The Origin of the Subtropical Anticyclones

Abstract

The origin of the Northern Hemisphere summer subtropical anticyclones is investigated using a linear quasigeostrophic model. It is found that the broad features in the model solutions forced by realistic heating fields acting on observed zonal flows agree well with those in the observations. The realistic features of the model solutions include the subtropical continental lows and oceanic highs in the lower troposphere, and continental ridges and midoceanic troughs in the upper troposphere. The forced responses are largest near the surface and the tropopause with a vertical node around 500 hPa as observed. The results indicate that the model stationary waves owe their existence largely to the Asian heat source. The authors thus propose that the observed low-level subtropical anticyclones over the North Pacific and North Atlantic Oceans be interpreted as a remote response of Rossby waves forced by the large-scale heat sources over Asia. The results support the existing theory that the observed low-level cyclones and upper-level anticyclones over Asia and North America are a local response to monsoonal latent heat release in the midtroposphere. The sensitivity of model solutions to the basic state is discussed, emphasizing the effects of the meridional and vertical shear of the zonal flow on the structure of the stationary waves.