Gaoyuan qixiang (Aug 2024)
Two Modes of Westerly Jet over the Asian Continent in Mid-Summer and Their Relationship with East Asian Atmospheric Circulation Anomalies
Abstract
Based on NCEP/NCAR reanalysis data， the leading modes of the westerly jet over the Asian continent during July-August from 1960 to 2019 and the associated mid-to-high latitude circulations has been investigated in this study.The results show that the first leading mode is characterized by an out-of-phase variation in the south-north direction along the jet axis， resulting in a north-south movement of the jet.The second leading mode exhibits an out-of-phase variation in the east-west direction along the Qinghai-Xizang （Tibetan） Plateau， as a southwest （northwest） - northeast （southeast） tilt of the jet stream axis， which is different from most previous studies emphasizing the intensity variations in the second mode.For the first leading mode， associated with the northward movement of the jet streams， the South Asian High （SAH） moves northward as well as the western Pacific subtropical high （WPSH） northward narrows and vice versa.Furthermore， this first leading mode is associated with the tripolar precipitation pattern over East Asia.Particularly， as the jet stream shifts northward （southward）， the precipitation decreases （increases） in the Jianghuai River and increases （decreases） in South China and North China.As well， the associated temperature variations also show a tripolar pattern over the Eurasian continent， with the boundary of around 20°N and 45°N.It suggests that when the jet stream moves northward， warm （cold） conditions cover East Asia， particularly in the central and eastern regions of China， the Korean Peninsula and Japan， while cold （warm） conditions dominant Lake Baikal， the Indian Peninsula and Indochina Peninsula.As for the second leading mode， the geopotential height in the east （west） side of the SAH increases due to the southwest （northwest）-northeast （southeast） tilt of the jet axis.Additionally， the second mode is related to the precipitation anomalies in the high latitudes of West Asia， Central Asia， and the Indian Peninsula.This mode is particularly important for the double dipole temperature pattern over the Eurasian continent.When the jet stream axis presents a southwest （northwest）-northeast （southeast） tilt， significant positive （negative） temperature anomalies are observed in the middle latitudes of East Asia and specifically in the high latitudes of West Asia.
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