13MJO Systematic Latitude Dependence of Convection-Circulation Coupling throughout the Northern Hemisphere Tropics

During the boreal summer, the northern hemispheric tropics experience strong intraseasonal variability of convection ranging from synoptic-scale activity (2-8 days) to low-frequency oscillations (30–70 days) in rainfall. The complex interactions within convection-circulation coupling associated with these intraseasonal events are poorly understood. Using ERA5 reanalysis, we study the interaction between moist convection (rainfall) and circulation (vorticity) on intraseasonal timescales.

We first identify intraseasonal convective events at different latitude zones (0-4°N, 5°N-9°N, 10°N-14°N, and 15°N-19°N) within major tropical ocean basins. We then demonstrate that away from the equator, convection and circulation coevolve more coherently both in space and time, with the respective maximum of the two variables occurring closer for events away from the equator. The correlation between vorticity and rainfall rises from near zero at the equator to about 0.4 at 20°N during the growing phase of these events. This highlights the latitude-dependent strength of convection-circulation coupling that has been previously overlooked in the literature.

The vertically resolved vorticity budget reveals larger magnitudes of the boundary layer vortex stretching for events away from the equator. This term is balanced by the meridional advection of relative vorticity in the boundary layer, while in the free troposphere, the vertical advection balances the horizontal advection and the vortex tilting. A spatial collocation of boundary layer stretching and free tropospheric vertical vorticity advection is observed away from the equator during the peak convection. Rapid nonlinear advection owing to the coincidence of stretching-induced vertical vorticity gradients and convection-induced vertical velocity leads to the barotropic structure of vorticity. This process intensifies away from the equator owing to larger planetary vorticity and, hence, larger stretching term. These systematic variations in convection-circulation coupling with latitude can serve as important benchmarks for testing general circulation models (GCMs) ability to accurately represent the intraseasonal variability in the tropics and provide pathways for model improvements.