How precipitation influences the carbon cycle through CO2 air-sea exchange: Results from an eddy correlation study

On board the R/V SONNE SPACES-OASIS (SO234-2/SO235) cruise during the summer Indian Monsoon, in July-August 2014, CO₂ air-sea exchange was measured directly using the eddy correlation technique (EC). The cruise encountered a range of wind speeds, sea surface temperatures, and the track crossed through both a large sink and a large source area for CO₂. The bulk concentration gradient measurements were in general agreement with the Takahashi et al. (2009) climatology. The EC measurements largely agreed with flux predictions based on the bulk air-sea concentration gradient measurements and the Nightingale et al. (2000) wind speed based gas transfer parameterization. However, there were distinct regions over the cruise track where there CO₂ flux into the surface ocean was found to be up to 4 times higher than predicted. These regions correspond to the large rainfall events during the monsoon. There is evidence that precipitation causes changes in the turbulence at the air-sea interface, which influences the gas transfer coefficient. Additionally, changes in salinity due to rainfall change the carbonate chemistry in the upper centimeters of the surface ocean, causing a larger gradient between air and sea than that measured in the bulk. The subsequent enhanced fluxes are undetectable without direct flux measurements. Since the ocean source/sink strength is believed to be better constrained than the land source/sink, it is used to calculate other components of the global carbon cycle. If the carbon from these precipitation enhanced fluxes is sequestered in the ocean mixed layer and beneath, there will be implications for the global carbon cycle. This study will use the directly measured fluxes and the GOTM model to understand the implications of these findings.