Structure and Formation Mechanisms of the Northern Hemisphere Summertime Subtropical Highs

Abstract

Abstract Three-dimensional structure and dynamics of the climatological-mean summertime subtropical highs over the North Pacific and Atlantic (i.e., the Azores high) are investigated. Each of the observed surface highs is accompanied by a meridional vorticity dipole aloft, exhibiting barotropic and baroclinic structures in its northern and southern portions, respectively, in a manner dynamically consistent with the observed midtropospheric subsidence. Each of the highs develops over the relatively cool eastern ocean, where a pronounced near-surface thermal contrast exists with a heated landmass to the east. The authors demonstrate through numerical experiments that those highs can be reproduced in response to a local shallow cooling–heating couplet associated with this thermal contrast, although the upper-level response is somewhat underestimated. The model experiments suggest that the near-surface thermal contrasts associated with those surface subtropical highs over the Pacific and Atlantic 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 and diverging toward downstream in the upper troposphere above each of the observed surface highs. The observed wave activity injection is significant into the Azores high but not at all into the Pacific high. Since each of the subtropical highs can be reproduced reasonably well, even for the premonsoon season (i.e., May), in response to a local shallow land–sea heating contrast, it is suggested that the monsoonal convective heating may not necessarily be a significant direct forcing factor for the formation of the summertime subtropical highs. In fact, the model response is quite weak if forced only by mid- and upper-tropospheric convective heating. The present study suggests the presence of a local land–sea–atmosphere feedback loop associated with a subtropical high and a continental low to its east, which may be triggered by increasing insolation over land from spring to summer.