Abstract The summer Eurasian westerly jet is reported to become weaker and wavier, thus promoting the frequent weather extremes. However, the primary driver of the changing jet stream remains in debate, mainly due to the regionality and seasonality of the Eurasian jet. Here we report a sharp increase, by approximately 140%, in the interannual variability of the summertime East Asian jet (EAJ) since the end of twentieth century. Such interdecadal change induces considerable changes in the large-scale circulation pattern across Eurasia, and consequently weather and climate extremes including heatwaves, droughts, and Asian monsoonal rainfall regime shifts. The trigger mainly emerges from preceding February North Atlantic seesaw called Scandinavian pattern (contributing to 81.1 ± 2.9% of the enhanced EAJ variability), which harnesses the “cross-seasonal-coupled oceanic-atmospheric bridge” to exert a delayed impact on EAJ and thus aids relevant predictions five months in advance. However, projections from state-of-the-art models with prescribed anthropogenic forcing exhibit no similar circulation changes. This sheds light on that, at the interannual timescale, a substantial portion of recently increasing variability in the East Asian sector of the Eurasian westerly jet arises from unforced natural variability. 

Atlantic origin of the increasing Asian westerly jet interannual variability

In the peak-summer of 2022, the Yangtze–Huaihe River Basin (YHRB) in China experienced its most extreme heatwaves since 1961. Notably, a rare heatwave also occurred in the YHRB in 2013. However, the Tibetan Plateau (TP) exhibited different thermal conditions during those two years. Observational analysis indicates that the 2013 and 2022 heatwaves were closely related to the local anomalous high pressure. The synergistic effects of the East Asian subtropical jet (EASJ), South Asian high (SAH), western Pacific subtropical high (WPSH), and divergence of moisture, supported the formation and maintenance of the high pressure. Compared to 2013, under the synergistic control of these intensified circulation systems in 2022, which led a broader descending motion and stronger heatwaves prevailed over the YHRB. Also, the surface air temperature of TP reached its highest value in 2022, whereas it was not distinctly anomalous in 2013. Corresponding to the TP thermal anomaly, an abnormal high pressure appeared throughout the troposphere over the northern YHRB. The TP heating contributed to the northward strengthening of the SAH and EASJ, which enhanced the anomalous westerly winds to the north of the anticyclone. Moreover, the WPSH extends westward during strong TP heating years. With the cooperation of high pressure and moisture, there was an anomalous meridional circulation pattern with ascending motion in North China and descending motion in the YHRB, accompanied by reduced cloud cover and increased insolation in the YHRB, leading to further intensification of local heatwaves in 2022. This study demonstrates the significant role of TP thermal anomaly in YHRB heatwaves. 

Comparative analysis of peak-summer heatwaves in the Yangtze–Huaihe River Basin of China in 2022 and 2013: Thermal effects of the Tibetan Plateau

Abstract This study explored the interannual characteristics of the out‐of‐phase pattern of summer precipitation over northern China during the past years, as well as the possible underlying mechanisms. The out‐of‐phase pattern is characterized by the positive precipitation anomaly in Northwest China (NWC) and the negative anomaly in North China (NC). Our analyses indicate that the variation of Asian westerly jet (AWJ) is found evidently associated with this out‐of‐phase pattern of summer precipitation. The meridional displacement of AWJ induces opposite trends in water vapour transport and circulation anomalies between NWC and NC, leading to the out‐of‐phase pattern. Further analyses suggest that the anomalies of Arctic sea ice concentration (SIC), North Atlantic and Northwest Pacific sea surface temperature (SST) can impact the variation of AWJ by triggering the Eurasian (EU), polar‐Eurasia (POL) and Pacific‐Japan (PJ) teleconnection patterns, respectively, which in turn modulate the formation of this pattern. The precipitation in NWC is primarily affected by the North Atlantic and Northwest Pacific SST anomalies, whereas the precipitation in NC is notably influenced by both SST anomalies and the Arctic SIC anomaly. However, the role of SST anomalies on precipitation in NWC and NC exhibits an evident contrast, leading to the out‐of‐phase pattern of summer precipitation. 

The out‐of‐phase pattern of summer precipitation over northern China and the possible mechanisms

Abstract The relationship between meridional displacement of the Asian jet and Indian summer monsoon (ISM) is examined in the observations and 40 state‐of‐the‐art Coupled Model Intercomparison Project, phase 6 (CMIP6) historical simulations. The southward displacement of the Asian jet (SWDAJ) is found to be linked with reduced rainfall over central and north India including monsoon trough region in both the observations and most of the CMIP6 models. In general, models are able to simulate the characteristics of the SWDAJ during summer with some differences in zonal asymmetry. Detailed analysis elucidated two observed pathways that connect the SWDAJ with ISM rainfall variability. The first is the west Asian jet–easterly jet pathway, where the SWDAJ is linked with the southeastward shift of the South Asian high, contributing to the weakening of tropical easterly jet. This pattern aligns with changes in low‐level ISM circulation and contributed to reduced rainfall over India. The west Asian jet–easterly jet pathway is well represented by most of the CMIP6 models. The second pathway involves the East Asian jet–Pacific Japan pattern, in which the SWDAJ over East Asia strengthens the western North Pacific anticyclone through the barotropic–baroclinic coupling. The anomalous westward extended easterlies originating from the western North Pacific anticyclone hinder the southwesterly monsoon flow and suppress the rainfall. It is found that CMIP6 models largely failed to capture the East Asian jet–Pacific Japan pathway, likely due to the models' tendency to overestimate the simulation of the Indian Ocean dipole, and westward extension of El Niño‐related sea‐surface temperature anomalies in most of the models. Further, the combined influence of El Niño and the Pacific decadal oscillation and El Niño alone on the SWDAJ and ISM rainfall is also discussed in detail. Results obtained in this study are useful to understand the ability of CMIP6 models in representing the midlatitude circulation associated with ISM rainfall variability. 

Association between the meridional displacement of the Asian jet and Indian summer monsoon rainfall in <scp>CMIP6</scp> models: Impact of El Niño and <scp>Pacific decadal oscillation</scp>

Prediction skill of Asian jet and Tropical Easterly jet in CMIP6 decadal prediction models

This study analyzes the frequency characteristics of tropical cyclone (TC) genesis in the southeastern part of the western North Pacific (SEWNP) during June–November from 1965 to 2019 and investigates the possible combined effect of the solar activity and El Niño-Southern Oscillation (ENSO). Results suggest that TCs generated in the SEWNP have the longest lifetime and greatest strength, and its frequency has apparent interannual and decadal variations, which is jointly affected by the solar activity and ENSO. In El Niño years during declining phases of solar cycle (1–3 years following the solar maximum), positive TC genesis frequency anomalies in the SEWNP are significantly strong and tend to occur in extremes. While the opposite is true for La Niña during ascending phases of solar cycle (1–3 years following the solar minimum). However, there exists no significant feature in the combined effect of La Niña (El Niño) and declining (ascending) phases of solar cycle. When declining (ascending) phases of solar cycle and El Niño (La Niña) are combined, the overlapping effect leads to apparently warmer (colder) sea surface temperature (SST) anomalies in the central equatorial Pacific and colder (warmer) SST anomalies in the western Pacific, so the SST anomalies gradient are stronger. It enhances low-level westerly (easterly) wind anomalies and upper-level easterly (westerly) wind anomalies, which is favorable for the further decrease (increase) of the vertical wind shear in the eastern (most) part of SEWNP. Moreover, the stronger and more westward low-level convergence (divergence) center appears in the Pacific, causing stronger low-level convergent (divergent) flow and upper-level divergent (convergent) flow anomalies, and strengthened (suppressed) ascending movement anomalies in the SEWNP. As a result, the TC genesis frequency in the SEWNP is much higher (lower). Further diagnoses show that absolute vorticity plays a leading role in El Niño years during declining phases of solar cycle, and its contribution is obviously much greater than other environmental factors. It is not perfectly symmetrical in La Niña years during ascending phases of solar cycle when the contribution of absolute vorticity is the greatest, but vertical wind shear is also important.

/pdf/combined-effect-of-the-solar-activity-and-enso-on-the-1hgdxje9.pdf

Combined effect of the solar activity and ENSO on the tropical cyclone genesis frequency in the southeastern part of the western North Pacific


This study investigates the impact of El Niño and Southern Oscillation (ENSO) on the seasonal evolution of the Asian–Pacific summer monsoon. The results show that during the summers when the SSTs in the equatorial eastern Pacific are above normal, i.e., the developing El Niño, the seasonal march of Asian–Pacific summer monsoon is delayed. Specifically, the stepwise northeastward extension of western North Pacific monsoon and the northward migration of East Asian and Indian monsoon are all delayed, resulting in rainfall deficiency in the western North Pacific and India, and more rainfall in the Yangtze River valley but less rainfall in North China due to the delayed northward migration and resultant prolonged rainy season in the Yangtze River valley. In addition, the seasonal march of the Asian upper-tropospheric westerly jet, which is mainly characterized by the northward migration during the monsoon development, is also delayed, concurrent well the delayed march of monsoonal rainfall. During La Niña summers, however, the seasonal march of the monsoon and Asian jet does not exhibit clear variations, suggesting the asymmetrical impacts between the El Niño and La Niña. Furthermore, we explain these variations in the seasonal evolution of rainfall and jet and the asymmetrical impacts between the El Niño and La Niña through the variations in tropical tropospheric temperatures and the resultant changes in meridional temperature gradient.

Warmer SSTs in the equatorial eastern Pacific delay the seasonal march of the Asian–Pacific summer monsoon onset

Interannual Meridional Displacement of the Upper-Tropospheric Westerly Jet over Western East Asia in Summer

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Combined effect of the solar activity and ENSO on the tropical cyclone genesis frequency in the southeastern part of the western North Pacific

Warmer SSTs in the equatorial eastern Pacific delay the seasonal march of the Asian–Pacific summer monsoon onset

Interannual Meridional Displacement of the Upper-Tropospheric Westerly Jet over Western East Asia in Summer