Impact of Different Ocean Conditions Present in the Bay of Bengal on Coupled TC Intensity Prediction

Coupled TC forecasts are performed using idealized representations of the ocean and atmosphere to study the impact of different oceanic conditions present in the Bay of Bengal (BoB) on predicted intensity. For all experiments, the atmospheric model is initialized by an idealized vortex with background conditions highly favorable for intensification. Eight different climatological mean temperature-salinity profiles in the BoB are first used to initialize the idealized, horizontally homogeneous ocean. These profiles represent pre- and post-monsoon conditions over the northern, central, and southern BoB, and also averaged over the entire BoB. Because the climatological ocean is highly favorable for intensification in all cases, it has only a modest impact on intensity evolution. The strongest storm formed in the southern BoB pre-monsoon case which had the highest initial SST and heat content relative to the 26C isotherm. In both the full-BoB and southern BoB cases, a weaker storm formed post-monsoon compared to pre-monsoon. In all cases, the ultimate intensity achieved was closely related to the enthalpy flux averaged over the inner-core region of the storm where this flux is largest. By contrast, intensity was less closely related to SST averaged over the same region, and poorly related to total SST cooling that occurred within the cold wake. These results suggest that the interplay between intensity and enthalpy flux is fundamentally coupled and that the ultimate intensity achieved by a storm cannot be accurately inferred solely by SST or SST cooling rate. These results also suggest that it is critically important to evaluate the physical realism of coupled prediction systems within this inner-core region, which will require simultaneous collection of high-quality three-dimensional observations of the atmosphere and ocean along with observations of surface waves and fluxes. Additional experiments are being performed to assess the impact of more extreme ocean conditions within the BoB, and also to assess the impact of freshwater barrier layers that frequently exist in the post-monsoon northern BoB; these results will be summarized in the presentation.