Mechanisms for an Early Spring Peak of Extratropical Cyclone Activity in East Asia

Abstract The frequency of extratropical cyclones in East Asia, including those traveling along the Kuroshio off the south coast of Japan, maximizes climatologically in early spring in harmony with local precipitation enhancement. Springtime cyclone activity is of great socioeconomic importance for East Asian countries. However, mechanisms for the early spring peak in the cyclone activity in East Asia have been poorly understood. This study aims to unravel the mechanisms, focusing particularly on favorable conditions for relevant cyclogenesis. Through a composite analysis based on atmospheric reanalysis data, we show that cyclogenesis that is enhanced around the East China Sea under anomalously strengthened cyclonic wind shear and temperature gradient, in addition to enhanced moisture flux from the south, is important for the early spring peak in the cyclone activity in East Asia. In early spring, climatologically strengthened cyclonic shear north of the low-level jet axis and associated frequent atmospheric frontogenesis in southern China and the East China Sea serve as favorable background conditions for low-level cyclogenesis. Through a set of linear baroclinic model experiments, we also demonstrate that climatologically enhanced diabatic heating around East Asia is pivotal in strengthening of the low-level jet. Our findings suggest the importance of the seasonal evolution of diabatic heating in East Asia for that of the climate system around East Asia from winter to spring, encompassing the early spring peaks in the climatological cyclone activity and precipitation. Significance Statement This study aims to elucidate mechanisms for an early spring peak in the frequency of low pressure systems in East Asia. We show that the climatological development of the low-level jet stream in southern China and the East China Sea in early spring is important for the more frequent generation of those low pressure systems. Climatological increase in diabatic heating in East Asia from winter to spring is found to strengthen the low-level jet in southern China and the East China Sea. Our findings suggest the importance of the seasonal evolution of diabatic heating in East Asia for the seasonal evolution of the East Asian climate system from winter to spring, including the early spring peaks in climatological low pressure system frequency and rainfall.