Distinct Responses of North Pacific and North Atlantic Summertime Subtropical Anticyclones to Global Warming

Chao He aGuangdong–Hong Kong–Macau Joint Laboratory of Collaborative Innovation for Environmental Quality, Institute for Environmental and Climate Research, Jinan University, Guangzhou, China

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Tianjun Zhou bState Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

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Abstract

The subtropical North Pacific and North Atlantic are controlled by basin-scale anticyclones in boreal summer. Based on a novel metric regarding the strengths of the rotational and the divergent circulation of anticyclones, we investigated the possible future responses in the intensity of these two subtropical anticyclones to global warming. While the North Atlantic subtropical anticyclone (NASA) is projected to strengthen, the North Pacific subtropical anticyclone (NPSA) is projected to weaken, in terms of both the rotational and the divergent circulation. The distinct responses of the NPSA and NASA are corroborated by the models participating in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP), under both intermediate and high emission scenarios. We further investigated the possible mechanism for their distinct responses by decomposing the effect of greenhouse gas forcing into the direct effect of increased CO2 concentration and the indirect effect through sea surface temperature (SST). The intensified NASA results from the CO2 direct forcing while the weakened NPSA is dominated by the SST warming. The CO2 direct forcing enhances the NASA by increasing land–ocean thermal contrast anchored by the largest subtropical continental area, the Eurasian–African continent. Both the uniform SST warming and the change in SST pattern act to weaken the NPSA by increasing the latent heating over the subtropical North Pacific basin, as suggested by atmospheric component model simulations. The distinct responses of the NPSA and the NASA may lead to zonal asymmetry of the subtropical climate change.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Chao He, hechao@jnu.edu.cn

Abstract

The subtropical North Pacific and North Atlantic are controlled by basin-scale anticyclones in boreal summer. Based on a novel metric regarding the strengths of the rotational and the divergent circulation of anticyclones, we investigated the possible future responses in the intensity of these two subtropical anticyclones to global warming. While the North Atlantic subtropical anticyclone (NASA) is projected to strengthen, the North Pacific subtropical anticyclone (NPSA) is projected to weaken, in terms of both the rotational and the divergent circulation. The distinct responses of the NPSA and NASA are corroborated by the models participating in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP), under both intermediate and high emission scenarios. We further investigated the possible mechanism for their distinct responses by decomposing the effect of greenhouse gas forcing into the direct effect of increased CO2 concentration and the indirect effect through sea surface temperature (SST). The intensified NASA results from the CO2 direct forcing while the weakened NPSA is dominated by the SST warming. The CO2 direct forcing enhances the NASA by increasing land–ocean thermal contrast anchored by the largest subtropical continental area, the Eurasian–African continent. Both the uniform SST warming and the change in SST pattern act to weaken the NPSA by increasing the latent heating over the subtropical North Pacific basin, as suggested by atmospheric component model simulations. The distinct responses of the NPSA and the NASA may lead to zonal asymmetry of the subtropical climate change.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Chao He, hechao@jnu.edu.cn

1. Introduction

The subtropical anticyclones in summer are planetary-scale atmospheric circulation systems in the lower troposphere. There are two major subtropical anticyclones over the Northern Hemisphere, the North Pacific subtropical anticyclone (NPSA) and the North Atlantic subtropical anticyclone (NASA). The poleward flow on the western flank of the subtropical anticyclone transports abundant moisture into eastern subtropical continental areas such as East Asia and the eastern United States (e.g., Chang et al. 2000; Zhou et al. 2009; L. F. Li et al. 2012), and the equatorward flow and the associated subsidence on the eastern flank of the subtropical anticyclone contribute to the arid and hot summers in subtropical deserts and Mediterranean climate zones over the western subtropical continents (Wu et al. 2009; W. Li et al. 2012; Shaw and Voigt 2015; Abd El-Ghani et al. 2017; Cherchi et al. 2018).

Since it is the most important atmospheric circulation system in the subtropics, the possible change in the subtropical anticyclones is essential to future climate change over the subtropics including the monsoon region, the subtropical desert, and the Mediterranean climate zones (W. Li et al. 2012; Polade et al. 2017; Tuel and Eltahir 2020). Indeed, the uncertainty in the projected subtropical hydroclimate change under global warming is dominated by the dynamic (rather than thermodynamic) component, that is, the uncertainty in the change of subtropical atmospheric circulation (Shepherd 2014; Zhou et al. 2018; Levine and Boos 2019; Chen et al. 2020; Monerie et al. 2020). The changes in the subtropical anticyclones also modulate the track and translation speed of the tropical cyclones by altering the steering flow (Zhao and Wu 2014; Kossin 2018; Sun et al. 2021), and affect the subtropical fishery by altering the alongshore wind and coastal upwelling (e.g., Garreaud and Falvey 2009; Yuan and Yamagata 2014; Aguirre et al. 2019).

The responses in the intensity of the subtropical anticyclones to global warming remain inconclusive. Geopotential height is the most widely used metric for subtropical anticyclones, but geopotential height increases systematically under global warming due to the thermal expansion of the atmosphere and it may show spurious intensification of high pressure systems (Yang and Sun 2003; He et al. 2015, 2018; Huang et al. 2015; Wu and Wang 2015). W. Li et al. (2012, 2013) suggested that all the subtropical anticyclones are projected to intensify under global warming scenarios, based on the change in streamfunction in the lower troposphere. But some other works suggested that the responses of NPSA and NASA to global warming may be different. Based on eddy streamfunction, Shaw and Voigt (2015) revealed an intensification and westward shift of the NASA but insignificant change of the NPSA under a future global warming scenario. He et al. (2017) argued that streamfunction may not accurately capture the local circulation change at basin scale, and revealed that the NASA is projected to intensify while the NPSA is projected to weaken, based on a variety of metrics including wind vectors, relative vorticity, divergence, and subsidence. As revealed by Cherchi et al. (2018), the responses of subtropical anticyclones are not uniform across different ocean basins under global warming, and the metrics are important in evaluating their changes.

The formation of the subtropical anticyclones is attributed to the zonal asymmetric diabatic heating related to the land–ocean distribution (e.g., Ting 1994; Chen et al. 2001; Rodwell and Hoskins 2001; Liu et al. 2001, 2004). The zonal asymmetry of sea surface temperature (SST) across the subtropical ocean basin interacts with the subtropical anticyclones, and acts as a positive feedback to intensify the subtropical anticyclones (Seager et al. 2003; Miyasaka and Nakamura 2005). Based on the formation mechanism of the subtropical anticyclones, W. Li et al. (2012, 2013) suggested that the subtropical anticyclones may be intensified by the enhancement of the land–ocean contrast in diabatic heating under global warming, including the enhanced oceanic longwave cooling and continental sensible and latent heating. However, the response of summertime circulation to global warming across the entire subtropical Northern Hemisphere is characterized by a zonal wavenumber-1 pattern (Shaw and Voigt 2015), although there are two continents and two ocean basins in subtropical Northern Hemisphere.

The increased greenhouse gas (GHG) concentration has an impact on the climate system through two physical processes, namely the direct radiative forcing due to the increased GHG (mainly CO2) concentration and the indirect effect through the SST warming (Shaw and Voigt 2015; Chadwick 2016; Ceppi et al. 2018; Baker et al. 2019; Zappa et al. 2020; Fahad and Burls 2021). The direct radiative effect of increased CO2 concentration acts to enhance the subtropical anticyclones and the Asian–African monsoon by enhancing the land–ocean thermal contrast (Shaw and Voigt 2015, 2016; He et al. 2020; Li et al. 2022), and the circulation response to direct CO2 forcing is dominated by the increased CO2 concentration over land (Shaw and Voigt 2016). The SST warming generally compensates the effect of CO2 direct forcing and weakens the subtropical anticyclones (Shaw and Voigt 2015, 2016), and the change in the spatial pattern of SST under global warming further alters regional climate (He et al. 2017; Levine and Boos 2019; Baker et al. 2019; Fahad and Burls 2021).

This study revisits the responses of the NPSA and NASA to global warming and focuses on the change in the intensity. The following two questions will be addressed: 1) Based on the local rotational and divergent circulation over each ocean basin, are the NPSA and the NASA projected to intensify or weaken? 2) What is the mechanism for the possibly different changes in the intensity of the NPSA and the NASA? The projected changes in these two subtropical cyclones will be investigated based on the coupled general circulation models (CGCMs) participating in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). Idealized simulations based on an ensemble of models participating in CMIP6 are also analyzed to separate the effects of CO2 direct forcing and indirect SST warming on the subtropical anticyclones.

The remainder of this paper is organized as follows. The model experiments and method adopted in this study are described in section 2. The responses of the NPSA and NASA to global warming scenarios are investigated in section 3, and the mechanisms therein are investigated based on a hierarchy of idealized model experiments in section 4. Finally, the major conclusions are summarized and discussed in section 5.

2. Model experiments and method

a. Model experiments

To investigate the climate change under global warming scenarios, the historical, SSP5–8.5, and SSP2–4.5 experiments based on 40 CGCMs participating in CMIP6 are adopted for analysis, and the names for these 40 models are listed in Table S1 in the online supplemental material. The historical, RCP8.5, and RCP4.5 experiments based on 30 CGCMs participating in CMIP5 are also adopted for analysis, and the names for these models are listed in Table S2. Only one realization for each model is selected to give them equal weight. Based on CMIP6 and CMIP5 experimental design, the historical experiment is forced by observed historical external forcing (CO2, aerosols, etc.) from 1850 to the early twenty-first century (Taylor et al. 2012; Eyring et al. 2016), and the 1950–99 period (referred to as “20C”) of the historical experiment is adopted as a baseline. The SSP5–8.5 and the RCP8.5 experiments are forced by future high-emission scenarios toward a radiative forcing of 8.5 W m−2 by the year 2100, and the emission pathways toward this radiative forcing differ slightly between SSP5–8.5 and RCP8.5 scenarios. The SSP2–4.5 and the RCP4.5 experiments are forced by an intermediate pathway toward a radiative forcing of 4.5 W m−2 by the year 2100. Future projections are assessed for the 2050–99 period (referred to as “21C”).

The projected change is obtained as the difference between the 21C period of the scenario-projection experiment with the 20C period of historical experiment for each model, and the multimodel median (MMM) among the 40 (30) models participating in CMIP6 (CMIP5) is interpreted as the forced climate response under the scenario. The intermodel consensus of the MMM-projected change is defined as the percentage of individual models that agree on the sign of the MMM-projected change. Based on Power et al. (2012), a 70% intermodel consensus is equivalent to the 95% confidence level according to Student’s t test, assuming independence among individual models. The spatial pattern for the MMM-projected change under the SSP5–8.5 scenario is shown in this article, and the robustness of the projected change is examined under the other three scenarios. Although the high-emission scenario may be unrealistic, it helps to extract the forced response from the internal variability and stochastic model error, and it is more comparable to the idealized experiments described below.

The preindustrial control (piControl) and abrupt-4xCO2 experiments based on 46 CGCMs participating in CMIP6 (Table S1) are analyzed in this study. The piControl experiment is performed by forcing the CGCMs with external forcing agents fixed at preindustrial level, and integrated for at least 500 years with a fixed CO2 concentration of 280 ppm. Branching from the piControl experiment, the abrupt-4xCO2 experiment is performed by abruptly raising the CO2 concentration to 1120 ppm and holding it constant for a 150-yr integration (Eyring et al. 2016), close to the CO2 concentration at the end of the twenty-first century under the SSP5–8.5 scenario. In the first year of the abrupt-4xCO2 experiment, the CO2 concentration is already quadrupled but the SST response is rather weak compared with the piControl experiment, and the MMM change in the first year of the abrupt-4xCO2 experiment relative to the last-50-year average of the piControl experiment indicates the direct climatic effect of increased CO2 concentration, referred to as the “fast” response in this study (Bony et al. 2013; Chadwick et al. 2014; He et al. 2020; Li et al. 2022). The MMM of the change in the last 50 years relative to the first year of the abrupt-4xCO2 experiment results from the indirect effect of increased CO2 through SST change (including the SST warming and the pattern of SST change), and is referred to as the “slow” response (Chadwick et al. 2014; He et al. 2020; Li et al. 2022).

The AMIP, AMIP-4xCO2, AMIP-p4K, and AMIP-future4K experiments performed by the atmospheric component model (AGCM) of 11 models participating in CMIP6 are also adopted (Table S1) to diagnose the climatic effects of CO2 direct forcing, uniform SST warming, and SST pattern changes. The AMIP experiment is forced by observed monthly SST from 1979 to 2014. The AMIP-4xCO2 experiment uses the same SST as AMIP, but with a quadrupled CO2 concentration. The AMIP-p4K experiment uses the same CO2 concentration as AMIP, but with a spatially uniform SST warming by 4 K. The AMIP-future4K experiment also uses the same CO2 concentration as AMIP, but with a patterned SST warming. The amplitude of globally averaged SST warming is 4 K in AMIP-future4K experiment and the spatial pattern of SST change is obtained from the response of CGCMs to CO2 quadrupling (Webb et al. 2017). A comparison between the AMIP-4xCO2 and AMIP experiments shows the effect of CO2 direct forcing, a comparison between the AMIP-p4K and AMIP experiments shows the effect of uniform SST warming, and a comparison between the AMIP-future4K and AMIP-p4K experiments shows the effect of SST pattern change. Table 1 overviews the CMIP6/CMIP5 experiments adopted in this study. All of the CMIP6 and CMIP5 model data are bilinearly interpolated onto a common 2.5° × 2.5° grid before analysis.

Table 1

Overview of the CMIP6/5 experiments adopted in this study.

Table 1

The Linear Baroclinic Model (LBM), developed by Watanabe and Kimoto (2000), is adopted in this study to examine the response of the atmospheric circulation to prescribed latent heating. The LBM is a primitive equation model, similar to the idealized GCM adopted by W. Li et al. (2012). Here, we run the LBM at a coarse resolution of T42 with 20 vertical levels in sigma coordinates, and linearize it at the summer mean state based on NCEP–NCAR reanalysis dataset (Kalnay et al. 1996). In this study, the LBM is forced by the three-dimensional latent heating anomaly over subtropical North Pacific (15°–45°N, 120°E–120°W) constructed based on precipitation anomaly. At each grid point over subtropical North Pacific, the column latent heating anomaly (H) is calculated as H = L × Pr where L = 2.5 × 106 J kg−1 and Pr is the precipitation anomaly, and a constant bowl-like profile with a maximum at 500 hPa is used to approximate the vertical structure of latent heating anomaly at all grid points (see Fig. S1 in the online supplemental material).

b. Method

The basic features of the subtropical anticyclones in the Northern Hemisphere are clockwise rotation and divergence in the lower troposphere and subsidence in the free troposphere. Hence, we adopt relative vorticity (ζ) and divergence (δ) at 925 hPa and vertical velocity (ω) at 700 hPa as the basic variables, to measure the subtropical anticyclones and their projected changes. Sea level pressure (SLP) is used in this study to identify the location of the subtropical anticyclones, and the domains of the NPSA and NASA are defined as the region enclosed by the SLP = 1018 hPa isobar based on the 20C period of historical experiment simulated by the MMM of the models participating in CMIP6 (purple contours in Figs. 1b–d). The 1018-hPa isobar is selected because it covers a relatively large fraction of the subtropical ocean but it does not intersect the complex topography (Fig. 1). The change in SLP is not used to measure the projected change in the subtropical anticyclones, since there is a systematic increase of SLP due to increased atmospheric water vapor content under a warming climate (Yang et al. 2016).

Fig. 1.
Fig. 1.

The simulated 20C mean state in boreal summer based on the MMM of the 40 models participating in CMIP6. (a) Precipitation (shading; unit: mm day−1) and sea level pressure (contours; unit: hPa). (b) Relative vorticity (unit: 10−6 s−1) and wind at 925 hPa. (c) Divergence at 925 hPa (unit: 10−6 s−1). (d) Vertical velocity at 700 hPa (unit: 10−2 Pa s−1). The purple curves in (b)–(d) indicate the SLP = 1018-hPa contour shown in (a), which represent the domains of the NPSA and the NASA.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

The intensity of a cyclone (anticyclone) can be measured in terms of its rotational strength and its convergent (divergent) strength based on the Stokes theorem. According to the Stokes theorem, the anticlockwise rotational circulation along a closed boundary of a domain equals the regional integrated ζ within the domain, and the divergent circulation out of the boundary of a domain is equal to the regional integrated δ within the domain (schematically illustrated in Fig. S2). In this study, the rotational intensity of the NPSA (NASA) is defined as the regional integrated −1 × ζ over the NPSA (NASA) domain, and the divergent intensity of the NPSA (NASA) is defined as the regional integrated δ within the NPSA (NASA) domain, where the NPSA and NASA domains are the oceanic regions enclosed by the 1018-hPa isobar based on the MMM of the CMIP6 models as shown in Figs. 1b–d. Unlike the contour-following approach used in He et al. (2017), this study uses a simpler definition on the domains of NPSA and NASA without considering the location shift, and this simplification does not change the conclusion (see section 3).

3. Projected changes in NPSA and NASA

Figure 1 shows the MMM-simulated mean state of SLP and precipitation, relative vorticity (ζ), and divergence (δ) at 925 hPa and ω at 700 hPa based on the 20C period of historical experiment, to demonstrate the basic characteristics of the subtropical anticyclones. High SLP centers are located over the subtropical North Pacific and North Atlantic in summer, associated with scarce precipitation over the eastern flank of the subtropical anticyclones (Fig. 1a). Negative values of ζ are seen across the subtropical North Pacific and North Atlantic within about 20°–40°N, consistent with the clockwise rotation of the wind vectors across the subtropical ocean basins (Fig. 1b). Different from the negative ζ across the subtropical ocean basins, the low-level divergence is primarily located at the eastern flank of the subtropical oceans (Fig. 1c), which has a similar spatial distribution to the subsidence at 700 hPa (Fig. 1d), and explains the scarce precipitation at the eastern ocean basin (Fig. 1a).

The projected changes of the precipitation, ζ, and δ at 925Pa and ω at 700 hPa are shown in Fig. 2, based on the MMM of 40 models participating in CMIP6 under the SSP5–8.5 scenario. Generally, precipitation increases over the subtropical North Pacific but decreases over the subtropical North Atlantic (Fig. 2a). Consistent with precipitation, the pattern of change in the atmospheric circulation also shows asymmetry between the North Pacific and the North Atlantic (Figs. 2b–d). The projected change in wind at 925 hPa is characterized by an anomalous cyclone over the North Pacific associated with a positive ζ anomaly (Fig. 2b), suggesting that the anticyclonic rotational circulation over the North Pacific is weakened. Similarly, the projected changes in δ at 925 hPa and ω at 700 hPa over the subtropical North Pacific are both characterized by negative anomalies, suggesting that the subsidence and low-level divergence over the North Pacific are projected to weaken. In contrast, the projected changes in δ and ω are positive over the subtropical North Atlantic (Figs. 2c,d), suggesting an enhancement of the divergent circulation and subsidence associated with the NASA. Over the subtropical North Atlantic, the projected change in ζ is negative at the western sector but positive at the eastern sector within the domain of the NASA (Fig. 2b). This pattern suggests a westward shift in the location of the NASA (Shaw and Voigt 2015, 2016; He et al. 2017), and a quantitative metric is required to evaluate the change in its intensity.

Fig. 2.
Fig. 2.

Projected changes as the difference between SSP5–8.5 and historical experiments based on the MMM of the 40 models participating in CMIP6. (a) Precipitation (unit: mm day−1). (b) Relative vorticity (unit: 10−6 s−1) and wind at 925 hPa. (c) Divergence at 925 hPa (unit: 10−6 s−1). (d) Vertical velocity at 700 hPa (unit: 10−2 Pa s−1). Stippling indicates that the MMM-projected changes are in agreement among more than 70% of the individual models. The purple curves indicate the SLP = 1018-hPa contours in 20C, which represent the domains of the NPSA and the NASA.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

The rotational intensity and divergent intensity of the NPSA and the NASA are calculated under multiple scenarios (see section 2b), and the MMM-projected fractional change in the intensity is shown in Fig. 3. The 30th–70th percentile range among the individual models is also shown in Fig. 3 to examine whether the sign of the MMM-projected change is agreed by more than 70% of the individual models. Based on the MMM-projected changes under the SSP5–8.5 scenario, the rotational intensity and the divergent intensity of the NPSA decrease by 5.2% and 22.5%, and the rotational intensity and the divergent intensity of the NASA increase by 2.5% and 12.2%, respectively. The MMM-projected changes are agreed by more than 70% of the individual models except that only 68% of the individual models agree on the increased rotational intensity of the NASA (Fig. 3a). Generally, the NPSA shows a significant weakening tendency while the NASA shows a strengthening tendency under the SSP5–8.5 scenario, and the sign of change in the rotational intensity is consistent with the divergent intensity.

Fig. 3.
Fig. 3.

Projected fractional changes in the rotational intensity and divergent intensity of the NPSA and the NASA, based on the regional integrated −1 × ζ and δ within the domain enclosed by the purple contour over each ocean basin in Fig. 2. (a) The MMM-projected fractional changes (unit: %; left y axis) in the rotational intensity (blue bar) and divergent intensity (pink bar) based on 40 models under SSP5–8.5 scenario, with the thin bars indicating the range between the 30th and 70th percentiles among the individual models. The MMM-projected fractional changes per degree of global mean surface warming is shown as purple diamonds (unit: % K−1; right y axis). (b) As in (a), but for the SSP2–4.5 scenario. (c),(d) As in (a),(b), but based on 30 models participating in CMIP5 under the RCP8.5 and RCP4.5 scenarios, respectively.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

Figures 3b–d also show the projected changes under the SSP2–4.5 scenario based on the 40 models participating in CMIP6 and under the RCP8.5 and RCP4.5 scenarios based on the 30 models participating in CMIP5. Similarly, they show a weakened NPSA and an enhanced NASA based on all these scenarios, and the amplitudes of changes are generally smaller under intermediate-emission scenarios (SSP2–4.5 and RCP4.5) compared with high-emission scenarios (SSP5–8.5 and RCP8.5). Scaled by the amplitude of global mean surface temperature warming, the projected changes under all of the four scenarios consistently suggest that the NPSA (NASA) weakens (strengthens) by about 1% K−1 in terms of its rotational intensity, and weakens (strengthens) by about 5% K−1 in terms of its divergent intensity (purple diamonds in Fig. 3).

The fractional change in the divergent intensity is greater than the rotational intensity for both NPSA and NASA (Fig. 3), probably because the 1018-hPa isobar covers the area with the strongest negative ζ but only a fraction of the area with the strongest positive δ (Figs. 1b,c). If the divergent intensity is defined as the regional integrated δ within the domain for the strongest climatological divergence, the fractional change in divergent intensity becomes smaller and comparable to the fractional change in rotational intensity (Fig. 3 in He et al. 2017). As the strongest divergence is located near the equator and deviates from the well-accepted core region of subtropical anticyclones (Fahad and Burls 2021), we define the domain for the subtropical anticyclones based on an isobar in this study. The MMM of the 1018-hPa isobar in 20C is adopted here but the 1018-hPa isobar differs among models and its location may change under global warming (Fig. S3). If the domains for the NPSA and NASA are selected based on the 1018-hPa isobars for each model in 20C and 21C respectively, the conclusion of enhanced NASA and weakened NPSA does not change (Fig. S4a). Indeed, the projected location shift of the NPSA is insignificant, and the centroid of NASA shifts westward by only 0.4° per degree of warming (Fig. S4b). Therefore, it is reasonable to define the intensity of the NPSA and NASA without considering the shift in their locations, which is simpler than the contour-following approach used by He et al. (2017).

In summer, the adiabatic warming due to descent in the subtropical anticyclone is largely compensated by net diabatic cooling, and the net diabatic cooling is dominated by the longwave cooling component (Rodwell and Hoskins 2001; Liu et al. 2004; W. Li et al. 2012; see also Fig. S5). Under a warmer climate, the increased atmospheric static stability acts to weaken the atmospheric circulation by enhancing the efficiency of adiabatic warming (cooling) over descent (ascent) regions (Ma et al. 2012; Kitoh et al. 2013; He et al. 2017; Wang et al. 2020, 2021), but it cannot explain the distinct changes in the intensity of the NPSA and NASA. Figure 4 examines the projected changes in the column total diabatic heating under the SSP5–8.5 scenario and the four components calculated based on surface and top-of-atmosphere fluxes, including latent heating (LH), surface sensible heating (SH), and shortwave (SW) and longwave (LW) radiative heating. Obviously, the change in the latent heating shows the strongest spatial heterogeneity with increased LH over North Pacific and decreased LH over North Atlantic (Fig. 4a). The sensible heating increases over most continental regions (Fig. 4b), and the net SW heating increases and the net LW cooling strengthens over almost all grid points (Figs. 4c,d), consistent with W. Li et al. (2012). The changes in SH and SW components do not show an evident contrast between the North Pacific and North Atlantic, while a stronger enhancement of the LW cooling appears over the subtropical North Atlantic (Fig. 4d), possibly due to reduced cloud associated with suppressed convection (Ardanuy et al. 1991; Chen et al. 2000; Gu and Zhang 2002). The increased total diabatic heating over the North Pacific and decreased total diabatic heating over North Atlantic (Fig. 4e) mostly result from the distinct changes in latent heating over these two ocean basins, and partly owe to the stronger enhancement of LW cooling over the subtropical North Atlantic.

Fig. 4.
Fig. 4.

Projected changes of column heating based on the MMM of the 40 models participating in CMIP6 under SSP5–8.5 scenario (unit: W m−2): (a) latent heating, (b) surface sensible heating, (c) shortwave heating (d) longwave heating, and (e) total diabatic heating as the sum of the above four components. Stippling indicates that the MMM-projected changes are agreed upon by more than 70% of the individual models.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

Given the importance of latent heating to the distinct changes in total diabatic heating between the North Pacific and North Atlantic, Fig. 5 further examines the intermodel correlation between the changes in the rotational intensity of the NPSA and NASA with the projected changes in the precipitation (proportional to latent heating), based on the 40 CGCMs participating in CMIP6 under the SSP5–8.5 scenario. The change in the intensity of the NPSA (NASA) shows significant negative correlation with the change in local precipitation over the subtropical North Pacific (Atlantic), but the correlation is relatively weak over the deep tropics and the midlatitudes. A similar pattern of intermodel correlation is seen as divergent intensity is adopted (figure not shown). Indeed, the intermodel correlation between changes in NPSA (NASA) intensity and the change in the regional averaged precipitation over the subtropical North Pacific (Atlantic) are statistically significant at the 95% confidence level (Fig. S6). The above evidence further suggests the essential role of the local latent heating in the changes of the subtropical anticyclone intensity over each ocean basin, from an intermodel perspective. However, it is worth noting that the changes in latent heating (precipitation) and atmospheric circulation are coupled with each other, and other types of idealized experiments are needed to investigate the mechanism therein.

Fig. 5.
Fig. 5.

(a) Intermodel correlation between projected change in the rotational intensity of the NPSA and the change in precipitation, based on the SSP5–8.5 scenario of the 40 models participating in CMIP6. (b) As in (a), but for the NASA. Stippling indicates that the correlation coefficient is statistically significant at the 95% confidence level according to a t test.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

4. Mechanisms for the distinct responses of the NPSA and NASA

a. Fast and slow responses of the CGCMs to abrupt quadrupling of CO2

To investigate the possible mechanism for the distinct responses in the intensity of the NPSA and NASA, the abrupt-4xCO2 experiment is adopted to decompose the total climate response into fast and slow responses (see section 2 for details). As seen in Fig. 6, the fast response is characterized by negative ζ anomalies and positive δ anomalies at 925 hPa over the subtropical North Atlantic, which suggests an enhancement of the NASA in terms of the clockwise rotation and the divergent circulation. But the fast response is rather weak over subtropical North Pacific with low intermodel consensus (Figs. 6a,b). The distinct circulation changes are consistent with the much weaker precipitation reduction over the North Pacific than the North Atlantic as a fast response to CO2 quadrupling (He and Soden 2017; He et al. 2020). The slow response is characterized by positive ζ anomalies and negative δ anomalies over the subtropical North Pacific associated with an anomalous cyclone (Figs. 6c,d), suggesting a weakened NPSA. But the slow response is much weaker over the subtropical North Atlantic than the subtropical North Pacific (Figs. 6c,d). The total response to CO2 quadrupling (Figs. 6e,f), as the sum of the fast and slow responses, shows a similar spatial pattern to the projected changes based on the SSP5–8.5 scenario (Fig. 2), with a pattern correlation coefficient of 0.90 for the ζ field and 0.91 for the δ field. This high similarity suggests the dominant role of the increased CO2 concentration (rather than aerosol or land use changes) in the projected change of the subtropical anticyclones.

Fig. 6.
Fig. 6.

The (a),(b) fast, (c),(d) slow, and (e),(f) total responses of the CGCMs to abrupt quadrupling of CO2 concentration, based on the MMM of 46 CGCMs. (a),(c),(e) Relative vorticity (unit: 10−6 s−1) and wind vectors at 925 hPa. (b),(d),(f) Divergence at 925 hPa (unit: 10−6 s−1). Stippling indicates that the MMM is in agreement among more than 70% of the individual models. The purple curves indicate the domain of the NPSA and NASA based on the SLP = 1018 hPa contour in 20C.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

The temporal evolution of the anomalous intensity of NPSA and NASA in the abrupt-4xCO2 experiment relative to piControl experiment is shown in Figs. 7a and 7b. Relative to the preindustrial control level, the intensity of the NASA shows positive anomalies during the entire 150-yr integration (blue curve in Figs. 7a,b). It shows that the intensification of the NASA occurs in the first summer after the abrupt CO2 quadrupling and no further enhancement is seen in the following 149 years, which confirms the essential role of CO2 direct forcing. Different from the NASA, the intensity of the NPSA changes little in the first several years of the abrupt-4xCO2 experiment, but decreases gradually in the following decades toward a robust weakening (pink curve in Figs. 7a,b). The gradual weakening of the NPSA may result from either systematic SST warming or the change in the pattern of SST, and idealized experiments by AGCMs are needed to disentangle their relative roles (Huang et al. 2013; Zhou et al. 2014; Chadwick 2016).

Fig. 7.
Fig. 7.

(a) Temporal evolution of the anomalous rotational intensity of NPSA (red curve) and NASA (blue curve) during the entire 150-yr abrupt-4xCO2 experiment relative to the piControl experiment. Logarithmic x coordinates are adopted to illustrate the first several years. The light shading indicates the range of ±1 standard deviation for the interannual variability of the intensity indices based on the piControl experiment. (b) As in (a), but for the anomalous divergent intensity. (c) The longitudinal profile for the MMM-simulated fast response in SLP (black curve; unit: hPa; left y axis) and 925-hPa meridional wind (purple curve; unit: m s−1; right y axis) averaged within 20°–40°N.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

Previous studies suggested that the direct forcing of increased CO2 concentration acts to enhance the land–ocean thermal contrast (Bony et al. 2013; Shaw and Voigt 2015, 2016; Li and Ting 2017). Figure 7c shows the 20°–40°N averaged changes in SLP and 925-hPa meridional wind in the fast response to CO2 quadrupling. A substantial decrease of SLP is seen only over Eastern Hemisphere continental areas (about 20°W–120°E); it is absent over the North American continent (black curve in Fig. 7c), which is characterized by a zonal wavenumber-1 pattern (Shaw and Voigt 2015), and it does not project on the climatological zonal wavenumber-2 SLP distribution (Fig. S7). The strongest decrease of SLP is located at about 0°–60°E out of the Asian monsoon region (Fig. 7c), and this pattern of change may not be explained by the “monsoon-desert” mechanism (Rodwell and Hoskins 1996) although Asian monsoon rainfall increases under CO2 direct forcing (He and Soden 2017; He et al. 2020; Li et al. 2022). In fact, the summertime circulation response to CO2 direct forcing is primarily dominated by the increased net energy input over land where there is less cloud and water vapor masking, which is determined by the Eastern Hemisphere continent because of its much larger extent than the North American continent (Shaw and Voigt 2016).

Forced by the above zonal pressure gradient anomaly on the western flank of the Eurasian continent, anomalous northerly wind appears over subtropical North Atlantic (purple curve in Fig. 7c). Therefore, anomalous subsidence and low-level divergence over the subtropical North Atlantic are generated by the anomalous northerly wind via negative moist enthalpy advection (Wu et al. 2017), and the precipitation and latent heating are suppressed over the North Atlantic in response to CO2 direct forcing (Kelly et al. 2018; He et al. 2020), which may further enhance the NASA by stimulating an anticyclonic Rossby wave response (Gill 1980). Consistent with the baroclinic feature of tropical circulation associated with latent heating, the response of subtropical circulation in summer is characterized by baroclinic vertical structure (Fig. S8), different from the barotropic structure of circulation response in winter related to zonal propagating planetary waves (Simpson et al. 2016; Seager et al. 2019; Tuel et al. 2021).

b. Responses of the AGCMs to the increased CO2 and changes in SST

A series of AGCM experiments performed by 11 CMIP6 models, including the AMIP, AMIP-4xCO2, AMIP-p4K, and AMIP-future4K experiments, are analyzed here to investigate the possible mechanisms for the zonal asymmetric climate response. The AMIP-4xCO2 and the AMIP-future4K experiments are compared with the AMIP experiment to confirm the roles of the CO2 direct forcing and the patterned SST warming. Based on the MMM of the AGCM simulations, the NASA is enhanced due to the quadrupling of atmospheric CO2 concentration under fixed SST (Figs. 8a,b), and the NPSA is weakened due to the change in SST (Figs. 8c,d), which are evident in the responses of ζ, δ, and wind vectors at 925 hPa (Figs. 8a–d). The responses to increased CO2 and changed SST based on the AGCMs resemble the fast and slow responses of the CGCMs to abrupt CO2 quadrupling, indicating that AGCMs can be used to investigate the mechanism for the responses of the NPSA and NASA.

Fig. 8.
Fig. 8.

AGCM-simulated climate responses to (a),(b) CO2 increase, (c),(d) patterned SST warming, (e),(f) uniform SST warming of 4 K, and (g),(h) pure SST pattern change without warming. (a),(c),(e),(g) Relative vorticity (unit: 10−6 s−1) and wind vectors at 925 hPa. (b),(d),(f),(h) Divergence at 925 hPa (unit: 10−6 s−1). Stippling indicates that the MMM is in agreement among more than 70% of the individual models. The purple curves indicate the domains of the NPSA and NASA based on the SLP = 1018 hPa contour in 20C.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

The relative effects of uniform SST warming and SST pattern change are further examined, based on the difference between the AMIP-p4K and AMIP experiments and the difference between the AMIP-future4K and AMIP-p4K experiments. The effects of uniform SST warming and SST pattern change are both characterized by weakened NPSA, in terms of positive ζ anomaly and negative δ anomaly over subtropical North Pacific (Figs. 8e–h), and their magnitudes are both weaker compared with the total effect of changed SST (Figs. 8c,d). Compared with the subtropical North Pacific, the responses of the atmospheric circulation to SST warming and SST pattern change are weak and noisy over the subtropical North Atlantic (Figs. 8e–h), which requires further quantitative evaluation.

The quantitative change in the intensity of the NPSA and NASA is evaluated based on the AGCM experiments and shown in Fig. 9. As consistently suggested by the rotational intensity and the divergent intensity indices, the effect of uniform SST warming and the effect of SST pattern change both act to the weaken the NPSA, and their contributions are almost equally important, whereas a small fraction of the weakening is canceled out by the CO2 direct forcing. In contrast, the enhancement of the NASA primarily results from the CO2 direct forcing, and the effects of the uniform SST warming and the SST pattern change on the NASA are negligible compared with the CO2 direct effect. The above results confirm the dominant role of the CO2 direct forcing on the enhanced NASA under global warming, and further indicate that uniform SST warming and the SST pattern change are both responsible for the weakened NPSA under global warming.

Fig. 9.
Fig. 9.

The fractional change in the (a) rotational intensity and (b) divergent intensity of NPSA and NASA forced by quadrupling of CO2 concentration (blue bar), uniform SST warming by 4 K (pink bar), and SST pattern change (yellow bar), based on the MMM of the 11 AGCMs. The thin black error bar indicates the range between the 30th and the 70th percentiles based on the individual models.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

Section 3 already showed that latent heating dominates the zonal asymmetric response of total diabatic heating, and the response of precipitation (proportional to latent heating) to uniform SST warming is shown in Fig. 10a. Even without any change in the SST pattern, a uniform warming of global SST by 4 K induces a substantial increase of precipitation over the subtropical North Pacific but much weaker change in precipitation over the subtropical North Atlantic (Fig. 10a). Since the climatological precipitation over the North Pacific (particularly the western North Pacific) is more abundant than over the North Atlantic (Fig. 1a), the zonal asymmetric pattern of the subtropical precipitation response to uniform SST warming may be rooted in the abundance of the mean state precipitation, which is consistent with the “wet-get-wetter” or “wettest-get-wetter” paradigm of precipitation response to global warming (Held and Soden 2006; Hsu and Li 2012). Based on the spatial pattern of the change in precipitation anomaly as shown in Fig. 10a, the three-dimensional latent heating anomaly is constructed by assuming a fixed vertical profile with a maximum in the midtroposphere (Fig. S1). Forced by the three-dimensional latent heating anomaly over the subtropical North Pacific within 15°–45°N, 120°E–120°W, the LBM simulates an anomalous cyclone over the subtropical North Pacific (vectors in Fig. 10a), suggesting that the NPSA is weakened by the enhanced latent heating over the subtropical North Pacific (Gill 1980).

Fig. 10.
Fig. 10.

(a) The response of precipitation (shading; unit: mm day−1) to uniform SST warming of 4 K based on the MMM of 11 AGCMs, and the LBM-simulated response of 925-hPa wind (vectors; unit: m s−1) to the latent heating anomaly over the subtropical North Pacific (15°–45°N, 120°E–120°W). (b) As in (a), but for the response of precipitation to change in SST pattern and the LBM-simulated wind response to the latent heating anomaly over the subtropical North Pacific. (c) The difference in SST (unit: K) between AMIP-future4K and AMIP-p4K experiments. (d) The difference in SST pattern between 21C in the SSP5–8.5 experiment and 20C in the historical experiment (global mean SST warming is removed to highlight the pattern). Stippling indicates that the MMM is in agreement among more than 70% of the individual models.

Citation: Journal of Climate 35, 24; 10.1175/JCLI-D-21-1024.1

Figures 10b and 10c show the difference in precipitation and SST between the AMIP-future4K and AMIP-p4K experiments. An increase of precipitation over the North Pacific is also seen in response to the SST pattern change but the change in precipitation over the subtropical North Atlantic is weak and noisy (Fig. 10b). Forced by the change in latent heating over the subtropical North Pacific (15°–45°N, 120°E–120°W) shown in Fig. 10b, the LBM simulation shows an anomalous cyclone over subtropical North Pacific (vectors in Fig. 10b), which suggests a weakened NPSA. The precipitation response to the SST pattern change conforms to the “warmer-get-wetter” mechanism (Xie et al. 2010), as stronger SST warming is seen over the subtropical North Pacific than over the North Atlantic (Fig. 10c). Although the pattern of SST change in the AMIP-future4K experiment shown in Fig. 10c is derived from the SST change based on the coupled models participating in CMIP3 (Webb et al. 2017), stronger SST warming over the subtropical North Pacific is also evident based on CMIP6 models under the SSP5–8.5 scenario (Fig. 10d). Therefore, both uniform SST warming and the stronger SST warming over the subtropical North Pacific are responsible for the projected weakening of the NPSA.

The vertical structure of diabatic heating is not a direct output of CGCMs, and it is typically estimated based on the thermodynamic equation using temperature and circulation data as input (Yanai and Tomita 1998; W. Li et al. 2012; He et al. 2017). To avoid circular reasoning, the above LBM experiments are forced by the latent heating anomaly rather than the total diabatic heating anomaly, and this is reasonable because the distinct changes in total diabatic heating between the North Pacific and North Atlantic are primarily controlled by latent heating. The LBM simulations here confirm that the enhanced latent heating over the subtropical North Pacific alone acts to weaken the NPSA, regardless of the change in sensible heating, shortwave heating, or longwave cooling. As there is a net diabatic cooling (heating) over subtropical oceans (continents) (see Fig. S5), the enhanced latent heating over subtropical North Pacific reduces the land–ocean contrast of total diabatic heating between the North Pacific and the surrounding continents. Therefore, our result supports the viewpoint that the intensity of summertime subtropical anticyclone is controlled by zonal land–ocean thermal contrast (Liu et al. 2004; W. Li et al. 2012, 2013; Shaw and Voigt 2015, 2016) in terms of total diabatic heating, but our result highlights that the future change in land–ocean thermal contrast is dominated by latent heating over the ocean, which was overlooked by W. Li et al. (2012).

5. Summary and discussion

This study revisited the response of the subtropical anticyclones in boreal summer to global warming, in terms of their rotational intensity and the divergent intensity defined based on the Stokes theorem. We further examined the change in the NPSA and NASA based on a series of idealized experiments, including the fast and slow responses of 46 CGCMs to abrupt quadrupling of CO2 concentration, the responses of 11 AGCMs to CO2 quadrupling, uniform SST warming, and SST pattern change. The major conclusions are summarized below and compared with previous works.

  1. The NPSA is weakened while the NASA is enhanced under global warming scenarios, in terms of both the rotational intensity and the divergent intensity. The distinct changes in the intensity of NPSA and NASA are consistent under the SSP5–8.5 and SSP2–4.5 scenarios based on CMIP6 models and the RCP8.5 and RCP4.5 scenarios based on CMIP5 models. Associated with weakened NPSA and enhanced NASA, the total diabatic heating increases (decreases) over the subtropical North Pacific (Atlantic), dominated by distinct changes in latent heating over these two ocean basins. W. Li et al. (2012) suggested an intensification of both NPSA and NASA under global warming, while Shaw and Voigt (2015) suggested enhanced NASA but an insignificant change of the NPSA based on eddy streamfunction. Our result is consistent with previous studies about the projected enhancement of NASA but inconsistent with these previous works about the NPSA intensity, possibly due to the chosen metric. Indeed, the local rotational wind component within an ocean basin is determined by the horizontal gradient of streamfunction, rather than the absolute value of (eddy) streamfunction (He et al. 2017). Although streamfunction well describes the overall pattern of climatological subtropical anticyclones (e.g., Ting 1994; Wang and Ting 1999; Chen et al. 2001; Rodwell and Hoskins 2001), the projected change in circulation is a much smaller quantity superimposed on the climatology and more accurate metrics are required to describe it.

  2. The enhancement of the NASA under global warming primarily results from the enhanced land–ocean thermal contrast anchored by the distribution of subtropical continents under CO2 direct forcing. The Eastern Hemisphere continent is the greatest subtropical continent, which strongly perturbs the energy input to the atmosphere under direct CO2 forcing (Shaw and Voigt 2016), and substantial decrease in sea level pressure occurs primarily over this subtropical continent as a direct response to increased CO2 concentration. On the western flank of the Eurasian continent, the anomalous northerly wind associated with the zonal pressure gradient stimulates anomalous subsidence over the subtropical North Atlantic through negative moist enthalpy advection, and the suppressed precipitation and latent heating over subtropical North Atlantic may in turn reinforce the NASA by stimulating anticyclonic Rossby waves. Land–ocean thermal contrast is crucial for the intensity of the subtropical anticyclones under global warming (W. Li et al. 2012, 2013), but enhanced land–ocean thermal contrast under CO2 direct forcing is anchored by the Eastern Hemisphere continent, possibly because of its much greater size than the North American continent (Shaw and Voigt 2015, 2016), which explains why CO2 direct forcing intensifies NASA but has little impact on NPSA.

  3. The weakening of the NPSA is an indirect effect of increased CO2 concentration through SST, via both uniform SST warming and the SST pattern change. Possibly due to the more abundant mean state precipitation over the North Pacific, a much stronger increase of precipitation occurs over subtropical North Pacific even if global SSTs warm uniformly, and the precipitation further increases over subtropical North Pacific since the amplitude of SST warming over the North Pacific is stronger than the zonal mean. As a result, the increase of latent heating over the subtropical North Pacific acts to weaken the NPSA by stimulating cyclonic Rossby waves. Shaw and Voigt (2015) suggested that the response of the NPSA to global warming is controlled by the “tug-of-war” effects between CO2 direct forcing on a shorter time scale and SST warming on a longer time scale, but our work suggests that the SST warming dominates the projected weakening of the NPSA whereas the CO2 direct forcing has negligible effect on NPSA. While W. Li et al. (2012) emphasized the roles of continental heating and oceanic longwave cooling in the future change of the subtropical anticyclones, our work suggests that the change in latent heating over the ocean dominates the change in the land–ocean contrast of total diabatic heating, and the projected changes of the NPSA and the NASA are consistent with the change in land–ocean contrast of total diabatic heating.

The response of subtropical atmospheric circulation to global warming has been widely studied in terms of the Hadley cell in the zonal mean sense (e.g., Hu and Fu 2007; Karnauskas and Ummenhofer 2014; Xia et al. 2020), but zonal asymmetry is also very important for subtropical climate change. The distinct changes in the intensity of the NPSA and NASA may imply distinct changes in hydroclimate between different subtropical continents (Shaw and Voigt 2015; He et al. 2017, 2020; Wang et al. 2020) and distinct changes in the track and translation speed of tropical cyclone between the two subtropical oceans (Zhao and Wu 2014; Kossin 2018; Sun et al. 2021). This study investigated the dynamics for the distinct responses in the NPSA and NASA based on a hierarchy of model simulations, but the decadal change in observational record is not addressed because of the uncertainty in decadal trend among reanalysis datasets (Chang and Yau 2015; Befort et al. 2016), particularly over subtropical oceans where observations are scarce. Future studies are needed to examine the observed decadal change in the subtropical anticyclones based on datasets from multiple sources, and to address whether the anthropogenic forced signal has already emerged from the natural variability (Zhang et al. 2007; Deser et al. 2012).

Acknowledgments.

The authors wish to acknowledge the editor, Dr. Mingfang Ting, and three anonymous reviewers for their insightful comments and suggestions that improved the quality of the research. This work was supported by National Key Research and Development Program of China (2020YFA0608904) and National Natural Science Foundation of China (42175024).

Data availability statements.

The CMIP6 and CMIP5 model data used in this study were accessed at the official website of the Earth System Grid Federation (https://esgf-node.llnl.gov) by searching the names for the models listed in Tables S1 and S2. The code and the NCEP–NCAR basic-state data for LBM were accessed at the LBM website (https://ccsr.aori.u-tokyo.ac.jp/∼lbm/sub/lbm_4.html).

REFERENCES

  • Abd El-Ghani, M. M., F. M. Huerta-Martínez, L. Hongyan, and R. Qureshi, 2017: Arid deserts of the world: Origin, distribution, and features. Plant Responses to Hyperarid Desert Environments, 1st ed., M. M. Abd El-Ghani et al., Eds., Springer, 1–7.

  • Aguirre, C., M. Rojas, R. D. Garreaud, and D. A. Rahn, 2019: Role of synoptic activity on projected changes in upwelling-favourable winds at the ocean’s eastern boundaries. npj Climate Atmos. Sci., 2, 44, https://doi.org/10.1038/s41612-019-0101-9.

    • Search Google Scholar
    • Export Citation
  • Ardanuy, P. E., L. L. Stowe, A. Gruber, and M. Weiss, 1991: Shortwave, longwave, and net cloud-radiative forcing as determined from Nimbus 7 observations. J. Geophys. Res., 96, 18 53718 549, https://doi.org/10.1029/91JD01992.

    • Search Google Scholar
    • Export Citation
  • Baker, H. S., T. Woollings, C. Mbengue, M. R. Allen, C. H. O’Reilly, H. Shiogama, and S. Sparrow, 2019: Forced summer stationary waves: The opposing effects of direct radiative forcing and sea surface warming. Climate Dyn., 53, 42914309, https://doi.org/10.1007/s00382-019-04786-1.

    • Search Google Scholar
    • Export Citation
  • Befort, D. J., S. Wild, T. Kruschke, U. Ulbrich, and G. C. Leckebusch, 2016: Different long-term trends of extra-tropical cyclones and windstorms in ERA-20C and NOAA-20CR reanalyses. Atmos. Sci. Lett., 17, 586595, https://doi.org/10.1002/asl.694.

    • Search Google Scholar
    • Export Citation
  • Bony, S., G. Bellon, D. Klocke, S. Sherwood, S. Fermepin, and S. Denvil, 2013: Robust direct effect of carbon dioxide on tropical circulation and regional precipitation. Nat. Geosci., 6, 447451, https://doi.org/10.1038/ngeo1799.

    • Search Google Scholar
    • Export Citation
  • Ceppi, P., G. Zappa, T. G. Shepherd, and J. M. Gregory, 2018: Fast and slow components of the extratropical atmospheric circulation response to CO2 forcing. J. Climate, 31, 10911105, https://doi.org/10.1175/JCLI-D-17-0323.1.

    • Search Google Scholar
    • Export Citation
  • Chadwick, R., 2016: Which aspects of CO2 forcing and SST warming cause most uncertainty in projections of tropical rainfall change over land and ocean? J. Climate, 29, 24932509, https://doi.org/10.1175/JCLI-D-15-0777.1.

    • Search Google Scholar
    • Export Citation
  • Chadwick, R., P. Good, T. Andrews, and G. Martin, 2014: Surface warming patterns drive tropical rainfall pattern responses to CO2 forcing on all timescales. Geophys. Res. Lett., 41, 610615, https://doi.org/10.1002/2013GL058504.

    • Search Google Scholar
    • Export Citation
  • Chang, C. P., Y. S. Zhang, and T. Li, 2000: Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part I: Roles of the subtropical ridge. J. Climate, 13, 43104325, https://doi.org/10.1175/1520-0442(2000)013<4310:IAIVOT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Chang, E. K. M., and A. M. W. Yau, 2015: Northern Hemisphere winter storm track trends since 1959 derived from multiple reanalysis datasets. Climate Dyn., 47, 14351454, https://doi.org/10.1007/s00382-015-2911-8.

    • Search Google Scholar
    • Export Citation
  • Chen, P., M. P. Hoerling, and R. M. Dole, 2001: The origin of the subtropical anticyclones. J. Atmos. Sci., 58, 18271835, https://doi.org/10.1175/1520-0469(2001)058<1827:TOOTSA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Chen, T., W. B. Rossow, and Y. Zhang, 2000: Radiative effects of cloud-type variations. J. Climate, 13, 264286, https://doi.org/10.1175/1520-0442(2000)013<0264:REOCTV>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Chen, X., T. Zhou, P. Wu, Z. Guo, and M. Wang, 2020: Emergent constraints on future projections of the western North Pacific subtropical high. Nat. Commun., 11, 2802, https://doi.org/10.1038/s41467-020-16631-9.

    • Search Google Scholar
    • Export Citation
  • Cherchi, A., T. Ambrizzi, S. Behera, A. C. V. Freitas, Y. Morioka, and T. Zhou, 2018: The response of subtropical highs to climate change. Curr. Climate Change Rep., 4, 371382, https://doi.org/10.1007/s40641-018-0114-1.

    • Search Google Scholar
    • Export Citation
  • Deser, C., A. Phillips, V. Bourdette, and H. Teng, 2012: Uncertainty in climate change projections: The role of internal variability. Climate Dyn., 38, 527546, https://doi.org/10.1007/s00382-010-0977-x.

    • Search Google Scholar
    • Export Citation
  • Eyring, V., S. Bony, G. A. Meehl, C. A. Senior, B. Stevens, R. J. Stouffer, and K. E. Taylor, 2016: Overview of the Coupled Model Intercomparison Project phase 6 (CMIP6) experimental design and organization. Geosci. Model Dev., 9, 19371958, https://doi.org/10.5194/gmd-9-1937-2016.

    • Search Google Scholar
    • Export Citation
  • Fahad, A. A., and N. J. Burls, 2021: The influence of direct radiative forcing versus indirect sea surface temperature warming on Southern Hemisphere subtropical anticyclones under global warming. Climate Dyn., 58, 23332350, https://doi.org/10.21203/rs.3.rs-449300/v1.

    • Search Google Scholar
    • Export Citation
  • Garreaud, R. D., and M. Falvey, 2009: The coastal winds off western subtropical South America in future climate scenarios. Int. J. Climatol., 29, 543554, https://doi.org/10.1002/joc.1716.

    • Search Google Scholar
    • Export Citation
  • Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447462, https://doi.org/10.1002/qj.49710644905.

    • Search Google Scholar
    • Export Citation
  • Gu, G., and C. Zhang, 2002: Cloud components of the intertropical convergence zone. J. Geophys. Res., 107, 4565, https://doi.org/10.1029/2002JD002089.

    • Search Google Scholar
    • Export Citation
  • He, C., T. Zhou, A. Lin, B. Wu, D. Gu, C. Li, and B. Zheng, 2015: Enhanced or weakened western North Pacific subtropical high under global warming? Sci. Rep., 5, 16771, https://doi.org/10.1038/srep16771.

    • Search Google Scholar
    • Export Citation
  • He, C., B. Wu, L. Zou, and T. Zhou, 2017: Responses of the summertime subtropical anticyclones to global warming. J. Climate, 30, 64656479, https://doi.org/10.1175/JCLI-D-16-0529.1.

    • Search Google Scholar
    • Export Citation
  • He, C., A. Lin, D. Gu, C. Li, B. Zheng, B. Wu, and T. Zhou, 2018: Using eddy geopotential height to measure the western North Pacific subtropical high in a warming climate. Theor. Appl. Climatol., 131, 681691, https://doi.org/10.1007/s00704-016-2001-9.

    • Search Google Scholar
    • Export Citation
  • He, C., T. Li, and W. Zhou, 2020: Drier North American monsoon in contrast to Asian–African monsoon under global warming. J. Climate, 33, 98019816, https://doi.org/10.1175/JCLI-D-20-0189.1.

    • Search Google Scholar
    • Export Citation
  • He, J., and B. J. Soden, 2017: A re-examination of the projected subtropical precipitation decline. Nat. Climate Change, 7, 5357, https://doi.org/10.1038/nclimate3157.

    • Search Google Scholar
    • Export Citation
  • Held, I. M., and B. J. Soden, 2006: Robust responses of the hydrological cycle to global warming. J. Climate, 19, 56865699, https://doi.org/10.1175/JCLI3990.1.

    • Search Google Scholar
    • Export Citation
  • Hsu, P., and T. Li, 2012: Is “rich-get-richer” valid for Indian Ocean and Atlantic ITCZ? Geophys. Res. Lett., 39, L13705, https://doi.org/10.1029/2012GL052399.

    • Search Google Scholar
    • Export Citation
  • Hu, Y., and Q. Fu, 2007: Observed poleward expansion of the Hadley circulation since 1979. Atmos. Chem. Phys., 7, 52295236, https://doi.org/10.5194/acp-7-5229-2007.

    • Search Google Scholar
    • Export Citation
  • Huang, P., S.-P. Xie, K. Hu, G. Huang, and R. Huang, 2013: Patterns of the seasonal response of tropical rainfall to global warming. Nat. Geosci., 6, 357361, https://doi.org/10.1038/ngeo1792.

    • Search Google Scholar
    • Export Citation
  • Huang, Y., H. Wang, K. Fan, and Y. Gao, 2015: The western Pacific subtropical high after the 1970s: Westward or eastward shift? Climate Dyn., 44, 20352047, https://doi.org/10.1007/s00382-014-2194-5.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437471, https://doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Karnauskas, K. B., and C. C. Ummenhofer, 2014: On the dynamics of the Hadley circulation and subtropical drying. Climate Dyn., 42, 22592269, https://doi.org/10.1007/s00382-014-2129-1.

    • Search Google Scholar
    • Export Citation
  • Kelly, P., B. Kravitz, J. Lu, and L. R. Leung, 2018: Remote drying in the North Atlantic as a common response to precessional changes and CO2 increase over land. Geophys. Res. Lett., 45, 36153624, https://doi.org/10.1002/2017GL076669.

    • Search Google Scholar
    • Export Citation
  • Kitoh, A., H. Endo, K. Krishna Kumar, I. F. A. Cavalcanti, P. Goswami, and T. Zhou, 2013: Monsoons in a changing world: A regional perspective in a global context. J. Geophys. Res. Atmos., 118, 30533065, https://doi.org/10.1002/jgrd.50258.

    • Search Google Scholar
    • Export Citation
  • Kossin, J. P., 2018: A global slowdown of tropical-cyclone translation speed. Nature, 558, 104107, https://doi.org/10.1038/s41586-018-0158-3.

    • Search Google Scholar
    • Export Citation
  • Levine, X. J., and W. R. Boos, 2019: Sensitivity of subtropical stationary circulations to global warming in climate models: A baroclinic Rossby gyre theory. Climate Dyn., 52, 48734890, https://doi.org/10.1007/s00382-018-4419-5.

    • Search Google Scholar
    • Export Citation
  • Li, L. F., W. H. Li, and Y. Kushnir, 2012: Variation of the North Atlantic subtropical high western ridge and its implication to southeastern US summer precipitation. Climate Dyn., 39, 14011412, https://doi.org/10.1007/s00382-011-1214-y.

    • Search Google Scholar
    • Export Citation
  • Li, T., and Coauthors, 2022: Distinctive South and East Asian monsoon circulation responses to global warming. Sci. Bull., 67, 762770, https://doi.org/10.1016/j.scib.2021.12.001.

    • Search Google Scholar
    • Export Citation
  • Li, W., L. Li, M. Ting, and Y. Liu, 2012: Intensification of Northern Hemisphere subtropical highs in a warming climate. Nat. Geosci., 5, 830834, https://doi.org/10.1038/ngeo1590.

    • Search Google Scholar
    • Export Citation
  • Li, W., and Coauthors, 2013: Intensification of the Southern Hemisphere summertime subtropical anticyclones in a warming climate. Geophys. Res. Lett., 40, 59595964, https://doi.org/10.1002/2013GL058124.

    • Search Google Scholar
    • Export Citation
  • Li, X., and M. Ting, 2017: Understanding the Asian summer monsoon response to greenhouse warming: The relative roles of direct radiative forcing and sea surface temperature change. Climate Dyn., 49, 28632880, https://doi.org/10.1007/s00382-016-3470-3.

    • Search Google Scholar
    • Export Citation
  • Liu, Y. M., G. X. Wu, H. Liu, and P. Liu, 2001: Condensation heating of the Asian summer monsoon and the subtropical anticyclone in the Eastern Hemisphere. Climate Dyn., 17, 327338, https://doi.org/10.1007/s003820000117.

    • Search Google Scholar
    • Export Citation
  • Liu, Y. M., G. X. Wu, and R. C. Ren, 2004: Relationship between the subtropical anticyclone and diabatic heating. J. Climate, 17, 682698, https://doi.org/10.1175/1520-0442(2004)017<0682:RBTSAA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Ma, J., S.-P. Xie, and Y. Kosaka, 2012: Mechanisms for tropical tropospheric circulation change in response to global warming. J. Climate, 25, 29792994, https://doi.org/10.1175/JCLI-D-11-00048.1.

    • Search Google Scholar
    • Export Citation
  • Miyasaka, T., and H. Nakamura, 2005: Structure and formation mechanisms of the Northern Hemisphere summertime subtropical highs. J. Climate, 18, 50465065, https://doi.org/10.1175/JCLI3599.1.

    • Search Google Scholar
    • Export Citation
  • Monerie, P.-A., C. M. Wainwright, M. Sidibe, and A. A. Akinsanola, 2020: Model uncertainties in climate change impacts on Sahel precipitation in ensembles of CMIP5 and CMIP6 simulations. Climate Dyn., 55, 13851401, https://doi.org/10.1007/s00382-020-05332-0.

    • Search Google Scholar
    • Export Citation
  • O’Neill, B. C., and Coauthors, 2016: The Scenario Model Intercomparison Project (ScenarioMIP) for CMIP6. Geosci. Model Dev., 9, 34613482, https://doi.org/10.5194/gmd-9-3461-2016.

    • Search Google Scholar
    • Export Citation
  • Polade, S. D., A. Gershunov, D. R. Cayan, M. D. Dettinger, and D. W. Pierce, 2017: Precipitation in a warming world: Assessing projected hydro-climate changes in California and other Mediterranean climate regions. Sci. Rep., 7, 10783, https://doi.org/10.1038/s41598-017-11285-y.

    • Search Google Scholar
    • Export Citation
  • Power, S. B., F. Delage, R. Colman, and A. Moise, 2012: Consensus on twenty-first-century rainfall projections in climate models more widespread than previously thought. J. Climate, 25, 37923809, https://doi.org/10.1175/JCLI-D-11-00354.1.

    • Search Google Scholar
    • Export Citation
  • Rodwell, M. J., and B. J. Hoskins, 1996: Monsoons and the dynamics of deserts. Quart. J. Roy. Meteor. Soc., 122, 13851404, https://doi.org/10.1002/qj.49712253408.

    • Search Google Scholar
    • Export Citation
  • Rodwell, M. J., and B. J. Hoskins, 2001: Subtropical anticyclones and summer monsoons. J. Climate, 14, 31923211, https://doi.org/10.1175/1520-0442(2001)014<3192:SAASM>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Seager, R., R. Murtugudde, N. Naik, A. Clement, N. Gordon, and J. Miller, 2003: Air–sea interaction and the seasonal cycle of the subtropical anticyclones. J. Climate, 16, 19481966, https://doi.org/10.1175/1520-0442(2003)016<1948:AIATSC>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Seager, R., T. J. Osborn, Y. Kushnir, I. R. Simpson, J. Nakamura, and H. Liu, 2019: Climate variability and change of Mediterranean-type climates. J. Climate, 32, 28872915, https://doi.org/10.1175/JCLI-D-18-0472.1.

    • Search Google Scholar
    • Export Citation
  • Shaw, T. A., and A. Voigt, 2015: Tug of war on summertime circulation between radiative forcing and sea surface warming. Nat. Geosci., 8, 560566, https://doi.org/10.1038/ngeo2449.

    • Search Google Scholar
    • Export Citation
  • Shaw, T. A., and A. Voigt, 2016: Land dominates the regional response to CO2 direct radiative forcing. Geophys. Res. Lett., 43, 11 38311 391, https://doi.org/10.1002/2016GL071368.

    • Search Google Scholar
    • Export Citation
  • Shepherd, T. G., 2014: Atmospheric circulation as a source of uncertainty in climate change projections. Nat. Geosci., 7, 703708, https://doi.org/10.1038/ngeo2253.

    • Search Google Scholar
    • Export Citation
  • Simpson, I. R., R. Seager, M. Ting, and T. A. Shaw, 2016: Causes of change in Northern Hemisphere winter meridional winds and regional hydroclimate. Nat. Climate Change, 6, 6570, https://doi.org/10.1038/nclimate2783.

    • Search Google Scholar
    • Export Citation
  • Sun, Y., Z. Zhong, T. Li, L. Yi, and Y. Shen, 2021: The slowdown tends to be greater for stronger tropical cyclones. J. Climate, 34, 57415751, https://doi.org/10.1175/JCLI-D-20-0449.1.

    • Search Google Scholar
    • Export Citation
  • Taylor, K. E., R. J. Stouffer, and G. A. Meehl, 2012: An overview of CMIP5 and the experiment design. Bull. Amer. Meteor. Soc., 93, 485498, https://doi.org/10.1175/BAMS-D-11-00094.1.

    • Search Google Scholar
    • Export Citation
  • Ting, M., 1994: Maintenance of northern summer stationary waves in a GCM. J. Atmos. Sci., 51, 32863308, https://doi.org/10.1175/1520-0469(1994)051<3286:MONSSW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Tuel, A., and E. A. B. Eltahir, 2020: Why is the Mediterranean a climate change hot spot? J. Climate, 33, 58295843, https://doi.org/10.1175/JCLI-D-19-0910.1.

    • Search Google Scholar
    • Export Citation
  • Tuel, A., P. A. O’Gorman, and E. A. B. Eltahir, 2021: Elements of the dynamical response to climate change over the Mediterranean. J. Climate, 34, 11351146, https://doi.org/10.1175/JCLI-D-20-0429.1.

    • Search Google Scholar
    • Export Citation
  • Wang, B., C. Jin, and J. Liu, 2020: Understanding future change of global monsoons projected by CMIP6 models. J. Climate, 33, 64716489, https://doi.org/10.1175/JCLI-D-19-0993.1.

    • Search Google Scholar
    • Export Citation
  • Wang, B., and Coauthors, 2021: Monsoons climate change assessment. Bull. Amer. Meteor. Soc., 102, E1E19, https://doi.org/10.1175/BAMS-D-19-0335.1.

    • Search Google Scholar
    • Export Citation
  • Wang, H., and M. Ting, 1999: Seasonal cycle of the climatological stationary waves in the NCEP–NCAR reanalysis. J. Atmos. Sci., 56 38923919, https://doi.org/10.1175/1520-0469(1999)056<3892:SCOTCS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Watanabe, M., and M. Kimoto, 2000: Atmosphere–ocean thermal coupling in the North Atlantic: A positive feedback. Quart. J. Roy. Meteor. Soc., 126, 33433369, https://doi.org/10.1002/qj.49712657017.

    • Search Google Scholar
    • Export Citation
  • Webb, M. J., and Coauthors, 2017: The Cloud Feedback Model Intercomparison Project (CFMIP) contribution to CMIP6. Geosci. Model Dev., 10, 359384, https://doi.org/10.5194/gmd-10-359-2017.

    • Search Google Scholar
    • Export Citation
  • Wu, B., T. Zhou, and T. Li, 2017: Atmospheric dynamic and thermodynamic processes driving the western North Pacific anomalous anticyclone during El Niño. Part I: Maintenance mechanisms. J. Climate, 30, 96219635, https://doi.org/10.1175/JCLI-D-16-0489.1.

    • Search Google Scholar
    • Export Citation
  • Wu, G. X., Y. Liu, X. Zhu, W. Li, R. Ren, A. Duan, and X. Liang, 2009: Multi-scale forcing and the formation of subtropical desert and monsoon. Ann. Geophys., 27, 36313644, https://doi.org/10.5194/angeo-27-3631-2009.

    • Search Google Scholar
    • Export Citation
  • Wu, L., and C. Wang, 2015: Has the western Pacific subtropical high extended westward since the late 1970s? J. Climate, 28, 54065413, https://doi.org/10.1175/JCLI-D-14-00618.1.

    • Search Google Scholar
    • Export Citation
  • Xia, Y., Y. Hu, and J. Liu, 2020: Comparison of trends in the Hadley circulation between CMIP6 and CMIP5. Sci. Bull., 65, 16671674, https://doi.org/10.1016/j.scib.2020.06.011.

    • Search Google Scholar
    • Export Citation
  • Xie, S.-P., C. Deser, G. A. Vecchi, J. Ma, H. Y. Teng, and A. T. Wittenberg, 2010: Global warming pattern formation: Sea surface temperature and rainfall. J. Climate, 23, 966986, https://doi.org/10.1175/2009JCLI3329.1.

    • Search Google Scholar
    • Export Citation
  • Yanai, M., and T. Tomita, 1998: Seasonal and interannual variability of atmospheric heat sources and moisture sinks as determined from NCEP–NCAR reanalysis. J. Climate, 11, 463482, https://doi.org/10.1175/1520-0442(1998)011<0463:SAIVOA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Yang, H., and S. Q. Sun, 2003: Longitudinal displacement of the subtropical high in the western Pacific in summer and its influence. Adv. Atmos. Sci., 20, 921933, https://doi.org/10.1007/BF02915515.

    • Search Google Scholar
    • Export Citation
  • Yang, J., W. R. Peltier, and Y. Hu, 2016: Monotonic decrease of the zonal SST gradient of the equatorial Pacific as a function of CO2 concentration in CCSM3 and CCSM4. J. Geophys. Res. Atmos., 121, 10 63710 653, https://doi.org/10.1002/2016JD025231.

    • Search Google Scholar
    • Export Citation
  • Yuan, C., and T. Yamagata, 2014: California Niño/Niña. Sci. Rep., 4, 4801, https://doi.org/10.1038/srep04801.

  • Zappa, G., P. Ceppi, and T. G. Shepherd, 2020: Time-evolving sea-surface warming patterns modulate the climate change response of subtropical precipitation over land. Proc. Natl. Acad. Sci. USA, 117, 45394545, https://doi.org/10.1073/pnas.1911015117.

    • Search Google Scholar
    • Export Citation
  • Zhang, X., F. W. Zwiers, G. C. Hegerl, F. H. Lambert, N. P. Gillett, S. Solomon, P. A. Stott, and T. Nozawa, 2007: Detection of human influence on twentieth-century precipitation trends. Nature, 448, 461465, https://doi.org/10.1038/nature06025.

    • Search Google Scholar
    • Export Citation
  • Zhao, H., and L. Wu, 2014: Inter-decadal shift of the prevailing tropical cyclone tracks over the western North Pacific and its mechanism study. Meteor. Atmos. Phys., 125, 89101, https://doi.org/10.1007/s00703-014-0322-8.

    • Search Google Scholar
    • Export Citation
  • Zhou, S., G. Huang, and P. Huang, 2018: Changes in the East Asian summer monsoon rainfall under global warming: Moisture budget decompositions and the sources of uncertainty. Climate Dyn., 51, 13631373, https://doi.org/10.1007/s00382-017-3959-4.

    • Search Google Scholar
    • Export Citation
  • Zhou, T. J., D. Y. Gong, J. Li, and B. Li, 2009: Detecting and understanding the multi-decadal variability of the East Asian summer monsoon—Recent progress and state of affairs. Meteor. Z., 18, 455467, https://doi.org/10.1127/0941-2948/2009/0396.

    • Search Google Scholar
    • Export Citation
  • Zhou, Z.-Q., S.-P. Xie, X.-T. Zheng, Q. Liu, and H. Wang, 2014: Global warming–induced changes in El Niño teleconnections over the North Pacific and North America. J. Climate, 27, 90509064, https://doi.org/10.1175/JCLI-D-14-00254.1.

    • Search Google Scholar
    • Export Citation

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  • Abd El-Ghani, M. M., F. M. Huerta-Martínez, L. Hongyan, and R. Qureshi, 2017: Arid deserts of the world: Origin, distribution, and features. Plant Responses to Hyperarid Desert Environments, 1st ed., M. M. Abd El-Ghani et al., Eds., Springer, 1–7.

  • Aguirre, C., M. Rojas, R. D. Garreaud, and D. A. Rahn, 2019: Role of synoptic activity on projected changes in upwelling-favourable winds at the ocean’s eastern boundaries. npj Climate Atmos. Sci., 2, 44, https://doi.org/10.1038/s41612-019-0101-9.

    • Search Google Scholar
    • Export Citation
  • Ardanuy, P. E., L. L. Stowe, A. Gruber, and M. Weiss, 1991: Shortwave, longwave, and net cloud-radiative forcing as determined from Nimbus 7 observations. J. Geophys. Res., 96, 18 53718 549, https://doi.org/10.1029/91JD01992.

    • Search Google Scholar
    • Export Citation