Abstract
The spatiotemporal structure of the recent decadal subsurface cooling trend in the North Atlantic Ocean is analyzed in the context of the phase reversal of Atlantic multidecadal variability. A vertically integrated ocean heat content (HC) Atlantic Multidecadal Oscillation index (AMO-HC) definition is proposed in order to capture the thermal state of the ocean and not just that of the surface as in the canonical AMO SST-based indices. The AMO-HC (5–657 m) index (defined over the area, 0°N–60°N, 5°W–75°W) indicates that: (1) The traditional surface AMO index lags the heat content (and subsurface temperatures) with the leading time being latitude-dependent. (2) The North Atlantic subsurface was in a warming trend since the mid-1980s to the mid-2000s, a feature that was also present at the surface with a lag of 3 years. (3) The North Atlantic subsurface is in a cooling trend since the mid-2000s with significant implications for predicting future North Atlantic climate. The spatial structure of decadal trends in upper-ocean heat content (5–657 m) in the North Atlantic prior to and after 2006 suggests a link with variability of the Gulf Stream–Subpolar Gyre system. The Gulf Stream leads variability in upper-ocean heat content over the Subpolar region by ∼13 years, and the lead increases from east to west from the Iceland Basin to the Irminger Sea to the Labrador Sea. The similarity between the structure of decadal mean anomalies, their change and trends in upper-ocean heat content and salinity in the North Atlantic and anomalies associated with a Gulf Stream index is striking. In this scheme, the displacements of the Gulf Stream–North Atlantic Current systems and their interactions with the Subpolar Gyre, as part of the meridional overturning circulation, have a decisive role for imposing decadal variability in both the Ocean and hydroclimate over the neighbouring continents.
Highlights
The state of the North Atlantic Ocean is a key element in understanding regional and global climate variability and change
The Atlantic Multidecadal Oscillation (AMO)-heat content (HC) index is shown for various depths, and a comparison of the surface and subsurface thermal evolution suggests that during the period of a notable surface cooling, the subsurface North Atlantic was already shifting to a warming phase since 1970, both the surface and subsurface were in phase during this period preceding the most pronounced warming in the 1990s
In this analysis a vertically integrated (5–657 m) ocean heat content AMO index has been proposed in an effort to capture the thermal state of the ocean and not just that of the sea surface as in the canonical AMO Sea surface temperatures (SST)-based indices, where both are defined over the same region
Summary
Modelling and empirical analyses indicate that heat transports by the Gulf Stream–North Atlantic Current (GS–NAC) system are important in order to have lowfrequency variability in the North Atlantic (Sutton and Allen, 1997; Delworth et al, 2017; Zhang, 2017). As such, cooling trends in the upper North Atlantic Ocean such as those after the 1970s (Rahmstorf et al, 2015) and after 2005 have recently been related to a weakening of the Atlantic Meridional Overturning Circulation (AMOC) by Robson et al (2016) The latter investigators found that the density anomalies became negative due to the pronounced 1995–2005 warming of the subpolar gyre, which, in turn, decreased the strength of the formation of North Atlantic deep waters and lead to a negative anomaly in the meridional heat transport and a cooling. The surface and subsurface thermal structures of the North Atlantic basin, especially its decadal-to-multidecadal variability are closely examined to provide some insights on the role of the GS–NAC–SPG system in the generation of the recent decadal cooling.
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