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

A comprehensive investigation of the climatology of and interannual variabilityand trends in the Asian summer monsoon anticyclone (ASMA) is presented,based on a novel area and moments analysis. Moments include centroidlocation, aspect ratio, angle, and “excess kurtosis” (measuring how farthe shape is from elliptical) for an equivalent ellipse with the same area asthe ASMA. Key results are robust among the three modern reanalyses studied.The climatological ASMA is nearly elliptical, with its major axis alignedalong its centroid latitude and a typical aspect ratio of 5–8. The ASMAcentroid shifts northward with height, northward and westward during development,and in the opposite direction as it weakens. New evidence finding noobvious climatological bimodality in the ASMA reinforces similar suggestionsfrom previous studies using modern reanalyses. Most trends in ASMAmoments are not statistically significant. ASMA area and duration, however,increased significantly during 1979–2018; the 1958–2018 record analyzedfor one reanalysis suggests that these trends may have accelerated in recentdecades. ASMA centroid latitude is significantly positively (negatively) correlatedwith subtropical jet core latitude (altitude), and significantly negativelycorrelated with concurrent ENSO; these results are consistent with and extendprevious work relating monsoon intensity, ENSO, and jet shifts. ASMA areais significantly positively correlated with the MEI ENSO index two monthspreviously. These results improve our understanding of the ASMA using consistentlydefined diagnostics of its size, geometry, interannual variability, andtrends that have not previously been analyzed.