Dry and Moist Atmospheric Circulation with Uniform Sea-Surface Temperature

Abstract

The steady and transient response of "dynamically" dry and moist atmospheres to uniform sea-surface temperature (SST) is studied. Specifically, the latent heat (Lv) of water vapor is varied, so that for small Lv, water substance is essentially a passive tracer from a dynamical point of view. Despite the lack of SST gradients, a general circulation with Hadley and Ferrel cells is observed for relatively stronger moist coupling. Organized precipitation patterns via equatorial waves appear to play a significant role in tropical ascent, and along with the equatorial deformation radius, the Hadley cell width increases with coupling strength. An abrupt switch to a much shallower tropical cell is noted when the system becomes completely passive. In all cases, the Hadley cell is thermally indirect and is influenced by eddy fluxes which are strong in the upper and lower troposphere. Moist static energy is transported equatorward in the tropics and a larger amount is directed poleward in the midlatitudes. Transient extratropical activity is seen in the form of intense warm-core vortices for strong coupling, and these systems become weaker and smaller as Lv decreases. The drift of these moist vortices results in the observed poleward energy transport in the midlatitudes. In the tropics, intraseasonal variability is dominant and systematically shifts to longer time periods with stronger coupling. In fact, large-scale, low-frequency Kelvin waves and MJO-like modes disappear as water vapor becomes passive in nature. Finally, extreme rainfall events associated with cyclonic storms vanish as water vapor becomes dynamically inactive. Tropospheric heating due to a saturation of the outgoing longwave radiation results in an increase in the stability of the atmosphere for strong coupling, and provides a plausible physical mechanism for interpreting the behavior of precipitation.