanet simulations. The aquaplanet simulations, which exclude the effects of topography and land–sea distribution, have been widely used in theoretical understanding of jet dynamics. Previous studies have focused on the zonal-mean jet response, so they often use an idealized SST or slab ocean to force the model and do not intend to mimic the realistic climatology of specific seasons/regions. In those experiments, the westerly jet is projected to shift poleward under warming, at least in the middle and lower levels (e.g., Lu et al. 2010, 2014; Medeiros et al. 2015; Voigt and Shaw 2016). Here, by manipulating the prescribed SST profiles [see details in section 2b(4)], we construct two sets of experiments that have distinct jet climatology in their control simulations and illustrate how climatology affects the jet responses to both uniform warming and EEW.
In the first experiment (prescribed SSTs in Fig. 10a), the jet climatology in the control simulation mimics the observations in the MJJ Asia–Pacific region. The jet is neither too strong nor too poleward, featuring a favorable climatology for the ETUW effect. In response to uniform warming (Fig. 10b), the ETUW from moist adiabat leads to a notable steepening of the subtropical temperature gradient. The jet change features a broadening with enhanced zonal winds at both flanks of the climatological jet, similar to the wind responses in Figs. 7d and 7e. With designed favorable climatology, the enhanced subtropical zonal winds extend to the lower levels, instead of confined to the upper troposphere as seen in previous studies (e.g., Chen et al. 2013). There is also notable eddy feedback that contributes to the enhanced subtropical zonal wind. In response to enhanced equatorial warming (Fig. 10c), the westerly jet shifts equatorward. This equatorward jet shift is consistently seen throughout the troposphere and is also contributed by eddy feedback. In combination (Fig. 10d), the total jet change features a clear equatorward jet shift. The enhanced zonal wind poleward of the climatological jet as seen in the response to uniform warming is dominated by the equatorward jet-shift change.
The jet responses in aquaplanet simulations with the control climatology similar to the MJJ Asia-Pacific region. (a) Zonal mean SST prescribed in the Aqua-Cntl, Aqua-p4KpCO2, and Aqua-p4KpCO2pEEW simulations. (b)–(d) Zonal mean responses of the (top) temperature, (middle) zonal wind, and (bottom) eddy-momentum flux convergence to uniform warming (Aqua-p4KpCO2 − Aqua-Cntl), to EEW (Aqua-p4KpCO2pEEW − Aqua-p4KpCO2), and to total warming (Aqua-p4KpCO2pEEW − Aqua-Cntl). In the top panels, the meridional temperature gradients are shown in the dashed lines and the increases in the meridional temperature gradient are shown in orange shading.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
In the second experiment (prescribed SSTs in Fig. 11a), the jet climatology in the control simulation mimics the observations in the NH autumn zonal mean. The jet is strong and away from the tropics, featuring an unfavorable climatology for the ETUW effect. In responses to uniform warming (Fig. 11b), the ETUW from moist adiabat does enhance the subtropical temperature gradient, but the peak of the climatological temperature gradient is too poleward to be affected by the subtropical influence. Consequently, the jet change is dominatedby the eddy-driven poleward jet shift. In response to EEW (Fig. 11c), the ETUW does enhance the subtropical zonal wind, but the effect is much muted compared to that in the first experiment (Fig. 10c). Also, the positive feedback from the eddy-momentum flux is absent. In combination (Fig. 11d), the enhanced subtropical zonal wind is confined to the upper level and the jet change is dominated by a poleward jet shift.
As in Fig. 10, but for the jet responses with the control climatology similar to the NH autumn zonal mean.
Citation: Journal of Climate 35, 16; 10.1175/JCLI-D-21-0723.1
These results resemble the jet responses to uniform warming and EEW in comprehensive climate models over favorable and unfavorable seasons/regions (Fig. 7) and support our hypothesis of the climatological-state dependency of the jet responses.
8. Summary and discussion
The responses of the westerly jets to global warming vary substantially across seasons and regions in the Northern Hemisphere. In contrast to the zonal mean poleward shift, the westerly jets are projected to shift equatorward in the early-summer Asia–Pacific region, in the late-winter America–Atlantic region, and in the winter and spring east Pacific. It is only in autumn that a zonally consistent poleward jet shift is projected. The opposite regional jet shifts between different seasons occur despite the fact that the large-scale SST warming pattern is similar between seasons (Fig. 9a). Here, we interpret the seasonally and regionally dependent jet shifts as a climatological-state-dependent competition between the poleward jet-shift mechanism from the changing midlatitudinal eddies and the equatorward jet-shift mechanism from the enhanced tropical upper-level warming (ETUW). When the climatological seasonal/regional meridional temperature gradient (or equivalently the climatological jet) is weak and peaks close to the tropics, the increased subtropical temperature gradient from the ETUW is effective in modulating the climatological temperature gradient and driving equatorward jet shifts. When the climatological seasonal/regional westerly jet is strong or peaks far away from the tropics, the jet regime is less susceptible to the influence of the ETUW and the westerly jet shifts poleward under the dominating effect of the midlatitudinal eddies. The ETUW is contributed by both tropical moist adiabat physics that amplifies the upper-level warming (even under uniform warming) and surface warming pattern that features enhanced equatorial warming. Based on AMIP experiments, we show that both the uniform warming and the enhanced equatorial warming contribute to the final equatorward jet shifts, and both their effects are seasonally and regionally dependent. Under favorable climatology in the early-summer Asia–Pacific and the late-winter America–Atlantic region, the ETUW substantially enhances the subtropical zonal wind, so the general poleward jet shifts under global warming become insignificant (featuring a broadening change with enhanced zonal wind on both flanks) and the equatorward jet shifts driven by the enhanced equatorial warming become most substantial. The dependency on regional/seasonal climatology is further demonstrated using aquaplanet simulations with designed climatological states.
We would like to highlight that the ETUW-induced equatorward jet shifts are most significant in the upper and middle troposphere (although equatorward shifts are also seen at 850 hPa). This is consistent with the fact that the ETUW enhances the subtropical meridional temperature gradient at the upper and middle levels (although the feedback from the eddy-momentum flux may help extend the effect to the lower troposphere). The eddy-driven poleward jet shift, on the other hand, is present throughout the troposphere. This is consistent with the fact that the eddy momentum flux convergence affects the column-integrated zonal momentum budget, which must be balanced by surface zonal drag from the low-level jet.
It is worthwhile to note that, besides the climatological state of the regional/seasonal westerly jet, the equatorward jet-shift effect of the ETUW may also depend on its the meridional extent/structure. Previous studies showed that, different from a narrow tropical warming, a broad tropical warming can lead to a poleward jet shift (Sun et al. 2013; Lu et al. 2014). These results were based on the idealized dry primitive equation model and the climatological jets in these simulations are located poleward of 40°N (i.e., unfavorable for the equatorward shift effect of the ETUW). Here, the enhanced equatorial warming has a narrow meridional extent, so like El Niño it drives a consistent equatorward jet shift. For uniform warming, in favorable seasons/regions the jet responses feature a broadening change with enhanced zonal winds on both flanks whereas in unfavorable seasons/regions the jet responses are denominated by a poleward jet shift.
Our study has focused on the regions with a notable climatological jet. Analyzed based on the sector-mean pressure–latitude cross section, the seasonally and regionally dependent jet shifts can be interpreted as a climatological-state-dependent response to two competing jet-shift mechanisms. It should however be noted that the longitudinal shift in the climatological global pattern may also contribute to regional jet changes, particularly over the regions with weak climatological zonal winds. For example, the winter zonal wind changes over the eastern Pacific can be interpreted as an ETUW-induced equatorward jet shift (gray dots in Fig. 6a) but may also be attributed to an eastward shift of the climatological global pattern (Simpson et al. 2016).
Our results motivate more research on future jet changes at regional and seasonal scales. Compared to the zonal or annual mean jet change, the seasonal and regional jet changes have more direct impacts on climate and weather. It is however more challenging to elucidate the underlying mechanisms of regional/seasonal jet changes as they involve complex interactions among regional factors and are harder to analyze (e.g., the spectral analysis of eddy momentum flux is not applicable at regional scales). Regional or local analysis frameworks, such as that proposed by Huang and Nakamura (2016) and Wang et al. (2021), may be helpful. A clear understanding of the regionally and seasonally dependent jet changes would lend confidence in projecting their impacts on the low-level circulation, regional climate, and extreme weather. The equatorward shift in the MJJ Asia–Pacific jet may be responsible for the intensification of the pre-mei-yu rainband (Chiang et al. 2019; Zhou et al. 2019). The poleward jet shift in the warm-season America-Atlantic region can drive seasonally dependent precipitation changes in the central United States, through its imprints on the low-level circulation and storm tracks (e.g., Zhou et al. 2022). In this study, we have focused on the ensemble-mean projection, but as indicated in Fig. 3 there is a notable spread in the projected degrees of jet shift among individual models. This leads to uncertainty in their impacts on regional climate and weather. Our results imply that uncertainties in the simulated regional climatology and the projected enhanced equatorial warming may play a role in explaining the intermodel spread of the jet shift. Future studies are needed to investigate the detailed mechanisms and provide potential constraints on the projection uncertainty.
Acknowledgments.
This study was supported by Office of Science, U.S. Department of Energy Biological and Environmental Research as part of the Regional and Global Model Analysis program area. We acknowledge the WCRP Working Group on Coupled Modeling, which is responsible for the CMIP. The Pacific Northwest National Laboratory (PNNL) is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract DE-AC02-05CH11231. The authors declare that they have no competing interests.
Data availability statement.
The CMIP6 outputs used in this study can be obtained from the CMIP6 archive at https://esgf-node.llnl.gov/projects/esgf-llnl. The ERA5 reanalysis dataset is available at https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5. The HadISST dataset is available at https://www.metoffice.gov.uk/hadobs/hadisst.
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