Numerical Study Of The Effects SSTA On Inter-decadal Variation Of East Asian Summer Monsoon

Used are the 1950-2000 observational SSTs from global, extratropical, tropical oceans, Indian Ocean-Pacific at tropics, tropical Indian Ocean and tropical Pacific, with each of them to conduct experiments by means of the NCAR Cam3 as the global atmospheric model, with the results compared to ERA-40 reanalysis in order to address the SSTA impact on inter-decadal change of East Asian summer monsoon (EASM) and the principal members of its circulation systems. The following are of note.1) The Cam3-simulated natural variability is dominantly the interannual variation in the main members of the summer monsoon and its circulations, with the monsoon and the south Asian high (western Pacific subtropical high and eastern hemispheric cross-equatorial flow) marked by more distinct (unremarkable) interdecadal variation and a close relation exists of the monsoon to atmospheric EU teleconnection wavetrain.2) The simulated EASM by means of observational SSTs of global/tropical oceans, Indian Ocean-Pacific at tropics and tropical Pacific shows an interdecadal weakening process to be in the mid-late 1970s, in better agreement with observations, thereby demonstrating that SSTA in these ocean regions play an active role in the interdecadal variability of the summer monsoon. The tropical Pacific is the key basin. The numerical experiment with extratropical SSTs indicates that the SSTA there has lower impact upon the interdecadal change of the monsoon. The modelings using tropical Indian Ocean SSTs demonstrates that the monsoon interdecadal variability is strengthened (weakened) when these SSTA are warmer (colder) than mean on an interdecadal basis with its variability opposite to that related to the tropical Pacific SSTs.3) Modelings show that variations in SSTs of global/tropical oceans, Indian Ocean -Pacific at tropics and tropical Indian Ocean exert pronounced impacts upon the interdecadal variability of western Pacific subtropical highs in summer, i.e., an interdecadal change happening to the highs'in the mid-late 1970s, after which the high's area is enlarged, its vigor is intensified and position is west- and/or southward of mean, all in better harmony with in situ observations. Also, change in tropic Pacific SST is innegligible to the interdecadal variation of the western Pacific subtropical high in summer, whose strength and area experience interdecadal variability in the late 1960s, with its westward extending point (southern boundary) undergoing an interdecadal variation early in the 1960s (mid-late 1970s), after which its intensity is amplified, its area is enlarged and position west- and/or southward compared to mean. In comparison, change in extratropical SSTs has little effect on the interdecadal variability of the subtropical high.4) Simulations show that change in tropical Pacific SSTs plays an important role the interdecadal variation of the south Asian high. The related EOF3 mode is the "sandwich" looking SST pattern, implying the change in equatorial eastern Pacific SST contrary to those in SSTs to the north and south of the eastern equatorial Pacific, which is marked by distinct interdecadal variation, in close association with that of the south Asian high.5) In the context of SSTs from global/tropical oceans, tropical Indian Ocean-Pacific and tropical Pacific the Cam3-reproduces the interdecadal variation of the cross-equatorial flows over Somali, 120°E and 150°E that change toward a strong phase in the middle - late 1970s, with the emulated Somalian airflow closer to observations, failing to simulate the interdecadal variation by using SSTs of extratropical and tropical Indian Ocean, thereby illustrating the prominent impacts on the interdecadal variability of the cross-equatorial air exerted by the change in SSTs over global/tropical oceans, tropical Pacific - Indian Ocean and the tropical Pacific, the tropical Pacific acting as the key region.6) As a background, the interdecadal variation of tropical Pacific SST has important effects upon the equivalent scale change in east Asian summer monsoon, western Pacific subtropical high, south Asian high and cross-equatorial air of the eastern hemisphere The tropical Indian SST as the interdecadal background has prominent influence upon the interdecadal variation of the western Pacific subtropical high in the following aspects. When tropical Pacific SSTs are in a warm background (1981-1998), East Asian summer monsoon reduces in vigor, the subtropical high increases in area, and is positioned south- and/or westward, the south Asian high is intensified, expanding its size with its position extending eastward, expanding westward and moved southward, in conjunction with reinforcement of summer cross-equatorial air at Somali, 120°E and 150°E, and v.v. to great extent when tropical Pacific SST is in a cold background (1958-1975). On the other hand, the western Pacific subtropical high is larger in size and positioned westward of mean when tropical Indian SST is in a warm background compared to tropical Indian SST in a cold background.