Impact of Typhoons on the Western Pacific: Temporal and Horizontal Variability of SST Cooling

Abstract

Abstract : A specific question that came out of a study of satellite imagery during the CBLAST project was What processes cause the cool SST in a hurricane wake to warm back toward pre-hurricane values? This SST warming (also called wake recovery) can be rapid. In the CBLAST Fabian case, the cool wake SST anomaly decayed (i.e., SST recovered back toward pre-hurricane values) with an e-folding time of about five days. In the CBLAST Frances case, the e-folding time was considerably longer, about two weeks. A model of this surface layer warming process was developed under a previous ONR grant (Price et al., 2008) and can be tested rigorously with the ITOP field data set (discussed below). This model is built around two hypotheses. The first, H1) SST warming is a local process driven in the main by an anomalous air-sea heat flux (anomalous compared to regions outside the cool wake). The heat flux over a cool wake is expected to be biased positive, typically by 40 - 80 W m 2 , simply because the SST in the wake is at first cooler than the surrounding regions (or summer conditions generally). This is a fairly direct inference from conventional air-sea heat flux parameterizations. The question then is How can such a small heat flux produce the observed, rapid warming rate? The second hypothesis is that H2) The evident shallow trapping (storage) depth of this anomalous heat flux is determined not by the anomalous heat flux itself (which is small) but rather by the much larger amplitude diurnal cycle of the surface heat flux that is typical of post-hurricane weather (light winds and clear skies).