Changing Spatial Patterns of Deep Convection in the Subpolar North Atlantic

Siren Rühs,Paul G Myers,Torge Martin,Arne Biastoch,Claus W Böning,Michael Dowd,Eric C J Oliver,Klaus Getzlaff

doi:10.1029/2021jc017245

Abstract

AbstractDeep convection and associated deep water formation are key processes for climate variability, since they impact the oceanic uptake of heat and trace gases and alter the structure and strength of the global overturning circulation. For long, deep convection in the subpolar North Atlantic was thought to be confined to the central Labrador Sea in the western subpolar gyre (SPG). However, there is increasing observational evidence that deep convection also has occurred in the eastern SPG south of Cape Farewell and in the Irminger Sea, in particular, in 2015–2018. Here we assess this recent event in the context of the temporal evolution of spatial deep convection patterns in the SPG since the mid‐twentieth century, using realistic eddy‐rich ocean model simulations. These reveal a large interannual variability with changing contributions of the eastern SPG to the total deep convection volume. Notably, in the late 1980s to early 1990s, the period with highest deep convection intensity in the Labrador Sea related to a persistent positive phase of the North Atlantic Oscillation, the relative contribution of the eastern SPG was small. In contrast, in 2015–2018, deep convection occurred with an unprecedented large relative contribution of the eastern SPG. This is partly linked to a smaller north‐westward extent of deep convection in the Labrador Sea compared to previous periods of intensified deep convection, and may be a first fingerprint of freshening trends in the Labrador Sea potentially associated with enhanced Greenland melting and the oceanic advection of the 2012–2016 eastern North Atlantic fresh anomaly.

Highlights

Oceanic deep convection and the associated deep water formation are key processes for regional to global climate variability, since they largely control the oceans uptake and storage of heat and trace gases such as CO2 (e.g., Rhein et al, 2017) and alter the structure and strength of the global overturning circulation (Kuhlbrodt & Griesel, 2007; Rhein et al, 2011)
While a long-term mean MLDa cannot be inferred from observations, a comparison of the simulated (SIMJRA) climatological March mixed layer depths (MLD) for the period 1990–2019 with that inferred from ARGO observations reveals a generally good agreement in the location and spatial extent of the deep convection (DCC)
In this study we employed hindcast simulations (1958–2019) with a realistic eddy-rich ocean model to assess how often and to what spatial extent deep convection and associated deep water formation in the subpolar North Atlantic occurs outside the primary deep convection region in the central Labrador Sea, that is, in the secondary deep convection regions south of Cape Farewell and in the Irminger Sea

Summary

Introduction

Oceanic deep convection and the associated deep water formation are key processes for regional to global climate variability, since they largely control the oceans uptake and storage of heat and trace gases such as CO2 (e.g., Rhein et al, 2017) and alter the structure and strength of the global overturning circulation (Kuhlbrodt & Griesel, 2007; Rhein et al, 2011). Deep convective mixing enables the transfer of oxygen and organic as well as inorganic matter from the well-ventilated euphotic zone to the deep ocean, and brings nutrients toward the surface (Severin et al, 2014), thereby largely shaping oceanic ecosystems. The subpolar North Atlantic constitutes one of the few oceanic regions where deep convection occurs in the open ocean, due to favorable wintertime conditions consisting of a generally weak interior stratification and a cyclonic subpolar gyre (SPG) circulation (associated with doming isopycnals that bring the weakly stratified waters closer to the surface), combined with strong sea-air buoyancy fluxes induced by cold and dry westerly winds (Lab Sea Group, 1998; Marshall & Schott, 1999). The resulting water masses, which are eventually exported southward via the Deep Western Boundary Current and interior pathways, are comprehensively referred to as North Atlantic Deep Water (NADW) and form the lower limb of the upper cell of the global overturning circulation in the Atlantic, that is, the Atlantic Meridional Overturning Circulation (AMOC)

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