The Processes of SST Cooling by Typhoon Passage and Case Study of Typhoon Rex with a Mixed Layer Ocean Model

Abstract

   The mechanisms by which sea surface temperatures (SSTs) decrease by passage of typhoons under various initial oceanic conditions and the atmospheric boundary conditions were investigated. A stronger wind stress, slower typhoon translation, excessive heat flux from the sea to the atmosphere, a thinner initial mixed layer, and a greater vertical gradient of the sea temperature in the thermocline layer, all have an effect on mixed layer temperature (MLT) cooling. MLT cooling occurs through intermingled processes by enhanced entrainment and upwelling. In contrast, horizontal advection produced by near-inertial oscillation has a slight effect on determining the distributions of MLTs. The depth in the mixed layer near the area along the typhoon track is determined by the magnitudes of the wind stresses and the translation speeds irrespective of the initial thickness of the mixed layer. In addition, under different wind stresses, translation speeds, and vertical profiles of sea temperatures, the MLT cooling is closely related to the ratio of maximum variation of depth by upwelling to the maximum thickness of the mixed layer. The ratio of maximum variation of depth by upwelling to the maximum thickness of the mixed layer under conditions of excessive heat fluxes, is the same as that under conditions of no heat flux. However, MLT cooling under conditions in which the maximum heat flux reaches 800W/m² is 0.7°C greater than those in no heat flux. In this case, seawater cooling at the transition layer caused by entrainment influences MLT cooling.
   Numerical simulations were conducted to elucidate the 3°C SST cooling by Typhoon Rex that was observed by R/V Keifu Maru of the Japan Meteorological Agency (JMA). The SST variation obtained by numerical simulation captures the aspects of observational SSTs reported by R/V Keifu Maru, in that the SST rapidly decreases and the maximum SST cooling reaches about 3°C. This rapid SST cooling is mainly caused by the stronger wind stresses and slower translation speeds of Typhoon Rex. The ocean heat contents, based on the model-computed sea temperature, are closely related to the intensities of Typhoon Rex.