The impact of tropical cyclones (TCs) on the CO 2 partial pressure at the sea surface (pCO 2sea ) and air-sea CO 2 flux (F CO2 ) in the Bay of Bengal (BoB) was quantified based on satellite data and in situ observations between November 2013 and January 2017. The in situ observations were made at the BoB Ocean Acidification mooring buoy. A weak time-mean net source of 55.78 ± 11.16 mmol CO 2 m À2 year À1 at the BoB Ocean Acidification site was estimated during this period. A wide range in increases of pCO 2sea (1.0-14.8 μatm) induced by TCs occurred in postmonsoon (October-December), and large decreases of pCO 2sea (À14.0 μatm) occurred in premonsoon (March-May). Large vertical differences in the ratio of dissolved inorganic carbon (DIC) to total alkalinity (TA) in the upper layer (ΔDIC/TA) were responsible for increasing pCO 2sea in postmonsoon. Relatively small values of ΔDIC/TA were responsible for decreasing pCO 2sea in premonsoon. Five TCs (Hudhud, Five, Kyant, Vardah, and Roanu) were considered. Hudhud significantly enhanced CO 2 efflux (18.49 ± 3.70 mmol CO 2 /m 2 ) in oversaturated areas due to the wind effect during the storm and wind-pump effects after the storm. Vardah insignificantly changed F CO2 (1.22 ± 0.24 mmol CO 2 /m 2 ) in undersaturated areas because of the counteraction of these two effects. Roanu significantly enhanced CO 2 efflux (19.08 ± 3.82 mmol CO 2 /m 2 ) in highly oversaturated conditions (ΔpCO 2 > 20 μatm) since the wind effect greatly exceeded the wind-pump effects. These five TCs were estimated to account for 55 ± 23% of the annual-mean CO 2 annual efflux, suggesting that TCs have significant impacts on the carbon cycle in the BoB.Plain Language Summary We examined the impact by five tropical cyclones on the sea surface pCO 2 and air-sea CO 2 exchange using Bay of Bengal Ocean Acidification moored buoy measurements over the Bay of Bengal. To our knowledge, this is the first study on the different effects of tropical cyclones and vertical differences in the ratio of dissolved inorganic carbon to total alkalinity in the upper layer on the sea surface pCO 2 and local air-sea CO 2 flux.

The typhoon Wind-Pump induced upwelling and cold eddy often promote the significant growth of phytoplankton after the typhoon. However, the importance of eddy-pumping and wind-driven upwelling on the sea surface chlorophyll a concentration (Chl-a) during the typhoon are still not clearly distinguished. In addition, the air-sea heat flux exchange is closely related to the upper ocean processes, but few studies have discussed its role in the sea surface Chl-a variations under typhoon conditions. Based on the cruise data, remote sensing data, and model data, this paper analyzes the contribution of the vertical motion caused by the eddy-pumping upwelling and Ekman pumping upwelling on the surface Chl-a, and quantitatively analyzes the influence of air-sea heat exchange on the surface Chl-a after the typhoon Linfa over the northeastern South China Sea (NSCS) in 2009. The results reveal the Wind Pump impacts on upper ocean processes: (1) The euphotic layer-integrated Chl-a increased after the typhoon, and the increasing of the surface Chl-a was not only the uplift of the deeper waters with high Chl-a but also the growth of the phytoplankton; (2) The Net Heat Flux (air-sea heat exchange) played a major role in controlling the upper ocean physical processes through cooling the SST and indirectly increased the surface Chl-a until two weeks after the typhoon; (3) the typhoon-induced cyclonic eddy was the most important physical process in increasing the surface Chl-a rather than the Ekman pumping and wind-stirring mixing after typhoon; (4) the spatial shift between the surface Chl-a blooms and the typhoon-induced cyclonic eddy could be due to the Ekman transport; (5) nutrients uplifting and adequate light were two major biochemical elements supplying for the growth of surface phytoplankton. tropical cyclones during the summer time [4]. Tropical cyclones have important "Wind Pump" impact on transporting and increasing the surface and subsurface Chl-a in oligotrophic ocean waters [5][6][7][8] through uplifting the nutrients by strong vertical mixing, upwelling, entrainment, as well as near inertial wave on the upper ocean layer, especially on the right-hand side of the storm track in the Northern Hemisphere [2,9,10]. Typhoons with slower translation speeds and stronger wind speeds have greater impact on Chl-a and the translation speeds play the more crucial role [4,11]. Tropical cyclones can also induce cyclone eddies (or reduce anti-cyclone eddies), and these eddy-pumping upwelling can further increase the surface and subsurface Chl-a [4,12]. These typhoon wind-driven physical processes and air-sea exchanges that subsequently affect the ocean's ecological status is defined as the "Wind Pump" [2,4,7,8,10,11].Biochemical conditions are essential in affecting the Chl-a through affecting its carbon/chlorophyll ratios [1,13]. Upwelling of the nutrient-rich waters from the deeper layer is the principal source of nutrients fueling the phytoplankton especially over the oligotrophic ocean like the NSCS [1,4]. The nitrogen is the maj...

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The Bermuda Testbed Mooring (BTM) and Bay of Bengal Ocean Acidification (BOBOA) mooring measurements were used to identify changes in the partial pressure of CO 2 at the sea surface (pCO 2sea) and air-sea CO 2 fluxes (F CO2) associated with passage of two tropical cyclones (TCs), Florence and Hudhud. TC Florence passed about 165 km off the BTM mooring site with strong wind speeds of 24.8 m s-1 and translation speed of 7.23 m s-1. TC Hudhud passed about 178 km off the BOBOA mooring site with wind speeds of 14.0 m s-1 and translation speed of 2.58 m s-1. The present study examined the effect of temperature, salinity, dissolved inorganic carbon (DIC), total alkalinity (TA), air-sea CO 2 flux, and phytoplankton chlorophyll a change on pCO 2sea as a response to TCs. Enhanced mixed layer depths were observed due to TCs-induced vertical mixing at both mooring sites. Decreased pCO 2sea (-15.16±5.60 μatm) at the BTM mooring site and enhanced pCO 2sea (14.81 ±7.03 μatm) at the BOBOA mooring site were observed after the passage of Florence and Hudhud, respectively. Both DIC and TA are strongly correlated with salinity in the upper layer of the isothermal layer depth (ILD). Strong (weak) vertical gradient in salinity is accompanied by strong (weak) vertical gradients in DIC and TA. Strong vertical salinity gradient in the upper layer of the ILD (0.031 psu m-1), that supply much salinity, dissolved inorganic carbon and total alkalinity from the thermocline was the cause of the increased pCO 2sea in the BOBOA mooring water. Weak vertical salinity gradient in the upper layer of the ILD (0.003 psu m-1) was responsible for decreasing pCO 2sea in the BTM mooring water. The results of this study showed that the vertical salinity gradient in the upper layer of the ILD is a good indicator of the pCO 2sea variation after the passages of TCs.

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Approximately 55% of anthropogenic carbon dioxide (CO 2 ) emissions were absorbed in the oceans (2.8 ± 0.4 GtC year −1 ) over the last decade (2011-2020) (Friedlingstein et al., 2022). The ability of the ocean to absorb CO 2 can be estimated by calculating the air-sea CO 2 flux (F CO2 ) at the sea surface (Takahashi et al., 1993). However, large differences between observations and models, as well as between models in annual F CO2 estimation, have been reported by previous studies in the Bay of Bengal (BoB) (Valsala & Maksyutov, 2010;Valsala et al., 2021;Sarma et al., 2013). These differences are mainly due to inadequate field measurements and poor understanding of the CO 2 partial pressure (pCO 2 ) at the sea surface (pCO 2sea ) and its controlling mechanism. Potential locations for collecting pCO 2sea measurements were identified by utilizing inverse modeling techniques (Valsala et al., 2021). It is crucial to continue and increase long-term high-resolution monitoring efforts to better understand the pCO 2sea controlling mechanism.

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