A southward withdrawal of the northern edge of the East Asian summer monsoon around the early 1990s

Abstract

The northern edge of the East Asian summer monsoon (EASM) is identified using the pentad total column water vapor obtained from ERA-Interim reanalysis data during 1979–2015. Empirical orthogonal function analysis is applied to study the meridional displacement of the northern edge of the EASM during the study period, and the results show an interdecadal southward shift around 1993/1994 and an indistinct northward displacement after 2007/2008. To focus on the interdecadal change around 1993/1994, composite analysis using the difference between 1979–1993 and 1994–2007 is employed. Through examination of the differences between these two periods, a significant anticyclonic anomaly is found over Mongolia, suggesting a pronounced interdecadal weakening of the Mongolian low during 1994–2007. Thus, northward advancement of the EASM may have been prevented by the anomalous northerly flow to the east of the weakened Mongolian low after 1993. Further study shows that the interdecadal weakening of the Mongolian low might be attributable to the meridional inhomogeneity of surface warming over the northern part of East Asia. Previous studies suggest that such meridional inhomogeneity would lead to a reduction in local atmospheric baroclinicity, and thus the suppression of extratropical cyclone activity over Mongolia, resulting in a southward withdrawal of the northern edge of the EASM on the interdecadal timescale.