Tropical Rainfall Prediction Based on Excitation and Regeneration by Mesoscale SST Gradients

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 explores intrinsic predictability based on the lifecycle of heavy rainfall events and coexistence with mesoscale SST gradients. 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 mechanism, including continual regeneration of rainfall and SST gradients, is explored. The regeneration mechanism is also examined for systematic behavior with respect to transient atmospheric forcings as a basis for statistical predictions at intraseasonal timescales. Forcings such as the MJO, Kelvin waves and other convectively-coupled disturbances are considered. 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 and their predictive potential.

Reference Li, Y. and R. E. Carbone, 2012: Excitation of Rainfall over the Tropical Western Pacific, J. Atmos. Sci., in press.

1 Corresponding author, carbone@ucar.edu, P.O. Box 3000, Boulder, CO 80307-3000