Modifications in the trace gases flux by a very severe cyclonic storm, Hudhud, in the coastal Bay of Bengal

Abstract

A very severe cyclonic storm, Hudhud, made landfall at Visakhapatnam city, the central east coast of India, on 12 October 2014 and it is the most destructive cyclone to ever hit the Indian subcontinent since the past two decades. In order to examine its impact on the flux of trace gases into the atmosphere, a study was made in the coastal Bay of Bengal, off Visakhapatnam, after the cyclone and compared with the pre-cyclone conditions. Hudhud suppressed the vertical mixing of the water column due to the occurrence of strong salinity stratification associated with torrential rainfall. The land run-off and precipitation brought significant amount of ammonium to the coastal waters, resulting in increased pH. The increased pH shifted the inorganic carbon equilibrium towards the formation of bicarbonate, resulting in decreased partial pressure of carbon dioxide (\(p\hbox {CO}_{2})\) after the cyclone Hudhud. The undersaturation of carbon dioxide (\(\hbox {CO}_{2})\) and nitrous oxide (\(\hbox {N}_{2}\hbox {O}\)) with respect to atmospheric equilibrium was observed during the post-cyclone period compared to the pre-cyclone period. About 80% of the post-cyclone decrease in \(\hbox {N}_{2}\hbox {O}\) (\({>}2\hbox { nM}\)) and \(p\hbox {CO}_{2}\) (150–\(200\,\upmu \hbox {atm}\)) was contributed by the dilution of coastal waters with the precipitated waters. In contrast, methane (\(\hbox {CH}_{4})\) concentrations were increased by 0.5–2.8 nM during the post-cyclone period than in the pre-cyclone period, and were attributed to the input of domestic sewage through land run-off. Dimethyl sulphide (DMS) and the total dimethyl sulphonio-propionate (\(\hbox {DMSP}_{t})\) concentrations decreased by 0.4–3.9 and 0.2–6.0 nM, respectively, during the post-cyclone period in comparison with the pre-cyclone period and it was consistent with lower phytoplankton biomass during the former than the latter. The sea-to-air flux of \(\hbox {CO}_{2}\), \(\hbox {N}_{2}\hbox {O}\) and DMS were 1.3 ± 0.5 mmol C \(\hbox {m}^{-2}\hbox {d}^{-1}\), \(0.9 \pm 0.3\,\upmu \hbox {mol m}^{-2}\hbox { d}^{-1}\) and \(5.8 \pm 3\,\upmu \hbox {mol m}^{-2}\,\hbox { d}^{-1}\) during the pre-cyclone period, respectively. The corresponding values during the post-cyclone period were lower at \(-2.0 \pm 1\,\hbox { mmol C m}^{-2}\hbox { d}^{-1}\), \(-0.4 \pm 0.1\,\upmu \hbox {mol m}^{-2}\hbox { d}^{-1}\) and \(2.8 \pm 2\,\upmu \hbox {mol m}^{-2}\hbox { d}^{-1}\), respectively. In contrast, the sea-to-air flux of \(\hbox {CH}_{4}\) increased from 0.6 to \(1.5\,\upmu \hbox {mol C m}^{-2}\hbox { d}^{-1}\) from the pre- to the post-cyclone period. This study suggested that the cyclone Hudhud modified the magnitude of the biogenic gas flux to the atmosphere from the coastal Bay of Bengal than hitherto hypothesised.