Is the Increasing Trend in the Environmental Upper-Ocean Heat Content in the Northwest Pacific Strengthening Tropical Cyclone Intensity?

Abstract

The IPCC Sixth Assessment Report (AR6) indicates that the global proportion of Category 3–5 tropical cyclones (TCs) has increased over the past four decades. One of the possible reasons is an increase in the upper-ocean heat content (UOHC). However, it is questionable to explain the long-term trend only based on the upper-ocean warming because Category 3–5 TCs enhance the ocean response in the form of sea surface cooling. Here we calculated a new heat content metric by using 2-m air and dew-point temperatures from 2000 to 2021 with oceanic reanalysis data under the consideration of atmospheric baroclinicity. Since 2000, the new heat content from July to December has not significantly increased in the tropical and subtropical Northwest Pacific (NWP). Although the median for conventional UOHC calculated based on the 26 °C-isotherm depth has increased significantly, the trends in the median and extreme values of TC central pressures obtained from multiple best track datasets were consistent with the trends of the new heat content metric. The results suggest that upper ocean warming alone cannot explain an increase in extremely intense TCs, while new heat content has a potentially closer relationship with TC activity than conventional UOHC.