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Satellite Evidence of Upper Ocean Responses to Cyclone Nilofar

ABSTRACT Cyclone Nilofar was the third-strongest cyclone in the Arabian Sea. It was a category 4 cyclone that arrived in last week of October 2014. It lingered in the Arabian Sea for 10 days and caused a significant phytoplankton bloom that was detected by satellite observation. In this study, the authors investigated the ocean biological and physical responses to cyclone Nilofar using chlorophyll a (chl a) concentration, sea surface temperature (SST), sea surface height anomaly (SSHA), sea surface wind measurements derived from different satellites, and in situ data. The maximum chl a concentration (>3.34 mg m−3) increased about 28 times in the Arabian Sea after the passage of the cyclone in comparison with the chl a concentration (0.12–0.92 mg m−3) before the passage of cyclone. Using the relationship of wind stress curl and upwelling, the authors found that during the cyclone the speed of upwelling increased more than 18 times compared with the speed in the period before the cyclone. Moreover, the mixed layer deepened by about 40 m. These findings reveal that the enhancement of chl a concentration was triggered by strong vertical mixing and upwelling. Along the track of the cyclone, the maximum sea surface cooling (4°–5°C) took place in the Arabian Sea along segments A–B and D–F where the translation speed of the cyclone was only 0.9–1.3 m s−1, and the mixed-layer depth was about 10 m in the pre-cyclone period. However, the drop in SST along segment C–D was only 1°–2°C, whereas in the pre-cyclone period the translation speed of the cyclone was 3.1–4.4 m s−1, and the mixed-layer depth was about 25 m in. So, the extent of the SST drop was probably due to the translation speed of the cyclone and the depth of the mixed layer. In addition, the region with the largest decline in SSHA can indicate the location of maximum cooling.