An optimization-based approach to track the Asian summer monsoon anticyclone across daily and interannual variability

Abstract. The definition of the boundary of the Asian summer monsoon anticyclone (ASMA) in the upper troposphere-lower stratosphere (UTLS) (350–410 K) is a known challenge that highly impacts the information about the anticyclone's behavior and affects the results when studying of its interannual variability. We present a novel method based on the absolute vortex moments that defines the ASMA boundary by solving an optimization problem. Here, we address the ASMA's climatology (1980–2023), interannual variability, the variability of the start and end dates and the duration of the anticyclone peak phase calculated with help of the defined novel method by using the ERA5 reanalysis provided by ECMWF. In addition, three individual years – 2017, 2022 and 2023 are highlighted during which aircraft campaigns took place to measure air inside the ASMA or its outflow (StratoClim, ACCLIP, PHILEAS). The interannual analysis is based on the anticyclone's centroid latitude and longitude, excess kurtosis, angle and aspect ratio using four isentropic surfaces: 350, 370, 390 and 410 K. Our findings show that the ASMA area decreases at 370, 390 and 410 K over the period 1980–2023 in contrast to previous studies. Further, we provide evidence of possible bimodality of the ASMA where the spatial distribution shows clustering of high-magnitude values (the Montgomery streamfunction values minus an optimized background value) around two centers on climatological data over 44 years as well as counting the number of days when two anticyclones (or two modes) where found simultaneously.