PNNL

Abstract

The northwestern tropical Pacific Ocean receives a substantial amount of rainfall and, consequently, has some of the freshest surface waters in the global tropics (Cravatte et al.,2009). With the hydrological cycle projected to strengthen in a warming world (Held and Soden, 2006), a corresponding amplification of the near-surface salinity stratification is expected (Cravatte et al. (2009), Durack et al. (2012)}. These changes have the potential to affect the intensity of `super typhoons', violent storms with deep upper-ocean mixing, strong sea surface cooling, and enhanced sensitivity to ocean stratification effects (Vincent et al., 2013). In this study, observations and coupled climate model simulations are used to examine the impacts of a changing hydrological cycle on the near-surface salinity stratification and the intensity of super typhoons in the northwestern Pacific. Analysis of 30 years of observations over the post-satellite period of 1979-2008 shows that the surface ocean freshened considerably at an average rate of about 0.06 psu/decade, causing a strengthening of upper-ocean salinity stratification and a reduction in the magnitude of super typhoon cold wakes. These changes increased the `Dynamic Potential Intensity (DPI)' (Balaguru et al., 2015), a variant of potential intensity that accounts for ocean stratification, for super typhoons. Averaged over the typhoon season and over the region 130E-150E and 5N-25N, the DPI increased at a rate of 0.13 m/s/decade, which is about 23% of the increasing trend due to changes in the upper-ocean thermal structure alone. Further, examination of the 100-year changes from a suite of coupled climate simulations under the RCP 8.5 scenario reveals a continued freshening of the upper ocean. These salinity changes act to reduce the magnitudes of cold wakes and increase the DPI for super typhoons by 0.14 m/s/decade throughout the 21st century.