Seasonal to decadal variability of the subpolar gyre (D2.2)

Observed ocean processes, mechanisms of subpolar gyre circulation and propagation of heat anomalies

Summary

The aims of this Blue-Action report are to investigate the propagation of warm ocean waters from the Atlantic subpolar gyre over the Greenland-Scotland Ridge (GSR) and towards the Arctic, assess the subpolar gyre circulation in order to quantify the atmospheric and oceanic mechanisms that influence its seasonal to decadal-scale variability, and establish the link between the warm and saline eastern waters and colder and less saline western waters and the mechanisms controlling the heat and freshwater transfer from the eastern subpolar gyre to the Greenland-Scotland. The work draws primarily on data from the OSNAP (Overturning in the Subpolar North Atlantic Program) moored array and associated CTD sections, Ocean Observatories Initiative observations in the Irminger Sea, and data from Argo floats, innovative glider observations in the eastern subpolar North Atlantic, new and historical observations of flows across the Greenland-Scotland Ridge, and altimeter datasets, integrating these with model analyses.

The first 21-month record from OSNAP, coupled with Argo float data, have been described as a “sea change” in our view of overturning in the subpolar North Atlantic, with the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins largely responsible for overturning and its variability in the subpolar basin. This is a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate meridional overturning circulation (MOC) variability. The observations also reveal: a highly variable MOC; the majority of the heat and freshwater transport across the OSNAP line, and its variability, is due to the variable overturning circulation rather than variations in temperature and salinity; and transports of heat northwards in the upper limb of the MOC are dominated by transports east of Reykjanes Ridge/Iceland Basin. Ongoing glider deployments from SAMS across the Rockall Trough and Hatton-Rockall plateau have made considerable progress in characterising the volume transports and seasonal variability within these North Atlantic Current branches in the Eastern Subpolar Gyre.

Further north, the structure and forcing of observed exchanges across the Greenland–Scotland Ridge have been examined. The observed variation of exchanges across the Greenland– Scotland Ridge largely reflect an overturning circulation on interannual time scales with a weaker horizontal circulation in the Nordic Seas on seasonal time scale. Considering buoyancy effects was found to be essential for the interannual time scales but not for the seasonal variability. The barotropic-like seasonal cycle of anomalous inflow and overflow following the rim of the Nordic seas can be explained by the direct influence of wind associated with changes in sea level pressure.

Volume budgets for the Arctic Mediterranean seas (AM) show overturning circulation at the Greenland-Scotland Ridge of 8.0 Sv. This compares to the mean estimate of 15.6 Sv at OSNAP_east. These suggest that over 7 Sv (> 45%) of MOC overturning measured at the OSNAP line is driven by processes occurring in the northeast section of the subpolar gyre between the OSNAP line and the GSR (Irminger Sea and Iceland Basin).  To examine the processes and mechanisms involved in this region we present detailed study of spatial and temporal variability and causes of deep convection at a site in the Irminger Sea. This shows the deep convection to be a result of a complex interaction of local atmospheric processes and more remote ocean processes of inflow and eddies.  Combining the observations with high resolution atmospheric reanalysis, the main source of multi-winter variability of deep convection at this site is shown to be changes in the frequency of occurrence Greenland tip jets. These tip jets are more common in periods of positive NAO, but a positive NAO only results in strong Irminger Sea heat loss when not dominated by the East Atlantic Pattern, as the latter leads to northerly flow and tip jet suppression. Improved representation of this coupled process in ocean-atmosphere models, including its complex relationship with the two main modes of North Atlantic atmospheric variability, may prove key to obtaining reliable projections of future changes in both the overturning and climate.

The observational time-series reported here are still relatively short (just two years of OSNAP overturning and transport estimates were available for this report, though more will be available before the end of Blue-Action), but combination with modelling results allows us to examine longer-term changes.

Model analysis of the role of atmospheric forcing with respect to the oceanic conditions prevailing at the end of the 1990s on the formation of the 2000s cooling of subpolar gyre show that the changes in the heat content cannot be linked to variations of the MOC or changes in the gyre strength but are rather associated with changes in the Labrador Sea Water pathway around the gyre.  In contrast, longer period climate modelling over many decades, finds anomalies in the heat content (OHC) tendency propagate around the subpolar North Atlantic on decadal time scales with a clear relationship to the phase of the AMOC. In the western subpolar North Atlantic, surface fluxes and SST appear to precede and cause AMOC changes, whereas in the east AMOC changes cause the changes in SST and surface fluxes.

Inter-annual to decadal-scale variability is also visible in biogeochemical tracers. The North Atlantic spring bloom, which is the primary food supplier to marine ecosystems in these subpolar waters, is terminated by silicate limitation every spring/summer. A new comprehensive compilation of data from the subpolar Atlantic Ocean shows clear evidence of a marked pre-bloom silicate decline of 1.5–2 μM throughout the winter mixed layer during the last 25 years. These marked fluctuations in pre-bloom silicate inventories will likely have important consequences for the spatial and temporal extent of diatom blooms, thus impacting ecosystem productivity and ocean-atmosphere climate dynamics.

Ongoing research shows that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, with a magnitude never seen before in 120 years of surface measurements. The freshwater anomaly in the Iceland Basin is now propagating into the Irminger and Labrador Seas along the pathway of the subpolar circulation, and into the Nordic Seas. These changes in salinity and stratification impact the extent of deep convection and contribute to changes in the overflow waters and hence the MOC; other results and dynamical arguments suggest that MOC changes may be driven more by the interaction of these anomalies with the eastern boundary. Examination of OSNAP and GSR overflow timeseries over the coming years should help elucidate the contributions of these multi-year processes. The far-reaching impact of eastern Atlantic salinity anomalies highlights the importance of understanding, and correctly simulating, interactions between the North Atlantic Ocean dynamics and the atmosphere circulation for future climate predictions.