Seasonal cycle in surface seawater isotopic composition

Data of seawater stable isotopologs in the quality-controlled database from the isotopic platform facility at LOCEAN (hereafter “CISE-LOCEAN”) provide an unprecedented sampling of the surface Atlantic Ocean and the southwest Indian Ocean. This investigation of  δ ¹⁸ O, δ ² H and deuterium excess (d-excess) in surface sea water for 20 subregions indicates a large variability of the regional regression lines with salinity for  δ ¹⁸ O and  δ ² H. Weakest slopes are found near the equator, and increase towards higher latitudes, but also to a lesser extent in the western Atlantic compared with the eastern Atlantic in the tropics and subtropics. These regressions often explain a large share of the variability in the data (40 % to 70 %), except for d-excess at high latitudes and  δ ¹⁸ O, δ ² H near the equator. The data are sufficient to extract a rather small 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 sea ice melt and depleted water of Arctic origin. A significant correlation between observed seasonal cycles in  δ ¹⁸ O, δ ² H, and d-excess and that of salinity is found in the central equatorial Atlantic. These seasonal cycles change between the vicinity of the equator and the region to its north, which is associated with the seasonal shift of the Inter-tropical Convergence Zone and with the seasonality of the equatorial upwelling. Elsewhere, the seasonal signal usually does not emerge from the noise. An independent over 7-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 surface salinity, δ ¹⁸ O, δ ² H and d-excess. Its interannual variability in  δ ¹⁸ O does not appear closely related to the one in salinity.