Changes in the deep and intermediate waters of the sub-polar gyre of the North Atlantic since the 1950s

A retrospective analysis of water mass properties along the northern boundary of the North Atlantic shows that the sustained freshening of the deep waters during the past 4 decades was preceded by a decade of increasing salinity (the late 1950s through the late 1960s). These decadal-scale signals can be traced from the various sills around the sub-polar gyre to the Newfoundland Basin, however the interannual oscillations seen in the Denmark Strait Overflow Water (DSOW) in the upstream regions fade out after the overflow passes south of Flemish Cap. This highlights a key role of the deep inter-gyre recirculation in the southward transport of cold and fresh waters. Successive hydrographic sections indicate that the volume of DSOW increases after entering the Newfoundland Basin while its vertical stratification decreases. Both this volume increase and the smoothing out of the interannual variation in properties are likely a consequence of further entrainment and mixing of DSOW in the Newfoundland Basin that is associated with recirculation in the deep layers. Maps of temperature and salinity on selected density surfaces for different decades show how the recent freshening of the deep layers is carried around the sub-polar gyre by the deep western boundary current and is then returned to the northeast by the North Atlantic Current. The freshening has extended into the sub-tropical gyre and is also being returned to the sub-polar gyre through the Gulf Stream - North Atlantic current exchange. While the dramatic salinity changes in the water mass properties are, to a large extent compensated by inverse changes in their temperatures, there have also been profound changes in the density distributions in the sub-polar gyre. The thickness of the density layer associated with the Labrador Sea waters have increased significantly between the 1960s and the 1990s. The waters in the upper 1.5 km have become denser while those deeper than 2 km have become less dense as this thick mass of intermediate water formed in the early 1990s has displaced the waters both lighter and denser than itself. Since these changes are largely confined to the sub-polar gyre, they impact the strength and deep structure of the North Atlantic current. By comparing volumetric T-S analysis of the sub-polar gyre from the 1960s and the 1990s, we estimate that the average gain of fresh water over the intermediate to bottom waters exceeds 4 metres. The Labrador Sea Water has gained about 2.52 metres, while the Northeast Atlantic Deep and DSOW have gained 0.63 m and 0.31 m, respectively.