Examining the Impact of Tropical Cyclones on Air‐Sea CO₂ Exchanges in the Bay of Bengal Based on Satellite Data and In Situ Observations

Abstract

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.

Variation of pCO2 concentrations induced by tropical cyclones “Wind-Pump” in the middle-latitude surface oceans: A comparative study

“…The ratio of DIC to TA (DIC/TA) slightly increased from 0.860 in the mixed layer to 0.863 below the ILD layer. The small DIC/TA difference corresponds to a pCO 2sea decrease of 15.16±5.60 μatm which agrees well with our previous study [14].…”

“…The CO 2 efflux takes away the gaseous CO 2 from the ocean which decreases the concentration of H 2 CO 3 , HCO À 3 and CO 2À 3 (DIC) through the thermodynamic equilibrium. The pCO 2sea variation due to the gas exchange of CO 2 can be estimated by the change of DIC in the mixed layer and the sum of the aqueous CO 2 and H 2 CO 3 [14,35]. Assuming the MLD of 50.2 m after TC Florence, the pCO 2sea variation induced by the F CO2 is about -3.42±0.39 μatm (Table 2), which is consistent with previous studies [36][37].…”

“…We summarize how the vertical salinity distributions affect the pCO 2sea in response to TCs. This study is a continuation of our previous study [14], and the first comparative study of TCs impact on the pCO 2sea variation in different water properties, which gives a consistent explanation for the pCO 2sea response to TCs in two cases.…”

“…where, S c is the Schmidt number calculated according to well established methodology [3]. The total uncertainty of F CO2 is 20% which is determined mostly by the uncertainty of k [14].…”

“…Each year tropical cyclones (TCs) visit both the subtropical North Atlantic Ocean and the Bay of Bengal (BoB)-a semienclosed sea in the north Indian Ocean. TCs were found to have much influence on the marine environment, such as cooling of sea surface temperature (SST) [8][9], increase of dissolved oxygen concentration [10][11] and increase of chlorophyll a (Chla) concentration both at the surface and subsurface [9,[12][13] due to the 'wind-pump' effects induced by TCs [13][14][15]. The amplitude of the surface cooling strongly depends on the initial upper-ocean conditions such as mixed layer depth (MLD) and stratification in the thermocline along with intensity and translation speed of TC [8,[16][17].…”