1. Ocean’s Impact on the Intensity of Three Recent Typhoons (Fanapi, Malakas, and Megi) – Results from the ITOP Field Experiment 1 I-I Lin (iilin@as.ntu.edu.tw; http://smart.as.ntu.edu.tw/) and 2 Peter G. Black (peter.black.ctr@nrlmry.navy.mil)iilin@as.ntu.edu.twhttp://smart.as.ntu.edu.tw/peter.black.ctr@nrlmry.navy.mil 1 Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan (R.O.C.) 2 SAIC, Inc and Naval Research Laboratory, Monterey, CA USA INTRODUCTION During the 20 August to 20 October 2010 ITOP field experiment, three typhoon cases, Fanapi, Malakas, and Megi were observed in great detail with aircraft, satellite, ships, buoys and profiling floats. Using ‘Combo Drops’ of GPS dropwindsondes and Airborne Expendable Bathythermographs (AXBTs) from WC-130J aircraft, in situ upper ocean thermal structure data from the Argo floats, satellite Sea Surface Temperature (SST) and altimetry data together with an ocean mixed layer model, the impact of the ocean’s thermal structure on the intensity and structure of these three typhoons were investigated. DISCUSSION All three typhoons passed over regions of similarly warm SST of ~ 29.5 ˚ C. However, large differences were found in the subsurface ocean structure. Typhoon Malakas, with maximum intensity of CAT2 on the Saffir-Simpson intensity scale, passed directly across the eddy-rich Southern Eddy Zone (Lin, et al., 2005) and into a region of shallow subsurface warm layer, as characterised by the depth of the 26 ˚ C isotherm (D26) of 37-40m and Upper Ocean Heat Content (UOHC) of ~ 38-44 kj/cm2. CAT3 Typhoon Fanapi passed over a region of moderate subsurface warm layer, with D26 of ~ 60-70m and UOHC of ~ 65-78 kj/cm2. CAT5 Super-Typhoon Megi passed over a region of deep subsurface warm layer, with D26 reaching 124-132m and UOHC reaching 136-138 kj/cm2. We hypothesize that these differences in the subsurface thermal structure played a 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 for Megi was found (typically < 1˚ C). This minimal negative feedback enabled ample air-sea enthalpy flux to support Megi’s intensification. Based on the preliminary report from the Joint Typhoon Warning Center (JTWC), Megi’s peak intensity was estimated at 160kts, although WC-130J surface wind measurements from dropsondes and the Stepped Frequency Microwave Radiometer (SFMR) as observed to be 180 kts, extreme winds not often observed even for CAT5 typhoons. In contrast, despite very warm pre-typhoon SST of ~ 29.5 ˚ C, the subsurface ocean condition for Malakas and Fanapi was much less favourable. As a result, the subsurface cold water much more easily entrained and upwelled to the surface, limiting the intensification for Malakas and Fanapi. CONCLUSION 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. We suggest that the La Nina event may have caused a larger than normal warm anomaly over the western North Pacific in October 2010. Figure 1: Tracks of the 3 ITOP cases (Megi, Fanapi, and Malakas) over-plotted on the cloud image of Megi (courtesy of Dr. Eric D’Asaro, UW). STY Megi (Case 1) had Vmax of 180 kt from dropsonde and SFMR on 17 Oct, 2010 and Pmin of 890mb (CAT5). Ty Malakas (Case 2) had Vmax of 85 kt on 24 Sept, 2010 and Pmin of 946 mb. Ty Fanapi (Case 3) had Vmax of 97 kt on 18 Sept, 2010 and Pmin of 937 mb. Figure 2: Intensity evolution of the 3 typhoons from the preliminary reports of the JTWC. 700 850 925 Megi Malakas Fanapi Figure 3a-3c: ‘Skew T-Log P diagram‘ observations showing the saturation condition at typhoon’s inner core for the 3 cases (based on the in situ WC-130J dropwindsonde data). SST ( ℃ ) D26 (m) UOHC (Kj/cm 2 ) T100 ( ℃ ) Megi29.5 ↔ 3084 ↔ 12893 ↔ 14627.9 ↔ 29.4 Fanapi29.1 ↔ 29.653 ↔ 9154 ↔ 9826.6 ↔ 28 Malakas29.5 ↔ 3041 ↔ 4741 ↔ 6024.6 ↔ 25.8 Figure 4: Pre-storm in situ depth-temperature profiles from the WC-130J AXBT data and Argo floats (Gould et al., 2004) along typhoon’s track (from Category-1 to peak). While SST for the 3 cases are similar, the subsurface structure varies greatly. Megi experienced the deepest subsurface warm ocean layer (highest heat content) with D26 of 100-160m. In contrast, the pre-typhoon D26 for Fanapi was 50-90m while that for Malakas was 40-50m. Table 1: Quantification of SST, D26, UOHC (Upper Ocean Heat Content, integration from SST to D26), and T100 (averaged ocean temperature from surface to 100m depth) for the 3 cases. Figure 5: Left Column: Pre-typhoon SST map for Megi (top), Fanapi (middle) and Malakas (bottom) based on TRMM and AMSR-E microwave SST observations. Right Column: Pre-typhoon UOHC map derived from satellite altimetry (Shay et al. 2000; Pun et al. 2007). Figure 6:Top: Inner-core SST for the 3 cases as simulated from the Mellor and Yamada Mixed layer model (Mellor and Yamada; Lin et al. 2008). Bottom: Corresponding enthalpy fluxes (method see Lin et al. 2008; 2009). Gould, J., and Coauthors, 2004: Argo profiling floats bring new era of in situ ocean observations. Eos, Trans. Amer. Geophys. Union, 85, 179, 190–191. Lin, et al, 2005: The Interaction of Supertyphoon Maemi (2003) with a Warm Ocean Eddy. Mon. Wea. Rev., 133, 2635-2649. 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