Storm‐induced changes in pCO₂ at the sea surface over the northern South China Sea during Typhoon Wutip

Abstract

In situ oceanographic measurements were made before and after the passage of Typhoon Wutip in September 2013 over the northern South China Sea. The surface geostrophic circulation over this region inferred from satellite altimetry data features a large‐size anticyclonic eddy, a small‐size cyclonic eddy, and smaller‐size eddies during this period. Significant typhoon‐induced changes occurred in the partial pressure of CO2 at the sea surface (pCO2sea) during Wutip. Before the passage of Wutip, pCO2sea was about 392.92 ± 1.83, 390.31 ± 0.50, and 393.04 ± 4.31 μatm over the cyclonic eddy water, the anticyclonic eddy water, and areas outside two eddies, respectively. The entire study region showed a carbon source (1.31 ± 0.46 mmol CO2 m−2 d−1) before Wutip. In the cyclonic eddy water after Wutip, high sea surface salinity (SSS), low sea surface temperature (SST), and high pCO2sea (413.05 ± 7.56 μatm) made this area to be a carbon source (3.30 ± 0.75 mmol CO2 m−2 d−1). In the anticyclonic eddy water after Wutip, both the SSS and SST were lower, pCO2sea was also lower (383.03 ± 3.72 μatm), and this area became a carbon sink (–0.11 ± 0.55 mmol CO2 m−2 d−1), in comparison with the pretyphoon conditions. The typhoon‐induced air‐sea CO2 flux reached about 0.03 mmol CO2 m−2 d−1. Noticeable spatial variations in pCO2sea were affected mainly by the typhoon‐induced mixing/upwelling and vertical stratifications. This study suggests that the local air‐sea CO2 flux in the study region was affected significantly by oceanographic conditions during the typhoon.

Chlorophyll Concentration Response to the Typhoon Wind-Pump Induced Upper Ocean Processes Considering Air–Sea Heat Exchange

“…where ρ a (=1.3 kg/m 3 ) is the air density, C D = 10 −3 × (0.6 + 0.07 U 10 ) is the drag coefficient, and U 10 is the wind speed at 10 m height above the sea surface as derived from the ASCAT wind product. Formula for calculating SST changes caused by sea surface net heat flux (NHF) is given by [20]:…”

“…The partial correlation analysis was used to investigate the role of the typhoon intensity and the SST on the surface Chl-a changes [2]. Ye also used the partial coefficients to reveal the contribution of temperature, salinity, and biological processes to the air-sea CO 2 exchange under typhoon conditions [20]. In this section, the partial correlation analysis was used to distinguish the possible contribution of these upper ocean physical processes on the surface Chl-a (Table 2 and Figure 7).…”

“…In addition, the temperature of~20-25 • C is usually phytoplankton favorable [1,36], so the typhoon-induced SST cooling especially in summer will be favored for the growth of the surface phytoplankton. The main processes for the typhoon-induced SST cooling include vertical mixing, Ekman pumping, and heat loss to the atmosphere [9,20], so quantitative analysis was made to classify the contribution of the typhoon-induced vertical motion (vertical mixing and the upwelling) and the air-sea heat exchange (net heat flux, NHF) in the SST (Figure 9). The NHF is an important indicator for measuring the exchange of air-sea heat exchange [17], and the SST change due to the NHF alone (∆SST nhf ) can be estimated from Formula (3) [20].…”

“…The main processes for the typhoon-induced SST cooling include vertical mixing, Ekman pumping, and heat loss to the atmosphere [9,20], so quantitative analysis was made to classify the contribution of the typhoon-induced vertical motion (vertical mixing and the upwelling) and the air-sea heat exchange (net heat flux, NHF) in the SST (Figure 9). The NHF is an important indicator for measuring the exchange of air-sea heat exchange [17], and the SST change due to the NHF alone (∆SST nhf ) can be estimated from Formula (3) [20]. After the typhoon Linfa, the ratios of the ∆SST nhf in the total SST changes (70.6%, 83.1%, and 78.9% during, one week after, and two weeks after the typhoon, respectively, Figure 9a) were much higher than that of the SST changes caused by the marine physical processes (29.4%, 16.9%, and 21.1% during, one week after, and two weeks after the typhoon, respectively, Figure 9a).…”

Hypoxic Bottom Waters as a Carbon Source to Atmosphere During a Typhoon Passage Over the East China Sea

“…Note that we will likely have more intense typhoons in a warmer future according to climate models (Webster et al, 2005). In shelf seas, CO 2 efflux during a typhoon is usually much higher than that of normal weather due to combined effect of mixing or upwelling of subsurface water with high dissolved inorganic carbon (DIC; Mathis et al, 2012;Ye et al, 2017) and extreme wind speed (Huang & Imberger, 2010;Nemoto et al, 2009). Such influence can be amplified when bottom shelf waters are hypoxic and enriched with CO 2 (Cai et al, 2011;Rabalais et al, 2014;Xue et al, 2015;Yu et al, 2014).…”