Beschreibung: The detection of multi-decadal trends in the oceanic oxygen content and its possible attribution to global warming is protracted by the presence of a substantial amount of interannual to decadal variability, which hitherto is poorly known and characterized. Here we address this gap by studying interannual to decadal changes of the oxygen concentration in the Subpolar Mode Water (SPMW), the Intermediate Water (IW) and the Mediterranean Outflow Water (MOW) in the eastern North Atlantic. We use data from a hydrographic section located in the eastern North Atlantic at about 48°N repeated 12 times over a period of 19 years from 1993 through 2011, with a nearly annual resolution up to 2005. Despite a substantial amount of year-to-year variability, we observe a long-term decrease in the oxygen concentration of all three water masses, with the largest changes occurring from 1993 to 2002. During that time period, the trends were mainly caused by a contraction of the subpolar gyre associated with a northwestward shift of the Subpolar Front (SPF) in the eastern North Atlantic. This caused SPMW to be ventilated at lighter densities and its original density range being invaded by subtropical waters with substantially lower oxygen concentrations. The contraction of the subpolar gyre reduced also the penetration of IW of subpolar origin into the region in favor of an increased northward transport of IW of subtropical origin, which is also lower in oxygen. The long-term oxygen changes in the MOW were mainly affected by the interplay between circulation and solubility changes. Besides the long-term signals, mesoscale variability leaves a substantial imprint as well, affecting the water column over at least the upper 1000 m and laterally by more than 400 km. Mesoscale eddies induced changes in the oxygen concentration of a magnitude that can substantially alias analyses of long-term changes based on repeat hydrographic data that are being collected at intervals of typically 10 years.

Beschreibung: This product contains daily snapshots from 1st of January 1993 to 5th of May 2016 of reconstructed salinity, temperature and geostrophic velocities (u and v components) profiles from the surface down to 1900 dbar (every 10 dbar) gridded on a ¼° Cartesian grid for the North Atlantic from 34.1250°N to 59.8750°N and 74.3750°W to 10.1250°W. Note that some of the regions within the dataset domain are not covered by the data (e.g. Labrador Sea, Irminger Sea and West European Basin). The reconstructed profiles are derived from satellite altimetry data and Argo floats using a Gravest Empirical Mode (GEM) technique. The GEM technique creates a transfer function between T/S profiles and dynamic height from Argo floats in order to parameterize salinity and temperature profiles as a function of dynamic height from the satellite altimetry. The GEM is built by interpolating with a cubic smoothing spline the salinity and temperature from all Argo floats available within a selected region, with the dynamic height referenced to 1900 from the surface until 1900 dbar, at intervals of 10 dbar. The interpolation between the salinity/temperature produces 191 splines interpolated onto a horizontal regular grid of 0.0005 dyn m every 10 dbar called look up table or transfer function. In order to have salinity and temperature vertical profiles whose temporal and spatial distribution correspond to those of the Sea Level Anomaly (SLA) from the satellite altimeter data (daily snapshot gridded on a 1/4° resolution from 1993 to 2016) a mean dynamic topography (MDT) must be added to the SLA in order to produce an absolute value (ADT) that corresponds after applying some adjustments to the dynamic height in the look up table. In this way for each corresponding ADT from satellite altimetry data we have a corresponding salinity and temperature profile from the look up table. Geostrophic velocities from the reconstructed temperature and salinity are calculated relative to the sea surface for each grid point and vertical layer. The absolute velocities are calculated by adding the surface geostrophic velocities from the MADT (Mapped absolute dynamic topography) product.

Beschreibung: Ventilation of Labrador Sea Water (LSW) receives ample attention because of its potential relation to the strength of the Atlantic Meridional Overturning Circulation (AMOC). Here, we provide an overview of the changes of LSW from observations in the Labrador Sea and from the southern boundary of the subpolar gyre at 47° N. A strong winter-time atmospheric cooling over the Labrador Sea led to intense and deep convection, producing a thick and dense LSW layer as, for instance, in the early to mid-1990s. The weaker convection in the following years mostly ventilated less dense LSW vintages and also reduced the supply of oxygen. As a further consequence, the rate of uptake of anthropogenic carbon by LSW decreased between the two time periods 1996–1999 and 2007–2010 in the western subpolar North Atlantic. In the eastern basins, the rate of increase in anthropogenic carbon became greater due to the delayed advection of LSW that was ventilated in previous years. Starting in winter 2013/2014 and prevailing at least into winter 2015/2016, production of denser and more voluminous LSW resumed. Increasing oxygen signals have already been found in the western boundary current at 47° N. On decadal and shorter time scales, anomalous cold atmospheric conditions over the Labrador Sea lead to an intensification of convection. On multi-decadal time scales, the ‘cold blob’ in the subpolar North Atlantic projected by climate models in the next 100 years is linked to a weaker AMOC and weaker convection (and thus deoxygenation) in the Labrador Sea.