Water Vapor Transport by an Equivalent-Barotropic Cyclonic Anomaly Corresponding to Extreme Austral Late Summer Precipitation in Southeast Australia during 2021

Abstract

Southeast Australia (SEA) experienced a wet February as well as an extremely wet March accompanied by devastating floods during 2021. Regional water vapor balance analysis at different levels indicates the leading role of water vapor inflow through zonal boundaries during February, and the dominant contribution of water vapor input through meridional boundaries during March, providing adequate anomalous moisture for abnormal precipitation. The horizontal distribution of vertically integrated water vapor flux is characterized as an anomalous cyclonic circulation pattern around the Tasman Sea and SEA, responsible for the intensified water vapor transport along northwesterlies from the tropical Indian Ocean and along anomalous onshore easterlies from the Tasman Sea during both months. Partition of the contributions of dynamic and thermodynamic processes to the anomalous atmospheric water vapor flux reveals the dominant role of the anomalous wind field, but the anomalous variation in the moisture field also plays a part in the water vapor convergence for SEA. The presence of upper and lower large-scale atmospheric circulations ascertains that cyclonic water vapor flux is attributed to a dominant equivalent-barotropic cyclone system over SEA. The plausible joint impacts of internal forcing from the positive southern annular mode (SAM) oscillation, and external forcing from La Niña, are further confirmed by composite analysis; a La Niña–induced low pressure system dominates the lower level over the Australian continent, and the SAM-caused anomalous cyclonic disturbance propagating from higher latitudes governs the higher level above southern Australia, leading to the important equivalent-barotropic cyclonic circulation just above the region of interest.