A multi-model comparison of the ocean contributions to multidecadal variability in the North Atlantic

In this study, we analyse the inter-relationships between the Labrador Sea densities, the boundary currents, the AMOC and, more generally, the wider climate of the North Atlantic across an ensemble of climate models. The study mostly relies on the analysis of two 300-year long high-resolution coupled control simulations (with HadGEM3-GC2 and HiGEM, respectively), and is completed with an ensemble of CMIP5 preindustrial control experiments to assess the model dependence of the results. We have characterised the main modes of Labrador Sea density (LSD) variability across these models. The leading EOF of LSD is reasonably consistent across the control experiments (i.e. HadGEM-GC2, HiGEM and CMIP5). All models show a fairly uniform vertical structure, with maximum positive density values near the surface that slowly decrease with depth. The associated Principal Components of Labrador Sea densities (PC1-LSD) is generally associated with multidecadal variability, and have enhanced variance between 12-30 years. There is also a good agreement on the density evolution in the ocean-only forced experiments; all depict an increase in the Labrador Sea densities from the 60s to the mid 90s, followed by a decreasing trend up to the present. These coherent Labrador Sea density changes are encouraging, but do not appear to translate to coherent changes in the AMOC strength in the forced historical runs. In the control experiments, decadal trends in PC1 do have a strong link with equivalent trends in the AMOC at 45°N, and to a lesser extent with the AMOC at 26°N (which tend to exhibit suppressed variability at interdecadal timescales). Interestingly, no link between PC1-LSD and AMOC at 26°N is observed when the Ekman transport signal is removed. The link with the AMOC at subpolar latitudes can be explained through an effect of LSDs on the western boundary currents. Indeed, PC1-LSD is tightly linked to the boundary densities at 45 and 57°N, but can show important discrepancies across models regarding the depths involved. Larger model discrepancies occur when extending the analysis to 35°N. Encouragingly, all models support a link between the multi-decadal trends in PC1-LSD and the delayed trends (by 3 to 10 years) in upper ocean temperature in the Eastern Subpolar Gyre (ESPG T700). This is a consistent result regardless of the particular model representation of the PC1-LSD links with the AMOC indices and the boundary densities, further supporting the role of the LSDs and the AMOC on the recent cold blob and its associated climate impacts.

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