A Modern Analog Analysis of the Relationship Between Northern and Southern East Asian Summer Monsoon Regions, with an Application to the Paleoclimate of the Past 1000 Years
Recent studies have identified a precipitation dipole (i.e. two regions in which precipitation changes are consistently out of phase with each other) between the Northern and Southern East Asian Summer Monsoon regions (NEASM/SEASM) during the past 1000 years. To explore the mechanism for this pattern, NCEP reanalysis data of the past 63 years (1948-2010) are used in a modern analog analysis. The precipitation rate shows a distinct shift around the year 1978, from high to low for NEASM, and in the opposite direction for SEASM. Differences in zonal flow (uwind/vwind), geopotential heights, vertical motion (omega) and sea-surface temperature (SST) in the western Pacific before and after 1978 were analyzed to identify underlying mechanisms. Based on this analysis, local changes in vertical motion in the two regions contributed to the out-of-phase pattern. Also significant to the rainfall anomalies is the SST of the Philippine Sea. A shift to lower SST in this region around 1978, similar to the SST anomaly observed there during El Nino events, served as the triggering factor for larger-scale changes in circulation patterns. First, a convection anomaly in the Northwest Pacific (NWP) affects the ascent in the Indian Summer Monsoon region (ISM), which has a teleconnection with the Mediterranean/Aral Sea regions via Rossby waves. Then, the Asian Jet transmits a potential vorticity anomaly from that region eastward to the NEASM. The SEASM, on the other hand, is affected mostly by the anomalous meridional wind originating from the SST anomaly in the Philippine Sea. The combination of local subsidence and large-circulation patterns explains the dipole pattern of NEASM and SEASM in recent decades. A decadal to multi-decadal scale dipole in aridity is also evident in high-resolution paleoclimatic reconstructions for the Medieval Warm Period in East Asia, and can also be explained with a similar mechanism.