Ocean's Impact on the Intensity of Three Recent Typhoons (Fanapi, Malakas, and Megi) – Results from the ITOP Field Experiment

During the 20 August to 20 October 2010 ITOP field experiment, three typhoon cases, Fanapi, Malakas, and Megi were studied. Using airborne C130 dropwindsonde data, C130 AXBT (Airborne Expendable Bathythermograph) data, in situ upper ocean thermal structure data from the Argo floats, satellite sea surface temperature and altimetry data together with an ocean mixed layer model, the impact of ocean's thermal structure to the intensity of these 3 typhoons are investigated. It is found that all three typhoons passed over regions of similarly warm sea surface temperature (SST) of ~ 29.5¢ªC. However, much distinction is found in the subsurface. Category-2 Typhoon Malakas passed over region of the shallowest subsurface warm layer, as characterised by the depth of the 26¢ªC isotherm (D26) of about 37-40m and Upper Ocean Heat Content (UOHC) of ~ 38-44 kj/cm2. Category-3 typhoon Fanapi passed over region of moderate subsurface warm layer, with D26 of ~ 60-70m and UOHC of ~ 65-78 kj/cm2. Category-5 typhoon Megi passed over region of the deepest subsurface warm layer, with D26 reaching 124-132m and UOHC reaching 136-138 kj/cm2. It is found that this distinction in the subsurface thermal structure played critical role in the intensification of the three typhoon cases. Due to the very deep D26 and high UOHC, very little typhoon-induced ocean cooling negative feedback (typically < 1¢ªC) for Megi was found. This very minimal negative feedback enabled ample air-sea enthalpy flux supply to support Megi's intensification. Based on the preliminary report from the Joint Typhoon Warning Center (JTWC), Megi's peak intensity reached 160kts, a very high intensity not often observed even for category-5 typhoons. In contrast, though with very warm pre-typhoon SST of ~ 29.5¢ªC, the subsurface ocean condition for Malakas and Fanapi was much less favourable. As a result, the subsurface cold water could be much easily entrained and upwelled to the surface to limit the intensification for Malakas and Fanapi. Finally, it was found that the very deep subsurface warm layer and high heat content over the region where Megi passed was about 10-30% higher than the climatological values. Preliminary results suggest the possible contribution of the La Nina event in causing such warm anomaly over the western North Pacific in October 2010.