ROLE OF LAND-SURFACE PROCESSES ASSOCIATED WITH INTERANNUAL VARIABILITY OF BROAD-SCALE ASIAN SUMMER MONSOON AS SIMULATED BY THE CCSR NIES AGCM/

The interannual variability of the South Asian summer monsoon and asso ciated land-surface processes over the Eurasian continent in a ten-yea r integration (1979-88) of an atmospheric general circulation model (A GCM) forced by observed sea surface temperatures (SSTs) is examined. T he AGCM has been developed jointly by the Center for Climate System Re search (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 year s. It is found that the simulated interannual variability of broad-sca le summer monsoon shows a good correlation with observations. Furtherm ore, distinct precursory signals, including the Eurasian snow in winte r and soil moisture anomalies in spring, have been found in the pre-mo nsoon seasons of weak and strong monsoon years. There is a sharp contr ast between weak and strong monsoon years; excessive snow over Eurasia south of 50 degrees N in winter and the increased soil moisture in sp ring are found prior to weak summer monsoon. These results are consist ent with evidence found in observational data analyses and some model experiments. A detailed analysis of surface heat budget shows that sno w-albedo feedback dominates over the Tibetan Plateau. On the other han d, to its west in the central Asia, the relatively lower land, the eff ective cloud albedo anomalies due to excessive rainfall and surface ev aporation influence the surface conditions. A numerical experiment wit h the Eurasian land surface initial conditions in spring, interchanged between weak and strong monsoon years, indicates positive roles playe d by the land surface processes in influencing the subsequent summer m onsoon circulations during the 10-year period. However, such land surf ace feedbacks are not strong enough to change the sign of the monsoon circulation anomalies. The direct influence of the El Nino/Southern Os cillation through the changes in Walker circulation appears to predomi nate.