A Limited Effect of Sub-Tropical Typhoons on Phytoplankton Dynamics

Abstract

Abstract. Typhoons are assumed to stimulate ocean primary production through the upward mixing of nutrients into the surface ocean, based largely on observations of increased surface chlorophyll concentrations following the passage of typhoons. This surface chlorophyll enhancement, seen on occasion by satellites, more often is 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 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. Contrary to the prevailing dogma, the results show that typhoons likely have limited effect on net ocean primary production. Observed surface chlorophyll enhancements during and immediately following typhoons in tropical and subtropical waters are more likely 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.

Extreme South Pacific Phytoplankton Blooms Induced by Tropical Cyclones

“…Still, as shown in Figure 1, the magnitude, location, and extremeness of TC-PBs vary depending on the satellite used. While it is suggested that some TC-PBs are the results of vertical homogenization of existing deep chlorophyll (Chai et al, 2020), we find that establishing a bloom threshold of 0.5 mg/m 3 rules out this process as generating the anomalous increases in Chl-a observed here. The complex interplay of upper-ocean stratification, wind-driven mixing, and rotationally driven Ekman suction is critical for understanding the response of the upper ocean and its ecosphere to TCs.…”

“…The cold wake contained high concentrations of surface chlorophyll, in places estimated to exceed 15 mg/m 3 . High surface values are not likely to be the result of mixing of a deep Chl-a maximum as may occur in the Northwest Pacific or in weak observed blooms (Chai et al, 2020).…”

“…Lin, 2012) and likely have little impact on carbon cycling, marine food webs, or sedimentation. Because tropical and subtropical regions often harbor deep subsurface chlorophyll maxima at depths of 100 m or more (Cullen, 2015), some post-TC increases in surface Chl-a may not be the signal of primary production but instead a vertical redistribution of preexisting biomass from wind-driven vertical mixing (Chai et al, 2020).…”

An extraordinary chlorophyll-a enhancement event jointly induced by two sequential tropical cyclones in the Kuroshio region south of Japan

“…A noteworthy feature in this case is that the weaker TC Neoguri rapidly generated an obvious augment of phytoplankton biomass in the upper ocean. A category 5 typhoon Trami in late September 2018 with average translation speed of 6.1 m/s barely caused net increase of primary production in the upper ocean observed by BGC-Argo float 2902750, in spite of the stronger intensity and slower translation speed compared to Neoguri (Chai et al, 2021). Huang et al (2016) reported that TC Jangmi caused a weaker surface bloom owing to the thick ML induced by TC Sinlaku that passed earlier.…”

“…The DCM is a ubiquitous phenomenon in worldwide oceans, formed by biological-chemicalphysical interaction. In the stratified open ocean, the vertical stratification below the ML suppresses the supply of nutrients from subsurface layer, generally leading to a DCM beneath the MLD and at the top of nutricline within the euphotic layer where there are adequate nutrients and light (Furuya, 1990;Cullen, 2015;Wang, 2020;Chai et al, 2021;Zhuang et al, 2021). On October 9, 2019, The DCM occurred at the depth of 59.9 m just above the nutricline.…”