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

Abstract

A comprehensive investigation of the climatology of and interannual variability and trends in the Asian summer monsoon anticyclone (ASMA) is presented, based on a novel area and moments analysis. Moments include centroid location, aspect ratio, angle, and ''excess kurtosis'' (measuring how far the shape is from elliptical) for an equivalent ellipse with the same area as the ASMA. Key results are robust among the three modern reanalyses studied. The climatological ASMA is nearly elliptical, with its major axis aligned along its centroid latitude and a typical aspect ratio of ;5-8. The ASMA centroid shifts northward with height, northward and westward during development, and in the opposite direction as it weakens. New evidence finding no obvious climatological bimodality in the ASMA reinforces similar suggestions from previous studies using modern reanalyses. Most trends in ASMA moments are not statistically significant. ASMA area and duration, however, increased significantly during 1979-2018; the 1958-2018 record analyzed for one reanalysis suggests that these trends may have accelerated in recent decades. ASMA centroid latitude is significantly positively (negatively) correlated with subtropical jetcore latitude (altitude), and significantly negatively correlated with concurrent ENSO; these results are consistent with and extend previous work relating monsoon intensity, ENSO, and jet shifts. ASMA area is significantly positively correlated with the multivariate ENSO index 2 months previously. These results improve our understanding of the ASMA using consistently defined diagnostics of its size, geometry, interannual variability, and trends that have not previously been analyzed.

Response of the Upper‐Level Monsoon Anticyclones and Ozone to Abrupt CO2 Changes

“…Our study also shows that quadrupling of the in coupled models leads to an equatorward shift of the ASM anticyclone, which is in agreement with a recent study by Manney et al. (2021), showing a strong positive correlation in the location between the anticyclone and subtropical westerly jet. Yet, the cause and effect remains unclear and is a subject of future investigations.…”

“…The strengthening and equatorward expansion of the zonal‐mean subtropical upper tropospheric westerly jet is a very robust result in models and is closely connected to the enhanced warming in tropical upper troposphere (Li et al., 2009, 2010; McLandress & Shepherd, 2009; Menzel et al., 2019). Previous studies of the upper tropospheric jets showed a close connection between variability of the subtropical westerly jet and the upper level anticyclones (Manney & Hegglin, 2018; Manney et al., 2014, 2021; Schiemann et al., 2009). Our study also shows that quadrupling of the in coupled models leads to an equatorward shift of the ASM anticyclone, which is in agreement with a recent study by Manney et al.…”

Persistence of moist plumes from overshooting convection in the Asian monsoon anticyclone

“…The 2017 Asian monsoon season was not marked by an anomalous dynamical behaviour (Manney et al, 2021); however, the campaign occurred during a break-to-active transition. The strongest convective activity took place in late July and early August above the southern slopes of the Himalayas and the Tibetan Plateau, as can be inferred from the low outgoing longwave radiation (OLR) displayed as dashed contours in Fig.…”

Transient Behavior of the Asian Summer Monsoon Anticyclone Associated With Eastward Eddy Shedding

“…The details of the bimodal distribution are sensitive to the use of different reanalysis data sets (Nützel et al., 2016), and bimodality is potentially driven by variations in convection (e.g., Garny & Randel, 2013), monsoonal heating (e.g., P. Zhang et al., 2016), orographic effects (Q. Zhang et al., 2002; Liu et al., 2007), and large‐scale dynamical variability (Amemiya & Sato, 2020). More recent analyses have highlighted that the AMA is subject to large dynamical variability on synoptic scales, constantly splitting, merging, and shedding anticyclonic eddies westward and eastward (Garny & Randel, 2013, 2016; Manney et al., 2021; Pan et al., 2016; P. M. Rupp & Haynes, 2020). C. J. Hsu and Plumb (2000) showed that an idealized monsoon anticyclone circulation periodically sheds secondary anticyclones due to dynamical instabilities, and observational confirmation of eddy shedding was first shown in Popovic and Plumb (2001).…”