Regularity and irregularity of the seasonal northward march of East Asian summer wet environment and the influential factors

Abstract

This study investigated the characteristics of regular and irregular seasonal northward march of East Asian summer wet environment (SNM_EASWE) over the continent during June–July over the past six decades and in the future. During 1961–2015 and in the future, the regular SNM_EASWE is defined mainly by an intensified climatic northward-moving rainband over eastern China, whereas the irregular SNM_EASWE is defined mainly by less clear features of northward-moving rainband. Further analyses indicate that the boreal summer intraseasonal oscillation (BSISO) played a dominant role in affecting the regular/irregular SNM_EASWE during 1961–1979, whereas both the BSISO and the seasonally varying western North Pacific (WNP) eddy may exert notable influences on the regular/irregular SNM_EASWE during 1980–2015 and in the future, where the WNP eddy refers to the anomalous anticyclone/cyclone over the WNP associated with the anomalous western Pacific subtropical high. During 1980–2015, a La Niña-like (El Niño-like) condition in the tropical Pacific during June–July may induce a regular (irregular) SNM_EASWE case because of the influence of anomalous convection over the Maritime Continent on the seasonally varying WNP eddy; an anomalous warming (cooling) in the tropical Indian Ocean during June–July may also partly induce a regular (irregular) SNM_EASWE case via influencing the seasonally varying WNP eddy. The increased influence of seasonally varying WNP eddy on the regular/irregular SNM_EASWE after 1979 are attributed to the increased interannual variability of convective activity over the Maritime Continent and of sea surface temperatures in the Arabian Sea after 1979.