1. Introduction
The western North Pacific (WNP) is the most active region of convection and tropical cyclone (TC) activity during boreal summer. The major large-scale circulations in this season, i.e., the monsoon trough and WNP subtropical high (WNPSH), modulate monsoon rainfall and TCs, bringing abundant precipitation (Hsu et al. 2014). The temporal and spatial variability of the WNPSH and monsoon trough influences extreme precipitation, TC activity, and drought events in the WNP and affects the weather and climate of the countries in this region. Exploring the knowledge of future WNPSH change provides an important clue to the potential changes in extreme events and their impacts on the surrounding countries against the backdrop of global warming.
Previous studies demonstrated certain degrees of uncertainty of the WNP monsoon trough and subtropical high system (Li et al. 2012; Shaw and Voigt 2015; He et al. 2017) and the seasonal dependency of the response of subtropical high in the future projection (e.g., Song et al. 2018). Chen et al. (2020) identified an intensified subtropical high in phase 5 of Coupled Model Intercomparison Project (CMIP5; Taylor et al. 2012) future projections after the correction based on observational constraints using the SST pattern.
The horizontal resolution of the climate model is a critical issue in resolving more realistic heavy precipitation associated with topography, typhoon, and mesoscale systems embedded in frontal systems in the WNP and East Asian region (Kitoh and Kusunoki 2008; Kobayashi and Sugi 2004; Li et al. 2015; Endo and Kitoh 2016; Wu et al. 2017). Arakane and Hsu (2020) applied a TC removal technique and demonstrated that the existence of TCs enhances the monsoon trough and weakens the WNPSH. They suggested that a climate model needs to properly resolve TCs to confidently simulate the circulation variability and change in the WNP. The finding implies limited confidence in the future projections of the WNP circulation by the CMIP atmosphere–ocean coupled general circulation models, because of their inadequate ability in simulating TCs due to coarse resolution (typically 100–200 km).
Ideally, it would need a high-resolution coupled climate model to resolve the issue, such as the High Resolution Model Intercomparison Project (HighResMIP) (Haarsma et al. 2016), although other issues such as inconsistent performance and projection between models may still exist. In the meantime, numerical experiments forcing high-resolution (few tens of kilometers) atmospheric GCM (AGCM), which can more properly resolve high-impact weather such as TCs, with projected SST changes have been conducted to project future climate changes, although it may underplay the effects of atmosphere–ocean coupling. Chen et al. (2019) analyzed the warming simulation results of two high-resolution AGCMs and identified decreased precipitation during the WNP typhoon season, accompanied by an anomalously enhanced WNPSH and a weakened monsoon trough. The decreased precipitation and anomalous circulation imply weakened TC activity, consistent with the findings of Tsou et al. (2016) and Hsu et al. (2021). These simulations revealed the impacts of future global ocean warming on the WNP monsoon, but the responsible processes and the relative contribution of SST changes by individual basins have not been properly diagnosed. By contrast, local and remote impacts of regional SST perturbation on present-day regional climate have been widely explored. Assuming that the physical processes underlying the influences of SST on regional climate remain similar in a warmer climate, the results derived from present-day climate would provide important clues for understanding the effect of SST changes in the warming future.
In the present-day climate, local and remote SST change is one of the key factors modulating the variability of the WNPSH (e.g., Wu et al. 2010; Chowdary et al. 2011; Yun et al. 2013; Chang et al. 2016; Hong et al. 2015, 2018). The warmer SST in the Indian Ocean (IO), Maritime Continent (MC), and the tropical Atlantic and the cooler SST in the WNP and eastern equatorial Pacific (e.g., during La Niña) can intensify the WNPSH. The warming in the IO associated with the anomalously cooler SST over the eastern equatorial Pacific (during La Niña or the mature and decaying phase of El Niño) may further enhance the westward extension of the WNPSH to the South China Sea (SCS). For example, the strong IO warming following the weak El Niño and La Niña in 2019/20 extended the western flank of the WNPSH and contributed to the Yangtze River flooding in 2020 (Takaya et al. 2020; Zhou et al. 2021). Hong et al. (2014, 2015) explored the influence of the tropical Atlantic warmer SST on the enhanced WNPSH based on observation and numerical simulations and found that the anomalously warmer Atlantic SST had a greater influence than La Niña did on the unprecedently strong WNPSH in the summer of 2010. Chang et al. (2016) demonstrated the enhanced triggering effect of the warmer tropical Atlantic after the 1980s on the prevailing coupled IO–WNP circulation pattern in summer (Wang et al. 2013).
Based on observational and numerical simulation evidence, Zhang et al. (2016) indicated that the Pacific Meridional Mode (PMM; Chiang and Vimont 2004; Chang et al. 2007) modulates the east–west shifts of TC tracks and genesis density in the WNP. Hong et al. (2016, 2018) demonstrated that a PMM-like anomalous SST induced an anomalous ascending (descending) motion and a negative (positive) change in vertical zonal wind shear over the central–eastern (subtropical western) North Pacific. The anomalous subsidence enhanced the western flank of the WNPSH, influenced the summer precipitation in East Asia, and caused an eastward shift of the major TC genesis region (e.g., the extreme event in 2015).
Whereas previous studies identified the impacts of remote SST variation on the WNPSH circulation and precipitation in the present-day climate, the potential influences of future regional SST warming on the changes in WNPSH have not been systematically explored, despite the uncertainty of SST projection among models (Xie et al. 2010; Christensen et al. 2013; Huang et al. 2013; Ma and Xie 2013; Chadwick et al. 2014). Clarifying the impacts of regional SST warming on atmospheric circulation change in the WNP region would certainly extend our understanding of future changes in WNP circulation and related extreme events. As reported by Chen et al. (2019), enhanced anomalous anticyclonic circulation and decreased precipitation in the WNP typhoon season were projected in high-resolution AGCM simulations. To extend the scope of their findings, the present study aims to answer the following two questions: 1) How would the SST warming in individual basins affect the WNP summer monsoon circulation? 2) What are the dominant regional SST changes that cause the enhanced WNPSH and weakened monsoon trough under future global warming scenario?
In this study, we used the High Resolution Atmospheric Model (HiRAM; Anderson et al. 2004; Zhao et al. 2009) to conduct a series of sensitivity experiments with SST warming in various basins to explore which parts of the SST warming pattern are the main factor leading to the projected WNP summer monsoon changes under global warming and the mechanisms underlying these changes. We show that the WNP climate is susceptible to SST changes in several basins, more than globally uniform SST warming, which increases the likelihood of reduced precipitation, enhanced WNPSH, and less TC activity in the warmer climate. Our study suggests that the characteristics of the mean state in the WNP region are a key factor for such response. Section 2 includes the descriptions of the HiRAM, the numerical simulation design, and the sensitivity experiments of SST warming. Section 3 presents the results for each experiment, followed by discussion in section 4 and conclusions in section 5.
2. Model and experimental design
a. Model
The HiRAM was developed based on Atmospheric Model, version 2 (AM2; Anderson et al. 2004; Zhao et al. 2009), with improvements in finer resolution and more explicit convection-related processes regarding the vertical transport of moisture and energy. These advancements helped HiRAM to simulate more realistic subgrid convection processes, rainfall distribution, and the relevant extreme precipitation, including those in WNP. In the present study, the horizontal resolution of the HiRAM is approximately 50 km.
The HiRAM has been demonstrated with good performance in the seasonal variability of the drought spell index, extreme precipitation, and their corresponding large-scale circulation over East Asia (Freychet et al. 2017); the seasonal evolution of the WNP and East Asian precipitation (Chen et al. 2019); the extreme precipitation indices during the spring and mei-yu seasons in East Asia (Chen et al. 2022); and atmosphere river and heavy precipitation in northeastern North America (Hsu and Chen 2020). In addition, HiRAM also has good skill in simulating the genesis frequency and location, seasonal cycle, track frequency, and intensity of TCs (Tsou et al. 2016; Hsu et al. 2021). These findings suggest that HiRAM is a good model for the purpose of this study.
b. Experiment design
In this study, we conducted time-slice experiments forced by prescribed SST (Bengtsson et al. 2009; Kusunoki and Mizuta 2013). The historical experiments (1985–2004, hereafter as His_Ctl) were conducted using SST extracted from the Met Office Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST1; Rayner et al. 2003) and historical greenhouse gas (GHG) concentration (Donner et al. 2011) as forcing. In the warming experiment (denoted as RCP8.5_Ens), the GHG concentration based on the representative concentration pathway 8.5 (RCP8.5) scenario (Meinshausen et al. 2011) and the projected ensemble-mean SST changes (Fig. 1a) in 2080–99 by 28 CMIP5 models (Mizuta et al. 2014) were used as forcing. The observed SST interannual variation during 1985–2004 was retained and added to the future SST changes. Under the condition of consistent interannual variation of SST, this approach enables us to explore how precipitation and circulation change in response to the forcings of RCP8.5 emission and warmer SST. Both experiments consist of four simulation members using different initial conditions, and the average of the four members is presented. We note that the impact of different initial conditions on the results of this study is relatively small (not shown). The differences in the 20-yr averages between 2080–99 and 1985–2004 focusing on July–October (i.e., the prevailing typhoon season) were diagnosed in this study.
Changes in annual mean SST (K) in (a) RCP8.5_Ens, (b) Uni_p25d, (c) RCP8.5_pat, (d) Trop_Ens, (e) TropAtl, (f) TropIO, (g) PMM-like, (h) TropWNP, (i) TropEP, and (j) RCP8.5_only experiments. SST in (c)–(j) is the warming relative to globally uniform warming of 2.5°C.
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
c. SST sensitivity experiment
In addition to His_Ctl and RCP8.5_Ens, we conducted a series of sensitivity experiments in which the SST changes in different basins (denoted as
List of experiments with different SST conditions and the RCP8.5 scenario. The 20-yr July–October averages during 1985–2004 and 2080–99 were examined. Experiments marked in bold are conducted under full regional SST warming.
A nine-point smoothing was applied to smooth the transition between prescribed regional SST warming and surrounding historical SST. All the sensitivity experiments adopt historical GHG concentration except for the experiment RCP8.5_only. In total, His_Ctl, RCP8.5_Ens, and fifteen sensitivity experiments were conducted (Table 1). Given the negligible effects of different initial conditions on the long-term means and the demanding computing resource needed for high-resolution simulations, one simulation member was conducted for each sensitivity experiment. The domains of SST change and the information pertaining to each experiment are listed in Table 1, and the spatial distributions of SST change are illustrated in Fig. 1.
3. Results
a. Responses in global warming experiment
The RCP8.5_Ens experiment projected a substantial precipitation reduction (Fig. 2a) in the WNP and the western region of the Pacific ITCZ and increased precipitation in the surrounding regions. The overall change is characterized by the weakening but expansion of the major tropical precipitation regions in the IO and WNP. Change in low-level atmospheric circulation displays a positive anomaly of streamfunction over the WNP, the SCS, and the Bay of Bengal, where the climatological monsoon trough prevails (Fig. 2b). The most notable feature is the significantly larger amplitude compared to other regions worldwide. The low-level anomalous anticyclonic circulation implies a westward-extended WNPSH and weakened monsoon trough, i.e., an unfavorable change for the prevailing convective activity and precipitation in the warming scenario. Increased precipitation exists along the edges of the extended subtropical high, ranging from the western IO to continental South Asia, East Asia, and the MC. Precipitation increases in these areas correspond to changes in the prevailing flow, e.g., the enhanced southwesterly in East Asia and the cyclonic circulation dipole straddling the equator in the subtropical Pacific.
(a) Spatial distribution of precipitation (mm day−1) in His_Ctl (contour; with the interval of 5 mm day−1) and differences between RCP8.5_Ens and His_Ctl (shaded colors) during July–October. (b) As in (a), but for the result of 850-hPa streamfunction (1 × 106 m2 s−1) in His_Ctl (contour: positive values only with an interval of 4) and changes in RCP8.5_Ens (shading). Vectors indicate the corresponding change in the wind field at 850 hPa (m s−1). Dotted areas in (a) mark significant differences at the 10% level.
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
We noted that the response of precipitation and circulation during this season in high-resolution AGCM simulation is not necessarily consistent with the results from coupled models. The ensemble of 24 CMIP5 models (Fig. S1a in the online supplemental material) indicates a statistically insignificant precipitation increase in the WNP, consistent with the change reported in IPCC_AR5 (Collins et al. 2013; Christensen et al. 2013). By contrast, no marked change in 850-hPa streamfunction is identified in the WNP (Fig. S1c). The insignificant and little changes in precipitation and streamfunction, respectively, are evidently due to the weak consensus between model projections (Figs. S1b,d). Whereas coupled model simulations showed marked inconsistency, the AMIP-type simulations based on HiRAM and MRI_AGCM yielded an enhanced WNPSH (Chen et al. 2019). On the other hand, a recent study by Chen et al. (2020) demonstrated that after bias correction coupled models projected more consistently an enhanced WNPSH, a result consistent with the projection of AGCMs. As indicated in previous studies (e.g., He et al. 2015, 2017; Li et al. 2012; Shaw and Voigt 2015; Chen et al. 2020), the change in WNPSH in the warmer future remains a challenging issue. We do not intend to resolve this challenging issue. Instead, we focus on understanding the physical processes leading to the projected enhancement of WNPSH in HiRAM.
b. Responses in sensitivity experiments with global-scale forcing
From the perspective of global-scale forcing, the Uni_p25d (Fig. 3a) simulated overall precipitation increase in the WNP and slight weakening of the WNPSH, which are clearly inconsistent with RCP8.5_Ens. The RCP8.5_pat (Fig. 3b) displays a spatial feature similar to the RCP8.5_Ens (Fig. 2a) with enhanced WNPSH and decreased precipitation over the WNP and increased precipitation over the equatorial region and Indian Ocean. The anticyclonic anomaly guides an enhanced southwesterly along the coastal areas of East Asia, explaining the slightly increased precipitation over India, the Indochina Peninsula, and eastern China.
Changes in precipitation (shading; mm day−1) and 850-hPa streamfunction (contour marked at ±0.5, ±1, and ±2; 1 × 106 m2 s−1) during July–October in (a) Uni_p25d, (b) RCP8.5_pat, and (c) RCP8.5_only experiments. Dotted areas mark significant differences at the 10% level.
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
The RCP8.5_only (Fig. 3c) resulted in reduced precipitation in the tropical western Pacific, which was orthogonal to the changes in the RCP8.5_Ens (Fig. 2a) and RCP8.5_pat (Fig. 3b), and a weaker anticyclonic change over the entire North Pacific. Whereas GHG increase could contribute, but with weaker amplitudes, to the reduced precipitation and enhanced anticyclone in the WNP, some regional responses, such as reduced precipitation in the equatorial Pacific and the Atlantic, are inconsistent with RCP8.5_Ens. The impacts of Uni_p25d provide uniform surface heating and destabilize the atmosphere, whereas the enhanced greenhouse effect RCP8.5_only is known to increase stability (Bony et al. 2013; Thorpe and Andrews 2014). The reversed impacts of the two forcings tend to offset each other (Figs. 3a,c). A sum (Fig. S2b) of the precipitation changes in these three experiments roughly accounts for the results of RCP8.5_Ens (Fig. 2a) with weaker magnitude and reflects the dominant impacts of the SST pattern.
c. Responses in sensitivity experiments with regional-scale forcing
Figures 4a–f further display the impacts of regional SST warming after subtracting 2.5°C. The Trop_Ens (Fig. 4a) generally captures the feature of the precipitation changes near the equator in the Pacific, IO, and Atlantic seen in the RCP8.5_Ens (Fig. 2) and RCP8.5_pat (Fig. 3b). However, it also simulated increased precipitation north of the equator in the tropical WNP, a feature not consistent with both RCP8.5_Ens and RCP8.5_pat, due to the prescribed SST warming in the region. As a result, precipitation reduction and subtropical high enhancement in the WNP appeared in a relatively restricted region close to East Asia, instead of over the Philippine Sea. The increased precipitation (Fig. 4a) with enhanced upward vertical velocity (not shown) along the equatorial SST warming belt resembles the structure of the deep-tropics squeeze (Lau and Kim 2015) and the narrowing of the ITCZ (Byrne and Schneider 2016), which partially contribute to the increased precipitation over the equatorial region but decreased precipitation off the equator (Fig. 2a).
As in Fig. 3, but for the results in (a) Trop_Ens, (b) TropAtl, (c) TropIO, (d) PMM-like, (e) TropWNP, and (f) TropEP experiments.
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
From the perspective of regional SST, the warming in the TropAtl, TropIO, and PMM-like (Figs. 4b–d) all causes anomalous anticyclonic circulation and decreased precipitation over the WNP and SCS regions. The elevated SST in the tropical Atlantic (Fig. 4b) heats the atmosphere and induces two anomalous cyclonic circulations over the eastern Pacific and the Atlantic, in conjunction with increased precipitation over the tropical Atlantic Ocean. By contrast, a pair of anomalous anticyclonic circulations extends eastward from western Africa and reaches the maximum strength over the WNP and the southwestern Pacific. The enhanced anticyclonic circulation provides a clear explanation for the considerable decline in precipitation over the northern IO, the Bay of Bengal, the entire WNP up to 45°N, and coastal East Asia.
In the TropIO experiment, the warmer SST (Fig. 4c) generates a set of cyclonic circulation changes accompanied by increased precipitation over the two flanks of the IO equator. On the other hand, an enhanced anticyclonic circulation pair over the Pacific and marked precipitation decrease over the WNP are induced in the TropIO. In the PMM-like experiment (Fig. 4d), the warm SST extended southwestward from the western coast of California, causing a cyclonic circulation change and more rainfall over the northeast Pacific and anticyclonic perturbation with decreased precipitation in the WNP–East Asian (EA) region.
In the three experiments mentioned above, the atmospheric responses over the WNP region are consistent with the findings in previous studies regarding the impact of the anomalous SST in these basins on the observed variability of the WNPSH (e.g., Hong et al. 2014; Chang et al. 2016; Hong et al. 2018). A Gill-type response forced by the regional anomalous heating explains the anomalous low-level cyclonic and anticyclonic circulation that was identified in these three experiments. Although the warming regions are different, the remote impacts of the warm SST in these three individual basin experiments lead to similar changes in circulation and precipitation in the WNP region. On the other hand, their impacts on the circulation and precipitation in other regions are not necessarily in the same sign and could be offset.
By contrast, the regional warmer SST in the WNP induced changes (Fig. 4e) that are opposite to those induced by remote SST forcing (e.g., Figs. 4a–d) and in RCP8.5_Ens. The TropWNP experiment (Fig. 4e) simulated a cyclonic anomaly and increased precipitation over the WNP region and an anticyclonic anomaly and decreased precipitation in the eastern Pacific and North Atlantic. Decreased precipitation was also simulated in the WNP-surrounding regions. The impacts of SST warming in the TropWNP experiment partially offset the forcing from other ocean basins and do not contribute to the enhanced WNPSH and decreased precipitation identified in the RCP8.5_Ens. The TropEP experiment (Fig. 4f), resembling an El Niño–like situation, produced a cyclonic anomaly over the eastern equatorial Pacific and weakened the WNPSH, shifting the rainy region eastward and leading to more precipitation over the central–eastern equatorial Pacific and decreased precipitation over the MC, eastern tropical Pacific, and IO. The impacts of the TropEP are mixed, with a negative contribution to the total changes in the equatorial western Pacific and a positive contribution in South Asia and oceanic East Asia. This result is consistent with previous studies (e.g., Hong et al. 2016, 2018) that demonstrated cooler SST associated with La Niña, not El Niño–like situation as in future warming condition, as a key factor in causing strong WNPSH. When considering the full increasing SST in the TropWNP and TropEP regions, similar distributions but with further enhanced responses with statistical significance are found (Figs. 5e,f).
As in Fig. 4, but for the simulation forced by full SST warming (i.e., without removing 2.5°C uniform warming).
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
As indicated in Figs. 4 and 5, regional SST warming in remote basins contributes rigorously to the enhanced WNPSH and decreased precipitation in the WNP during July–October, except those in the tropical WNP and partially in the tropical eastern Pacific (EP). This result is consistent with the findings in previous studies on known extreme climate events (e.g., Hong et al. 2015, 2018; Takaya et al. 2020; Zhou et al. 2021). Experiments also show that the effects of SST warming from remote ocean basins are stronger than those from the WNP. The precipitation change estimated from the summation of the subbasins in the tropical region also generally resembles that shown in Trop_Ens (Fig. 4a), indicating the change in Trop_Ens can be roughly approximated by the contribution of SST changes in various tropical basins (Fig. S2d).
The enhanced WNPSH and decreased precipitation are also reflected in TC activity. A markedly reduced TC genesis frequency in the WNP (Fig. 6b) was simulated in RCP8.5_Ens, along with the decrease in the southwestern Pacific and the tropical North Atlantic and the increase in the tropical central and eastern Pacific. The changes in TC track density simulated in
(a) TC track density (frequency year−1) in HiRAM His_Ctl and the difference in (b) RCP8.5_Ens during July–October.
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
A summation of TC activity changes in Uni_p25d, RCP8.5_pat, and RCP8.5_only (Fig. S4b) shows discrepancies from RCP8.5_Ens, e.g., TC increases in the eastern WNP, instead of an overall decrease, and a less coherent pattern in the eastern North Pacific and Atlantic region. Evidently, a combination of three individual forcing experiments could not reproduce the results of the RCP8.5_Ens experiment, reflecting the nonadditive nature of the changes. The sum of five
We note that although the changes shown in Fig. 4 are not as statistically significant as that in RCP8.5_Ens, the experiments with full regional SST warming (i.e., without subtracting 2.5°C uniform warming) simulated similar but much stronger responses as those presented in Fig. 4 with statistical significance (Fig. 5). This result suggests that whereas separation of full SST warming into globally uniform warming and pattern (i.e., deviation of regional warming from uniform warming) has been a common practice in recent research approach (e.g., He and Soden 2015; Chadwick et al. 2017; He et al. 2020) for studying global precipitation and hydrological cycle changes, the full SST warming experiment is what that reflects the full impact of a warmer basin, not just the impacts relative to the global uniform warming. In view of weak model responses to the relative ocean basin warming and nonadditive nature of these experiments discussed above, we focus in the following discussion on the responses to total SST warming in individual basins, which is more relevant to the aims of this study: 1) How would the SST warming in individual basins affect the WNP summer monsoon circulation? 2) What are the dominant regional SST changes that cause the enhanced WNPSH and weakened monsoon trough under future global warming scenario?
d. Atmospheric circulation response in
The effects of regional SST warming on changes in the vertical circulation over the WNP are consistent with those seen in the low-level atmospheric circulation. In the RCP8.5_Ens (Fig. 7a), positive anomalies (red shading) are evident over the WNP, where the prevailing upward motion exists in the present climate (blue dashed contour). This indicates a weakened atmospheric vertical circulation in the WNP region.
Changes in vertical velocity (shaded color; 102 Pa s−1; vector; 103 Pa s−1) and meridional circulation (vector; m s−1) averaged from 10° to 25°N in (a) RCP8.5_Ens, (b) Trop_Ens, (c) TropAtl, (d) TropIO, (e) PMM-like, (f) TropWNP, and (g) TropEP experiments. Contours denote vertical velocity in HiRAM His_Ctl (marked at ±0.5, ±1, and ±2).
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
The Trop_Ens_Full (Fig. 7b) displays a response similar to that revealed in the RCP8.5_Ens at the WNP region but with an opposite sign over the west of 90°E and the eastern Pacific and the Atlantic regions. The TropAtl_Full (Fig. 7c), TropIO_Full (Fig. 7d), and PMM-like_Full (Fig. 7e) simulate upward anomalies over the SST warming region and prominent downward anomalies at 120°–150°E. However, these three experiments only partially reproduced the circulation changes seen in Fig. 7a in the rest of the subtropical regions. The TropWNP_Full (Fig. 7f) generates enhanced upward motion over the WNP region and consistently shows opposite signs to RCP8.5_Ens in the rest of the subtropical areas. The TropEP_Full produced a weak upward anomaly over the WNP at lower troposphere and enhanced upward motion at the equatorial central Pacific. In contrast to TropWNP_Full, other
The responses of atmospheric circulation in the WNP region to different
The dynamic teleconnection path can be seen clearly from the changes in upper-level velocity potential (Fig. 8). A positive/negative dipole in upper-level velocity potential and anomalous divergent winds were simulated in RCP8.5_Ens (Fig. 8b). The collocation of the dipole centers with the present-day climatology (Fig. 8a), which is characterized by the strong convection and upper-level divergence over the WNP, indicates a weakened global atmospheric divergent circulation in the future warmer climate. The anomalous upper-level convergence over the WNP, accompanied by anomalous lower-level divergence (not shown), corresponds to the enhanced WNPSH, reduced precipitation, and TC activity in the WNP.
Velocity potential (shaded color; 106 m2 s−1) and divergence wind (vector) at 200 hPa in (a) HiRAM His_Ctl and their differences in (b) RCP8.5_Ens. As in (b), but for (c) Trop_Ens_Full, (d) TropAtl_Full, (e) TropIO_Full, (f) PMM-like_Full, (g) TropWNP_Full, and (h) TropEP_Full experiments.
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
The Trop_Ens_Full (Fig. 8c) reveals a convergence anomaly over the WNP region, but with the center shifting northwestward compared with RCP8.5_Ens, and a broader divergence anomaly extending from the eastern Pacific to western Africa, resembling the general feature shown in RCP8.5_Ens. The TropAtl_Full (Fig. 8d), TropIO_Full (Fig. 8e), and PMM-like_Full (Fig. 8f) consistently produce anomalous convergent responses over the WNP region, accompanied by strong upper-level divergence anomalies over the warmer Atlantic Ocean, IO, and eastern Pacific, respectively. The simulated responses in these three experiments resemble those shown in Trop_Ens_Full, but with stronger magnitude and broader extension over the forcing regions.
The TropWNP_Full (Fig. 8g) experiment shows a divergent response directly over the warmer WNP and yields an enhancement, instead of weakening, of present-day divergence circulation. The SST warming in TropEP_Full induces a divergence over the warming region and a convergence center over the Indochina Peninsula and the MC, i.e., a westward shift relative to the pattern identified in RCP8.5_Ens, which is consistent with the response to an El Niño event. Figure 8 suggests the high sensitivity of the climatological convection and circulation over the WNP to anomalous SST in various basins, and more interestingly, a consistent response may be viewed as an intrinsic pattern determined by the climatological mean state.
e. Change in Rossby wave source in
Figure 9 presents the results of the Rossby wave source (RWS; Sardeshmukh and Hoskins 1988) analysis at 200 hPa. RWS analysis is often used to interpret how local heating modulates regional circulation. In this study, it is used to explore the teleconnections between
As in Fig. 8, but for the results of the RWS (10−10 s−2) at 200 hPa.
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
In the historical experiment, the 200-hPa RWS shows a more substantial magnitude and broader extent of negative values over Asia and the western part of the North Pacific during July–October (JASO), especially over the WNP, near the midlatitude North Pacific storm tracks and the ITCZ (Fig. 9a). We note that the regions of negative RWS values collocate with the regions of negative velocity potential (Fig. 8a), reflecting the essential role of the divergent flow (Fig. 9a). In addition, the WNP is the region with the strongest RWS in the tropics. The large magnitude of RWS in the low latitude is generally contributed by the stretching term associated with convective heating and strong divergence in the upper troposphere, such as those over the WNP and near the ITCZ. Vorticity advection driven by the divergent wind often prevails in the midlatitude where strong absolute vorticity gradient exists, e.g., near the North Pacific jet stream (detailed diagnostics are not shown). By contrast, positive RWS values appear in the regions with positive velocity potential and convergent flow (Figs. 8a and 9a), such as the Mediterranean, the Middle East, the eastern Pacific, and the Atlantic. Negative and positive RWSs tend to respectively induce negative and positive vorticities which respectively lead to anticyclonic and cyclonic circulation in the upper troposphere. The global distribution of RWS matches the distribution of upper-level ridge and trough (figures not shown; Sardeshmukh and Hoskins 1988). These spatial structures in the HiRAM historical experiment are generally consistent with observation and previous studies (e.g., Lu and Kim 2004; Shimizu and de Albuquerque Cavalcanti 2011). The global wave-one-like spatial distribution of divergent circulation and RWS suggests that the western North Pacific, where the WNP–EA monsoon and TCs prevail, is the key region driving global circulation during July–October.
In RCP8.5_Ens (Fig. 9b), the changes in RWS show the opposite sign to the historical climatology in Fig. 8a, indicating the weakening of global divergent flow. Positive anomalies are evidently found over the western Pacific, the northeastern region of Eurasia, and the southern IO. By contrast, negative changes are identified around the Mediterranean, the eastern equatorial Pacific, and the South Pacific. The spatial contrast of the RWS source and sink becomes weaker in RCP8.5_Ens, again reflecting the weakening of the seasonal mean circulation driven by global SST warming.
In Trop_Ens_Full, TropAtl_Full, TropIO_Full, and PMM-like_Full experiments (Figs. 9c–f), positive anomalies are generally identified over the WNP region, despite several local discrepancies over other areas. The TropEP drives a westward shift in the RWS source and sink relative to those in experiments mentioned above, which is consistent with velocity potential change in Fig. 8. In TropWNP_Full, the response is out of phase with that identified in RCP8.5_Ens, which reveals a negative RWS anomaly mostly over the Pacific basin. These results indicate that the SST warming in remote basins acts to weaken the global divergent circulation, which is essentially driven by convection and heating in the WNP. The aforementioned remote warmer SST cases indirectly weaken the monsoon trough and TC activity and enhance the WNPSH. This compound effect of SST warming in remote basins overwhelms the enhancing effect of in situ SST warming over the WNP and partially from the eastern equatorial Pacific.
The RWS′ was further diagnosed by decomposing the RWS into four contributing terms [Eq. (2)]. The RWS′ in RCP8.5_Ens (Fig. S7) is mainly contributed by vortex stretching associated with the divergence change (Fig. S7), capturing the general spatial structures in the WNP, Mediterranean, and the eastern and southern Pacific shown in Fig. 9b. The vorticity advection change (Fig. S7) due to the change in divergent wind marks the local spatial pattern where the vorticity gradient is more substantial in the historical experiment and contributes to the positive and negative anomalies identified in Fig. 9b. The contributions from the changes in relative vorticity in both stretching and advection terms are small, particularly over the WNP region (Fig. S7). The contribution of each component is generally consistent with previous studies (e.g., Lu and Kim 2004; Shimizu and de Albuquerque Cavalcanti 2011).
The zonal average over 110°E–180° clearly indicates the maximum of positive RWS′ (Fig. 10a). These structures and signs of the RWS′ between 10° and 30°N are relatively consistent among the
Changes in (a) RWS, (b)
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
4. Effect of remote SST warming
In observed present-day climate and historical experiment, the WNP is the region with the warmest SST and the most active convection (as indicated by the strongest divergence at 200 hPa; Fig. 8a) during July–October. The TropWNP_Full experiment shows a strengthening of atmospheric circulation and increased precipitation, reflecting the direct response to the in situ SST warming. On the other hand, the RCP8.5_Ens and most of the
Prescribed SST in SST experiments (x axis; °C) versus the corresponding 200-hPa divergence over the WNP region (y axis; 10−6 s−1). Blue capitalized abbreviations represent the regional averaged results estimated in the HiRAM His_Ctl experiment, and pink capitalized abbreviations represent the results estimated in RCP8.5_Ens. Brown and purple capitalized abbreviations are the results retaining 2.5° warming in the Trop_WNP and those specific
Citation: Journal of Climate 37, 20; 10.1175/JCLI-D-23-0403.1
In the boreal warm season, the northern subtropical region is prevailed by low-level anticyclonic circulation, with the exception of South Asia and the WNP, where convectively driven cyclonic circulation prevails (Figs. 2, 8a, and 9a). As seen in Fig. 8a, the WNP serves as the epic center of deep convection and cyclonic circulation, actively teleconnecting with the major subsidence and anticyclonic regions across the globe. We hypothesize that the inherent structure of this global circulation, which is shaped by land–sea distribution, topography distribution, and SST distribution, is sensitive to the SST change in other ocean basins. This relative SST fluctuation allows SST change in one basin to efficiently modify the global divergent and rotational circulation, consequently impacting the climate in remote regions. On the other hand, whether the WNP is the most sensitive region in nature remains uncertain if considering more complicated processes (e.g., atmosphere–ocean coupling). This is an intriguing issue that warrants further investigation.
5. Conclusions
This study explores the impact of local and remote SST changes on the enhanced WNPSH in global warming simulations. We found that the WNPSH can be enhanced consistently by SST warming in various basins, such as the Indian Ocean, the tropical Atlantic, the subtropical northeastern Pacific where the PMM prevails, and the entire tropical ocean belt. These
The large-scale responses to various regional SST forcing were well reproduced in a series of ensemble simulations with nine sets of random diabatic forcing by using Simplified Parameterizations, Primitive-Equation Dynamics (SPEEDY; Kucharski et al. 2013), an intermediate-complexity AGCM with structure of eight vertical levels and a horizontal resolution of T30, demonstrating the robustness of the findings revealed in HiRAM simulations (Figs. S8–S11).
In terms of the RWS analysis, most experiments demonstrated a similar response with positive RWS′ changes over the WNP where the negative RWS prevails in the historical experiment. The decomposition revealed that the RWS′ is dominated by the change in vortex stretching associated with divergence change. This result is found in all experiments, except in the TropWNP and TropEP that display opposite or mixed changes in the WNP. We suggest that the consistent upper-level convergent anomaly found in the
This study reveals that the WNPSH, being the epic center of the global teleconnection, is susceptible to SST changes in various basins by inducing large to global-scale divergent and rotational circulation anomaly. It is hypothesized that the WNP climate, which is influenced by the monsoon trough and subtropical high, could undergo more dramatic change than that in other basins. This is because the global ocean warming tends to suppress the cyclonic circulation and convection in the WNP and overwhelm the enhancement by the warming WNP and eastern equatorial Pacific. Although this hypothesis based on AMIP-type simulations does not consider the effects of ocean–atmosphere interactions that might damp the remote impacts discussed here, the understanding of how SST warming in various basins affects the WNP climate provides important clues on how the future WNP climate might evolve in the advancing warming future.
One final note is about the simulation approach taken in this study. Separating the full SST change into spatially uniform SST (considering the relatively uniform radiative effect of greenhouse gases) and SST pattern change has been applied in recent studies for understanding the impacts of global warming on global precipitation and hydrological cycle, focusing especially on local processes. Our study has a different aim, which is to understand how the SST changes in various basins affect the AGCM-simulated large circulation and precipitation changes in the WNP. The approach adopted in the study was inspired by many previous studies revealing that climate variability and extremes in the WNP, not necessarily controlled by in situ SST, were often affected by SST warming or cooling in remote ocean basins through far-reaching dynamic processes. Using full SST anomaly as forcing has been a common practice in the studies of observed extreme climate events and interannual–interdecadal fluctuations, based on AMIP-type and pacemaker experiments (e.g., Lau and Nath 1994; Hoerling and Kumar 1997; Kosaka and Xie 2016; Qian et al. 2019; Takaya et al. 2020; Hong and Hsu 2023; Beniche et al. 2024). Taking the same approach would also help reveal key dynamic processes in the impact study of future SST changes. Prescribing relative, instead of total, warming in individual basin would seriously underestimate the amplitudes of the responses, both locally and remotely, and undermine the effort to understand the impacts of individual basin warming.
Extremes such as flooding, drought, and heatwave events in the Asian region are often associated with enhanced WNPSH triggered by the anomalous regional SST (Hong et al. 2015; Zhou and Yu 2005; Kosaka et al. 2013; Takaya et al. 2020; Tseng et al. 2020; Zhou et al. 2021). Enhanced WNPSH variability had been detected from the observational evidence (e.g., Choi and Kim 2019) and global warming projections (Yang et al. 2022). A slight change in the WNPSH edges usually brings extreme events to the surrounding countries. Increased variability of the WNPSH also raises the risk of more frequent and severe extreme climate conditions in the warmer future (Choi and Kim 2019; Yang et al. 2022). Our study demonstrates that the WNP is a sensitive region to various remote SST warming, implying higher risk and challenges in managing water resources and disaster associated with extreme events in the warmer climate.
Acknowledgments.
We thank three anonymous reviewers whose valuable comments improved this paper. We thank the Taiwan Climate Change Projection Information and Adaptation Knowledge Platform (TCCIP) for providing the computing resources in the National Center for High-Performance Computing. The HiRAM simulations were conducted at the National Center for High-performance Computing, Taiwan. The HiRAM simulations are available at https://rcec.sinica.edu.tw/?action=research&cid=1&ntype=4. This work was supported by the Ministry of Science and Technology (MOST) of Taiwan under MOST 111-2123-M-001-007 – and National Science and Technology Council (NSTC) 110-2111-M-865-001-MY3.
Data availability statement.
The HiRAM simulations are available at https://rcec.sinica.edu.tw/?action=research&cid=1&ntype=4.
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