Interannual variability of the Asian summer monsoon anticyclone

Abstract. The definition of the boundary of the Asian summer monsoon anticyclone (ASMA) in the upper troposphere and 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 boundaries by solving an optimization problem. A 44-year ASMA climatology (1980–2023) will be shown using the ERA5 reanalysis provided by ECMWF. 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. 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, aspect ratio using 4 isentropic surfaces: 350, 370, 390 and 410 K. Our findings show that the ASMA area decreases over the period 1980–2023 in contrast to previous studies. Further, we provide evidence of possible bimodality of the ASMA by showing clustering of 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.