Upper-Ocean Thermal Structure and the Western North Pacific Category 5 Typhoons. Part I: Ocean Features and the Category 5 Typhoons’ Intensification

TLDR

Category 5 cyclones are the most intense and devastating, and recent observations show they intensify over warm ocean features, prompting investigation into the ocean’s role in their intensification. The study systematically examined 30 western North Pacific category 5 typhoons from 1993‑2005 to assess how ocean features influence intensification. Using 13 years of satellite altimetry, in‑situ and climatological upper‑ocean thermal data, JTWC best‑track records, and an ocean mixed‑layer model, the authors analyzed the typhoons’ interactions with ocean features. They found two regimes: in the shallow‑warm SEZ/Kuroshio zone, positive features are essential to deepen the warm layer and restrain self‑induced cooling (≈27 % of typhoons), whereas in the deep‑warm gyre central region, positive features are unnecessary because the background warm layer already limits cooling.

Abstract

Abstract 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 ∼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 ∼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°–21°N, 121°–170°E) where the background climatological warm layer is deep (typically the depth of the 26°C isotherm is ∼105–120 m and the upper-ocean heat content is ∼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.