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Impact of Tropical Cyclones on Sea Surface Chlorophyll

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Tiny organisms in the ocean called phytoplankton consume CO2, affecting its air-sea exchange rate and the efficiency with which carbon is sequestered in the deep ocean. The intense vertical mixing induced by some tropical cyclones has been shown to significantly enhance surface chlorophyll a (chl a) concentration, which influences CO2 uptake through photosynthesis. While individual hurricanes have been shown to induce sea surface chlorophyll blooms along their tracks, some studies suggest that their cumulative impact may not be significant.A team of scientists–including two U.S. Department of Energy scientists at Pacific Northwest National Laboratory–assessed the tropical cyclone-induced chl a enhancement in the subtropical North Atlantic Ocean during 1998-2011. Using an integrated and comprehensive approach that includes quantification of cyclone-induced changes in the mixed layer depth, the team found that tropical cyclones indeed have a substantial impact on integrated chlorophyll. The accumulated cyclone energy explains 22% of the interannual chl a variance during the hurricane season (June–November), after removing the influence of the North Atlantic Oscillation. The variance of chlorophyll concentrations explained by tropical cyclones is thus about 70% of that explained by the NAO, which has well-known impacts in this region. Thus the team found that tropical cyclones contribute significantly to interannual variations of primary productivity in the western subtropical North Atlantic during the hurricane season. Tropical cyclone-induced changes in chl a concentration may also affect the survival rates of fish larvae and their advancement to adulthood.

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Yannick Le Page
Pacific Northwest National Laboratory (PNNL)
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G.F. was supported by base funds to NOAA/AOML. K.B. and L.R.L. were supported by the U.S. Department of Energy (DOE) Office of Science Biological and Environmental Research as part of the Regional and Global Climate Modeling program. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830. We thank two anonymous reviewers for their helpful suggestions. All data used to produce the results of this paper are freely available from the URLs supplied in section 2.