Asian summer monsoon anticyclone: trends and variability

Ghouse Basha,M Venkat Ratnam,Pangaluru Kishore

doi:10.5194/acp-20-6789-2020

Abstract

Abstract. The Asian summer monsoon anticyclone (ASMA) has been a topic of intensive research in recent times regarding its variability in dynamics, chemistry and radiation. This work explores the spatial variability and the trends of the ASMA using observational and reanalysis data sets. Our analysis indicates that the spatial extent and magnitude of the ASMA is greater during July and August than in June and September. The decadal variability of the anticyclone is very large at the edges of the anticyclone compared with the core region. Significant decadal variability is observed in the northeastern and southwestern parts of the ASMA with reference to the 1951–1960 period. The strength of the ASMA shows a drastic increase in zonal wind anomalies in terms of temporal variation. Furthermore, our results show that the extent of the anticyclone is greater during the active phase of the monsoon, strong monsoon years, and La Niña events. Significant warming with strong westerlies is observed exactly over the Tibetan Plateau from the surface to the tropopause during the abovementioned periods. Our results support the existence of transport process over the Tibetan Plateau and the Indian region during active, strong monsoon years and during strong La Niña years. Therefore, it is recommended that the different phases of the monsoon be taken into account when interpreting the variability of pollutants and trace gases in the anticyclone.

Highlights

The Asian summer monsoon anticyclone (ASMA) is a dominant circulation in the Northern Hemisphere (NH) summer that extends from Asia to the Middle East in the upper troposphere and lower stratosphere (UTLS)
During the months of September and June, the geopotential height (GPH) values, which represent the spatial extent of the anticyclone, are low compared with July and August
The spatial extent of the anticyclone circulation is clearly evident in the grid from 15 to 45◦ N and from 30 to 120◦ E at 100 hPa, and the climatological averaged GPH values vary from 16.5 to 17 km in the National Center for Environmental Prediction (NCEP) reanalysis between 1951 and 2016

Summary

Introduction

The Asian summer monsoon anticyclone (ASMA) is a dominant circulation in the Northern Hemisphere (NH) summer that extends from Asia to the Middle East in the upper troposphere and lower stratosphere (UTLS). The ASMA circulation responds to heating corresponding to the deep convection of the south Asian monsoon (Hoskins and Rodwell, 1995; Highwood and Hoskins, 1998) This strong anticyclone circulation isolates the air and is tied to the outflow of deep convection, which has distinct maximum characteristics in terms of dynamic and chemical variability (Randel and Park, 2006; Park et al, 2007). The anticyclone circulation in UTLS has been paid more attention by researchers in order to understand the dynamics, chemistry and radiation of the region. This problem has been discussed by several authors (e.g., Park et al, 2007; Fadnavis et al, 2014; Glatthor et al, 2015; Vernier et al, 2015; Santee et al, 2017).

Data and methodology

IMD gridded precipitation data

Results and discussion

Summary and conclusions