Formation and Recovery of a Cold Wake from Typhoon Fanapi (2010)

Cold anomaly of sea surface temperature (SST) is often created after the passage of a moving hurricane or typhoon. The SST reduction within these cold anomalies or cold wakes may reach 2„a to 4„aC. The cold wakes may have important impact on the development of a tropical cyclone due to their control on the surface energy fluxes. This work is aimed at understanding the evolution of cold wake and its impacts on the boundary layers on both sides of the air-sea interface. During 2010 typhoon season, Naval Research Laboratory coupled COAMPS-Tropical Cyclone was used to provide real-time forecasts for ITOP (Impact of Typhoons on the Ocean in the Pacific) field experiment. Typhoon Fanapi started as a tropical depression on September 14, and turned into a Category 4 typhoon on September 18. Along its passage, Typhoon Fanapi produced a large area of cold wake, leading to about 2„aC reductions in SST. The coupled COAMPS-TC realistically predicted the cold wake formation and recovery as well as the typhoon's track and intensity in general. We use combined coupled COAMPS-TC prediction and observation data collected during the ITOP IOP to investigate the characteristics of the cold wake evolution, evolution of atmospheric as well as oceanic boundary layers. The cold wake was predicted by the model on the right hand side of the storm track; it is driven by the strong shear mixing in the ocean mixed layer. The predicted maximum SST reduction within the wake is 2.5„a C, a value very close to the AXBT and satellite observations. Because of this decrease in SST, a stable atmospheric boundary layer is formed, leading to decreases in the surface wind speed, sensible and latent heat fluxes. The predicted warming rate in the cold wake recovery process is comparable with the satellite observation, even though diurnal signal is much more significant in the model prediction. An important question is what determines the recovery time scale. Given the similar solar warming rate between the cold wake and undisturbed environment, this time scale should depend on the differences in the surface turbulence and longwave radiative fluxes and the depth of the ocean surface layer. Currently, we are investigating this issue by analyzing surface energy budget from observation data and model results.