Labrador Sea sub-surface density as a precursor of multi-decadal variability in the North Atlantic: a multi-model study

Abstract

Abstract. The Subpolar North Atlantic (SPNA) is a region with prominent decadal variability that has experienced remarkable warming and cooling trends in the last few decades. These observed trends have been preceded by slow-paced increases and decreases in the Labrador Sea density (LSD), which are thought to be a precursor of large scale ocean circulation changes. This article analyses the inter-relationships between the LSD and the wider North Atlantic across an ensemble of coupled climate model simulations. In particular, it analyses the link between subsurface density and the deep boundary density, the Atlantic Meridional Overturning Circulation (AMOC), the Subpolar Gyre (SPG) circulation, and the upper ocean temperature in the eastern SPNA. All simulations exhibit considerable multidecadal variability in the LSD and the ocean circulation indices, which are found to be interrelated. LSD is strongly linked with the strength of subpolar AMOC and gyre circulation, and is also linked with the subtropical AMOC, although the strength of this relationship is model dependent. The connectivity of LSD with the subtropics is found to be sensitive to different model features, including: the mean density stratification in the Labrador Sea; the strength and depth of the AMOC; and the depth at which the LSD propagates southward along the western boundary. Several of these quantities can also be computed from observations, and comparison with these observation-based quantities suggests that models representing a weaker link with the subtropical AMOC may be more realistic. This would imply that RAPID AMOC measurements might not be adequate to represent decadal to multidecadal changes in the subpolar overturning circulation.

The evolution of the North Atlantic Meridional Overturning Circulation since 1980

“…While many AMOC anomalies remain localized, some are communicated meridionally, giving rise to large-scale AMOC variations. Large-scale AMOC anomalies are thought to be generated in the subpolar gyre and communicated southwards into the subtropical gyre 32,86 , either slowly through advection of deep subpolar density anomalies 87 , or more quickly through boundary waves 86,88,89 . There is strong mixing of these dense waters before reaching the subtropics [90][91][92] , meaning that perhaps only large and persistent AMOC anomalies succeed in being communicated southwards 93,94 .…”

“…However, the strong subpolar AMOC in the mid-1990s has also been linked to a strong subtropical AMOC in the late 1990s, with a faster propagation time 30,112 . Although some models also suggest meridional linkages of AMOC variability [86][87][88] , the processes and timescales vary, and it might be that some subpolar signals do not reach the subtropics 90,91,93 .…”

“…While AMOC variability is an important driver of North Atlantic temperature and salinity, as suggested by models 3,87 and observations 150 , uncertainties about its relative contribution exist. Indeed, other processes can also be important, including local atmospheric forcing 151,152 and external forcing 2,153 .…”

Role of air-sea fluxes and ocean surface density on the production of deep waters in the eastern subpolar gyre of the North Atlantic

“…Further exploration is needed before a specific attribution for the large transformation in winter 2014-2015 is made, however we do note that a number of recent papers have documented strong advective changes in the Iceland Basin over the past few years. In one study (Ortega et al, 2020) the NAC is shown to drive large salinity and temperature property changes at the surface and subsurface, while in another study (Holliday et al, 2020), strong property changes are attributed to changes in the fraction of water from the Labrador Sea that reaches the Iceland Basin. These studies, highlighting the importance of ocean advection, are consistent with a previous paper that showed large hydrographic property changes for SPMW localized over the Reykjanes Ridge that cannot only be explained by the variability of the local air-sea fluxes (Thierry et al, 2008).…”

Impact of Initialization Methods on the Predictive skill in NorCPM - An Arctic-Atlantic Case Study

“…The Atlantic Meridional Overturning Circulation (AMOC) is pointed out as a key source of decadal to multidecadal climate predictability in the SPNA. However, recently the Labrador Sea Water thickness anomalies have been indicated as precursor of upper ocean heat content predictability in this area, as well as an important driver influencing the southward ocean circulation (Ortega et al, 2020;Yeager, 2020). The Labrador Sea Water is formed by convective processes driven by high oceanic heat losses during winter.…”