Biases in AMIP model simulations of the east China monsoon system

AMIP model simulations of the east China (5-50 degreesN; 105-122 degreesE) monsoon system are analyzed to study coherent relationships between rainfal l and wind annual cycle biases. A comparison with observed interannual vari ability patterns is carried out to identify the physical processes that exp lain the biases. The analyses show that poleward displacement of the simula ted east Asian jet stream causes the ascending branch of the jet-induced tr ansverse circulation to move north and, as a consequence, produces negative (positive) rainfall biases occur in central (northeast) China. The model s imulations show decreased southwesterly flow and ITCZ rainfall over the Sou th China Sea when weaker (versus observations) summer Hadley and Walker cir culations are present. This results from diminished model tropical disturba nce activity, and highlights the importance of air-sea interactions. In add ition, during October-January, intensified model low-level easterlies enhan ce moisture transport and produce positive local rainfall biases over centr al and northeast China. Biases in the east China monsoon system are concurr ently reflected in the planetary circulation. Enhanced northeast China rain fall results from increased surface pressure over the North Pacific and an amplified zonal pressure gradient along the east China coast. This bias pat tern is associated with differences in model representations of topography. On the other hand, the South China Sea experiences an extensive elongated meridional rainfall bias dipole structure that straddles the equator. This is accompanied by a baroclinic vertical pattern over the tropics as well as a barotropic wave train that extends from Australia to the Antarctic, wher e the teleconnection is likely a direct atmospheric response to tropical co nvective heating.