Drivers of mesoscale convective aggregation and spatial humidity variability in the tropical western Pacific

Abstract Idealized models show aggregation of tropical convection on the mesoscale driven by radiative-cloud feedbacks. We apply a multivariate analysis of column humidity, precipitation and sea surface temperature (SST) observations to mesoscale domains in the tropical western Pacific to demonstrate that in boreal summer and autumn, convection remains spatially random despite radiative-feedbacks acting to aggregate convection. Instead, in winter and spring, a weak meridional SST gradient exists and convection is usually clustered over the regions of warmest SSTs, with significant meridional humidity gradients. However, this is sporadically interrupted by episodes of convection migration to the coldest SSTs and limited spatial humidity variance. These episodes are the result of westward propagating equatorial Rossby waves, which act to remove meridional humidity gradients. It thus appears that the drivers of mesoscale convective clustering and humidity variability in the Pacific warm pool are the SST gradients and equatorial wave dynamics.