Large-Scale Atlantic Salinity Changes over the Last Half-Century: A Model–Observation Comparison

Abstract

Large-scale freshening of the northern Atlantic, and concurrent salinity increases in the low-latitude Atlantic upper layers, have been widely reported for the second half of the twentieth century. The role of anthropogenic and/or unforced variability processes in these changes, and the potential for the high-latitude freshening to slow the Atlantic meridional overturning circulation (MOC), have been the subject of debate. These issues are investigated by comparing observed and simulated changes, using the Third Hadley Centre Coupled Model (HadCM3). This analysis suggests that a substantial part of the observed trends could be related to multidecadal variability of the MOC. Using an SST-derived proxy for historical MOC changes, in conjunction with model internal variability relationships, suggests that much of the observed evolution of northern Atlantic freshwater content can be explained as being driven by unforced MOC variability.HadCM3 simulations with "external" historical time-varying forcings show anthropogenically forced increases in the main hydrological cycle over the Atlantic: an increase in net precipitation at high latitudes and in net evaporation in the subtropics. In the northern Atlantic the freshening from additional surface freshwater is counteracted by changes in ocean freshwater transport. A similar ocean compensation is absent at lower latitudes, where there is decreasing freshwater content. It is suggested that in the recent historical period this externally forced trend is likely to have led to anomalies exceeding the unforced variability range.

A multimodel comparison of centennial Atlantic meridional overturning circulation variability

“…This is at zero lag in HadCM3 and KCM and at 1 year lag in MPI-ESM. This anomaly appears to be gradually advected northwards into the subpolar gyre (defined as a box 45°W to the prime meridian and from 45°N to 75°N) on a timescale of a few decades in agreement with the analysis of Pardaens et al (2008) and VW2004 (also see Sect. 2 of this paper).…”

Intergovernmental Panel for Climate Change (IPCC) and Attribution and Prediction of Climate: Progress since the Fourth Assessment

“…While freshening at high latitudes in the Atlantic is consistent with observed increases in precipitation at high latitudes [6], climate model studies suggest that Atlantic freshening could be associated with changes in northward advection associated with variability of the meridional overturning circulation [74]. An optimal detection analysis of Atlantic salinity changes [44] detected a human influence on the observed increases in salinity at low latitudes but found that high latitude changes, including a recent reversal of the freshening observed previously, are consistent with internal variability.…”

Chapter 1 Impacts of the Oceans on Climate Change

“…However, because of the ability of the ocean circulation to transport large amounts of fresh water, the interpretation of the salinity observations remains a matter of debate (e.g. Pardaens et al, 2008;Wu and Wood, 2008). New datasets from satellites suggest a stronger response of the hydrological cycle to temperature changes than is seen in climate models (Wentz et al, 2007), but the datasets are still new and require further scrutiny.…”