Spatio-temporal variability in the freshwater input to the surface water of Southern Ocean

Abstract:

Ocean heat content is rising rapidly in high-latitude regions of both hemispheres as a consequence of global warming (e.g., Gille 2002; Karcher et al. 2003; Bindoff et al. 2007; Purkey and Johnson 2010). Recent warming and freshening of Southern Ocean has affected hydrological cycle in terms of increasing tendency of precipitation as liquid water instead of snow. Limited data is available on the extent of fresh water flux by precipitation and sea ice melting to the surface ocean. The spatial extent of sea ice formation is documented based on remote sensing observation. We investigate here spatial variability in freshwater inputs to the Indian sector of Southern Ocean region using combined observation of oxygen isotopes ratios and salinity of surface water during the summer of 2011, 2012 and 2013. Together with this, the measured isotopic ratios of meteoric water and sea ice melt were used in the mass balance equation for deriving the contribution of both of these components in the surface water of southern ocean. The three component mixing equations (Meredith et al., 2013) allowed estimation of fractional contribution of rain over the years. The δ¹⁸O of meteoric water followed the pattern nearly similar to the observation documented in the continental stations (Global Network of Isotopes in Precipitation, GNIP) located in the southern hemisphere. However, a slight but consistent heavier composition was documented in rainwater as compared to the GNIP stations. Our observation suggests that the meteoric water is the dominant freshwater source over the ocean, accounting for up to 10-15% of the water present in the surface ocean during the austral summer whereas Sea-ice melt accounts for a much smaller percentage (maximum around 1%). Our observation is consistent with previous studies where similar magnitude of fresh water input was proposed based on observation from coastal region (Meredith et al., 2013).