Holocene westerly jet dynamics and the regional hydroclimatic barrier

Jet streams on Earth are narrow bands of zonal westerly winds that circulate at high speeds, reaching over 400 km/h near the tropopause. These jet streams have significant impacts on global and regional climates under current climate conditions. In the Southern Hemisphere, the higher speed of the westerly jet exerts a stormier influence on relatively large- and global-scale climate systems (1). The Southern Hemisphere westerly jet and prevailing westerly winds play a crucial role in rapidly distributing heat and moisture globally by regulating meridional heat flow and atmospheric carbon dioxide concentration (2). In contrast, the climatic influences of the Northern Hemisphere westerly jet are complex and still not fully understood due to the intricate distribution of geography and air pressure patterns (1). Specifically, the Northern Hemisphere subtropical westerly jet significantly impacts the hydroclimate of midlatitude arid regions, leading to fluctuations in both rainfall and drought patterns and contributing to conflicts over water resources in the region (3). Over the last few decades, the importance of jet streams in shaping climate has been highlighted in paleoclimate studies (4–7). These studies are essential for gaining a deeper understanding of climate dynamics beyond the modern era, encompassing centennial to orbital timescales, and establishing clearer links between regional climate systems and jet streams.

One aspect that remains ambiguous is the precise role of the Northern Hemisphere jet stream in shaping precipitation patterns within a narrow geographical area. In PNAS, Tan et al. (8) pinpoint an invisible hydroclimatic barrier caused by westerly jet dynamics in the midlatitude arid region of Central Asia throughout the Holocene (Fig. 1). This study is particularly significant as proxy data for this region have been scarce during the Holocene.

Fig. 1.

Schematic illustration depicting orbital-scale Holocene westerly jet dynamics and their influence on arid Central Asian hydroclimates. The upper image displays the trajectories of the summer and winter westerly jet over the Central Asian region during the early Holocene. Notable features such as the winter storm track, hydroclimatic barrier, and climatic symbols denoting winter precipitation dominance in Western Central Asia and summer precipitation dominance in Eastern Central Asia are annotated. The lower image mirrors the upper image but represents the present-day setting. Inset schematic diagrams illustrate insolation changes and seasonal latitudinal shifts of the westerly jet path throughout the Holocene. Refer to the main text and ref. 8 in PNAS for comprehensive details.

One of the key subsystems of Earth’s jet streams, the Asian subtropical westerly jet (ASWJ), is recognized as a significant hydroclimate regulator with substantial impacts on many mid- to high-latitude regions across the Eurasian continent (9, 10). This subsystem also exhibits teleconnectivity, influencing weather and climate patterns in the North Pacific and North American regions, and at times facilitating the conveyance of atmospheric rivers to the western coast of North America, resulting in intense precipitation (11). Therefore, delving into the long-term paleoclimate variations orchestrated by the ASWJ in the central Eurasian region serves as a fundamental step in unraveling the effects of the subtropical westerly jet on midlatitude Northern Hemisphere climates.

The Central Asian region has emerged as an area facing pressing water supply challenges stemming from recent shifts in hydroclimates (12). However, the substantial discrepancy in the availability of paleohydroclimatic datasets between the western and eastern sectors hampers our ability to comprehend the long-term interplay between regional hydroclimates and potential influences from the westerly jet (13, 14). Various Holocene hydroclimatic datasets from approximately the longitude of 74E in the eastern sector have indicated a gradual transition toward wetter conditions throughout the Holocene (8). In contrast, there is a notable lack of paleohydroclimatic data in the western region beyond 74E.

In regions where paleoclimate data are lacking, such as the arid Western Central Asian (WCA) region, speleothem proxy records can serve as valuable sources of information. While the primary advantage of paleoclimatic studies utilizing speleothems has traditionally been attributed to precise U-series dating, an aspect that has received less attention is the broad geographical distribution of karst caves and speleothems themselves, spanning from polar to tropical regions (15). Tan et al. (8) effectively leveraged this characteristic of speleothems in their study. Their investigation revealed that speleothem records in the WCA region clearly depict a decline in precipitation over the course of the Holocene, contrasting with trends observed in the Eastern Central Asian (ECA) region. These divergent changes have led to the emergence of contrasting hydroclimatic patterns between the adjacent WCA and ECA regions throughout the Holocene, and this feature appears to be an invisible barrier between both regions (Fig. 1). Then, how can we comprehend the hydroclimatic barrier that manifests between such closely situated areas?

Conventionally, oxygen isotope data derived from speleothem calcites in subtropical to tropical monsoon regions have served as a crucial proxy record (16). However, the effectiveness of speleothem oxygen isotope data in Central Asia as a proxy for distinguishing the Holocene history of regional hydroclimatic changes is limited due to variations in moisture sources between adjacent regions and/or distinct large-scale atmospheric circulation processes, including differences in upstream areas (17). To elucidate and distinguish the regional differences in Holocene hydroclimates between WCA and ECA, Tan et al. (8) introduced additional proxies such as changing patterns of carbon isotope composition, strontium/calcium (Sr/Ca) ratio, and initial ²³⁴U/²³⁸U activity ratio, all of which are reliable indicators of regional precipitation. Under drier climatic conditions, speleothems receive cave drip waters with higher ¹³C, Sr, and ²³⁴U relative to ¹²C, Ca, and ²³⁸U. This phenomenon occurs because lower precipitation and subsequent reduced soil moisture lead to decreased soil respiration, increased precipitation of calcium carbonate minerals above the caves, and diminished weathering of rocks in the epikarst zone (18). Consistent changes observed throughout the Holocene in all these proxy data indicate a gradual transition of WCA to a drier climate regime on an orbital timescale. In contrast, identical proxy records from various regions in ECA suggest a shift toward a more humid climate, highlighting a distinct antiphasing trend between adjacent WCA and ECA regions throughout the Holocene.

“In PNAS, Tan et al. pinpoint an invisible hydroclimatic barrier caused by westerly jet dynamics in the midlatitude arid region of Central Asia throughout the Holocene.”

Presently, West Central Asia (WCA) is directly and indirectly influenced by winter cyclonic storms originating from the Mediterranean region, known as Western Disturbances (WD). These winter storms transport significant moisture along the path of the subtropical westerly jet (19). Despite being arid, WCA’s total annual precipitation is primarily controlled by winter moisture supply, predominantly from WD. Conversely, the neighboring ECA region experiences the majority of its total annual precipitation from summer moisture sources, likely from adjacent continental and monsoonal systems (13). Given these distinct climatic conditions, dynamic shifts in the path of the jet stream can be attributed to the antiphasing Holocene precipitation records observed between WCA and ECA (Fig. 1), as outlined by Tan et al. (8). Indeed, simulations of the latitudes of the westerly jet in these regions have shown opposing trends (20). On an orbital timescale, solar insolation acts as a primary driver of the latitudinal displacements of the westerly jet and westerlies (2). Throughout the Holocene, increasing winter insolation in the Northern Hemisphere has led to a gradual northward shift in the average latitude of the westerly jet during the winter season, resulting in weaker winter storms and reduced winter precipitation in WCA (Fig. 1). Conversely, decreasing summer insolation has driven the summer westerly jet closer to Central Asia, thereby increasing summer precipitation in ECA. Consequently, while winter precipitation in WCA has steadily decreased, summer precipitation in ECA has shown an increasing trend since the early to middle Holocene. These dynamics of the westerly jet serve as an invisible hydroclimatic barrier between the two adjacent regions.

Tan et al. (8) offer a compelling depiction of the influence exerted by the jet stream within highly specific geographic locales, leveraging multiproxies alongside high-precision dating results to track fluctuations in precipitation. To affirm the findings of them, further studies are necessary, not only in the same Central Asian region but also in other midlatitude regions, including Far East Asia, which lies along the path of the subtropical westerly jet in the Northern Hemisphere. The widespread distribution of speleothems makes them particularly suitable for this international collaborative effort (15). Additionally, it is imperative to analyze various types of paleoclimate records to track changes in other climatic components along the westerly jet path (6, 7). Through such endeavors, we can gain a deeper understanding of the fundamental teleconnections among regional climate systems.

Recent studies have implicated global warming in the dynamics of the jet stream, particularly over the past few decades (21, 22). Further global warming and increasing winter insolation may lead to a further northward migration of the winter westerly jet in WCA, potentially resulting in drier conditions in this region but wetter conditions in ECA. The ongoing global warming is poised to bring significant changes to the hydrological cycle and water demand for many countries in the future, potentially escalating international conflicts (12). However, our current understanding of the impact on long-term regional climate changes remains inadequate. The findings of Tan et al. (8) underscore the importance of paleoclimate research, particularly with the incorporation of various regional hydroclimatic proxies, in advancing our detailed understanding of current climate changes.