Seasonal cycle in surface seawater isotopic composition

Abstract. Data of stable isotopologs of water in the quality-controlled CISE-LOCEAN database provide an unprecedented sampling of the surface Atlantic Ocean and the southwest Indian Ocean. This investigation of δ¹⁸O, δ²H and deuterium excess (d-excess) in surface sea water indicates large variability of the regional regression lines with salinity as a function of latitude for δ¹⁸O and δ²H, with weakest slopes near equator, but also to a lesser extent with weaker slopes in the western Atlantic compared with the eastern Atlantic at low and mid-latitudes. These regressions often explain a large share of the variability in the data (40 to 70 %), except for d-excess at the higher latitudes and δ¹⁸O, δ²H near the equator. The data are sufficient to extract a seasonal variability in these relationships and an average seasonal cycle in some regions of the North Atlantic subpolar gyre with signals probably related to changes in the input of heavier sea ice melt and depleted water of Arctic origin. A seasonal cycle is also found in the central equatorial Atlantic with the strong contrast between the vicinity of the equator and the region to its north, associated with the seasonal shift of the precipitation belt and with the seasonality of the equatorial upwelling. In those regions there is also some evidence in seasonal variability of the d-excess that is not currently explained.  Elsewhere, the seasonal signal usually does not come out of the noise. A 7–8 years long time series of nearly monthly surface sampling at the BATS site near Bermuda (31°50’ N, 64°10’ W) also indicates a rather well-defined seasonal cycle of SSS, δ¹⁸O, δ²H and deuterium excess. There is also some indication of interannual variability, which might not be closely related to salinity.