Large-scale atmosphere-ocean-land interaction in the formation of summertime subtropical highs

In climatology, each of the surface subtropical highs over the North Pacific and Atlantic (i.e., the Azores high) develops over the relatively cool eastern ocean, where a pronounced near-surface thermal contrast exists with a heated landmass to the east. It is demonstrated through our model experiments that it is the shallow diabatic cooling/heating couplet associated with this thermal contrast, rather than deep convective heating, that primarily forces the highs. Although the upper-level response is somewhat weaker, the observed structure of the high is reproduced reasonably well in the model. The model experiments suggest that the near-surface thermal contrasts associated with the subtropical highs can act as sources of the observed planetary waves over the western hemisphere. In fact, a wave-activity flux for stationary Rossby waves is distinctively upward above each of the observed surface highs, while no significant wave-activity injection is observed into the Pacific high. Since each of the subtropical highs can be reproduced reasonably well even for May in response to the local shallow heating contrast, it is concluded that the monsoonal convective heating is not the primary factor for the formation of the summertime subtropical highs but it may indirectly contribute by keeping the subtropical landmass upstream of the heating in hot and dry condition. Interaction of the subtropical highs with the underlying ocean and the associated feedbacks are suggested. They are manifested, for example, as the enhancement of surface evaporative cooling observed over the eastern subtropical ocean from early to mid-summer.