A Moments View of Climatology and Variability of the Asian Summer Monsoon Anticyclone

Understanding the climatology and variability of the Asian summer monsoon anticyclone (ASMA) is essential to diagnosing transport and composition in the upper troposphere / lower stratosphere in summer. Herein, we follow Santee et al. (2017) in defining the ASMA boundaries on isentropic surfaces using contours of Montgomery streamfunction. We calculate diagnostics of ASMA shape, size, and position using a moments analysis to show the climatology and variability of geometrical and dynamical features of the ASMA, and to explore correlations with other phenomena, such as subtropical jet variations and ENSO. The analyses are done and compared using the MERRA-2, ERA-Interim, and JRA-55 reanalyses for 1979--2018; the JRA-55 reanalysis is also analyzed for 1958--2018. While the climatological seasonal cycles agree qualitatively among the reanalyses, there are several notable quantitative differences, including larger area and a longer duration in the MERRA-2 than in the other reanalyses. The ASMA area and duration show increasing trends that are significant in all the reanalyses for some isentropic levels and some parts of the season. We analyze variability and trends in dynamical fields (e.g., PV and PV gradients, temperatures, tropopause heights) to help understand the changes in ASMA area. The relationship of these dynamical features to assimilated and MLS ozone in the ASMA will also be explored.

Reference:

Santee, M.L., G.L. Manney, N.J. Livesey, M.J. Schwartz, J.L. Neu, and W.G. Read, A comprehensive overview of the climatological composition of the Asian summer monsoon anticyclone based on 10 years of Aura Microwave Limb Sounder measurements, J. Geophys. Res., 122, 5491–5514, doi:10.1002/2016JD026408., 2017.