Upper-ocean thermal structure and the western North Pacific category 5 typhoons. Part I: Ocean features and the category 5 typhoons' intensification

Category 5 cyclones are the most intense and devastating cyclones on earth. With increasing observations of category 5 cyclones, such as Hurricane Katrina (2005), Rita (2005), Mitch (1998), and Supertyphoon Maemi (2003) found to intensify on warm ocean features (i. e., regions of positive sea surface height anomalies detected by satellite altimeters), there is great interest in investigating the role ocean features play in the intensification of category 5 cyclones. Based on 13 yr of satellite altimetry data, in situ and climatological upper-ocean thermal structure data, best-track typhoon data of the U. S. Joint Typhoon Warning Center, together with an ocean mixed layer model, 30 western North Pacific category 5 typhoons that occurred during the typhoon season from 1993 to 2005 are systematically examined in this study. Two different types of situations are found. The first type is the situation found in the western North Pacific south eddy zone (SEZ; 21-26 N, 127-170 E) and the Kuroshio (21-30 N, 127-170 E) region. In these regions, the background climatological warm layer is relatively shallow (typically the depth of the 26 C isotherm is around 60 m and the upper-ocean heat content is similar to 50 kJ cm(-2)). Therefore passing over positive features is critical to meet the ocean's part of necessary conditions in intensification because the features can effectively deepen the warm layer (depth of the 26 C isotherm reaching 100 m and upper-ocean heat content is similar to 110 kJ cm(-2)) to restrain the typhoon's self-induced ocean cooling. In the past 13 yr, 8 out of the 30 category 5 typhoons (i. e., 27%) belong to this situation. The second type is the situation found in the gyre central region (10 degrees-21 degrees N, 121 degrees-170 degrees E) where the background climatological warm layer is deep (typically the depth of the 26 C isotherm is similar to 105-120 m and the upper-ocean heat content is similar to 80-120 kJ cm(-2)). In this deep, warm background, passing over positive features is not critical since the background itself is already sufficient to restrain the self-induced cooling negative feedback during intensification.