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1. Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic

   https://doi.org/10.1038/s41467-020-14474-y  

   Holliday, N. Penny ; Bersch, Manfred ; Berx, Barbara ; Chafik, Léon ; Cunningham, Stuart ; Florindo-López, Cristian ; Hátún, Hjálmar ; Johns, William ; Josey, Simon A. ; Larsen, Karin Margretha H. ; et al ( January 2020 , Nature Communications)

   The Atlantic Ocean overturning circulation is important to the climate system because it carries heat and carbon northward, and from the surface to the deep ocean. The high salinity of the subpolar North Atlantic is a prerequisite for overturning circulation, and strong freshening could herald a slowdown. We show that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, with a magnitude never seen before in 120 years of measurements. The cause was unusual winter wind patterns driving major changes in ocean circulation, including slowing of the North Atlantic Current and diversion of Arctic freshwater frommore »the western boundary into the eastern basins. We find that wind-driven routing of Arctic-origin freshwater intimately links conditions on the North West Atlantic shelf and slope region with the eastern subpolar basins. This reveals the importance of atmospheric forcing of intra-basin circulation in determining the salinity of the subpolar North Atlantic.

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2. Nordic Seas Hydrography in the Context of Arctic and North Atlantic Ocean Dynamics

   https://doi.org/10.1175/JPO-D-20-0071.1  

   Kenigson, J. S. ; Timmermans, M.-L. ( January 2021 , Journal of Physical Oceanography)

   Abstract The hydrography of the Nordic seas, a critical site for deep convective mixing, is controlled by various processes. On one hand, Arctic Ocean exports are thought to freshen the North Atlantic Ocean and the Nordic seas, as in the Great Salinity Anomalies (GSAs) of the 1970s–1990s. On the other hand, the salinity of the Nordic seas covaries with that of the Atlantic inflow across the Greenland–Scotland Ridge, leaving an uncertain role for Arctic Ocean exports. In this study, multidecadal time series (1950–2018) of the Nordic seas hydrography, Subarctic Front (SAF) in the North Atlantic Ocean [separating the water massesmore »of the relatively cool, fresh Subpolar Gyre (SPG) from the warm, saline Subtropical Gyre (STG)], and atmospheric forcing are examined and suggest a unified view. The Nordic seas freshwater content is shown to covary on decadal time scales with the position of the SAF. When the SPG is strong, the SAF shifts eastward of its mean position, increasing the contribution of subpolar relative to subtropical source water to the Atlantic inflow, and vice versa. This suggests that Arctic Ocean fluxes primarily influence the hydrography of the Nordic seas via indirect means (i.e., by freshening the SPG). Case studies of two years with anomalous NAO conditions illustrate how North Atlantic Ocean dynamics relate to the position of the SAF (as indicated by hydrographic properties and stratification changes in the upper water column), and therefore to the properties of the Atlantic inflow and Nordic seas.« less
3. Submesoscale modulation of deep water formation in the Labrador Sea

   https://doi.org/10.1038/s41598-020-74345-w  

   Tagklis, Filippos ; Bracco, A. ; Ito, T. ; Castelao, R. M. ( October 2020 , Scientific Reports)

   Submesoscale structures fill the ocean surface, and recent numerical simulations and indirect observations suggest that they may extend to the ocean interior. It remains unclear, however, how far-reaching their impact may be—in both space and time, from weather to climate scales. Here transport pathways and the ultimate fate of the Irminger Current water from the continental slope to Labrador Sea interior are investigated through regional ocean simulations. Submesoscale processes modulate this transport and in turn the stratification of the Labrador Sea interior, by controlling the characteristics of the coherent vortices formed along West Greenland. Submesoscale circulations modify and controlmore »the Labrador Sea contribution to the global meridional overturning, with a linear relationship between time-averaged near surface vorticity and/or frontogenetic tendency along the west coast of Greenland, and volume of convected water. This research puts into contest the lesser role of the Labrador Sea in the overall control of the state of the MOC argued through the analysis of recent OSNAP (Overturning in the Subpolar North Atlantic Program) data with respect to estimates from climate models. It also confirms that submesoscale turbulence scales-up to climate relevance, pointing to the urgency of including its advective contribution in Earth systems models.

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4. The Mechanisms of the Atlantic Meridional Overturning Circulation Slowdown Induced by Arctic Sea Ice Decline

   https://doi.org/10.1175/JCLI-D-18-0231.1  

   Liu, Wei ; Fedorov, Alexey ; Sévellec, Florian ( February 2019 , Journal of Climate)

   We explore the mechanisms by which Arctic sea ice decline affects the Atlantic meridional overturning circulation (AMOC) in a suite of numerical experiments perturbing the Arctic sea ice radiative budget within a fully coupled climate model. The imposed perturbations act to increase the amount of heat available to melt ice, leading to a rapid Arctic sea ice retreat within 5 years after the perturbations are activated. In response, the AMOC gradually weakens over the next ~100 years. The AMOC changes can be explained by the accumulation in the Arctic and subsequent downstream propagation to the North Atlantic of buoyancy anomaliesmore »controlled by temperature and salinity. Initially, during the first decade or so, the Arctic sea ice loss results in anomalous positive heat and salinity fluxes in the subpolar North Atlantic, inducing positive temperature and salinity anomalies over the regions of oceanic deep convection. At first, these anomalies largely compensate one another, leading to a minimal change in upper ocean density and deep convection in the North Atlantic. Over the following years, however, more anomalous warm water accumulates in the Arctic and spreads to the North Atlantic. At the same time, freshwater that accumulates from seasonal sea ice melting over most of the upper Arctic Ocean also spreads southward, reaching as far as south of Iceland. These warm and fresh anomalies reduce upper ocean density and suppress oceanic deep convection. The thermal and haline contributions to these buoyancy anomalies, and therefore to the AMOC slowdown during this period, are found to have similar magnitudes. We also find that the related changes in horizontal wind-driven circulation could potentially push freshwater away from the deep convection areas and hence strengthen the AMOC, but this effect is overwhelmed by mean advection.

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5. Atlantic Multidecadal Variability and Associated Climate Impacts Initiated by Ocean Thermohaline Dynamics

   https://doi.org/10.1175/JCLI-D-19-0530.1  

   Kim, Who M. ; Yeager, Stephen ; Danabasoglu, Gokhan ( January 2020 , Journal of Climate)

   The sea surface temperature (SST) signature of Atlantic multidecadal variability (AMV) is a key driver of climate variability in surrounding regions. Low-frequency Atlantic meridional overturning circulation (AMOC) variability is often invoked as a key driving mechanism of AMV-related SST anomalies. However, the origins of both AMV and multidecadal AMOC variability remain areas of active research and debate. Here, using coupled ensemble experiments designed to isolate the climate response to buoyancy forcing associated with the North Atlantic Oscillation in the Labrador Sea, we show that ocean dynamical changes are the essential drivers of AMV and related climate impacts. Atmospheric teleconnectionsmore »also play an important role in rendering the full AMV pattern by transmitting the ocean-driven subpolar SST signal into the rest of the basin, including the tropical North Atlantic. As such, the atmosphere response to the tropical AMV in our experiments is limited to a relatively small area in the Atlantic sector in summertime, suggesting that it could be overestimated in widely adopted protocols for AMV pacemaker experiments.

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