Dynamics of Seasonally-Varying Tropical Convergence Zones

Abstract

In the tropics, rain tends to be organized along concentrated rain bands, with the Intertropical Convergence Zone (ITCZ) over the ocean and monsoonal convergence zones over tropical land masses being particularly prominent features affecting hundreds of millions of people. What fundamentally controls the spatial and temporal distribution of these tropical rainbands remains an outstanding question in the literature. This thesis aims to enhance our understanding of the dynamics of seasonally migrating convergence zones over the ocean and in the South Asian monsoon region. First, we explore to what extent energetic arguments that have provided insight into the position and shifts of the annual and zonal mean ITCZ can also be applied on shorter timescales. Idealized aquaplanet simulations show that the energy flux equator (EFE) always leads the ITCZ, leading to a breakdown of the commonly assumed anti-correlation between the ITCZ position and the cross-equatorial energy transport. At times during which the EFE and the ITCZ reside on opposite sides of the equator, the required energy transport is in fact achieved by the Hadley cell, in which the ITCZ is embedded, changing its vertical structure into one of negative gross moist stability. One way in which this is accomplished is through the development of a shallow return flow at levels near minimum moist static energy. While the relationship between the EFE and the ITCZ in the observed seasonal cycle is more complex than what is seen in the idealized simulations, the development of bottom-heavy circulations is a common feature both in the zonal mean and in individual longitudinal sectors at times when the EFE and the ITCZ are in opposite hemispheres. In the last chapter of this thesis, we explore changes in the South Asian monsoon as topography over Africa is removed in the full-physics GFDL AM2.1 GCM. Against expectations, the removal of the African topography is accompanied by a strengthening of the precipitation over India despite a weakening of the Somali jet. This counter-intuitive precipitation increase is associated with the development of a lower-level cyclonic wind anomaly, and associated meridional moisture flux convergence, over the Indian peninsula. Potential vorticity (PV) budget analyses following air parcel trajectories show that this cyclonic anomaly arises because, in the absence of the blocking effect of the African topography, air particles that reach the Arabian Sea originate at higher latitude and hence have a higher background planetary vorticity.