A study of a Hurricane Katrina–induced phytoplankton bloom using satellite observations and model simulations

Abstract

Satellite observations of sea surface temperature (SST) and chlorophyll‐a concentration from the Moderate Resolution Imaging Spectroradiometer (MODIS) on Aqua revealed a phytoplankton bloom event in the vicinity of the Loop Current (LC) in the Gulf of Mexico after Hurricane Katrina's passage in August 2005. The sea surface height (SSH) anomaly data derived from satellite altimetry measurements indicated that the phytoplankton bloom coincided with a LC frontal eddy and thus suggested that the cold‐core eddy may have played a critical role in the phytoplankton bloom event. Using the Hybrid Coordinate Ocean Model (HYCOM), we have carried out numerical simulations to understand the mechanism that triggers the phytoplankton bloom after Katrina passage. The model study shows that the preexisting cold‐core eddy strengthens the upper ocean dynamics and nutrient responses with significant increase of nutrient concentration after the passage of Hurricane Katrina, because of the uplift of nutrient fields within the cold‐core eddy. The Hurricane Katrina–triggered phytoplankton bloom is attributed to the preexisting cold eddy as well as the slowly passing hurricane. The nutrient distribution after Hurricane Katrina indicates that the increase of nitrate concentration plays a dominant role in the development of the phytoplankton bloom.