A case study of a phytoplankton bloom triggered by a tropical cyclone and cyclonic eddies

Abstract

Strong tropical cyclone (TC) Ockhi occurred in the southeastern Arabian Sea (AS) in 2017. Ockhi greatly changed the oceanic conditions and induced large variation in chlorophyll-a (Chl-a). The dynamic mechanisms of the long-term phytoplankton bloom after the passage of the TC were investigated in this study. Prominent surface ocean responses, e.g., decreasing temperature and salinity, were identified from Argo data by comparing the pre-and post-conditions of the TC. A phytoplankton bloom was observed in southeastern AS after the passage of TC Ockhi within the area of (11˚N-14˚N, 67˚E-70˚E) and lasted for seven days. Interestingly, there were two weaker cyclonic eddies, with an average vorticity of less than 0.14 s-1 , on the TC trajectory from November 28 to December 2. As Ockhi approached, strong vertical mixing occurred on December 3, increasing the eddy vorticity to 0.26 s-1. After the passage of Ockhi, both eddies, with a two-day oscillation period, were substantially enhanced. Especially from December 11 to 16, the vorticity above 70 m was as high as 0.2 s-1 in the thermocline. Because of the high photosynthetically available radiation (PAR) and low precipitation, the enhanced cyclonic eddies induced upwelling for the entire thermocline for over ten days and uplifted nitrates into the mixed layer. This study offers new insights on the influence of eddies in regulating the impacts of typhoons on Chl-a, and the results can help evaluate typhoon-induced biological responses in the future.

Pico‐ and nanoplankton communities on a near to offshore transect along the continental shelf of the northwestern Gulf of Mexico in the aftermath of Hurricane Harvey

“…While understanding the effects of tropical cyclones on phytoplankton is an ongoing area of research, much of that work has quantified only biomass (e.g., Zheng and Tang 2007; Han et al 2012; Lü et al 2020, and others). Fewer studies have resolved effects on composition of phytoplankton communities, with important exceptions (e.g., Chang et al 1996; Peierls et al 2003; Anglès et al 2015; Paerl et al 2020; Wachnicka et al 2020).…”

Tropical cyclones: what are their impacts on phytoplankton ecology?

“…Cooling and mixing result in a deepening of the surface mixed layer and injection of nutrients from depth ( Babin et al., 2004 , Song et al ., 2007). There is also growing evidence that TCs interacting with eddies of a similar direction of rotation strongly induces upward transport of nutrients into the euphotic zone ( Lü et al., 2020 ).…”

A review of ocean-atmosphere interactions during tropical cyclones in the north Indian Ocean

“…During Cyclone Nilofar, a twenty eight fold increase in the chlorophyll was observed after the passage of the cyclone (Ali et al 2020). Cyclone Ockhi in the 2017 post-monsoon season, induced chlorophyll blooms in the southeast Arabian Sea, which lasted for seven days after cyclone passage (Lü et al 2020). The presence of cyclonic eddies along the path of Cyclone Ockhi enhanced the cyclone induced chlorophyll blooms in the southeast Arabian Sea (Lü et al 2020).…”

“…Cyclone Ockhi in the 2017 post-monsoon season, induced chlorophyll blooms in the southeast Arabian Sea, which lasted for seven days after cyclone passage (Lü et al 2020). The presence of cyclonic eddies along the path of Cyclone Ockhi enhanced the cyclone induced chlorophyll blooms in the southeast Arabian Sea (Lü et al 2020). Similarly, in the Bay of Bengal, the existence of a cyclonic eddy before Cyclone Hudhud facilitated chlorophyll blooms (Chacko 2017).…”