Ocean-atmosphere responses to the coexistence of mesoscale SST gradients and convective rainfall

Tropical oceanic regions such as the western Pacific warm pool and the Indian Ocean are well known for high SST and the occurrence of copious convective rainfall. Such conditions are often regarded as synonymous at regional and global scales of analysis because their coexistence is so extensive. Upon examination of SST structure over a four year period, Li and Carbone (2012) revealed a stronger association between mesoscale SST gradients and the excitation of rainfall than SST itself. They showed that gradients of order 50-200 km dimension are ubiquitous; associated with 75% of all rainfall events; and exhibit SST variability that exceeds climatological variation for the Pacific warm pool region. Rainfall excitation locations show an SST bias of only +0.25C in a field of ~3C variability. The correlation of total rainfall with SST is only 0.42 when examined at relevant temporal and spatial scales.

Recognizing that rainfall events both generate and are generated by SST gradients, this presentation statistically examines the lifecycle of heavy rainfall events and SST gradients over a period of four years. A timeseries analysis illustrates mutual dependencies between upper ocean thermal structure (gradient and Laplacian of SST) and the occurrence of rainfall. The existence of a self-propagating, continual regeneration of rainfall-SST gradients is explored. The regeneration mechanism is also examined for systematic behavior with respect to transient atmospheric forcings, such as the MJO, Kelvin waves and other convectively-coupled disturbances. Relative phases, phase speeds and amplitudes of both SST structure and rainfall events will be presented to infer the respective influences of one medium on the other.