Zonal Extension of the Middle East Jet Stream and Its Influence on the Asian Monsoon

Abstract

Abstract Investigation into the interannual variation of the Middle East jet stream (MEJS) and its influence on the Asian monsoon indicates that the eastward extension of MEJS is closely related with a wetter and colder winter in southern China and a later onset of the subsequent Asian summer monsoon, compared with normal conditions. When the MEJS extends eastward, a significant barotropic anomalous anticyclone is located over the Arabian Sea (AS), associated with the southeastward-propagating wave train from Europe. Intense divergence in the southwest of the AS anomalous anticyclone favors more convection over the western tropical Indian Ocean, which excites an anomalous upper-level anticyclone to the north as a Rossby wave response, further intensifying the AS anticyclonic anomaly. This positive feedback loop maintains the AS anomalous anticyclone and results in the eastward extension of the MEJS. Accordingly, intense northeasterly anomalies over the Mediterranean Sea and the subtropical westerly anomalies bring abundant cold air from the mid- to higher latitudes to subtropical regions, resulting in a widespread cooling in subtropical Eurasia including southern China. Barotropic anomalous westerlies occur around the Tibetan Plateau in the south and deepen the India–Burma (now Myanmar) trough, favoring more water vapor transport from the Bay of Bengal to southern China. These wetter and colder conditions in subtropical Eurasia can persist from winter to spring, leading to the much later onset of the Asian summer monsoon. Therefore, the winter MEJS variability can be considered an important indicator for the Asian monsoon. Significance Statement Diabatic heating over the western tropical Indian Ocean exerts significant influence on the wintertime Middle East jet stream (MEJS), whose eastward extension leads to a wetter and colder winter in southern China and a later onset of the Asian summer monsoon. When the MEJS extends eastward, an anomalous anticyclone appears over the Arabian Sea and strengthened northeasterlies occur over the Mediterranean Sea. The westerly anomalies bring cold air from the high latitudes, resulting in cooling in subtropical Eurasia. Anomalous westerlies also occur around the Tibetan Plateau and deepen the India–Burma trough, favoring water vapor transport from the Bay of Bengal. These wetter and colder conditions persist from winter to spring, leading to later onset of the Asian summer monsoon.