Controls on tropical mean state and intraseasonal precipitation variability in an idealized moist atmospheric model

Abstract

Through latent heat released by condensation and feedbacks involving radiation, physical processes related to water vapor play key roles in tropical dynamics. While some understanding of aspects of tropical climate can be obtained using dry general circulation models (GCMs), e.g. theories for what controls the width of the Hadley circulation, understanding of other aspects requires, or at the very least can be refined by, the inclusion of the influence of moist processes. A natural intermediate step in the model hierarchy between a dry GCM and a comprehensive GCM is the idealized moist model, which includes simplified treatments of moist processes. In this dissertation, we make use of this model, in this case coupled to a full radiative transfer code to additionally include water-vapor-radiation feedbacks, to study aspects of tropical mean state and transient precipitation in simplified setups.

We begin by quantifying the impact of including the interaction between water vapor and radiation on the sensitivity of the latitude of the intertropical convergence zone (ITCZ) to a cooling in the Northern Hemisphere (NH). We do so by applying varying magnitude cooling forcings in the NH in two model configurations: one with interactive water vapor and radiation, and one with the water vapor field seen by the radiation code prescribed as that from a hemispherically-symmetric control simulation. We find that the ITCZ shifts roughly twice as far for a given forcing in the interactive cases versus the prescribed cases.

We then move on to studying transient disturbances, starting with monsoon low pressure systems (MLPSs). To study these we add a simplified treatment of land to the model. With realistic continental geometry we show using regression and tracer budget analysis that the model supports MLPS-like storm systems in the South Asian monsoon region, whose dynamical properties share some commonalities with monsoon depressions found in reanalysis datasets. Finally we turn to equatorial waves. Through aquaplanet experiments in which heating perturbations with varying magnitude and vertical structures are added, we show that MJO-like variability can be preferentially enhanced if the net heating of the deep tropical atmosphere is increased.