Abstract
Abstract. The spatial pattern of subseasonal variability of the Asian monsoon anticyclone is analyzed using long-term reanalysis data, focusing on the large-scale longitudinal movement. The air inside the anticyclone is quantified by a thickness-weighted low-PV (potential vorticity) area on an isentropic surface. It is shown that the longitudinal movement of the air inside the Asian monsoon anticyclone has a timescale of 1 to 2 weeks, which is shorter than the monthly dominant timescale of the variability in the anticyclone intensity. The movement of the anticyclonic air is suggested to be largely controlled by passive advection. The typical time evolution of the variability pattern, explained by two leading empirical orthogonal function (EOF) components of 100 hPa geopotential height, shows large-scale geopotential anomalies moving westward spanning from low to middle latitudes. This corresponds well with the rapid westward movement of low-PV air known as “eddy shedding” and following the eastward retreat of the anticyclonic air. The two EOF components can also explain the bimodal longitudinal distribution of geopotential maximum location.
Highlights
The Asian monsoon anticyclone is characterized by a planetary-scale anticyclonic circulation, which persists in the upper troposphere and lower stratosphere (UTLS) region over the Eurasian continent throughout the northern summer
The Asian summer monsoon region is considered one of the most important pathways of tropospheric tracers entering into the stratosphere
While the whole picture to explain the budget of the low-potential vorticity (PV) area at these levels is more complicated than what is seen at a single level, we consider that the longitudinal oscillatory behavior with a submonthly timescale is one of the important features of the variability of AMA and the zonal flux Fλ=60◦ at 370 K as a representative variable for it
Summary
The Asian monsoon anticyclone (hereafter AMA; known as the South Asian high or the Tibetan high) is characterized by a planetary-scale anticyclonic circulation, which persists in the upper troposphere and lower stratosphere (UTLS) region over the Eurasian continent throughout the northern summer. It contributes to the tracer transport from the AMA to the midlatitude lower stratosphere (Vogel et al, 2016; Fadnavis et al, 2018) Another important spatial characteristic of the variability of the AMA is the longitudinal movement of the anticyclone center. The purpose of this study is to give a unified view of the dominant pattern of the variability of the AMA, incorporating the existing descriptions, namely, the event-like westward shedding of anticyclonic vortex with low-PV air and the longitudinal movement of the maximum geopotential height location, each of which has been separately discussed in different contexts so far.
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