[A-AS04] Advances in Tropical Cyclone Research: Past, Present, and Future

[AAS04-10] Interannual variability in potential impacts of upper ocean salinity on sea surface cooling induced by tropical cyclones

*Rintaro Miyagi¹, Tomoki Tozuka¹ (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo)

Keywords:tropical cyclone, ENSO, oceanic stratification

The ocean plays a crucial role in the evolution of tropical cyclones (TCs) through TC-induced sea surface temperature (SST) cooling, which inhibits development of TCs. Since the SST cooling is expected to be strongly influenced by the oceanic stratification, it is important to investigate the upper ocean structure prior to the passage of TCs. To investigate potential impacts of upper ocean on the SST cooling, this study uses the measure that is based on the change in upper ocean potential energy caused by TCs, which can incorporate effects of salinity stratification and uses “vertically averaged temperature” that is more appropriate for the SST cooling associated with vertical mixing. Using that measure, some studies investigated the haline effect or interannual variations in oceanic stratification associated with El Niño-Southern Oscillation (ENSO). However, the relative importance of thermal and haline stratification associated with the developing and decaying years of El Niño and La Niña is not quantified in the northwestern Pacific, where many TCs develop. Motivated by the above, using an ocean reanalysis product, interannual variations in potential impacts of the upper ocean stratification on TC-induced SST cooling associated with the evolution of El Niño and La Niña events are investigated, with a special focus on the role of haline stratification. It is found that the haline stratification could suppress the SST cooling by as much as 20% to the south of 15°N in June-August, and this contribution may vary by up to 10% associated with the ENSO. More specifically, it is shown that the potential impacts and the effect of anomalous haline stratification are different between the developing and decaying years of El Niño and La Niña. Anomalies are larger and more prevalent in the decaying years, and regions to the west of 165°E are more susceptible to the SST cooling during El Niño decaying years, while the SST cooling is suppressed to the west of 150°E during La Niña decaying years. Further analyses have revealed that an anomalously dry condition in the western tropical Pacific in El Niño decaying years lead to near-surface positive salinity anomalies and destabilize the upper ocean, while the opposite is true for La Niña decaying years. Although the effect of haline stratification has been found less important than that of thermal stratification as in previous works, potential impacts of upper ocean salinity on TC-induced SST cooling associated with the ENSO have been quantitatively estimated for the first time.