Role of Land Surface Processes Associated with Interannual Variability of Broad-scale Asian Summer Monsoon as Simulated by the CCSR/NIES AGCM

Abstract

The interannual variability of the South Asian summer monsoon and associated land-surface processes over the Eurasian continent in a ten-year integration (1979-88) of an atmospheric general circulation model (AGCM) forced by observed sea surface temperatures (SSTs) is examined. The AGCM has been developed jointly by the Center for Climate System Research (CCSR), the University of Tokyo, and the National Institute for Environmental Studies (NIES). A monsoon intensity index, based on the magnitude of summer-mean vertical shear of zonal wind over the south Asian monsoon region, is used to classify weak and strong monsoon years. It is found that the simulated interannual variability of broad-scale summer monsoon shows a good correlation with observations.
Furthermore, distinct precursory signals, including the Eurasian snow in winter and soil moisture anomalies in spring, have been found in the pre-monsoon seasons of weak and strong monsoon years. There is a sharp contrast between weak and strong monsoon years; excessive snow over Eurasia south of 50°N in winter and the increased soil moisture in spring are found prior to weak summer monsoon. These results are consistent with evidence found in observational data analyses and some model experiments. A detailed analysis of surface heat budget shows that snow-albedo feedback dominates over the Tibetan Plateau. On the other hand, to its west in the central Asia, the relatively lower land, the effective cloud albedo anomalies due to excessive rainfall and surface evaporation influence the surface conditions.
A numerical experiment with the Eurasian land surface initial conditions in spring, interchanged between weak and strong monsoon years, indicates positive roles played by the land surface processes in influencing the subsequent summer monsoon circulations during the 10-year period. However, such land surface feedbacks are not strong enough to change the sign of the monsoon circulation anomalies. The direct influence of the El Niño/Southern Oscillation through the changes in Walker circulation appears to predominate.