Abstract
Abstract. Typhoons are assumed to stimulate primary ocean production through the upward mixing of nutrients into the ocean surface. This assumption is based largely on observations of increased surface chlorophyll concentrations following the passage of typhoons. This surface chlorophyll enhancement, occasionally detected by satellites, is often undetected due to intense cloud coverage. Daily data from a BGC-Argo profiling float revealed the upper-ocean response to Typhoon Trami in the northwest Pacific Ocean. Temperature and chlorophyll changed rapidly, with a significant drop in sea surface temperature and a surge in surface chlorophyll associated with strong vertical mixing, which was only partially captured by satellite observations. However, no net increase in vertically integrated chlorophyll was observed during Typhoon Trami or in its wake. In contrast to the prevailing dogma, the result shows that typhoons likely have a limited effect on net primary ocean production. Observed surface chlorophyll enhancements during and immediately following typhoons in tropical and subtropical waters are more likely to be associated with surface entrainment of deep chlorophyll maxima. Moreover, the findings demonstrate that remote sensing data alone can overestimate the impact of storms on primary production in all oceans. Full understanding of the impact of storms on upper-ocean productivity can only be achieved with ocean-observing robots dedicated to high-resolution temporal sampling in the path of storms.
Highlights
The western North Pacific Ocean is a highly energetic region on the globe (Gray, 1968) and is where nearly onethird of tropical cyclones originate (Needham et al, 2015)
The BGC-Argo profiling float (ID: 2902750), an unmanned observational platform (Bishop et al, 2002; Boss et al, 2008; Claustre et al, 2010; Mignot et al, 2014; Chacko, 2017), was deployed by the State Key Laboratory of Satellite Ocean Environment Dynamics (SOED) of China in early September of 2018 in the northwest Pacific. It was equipped with a conductivity–temperature–depth (CTD) instrument (SBE41CP manufactured by Seabird) measuring temperature and salinity and an optical sensor package (ECO Triplet manufactured by WET Labs) measuring chlorophyll a concentration, fluorescent dissolved organic matter (FDOM) and particulate backscattering coefficient
The daily profiling frequency capture of the rapid response of the ocean surface to typhoon Trami and our findings show substantial increases in vertical mixing and surface phytoplankton biomass, similar to past studies
Summary
The western North Pacific Ocean is a highly energetic region on the globe (Gray, 1968) and is where nearly onethird of tropical cyclones originate (Needham et al, 2015). The strong tropical cyclones in this region, referred to as typhoons, are highly dangerous and have caused great loss of life and property throughout history (Frank and Husain, 1971; Dunnavan and Diercks, 1980; Kang et al, 2009; Needham et al, 2015). Typhoons extract their energy from warm surface ocean waters; the heat content in the upper ocean (quantified by the sea surface temperature (SST) as the indicator) has a key role in development of typhoons (Emanuel, 1999).
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