Upper Ocean Response To Typhoon In The Tropical Pacific

Understanding the mechanisms of the upper ocean response to tropical cyclone (TC) on both local and basin-wide scales can help us to improve TC forecasting and to elucidate its role in the Earth’s climate system. In this study, the characteristics and physical processes of the oceanic response to TCs in the tropical Pacific are investigated based on satellite and Argo observations, as well as an ocean general circulation model (OGCM). The OGCM adopted in this work is a layered model with a hybrid vertical mixing scheme.First, the local response of the upper ocean to TC is studied. The TC wind stress fields are extracted from the 6-hourly Cross-Calibrated Multi-Platform (CCMP) satellite scatterometer product using a locally weighted regression (Loess) method. Then by comparing the model experiments with and without TC wind forcing, the effects of TC on the ocean are isolated and examined. In a case study of typhoon Rammasun, which passed through the northwestern tropical Pacific in May,2008, it is demonstrated that the local response is characterized by a quick deepening of the surface mixed layer, a strong latent heat loss to the atmosphere, and an intense upwelling near the center of typhoon, leading to a cooling of the oceanic surface layer that persists as a cold wake along the typhoon track. A heat budget analysis for the surface mixed layer indicates that vertical mixing and upwelling are dominant processes responsible for the surface cooling, while the surface heat flux also play a non-negligible role. More interestingly, due to the combined effects of mixing and upwelling, the upper ocean response exhibits a complicated multi-layer thermal structure.Next, the remote effects of TC wind forcing on the tropical Pacific Ocean are investigated using the OGCM. The seasonally varying TC wind fields in year 2008 are extracted from satellite data using the same Loess method, and the extracted TC winds are prescribed as a repeated annual cycle over the western Pacific regions off the equator (poleward of 10°N/S). Two sets of long-term OGCM experiments are performed and compared, one forced with climatological winds and the other with the TC wind forcing added. Large, persistent thermal perturbations are induced locally by TCs in the western tropical Pacific, which are seen to spread along the mean ocean circulation pathways around the tropical Pacific basin. In particular, a remote effect emerges in the eastern equatorial Pacific in response to the prescribed western Pacific, off-equatorial TC forcing, characterized by a cooling in the mixed layer and a warming in the thermocline. A heat budget analysis shows that vertical mixing is the dominant process responsible for the SST cooling in the eastern equatorial Pacific, indicating a weakened stratification in the subsurface water from the west.The main new findings of this study are twofold. First, the local response of the upper ocean to TC exhibits a four-layer anomalous thermal structure, including a cooling layer near the surface and a warming layer right below, accompanied by another pair of cooling/warming layers in the thermocline. This suggests that the oceanic response is much more complicated than previously thought. Second, the remote response of the eastern equatorial Pacific to the observed TCs in the western Pacific shows a surface cooling, in contrast to the warming obtained in a number of previous studies that parameterized the effect of TC in terms of vertical mixing alone. These findings indicate that the commonly accepted "heat pump" theory is an oversimplification of TC’s impact on the ocean, and that the estimates of TC’s influence on global ocean circulation and heat transport based on such a theory need to be largely revised.