Master´s Thesis Defense by Pernille Kirstein Hansen – Niels Bohr Institutet - Københavns Universitet

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Master´s Thesis Defense by Pernille Kirstein Hansen

Ocean response to Tropical Cyclones, with special emphasis on Hurricane Nadine

Tropical cyclones (TCs) have long been known to leave a trace of cold water in their wake. This is caused by an increase in vertical mixing induced by the strong wind-stress exerted on the ocean surface by the TCs. In this study Argo buoys from the Argo Mission are used to investigate the ocean's response to the long-lasting Hurricane Nadine (10 September to 4 October, 2012), which traveled in a loop in the North Eastern Atlantic Ocean. The Argo buoys provide information about the oceans temperature and salinity distributions, and the large amount of available data makes it possible to examine the ocean structure before and after the passage of Hurricane Nadine. The track of Nadine is divided into nine areas in order to analyze the ocean's response through the whole evolution of the hurricane. The temperature and salinity distributions are also affected by the induced precipitation, and in order to determine this effect, data from the Tropical Measuring Mission TRMM are used.

The looped structure of Nadine's track resulted in a distinctive area of cold surface water. This cold surface area is still present over a month after the dissipation of the hurricane as observed in the NOAA High-Resolution Blended Analysis of Daily SST and Ice. The Argo data showed a sea surface cooling range between 0.2 degrees and 3.2 degrees C, with these values being lower bounds due to time between measurements. The salinity response showed a tendency toward fresher water in the cold wake, and the mixed layer (ML) showed a deepening of up to 30 m. The Argo data also showed the presence of barrier layers (BLs) in the first three areas of Nadine's natural intensification period, which damped the vertical mixing induced by Nadine. This resulted in a reduction of the sea surface cooling, indicating a positive contribution from the BLs to the further intensification of Nadine.

The overall effect from precipitation was small and mostly visible by changes in the salinity distributions. In association with the induced negative sea surface temperature anomaly, a warm anomaly was found between the initial ML and the new ML after passage of Nadine. In one of the areas along the track the warm anomaly is present down to around 100 m, and this might contribute to a warmer ocean in the following season.