The Northern Hemisphere's surface features two primary continents, Eurasia and North America, along with two significant bodies of water, the North Pacific and North Atlantic Oceans. Land and sea exhibit distinct thermal heat capacities, which play a central role in shaping planetary-scale atmospheric patterns during both winter and summer seasons7. In summer, quasi-stationary high-pressure systems dominate the North Pacific and North Atlantic oceans, while continental low-pressure systems prevail over East Asia and North America8. The juxtaposition of these continental lows and oceanic highs results in diverse summer climates across regions situated at similar latitudes. For instance, East Asian summers are characterized by high temperatures, humidity, and heavy precipitation, constituting what is commonly referred to as the East Asian monsoon9. In contrast, California experiences relatively dry and less warm conditions during the summer months10. The continental low situated to the west and the NPSH located to the east interact to produce low-level southerly winds that transport hot and humid air towards East Asian countries11,12,13. Conversely, in California, the interaction between the western subtropical high and the eastern continental low generates low-level northerly winds, which bring dry and cold conditions to the area14.
In recent times, Southeast and East Asia have been grappling with severe flooding caused by anomalous wet weather and extreme heat events, posing a significant threat to social resources and human lives during the summer months15. These occurrences appear to be closely associated with the behavior of the NPSH, which exerts influence over the region's wind patterns14. The intensity of the subtropical high plays a pivotal role in shaping the strength and direction of low-level winds, consequently affecting the distribution of meteorological phenomena in Southeast and East Asia. Research conducted using General Circulation Models (GCMs) suggests that the intensity of the NPSH is likely to increase as a result of global warming17. Additionally, other studies propose that the variability of the subtropical high pressure may also amplify due to the effects of global warming18. A critical question arises regarding the establishment of a fluid-dynamical connection between global warming and the intensity of the oceanic high, driven by land-sea temperature contrasts.
The theoretical framework for understanding the dynamics of the large-scale atmosphere in mid-latitudes has been established based on a conceptual model featuring zonal symmetry19. This model envisions an Earth entirely covered by oceans, devoid of landmasses, wherein the behavior of the large-scale atmosphere in mid-latitudes is primarily influenced by latitudinal heat flux imbalances. Under the assumption of radiative-convective equilibrium, westerly winds prevail in mid-latitudes. However, the zonal wind in these regions exhibits baroclinic instability20, leading to the formation of synoptic storms. These storms play a crucial role in redistributing energy from low latitudes to high latitudes during their life cycles21, thereby giving rise to what is known as baroclinic eddies22,23—the fundamental driver of mid-latitude weather phenomena. This theoretical framework has become integral to the field of daily weather forecasting. Despite being developed within a simplified context, the fundamental features of synoptic storms predicted by this theory hold true even in more realistic scenarios incorporating land-sea contrasts. Consequently, the quasi-geostrophic potential vorticity equation, an approximation of the full fluid-dynamical equations governing the atmosphere, has emerged as a primary framework for describing synoptic eddies in mid-latitudes24. However, the occurrence of extreme flooding and heat events in certain longitudinal locations during summer cannot be fully elucidated by the zonally symmetric dynamics of the large-scale atmosphere. Instead, these phenomena are often linked to the zonally asymmetric response of the atmosphere to planetary-scale surface thermal forcing. Moreover, relying solely on the quasi-geostrophic potential vorticity equation may be inadequate for understanding the planetary-scale atmospheric response, particularly in regions where relative vorticity is relatively weak25.
A recent development in atmospheric dynamics introduces a novel framework that considers zonal asymmetry induced by land-sea contrasts26. Building upon the theoretical notion that two asymptotic scales—planetary and synoptic scales—coexist within the large-scale atmosphere, it is proposed that planetary-scale motion acts as a mean field, while synoptic-scale motion emerges as eddies generated from the mean field via baroclinic instability26,27. The planetary-scale motion, characterized by horizontal length scales comparable to the Earth's radius, maintains hydrostatic and geostrophic balances, including the Sverdrup relation26. According to a simple theoretical analysis, the planetary-scale response of the atmosphere to zonally asymmetric thermal surface forcing intensifies as the zonal mean wind weakens28. Particularly noteworthy is the existence of a threshold in zonal mean wind speed, beyond which the vertical structure of the response transitions from exponential decay to wave-like propagation. In other words, steady-state responses exhibit wave-like behavior when the zonal mean zonal wind is weaker. This new framework offers a promising approximation for describing planetary-scale atmospheric flow phenomena such as the NPSH.
The theoretical framework, which relies on an approximation of the primitive equations, requires validation through numerical simulations based on either the full equation or reanalysis data. Initially, an idealized Global Climate Model (GCM) dynamic core is employed to assess the reliability of the theory's predictions29. While this numerical model comprises primitive equations with simplified parameterizations, it is adequate for verifying the theoretical propositions concerning large-scale atmospheric dynamics. By manipulating surface thermal heat flux, it becomes feasible to modulate the overall intensity of the jet stream in mid-latitudes30. This is achieved by introducing additional positive or negative surface heat flux to high latitudes, thereby adjusting the thermal wind balance. Consequently, this adjustment induces a decrease or increase in the zonal mean zonal wind, as illustrated in Figure 1a. Furthermore, to explore the response of the planetary-scale atmosphere to land-sea contrasts, a zonally asymmetric surficial thermal forcing akin to the land-sea contrast in the Northern Hemisphere is applied. This zonal asymmetry is depicted by the gray line contours in Figure 1b and c. The objective is to observe how the planetary-scale atmosphere responds to the land-sea contrast under varying degrees of Arctic amplification.
Figure 1 compares two contrasting cases derived from numerical simulations and reanalysis data. Upon the addition or subtraction of zonally symmetric surface thermal forcing in high latitudes, the magnitude of the zonal mean zonal wind undergoes weakening or strengthening, respectively, in accordance with the thermal wind balance (see Fig. 1a). Removing zonal mean fields allows for the observation of how the mid-latitude atmosphere responds to zonally asymmetric surface thermal forcing. As anticipated from theory, the 850-hPa response is more pronounced (less pronounced) (see Fig. 1b and c) when the zonal mean zonal wind is weaker (stronger). To investigate this response in reanalysis, two boreal summer composites were constructed based on the magnitude of the zonal mean wind in Eurasia and the North Pacific using JRA 55 data from 1979 to 202231 (see Fig. 1d, e, f). Focusing on the North Pacific region, the composites clearly show that the intensity of the subtropical high is stronger (weaker) (Fig. 1e and f) when the zonal mean wind is weaker (stronger) (Fig. 1d). These findings from numerical simulations and reanalysis data align with theoretical predictions, affirming the consistency of the theoretical argument based on planetary geostrophic motion28. Even in the presence of synoptic eddies, the overall structure of the planetary response closely resembles the analytical solutions derived from linearized planetary geostrophic motion28.
The response of the atmosphere at the planetary scale to zonally asymmetric surface thermal forcing exhibits a threshold behavior, wherein its increase is particularly pronounced as the background zonal wind weakens28. This behavior has been observed to undergo a transition from exponential decay to wave-like propagation when the zonal mean zonal wind falls below a certain threshold value, as documented in previous studies28. Moreover, the sensitivity of the atmospheric response magnitude to a decrease in zonal mean zonal wind is notably heightened when the wind speed becomes smaller than this threshold. To further explore this phenomenon, a numerical sensitivity test is conducted to assess how the magnitude of the planetary-scale atmospheric response to surface thermal forcing changes with decreasing zonal mean zonal wind (see Fig. 2a). Several numerical simulations are performed with varying levels of high latitude surface thermal forcing. The results demonstrate that the magnitude of the stream function at upper levels increases as the zonal mean zonal wind decreases. Furthermore, a notable observation is made: when the zonal mean zonal wind (U) drops below approximately 12 m/s, the rate of increase of the magnitude of the stream function intensifies, exhibiting behavior akin to the threshold phenomenon previously described.
To validate the relationship between the averaged zonal wind and the intensity of the planetary-scale atmospheric response, weakly averaged geopotential height and zonal wind data from reanalysis sources are analyzed (refer to Fig. 2b). Daily reanalysis data spanning from May to August, collected between 1979 and 2022, are utilized to construct the weakly averaged fields, focusing on the mid-latitude region covering Eurasia and the North Pacific. A scatter plot is generated to illustrate the relationship, with the intensity of the planetary-scale atmospheric response quantified by the difference in geopotential height between Eurasia and the North Pacific, normalized by the temperature difference between these regions. Given potential variations in thermal forcing across the area, the geopotential height difference is normalized by the temperature difference, which directly reflects the disparity in surface thermal forcing between land and sea. Despite potential variability stemming from various unknown sources, the observed relationship between zonal mean zonal wind and the intensity of the planetary-scale atmospheric response to land-sea contrast closely resembles that observed in idealized numerical simulations (see Fig. 2b). Specifically, as the zonal mean zonal wind weakens, the intensity of the planetary-scale atmospheric response to a zonally-asymmetric thermal forcing strengthens. Notably, when the background zonal wind falls below approximately 12 m/s, the sensitivity of the atmospheric response becomes more pronounced, suggesting a dynamic regime change consistent with theoretical predictions.
We can investigate whether the changes observed in the North Pacific subtropical high are connected to ongoing global warming. A critical inquiry revolves around whether the weakening of the zonal mean zonal wind is attributable to global warming. According to the principles of the thermal wind balance, a fundamental dynamic characteristic of the large-scale atmosphere, the magnitude of the zonal wind is primarily governed by the meridional temperature gradient. The temperature contrast between low and high latitudes is roughly proportional to the overall strength of the zonal mean zonal wind in mid-latitudes. Consequently, any weakening of the zonal wind in mid-latitudes should be correlated with changes in high-latitude ocean conditions due to global warming. A prominent feature of contemporary global warming is Arctic Amplification, where temperatures in high latitudes rise at a faster rate than those in low latitudes. This phenomenon is particularly pronounced in the Arctic Ocean, leading to the rapid decline of sea ice due to the well-known positive feedback mechanism, the sea ice albedo feedback32. This trend is evident in the declining trend of Arctic Sea ice concentration (refer to Fig. 3a). Additionally, Fig. 3b depicts the distribution of linear trends in sea surface temperature (SST) across the Northern Hemisphere, revealing a faster rise in SSTs in high latitudes (red shading), especially evident in July. Consistent with the expectations derived from the thermal wind balance, the trend of zonal wind in the North Pacific demonstrates a negative trajectory (see Fig. 3c). Consequently, we can infer that under the influence of global warming, the intensity of the NPSH is likely to increase.
The probability density function (PDF) analysis of the zonal wind and the magnitude of the NPSH over the recent decade (2013 to 2022) reveals a notable increase in the probability density for weaker zonal winds and stronger subtropical highs compared to the preceding decade (1979 to 1988) (see Fig. 3d). Concurrently, there is an expansion in the width of the probability density function, indicating an increase in the variability of the subtropical ocean high. While extreme cases become more frequent, their trend aligns with the intensification of the subtropical high alongside weaker zonal mean zonal winds. These findings suggest that the zonal mean wind in the North Pacific has gradually weakened amidst ongoing global warming, consequently rendering the planetary-scale atmosphere more sensitive to land-sea contrasts during the summer season. This heightened sensitivity may contribute to the observed increase in the probability of occurrences characterized by a stronger subtropical high alongside weaker zonal winds.
When considering the response of the planetary-scale atmosphere to land-sea contrasts, the establishment of quasi-stationary oceanic highs and continental lows introduces intriguing dynamic features, particularly in the regions situated between high and low pressure systems. In East Asia, for instance, the continental low is positioned in the west, juxtaposed with an oceanic high in the east. This configuration prompts low-level southerly winds to transport hot and moist air from tropical regions to East Asia. Conversely, in western North America, the northerly winds carrying cold and dry air from the Arctic region are instigated by the oceanic high in the west and the continental low in the east.
According to the Sverdrup relation33,34, which represents a fundamental balance in the planetary-scale atmosphere, the presence of low-level southerly winds (Fig. 4b) necessitates corresponding upward motion (Fig. 4a). Consequently, hot and moist air moves northward and ascends in Southeast Asia. This low-level southerly wind, coupled with upward motion, creates favorable conditions for regional precipitation as moisture is lifted upward. Therefore, the strengthening of low-level southerly winds results in increased vertical velocity, dynamically linked to heightened precipitation in Southeast Asia. Comparatively, weaker westerly cases (Fig. 4d) exhibit more intensified precipitation than stronger westerly cases (Fig. 4e) in Southeast and East Asia, as evident in their discrepancy (Fig. 4f). Notably, precipitation in Southeastern countries and the South China region is more pronounced in weaker westerly scenarios. Recent observations indicate a preference for weaker zonal mean zonal winds over historical trends (Fig. 4c). The joint PDF of precipitation and background zonal wind over the past 15 years illustrates a shift towards weaker zonal mean zonal winds and higher precipitation in Southeastern countries, as depicted in Fig. 4c.
The intensification of large-scale convection in the subtropical Pacific Ocean has the potential to induce latitudinal Rossby wave propagation35. It is well-established that latent heat released from organized convection over extensive subtropical areas can serve as a driving force for Rossby wave-like responses36,37, propagating towards high latitudes38,39,40. The dipole pattern of atmospheric pressure observed over the South China/Philippine Seas (low) and Korea/Japan (high) is a good demonstration of these propagating Rossby waves. The occurrence of high pressures over Korea and Japan often causes a northwestward expansion of the NPSH. This atmospheric configuration is known as the Pacific-Japan (PJ) pattern, a prominent quasi-stationary pattern observed during the boreal summer, exerting significant influence on weather and climate variabilities in East Asia41,42. Recent research has highlighted the strong association between the PJ-related Rossby waves and extreme heat events experienced in Korea and Japan43.
The difference of geopotential height between the weaker (Fig. 5a) and the stronger (Fig. 5b) westerly cases presents atmospheric pressure pattern associated with the weak background westerly and stronger precipitation in Southeast Asia (Fig. 4f). It shows hints of Rossby waves occurring from the subtropical region, which is also reminiscent of the Pacific Japan pattern (Fig. 5c). This intensification of the precipitation in Southeast Asia likely provides an additional source of the northward propagating Rossby waves. Similarly, the difference of surface air temperature between the two cases shows higher temperature anomalies in East Asia (Fig. S2). The composite of negative PJ modes in recent years is more intensified than that in past years (Figs. 5d and e). According to the joint PDF of daily maximum temperature and PJ indices, the recent one is shifted toward higher temperature and more negative PJ indices compared with the past one (Fig. 5f). Following the recent research, it can be deduced that extreme heat events in East Asia are more preferred in recent years due to the intensification of the negative PJ.
The phenomenon of Arctic Amplification leads to a reduction in the latitudinal temperature gradient, consequently weakening the westerlies in mid-latitudes to maintain thermal wind balance. This weakening of the westerlies prompts a stronger planetary-scale atmospheric response to land-sea thermal contrasts, equivalent to an intensification of the NPSH. In regions situated between continental lows and oceanic highs, southerly winds strengthen while adhering to the Sverdrup relation. This leads to intensified large-scale convective heating in the western subtropical Pacific, acting as a source for the formation of synoptic-scale highs covering East Asia. This fluid-dynamical connection originating from Arctic Amplification suggests that the recent surge in extreme flooding and heat events in Southeastern Asia and East Asia is linked to ongoing global warming. While a more detailed dynamical description is warranted in the future, it appears increasingly plausible that planetary-scale changes due to global warming are dynamically linked to synoptic-scale phenomena responsible for generating extreme events.