Impact of a tropical cyclone on biogeochemistry of the central Arabian Sea

Remotely sensed data are combined with shipboard measurements to investigate biogeochemical changes caused by a moderate tropical cyclone in the central Arabian Sea in December 1998. The sea surface temperature decreased by similar to 4 degrees C, whereas surface nitrate and chlorophyll concentrations increased by > 5 mu M and up to 4 mg m(-3), respectively, over a large area affected by the cyclone. Nutrient enrichment in the surface layer of the cyclone-affected zone is estimated to have supported a new production of similar to 4.2 Tg C, approximately 5% of the annual organic carbon export to the deep sea ( beyond the continental margin) for the entire Arabian Sea. Entrainment of nitrous oxide from the thermocline led to more than doubling of its concentration in the mixed layer. The cyclone also resulted in an increase in nitrous oxide inventory within the oxygen minimum zone. Our results imply that, should there be an increase in the frequency and intensity of tropical cyclones as a result of global warming, as projected in some recent reports, carbon production and respiration, and redox processes within the oxygen minimum zones, such as the production of nitrous oxide through nitrification/denitrification, and of molecular nitrogen through denitrification/anaerobic ammonium oxidation, may be significantly impacted.