Role of the Midlatitude Air-Sea Interaction in Orographically Forced Climate

Abstract

Possible feedbacks between the midlatitude atmosphere and ocean relevant to maintain the mean climatological state in January are examined by using a coupled model with intermediate complexity (ICM), which consists of a 2-layer atmospheric general circulation model (AGCM) and a 2.5-layer shallow water ocean model covering the Pacific Ocean. Two series of the numerical experiments including the seasonal cycle were conducted: the experiments using the coupled model (ICM run) and the experiments using the AGCM (AGCM run). In each series of the experiments, the global mountain height was systematically changed; a parameter α that is multiplied to the surface elevation was varied from 0 to 1.4 with the interval of 0.2. The ICM run with α = 1 is regarded as the control run. Comparison of the January climatology between the ICM and the AGCMshows that the upper-level wind and low-level baroclinicity over the western Pacific Ocean increase linearly with α in both models while the sensitivity to change in α is higher in the ICM. This appears to be explained by the following positive feedback; in the coupling system, orographically forced stationary waves at larger α spin up ocean gyres, transporting the warm water near the western boundary that results in an enhancement of local evaporation and precipitation. The latent heat release works to intensify the stationary waves which are responsible to maintain the low-level baroclinicity. The low-level wind, SST and the storm track activity do not change linearly with increasing α. In particular, the storm track strengthens with mountain uplift until α = 0.6 while weakening afterwards. This weakened storm track decelerates the low-level wind downstream, which reduces the wind-driven circulation and the increase in SST around the gyre boundary at the heigher mountain.