Characteristics of precipitation in East Asia associated with Okhotsk Sea blocking in June and July

Abstract

This study investigates the influence of Okhotsk Sea blocking (OKB) on summer precipitation in East Asia in terms of the movement, enhancement and extension of the monsoon rainband. For this purpose, a composite analysis is conducted for nine OKB events that occur in June and July from 2000 to 2022. The results show that the OKB events that occur in June are associated with decreased precipitation in East Asia, whereas those that occur in July are related to increased precipitation in the same region. The long-lasting OKB can continuously maintain and intensify anomalous cyclonic circulations in northeastern Asia, leading to enhanced vertically integrated moisture flux convergence (VIMFC) along the northern edge of the Western North Pacific subtropical high. However, there are differences in spatial distribution and displacement of anomalous cyclonic circulations between June and July. The OKB occurring in June is related to simultaneously developing and progressively strengthening anticyclonic circulation in the Okhotsk Sea as well as cyclonic circulation in northeastern Asia. Under the displacement of an intensified and southwardly extended cyclonic circulation, the VIMFC shifts southward and into a southeast-to-northwest orientation, compared to climatology, resulting in decreased precipitation in East Asia, particularly in southern China and southern Japan. On the other hand, when the OKB occur in July, anticyclonic circulation in the Okhotsk Sea is developed initially followed by cyclonic circulation in northeastern Asia, amplifying several days after onset. Anomalous cyclonic circulation is much weaker and located more northward than in June, which is favourable for stagnating and enhancing the VIMFC in East Asia. As a result, precipitation can increase in the region, particularly on the Korean Peninsula and in Japan. This study investigates the influence of Okhotsk Sea blocking (OKB) on summer precipitation in East Asia. The results show that the OKB events that occur in June are associated with decreased precipitation in East Asia, whereas those that occur in July are related to increased precipitation in the same region. image