Trends in anthropogenic CO2 in water masses of the Subtropical North Atlantic Ocean

摘要

The variability in the storage of the oceanic anthropogenic CO2 (C-ant) on decadal timescales is evaluated within the main water masses of the Subtropical North Atlantic along 24.5 degrees N. Inorganic carbon measurements on five cruises of the A05 section are used to assess the changes in C-ant between 1992 and 2011, using four methods (Delta C*, TrOCA, phi C-T(0), TTD). We find good agreement between the C-ant distribution and storage obtained using chlorofluorocarbons and CO2 measurements in both the vertical and horizontal scales. C-ant distribution shows higher concentrations and greater decadal storage rates in the upper layers with both values decreasing with depth. The greatest enrichment is obserbed in the central water masses, with their upper limb showing a mean annual accumulation of about 1 mu mol kg(-1) yr(-1) and the lower limb showing, on average, half that value. We detect zonal gradients in the accumulation of C-ant. This finding is less clear in the upper waters, where greater variability exists between methods. In accordance with data from time series stations, greater accumulation of C-ant is observed in the upper waters of the western basin of the North Atlantic Subtropical Gyre. In intermediate and deep layers, the zonal gradient in the storage of C-ant is more robust between methods. The much lower mean storage rates found along the section (<0.25 mu mol kg(-1) yr(-1)) become more obvious when longitudinal differences in the C-ant accumulation are considered. In particular, west of 70 degrees W the ventilation by the Labrador Sea Water creates a noticeable accumulation rate up to similar to 0.5 mu mol kg(-1) yr(-1) between 1000 and 2500 dbar. If a Transient Stationary State of the C-ant distributions is considered, significant bi-decadal trends in the C-ant, storage rates in the deepest North Atlantic waters are detected, in agreement with recent estimations. (C) 2014 Elsevier Ltd. All rights reserved.