Subsurface temperature maxima in the Labrador Sea and the subpolar North Atlantic

Deep and shallow subsurface temperature maxima (Tmax) in the Labrador Sea are found to be the result of anomalous freshwater input during past decades in particular during the early 1990s. The deep Tmax is associated with a specific water mass imported into the Labrador Sea. The shallow Tmax is of local origin and created by anomalous heat input in 1999, not eroded by surface buoyancy forcing in recent years. Both are associated with stability maxima: the deep Tmax is a barrier for maximum convection depth, the shallow Tmax separates the water in a layer ventilated by overturning and a layer modified through lateral fluxes only. The shallow Tmax is exported into the subpolar gyre. A complementary shallow Tmax in the Greenland Sea suggest a concerted response of deep convection regions to anomalous freshwater input: diminished vertical mixing and dominance of lateral heat/salt fluxes underneath Tmax, shallow convection above it

Interannual variability of newly formed Labrador Sea Water from 1994 to 2005

Mooring records collected in the central Labrador Sea are evaluated regarding the variability of the hydrographic properties of newly formed Labrador Sea Water (LSW) between 1994 and 2005. This time series is longer and of significantly higher temporal resolution than any discussed before in the context of decreasing convection activity. For the upper 1500 m depth range two distinct warming periods are identified from 1997 to 1999 and from 2003 to 2005 leading to a substantial temperature increase of 0.6°C over the recent decade. The time series of LSW source water properties suggest that ocean transport of heat and freshwater anomalies play a significant role in determining the ultimate convection depth. In 2005 the LSW temperature and salinity had reached high values comparable to those from the early 1970's, shortly after the passage of the Great Salinity Anomaly

Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008.

In the process of open-ocean convection in the subpolar North Atlantic Ocean, surface water sinks to depth as a distinct water mass, the characteristics of which affect the meridional overturning circulation and oceanic heat flux. In addition, carbon is sequestered from the atmosphere in the process. In recent years, this convection has been shallow or non-existent, which could be construed as a consequence of a warmer climate. Here we document the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008. We use profiling float data from the Argo programme to document deep mixing. Analysis of a variety of in situ, satellite and reanalysis data shows that contrary to expectations the transition to a convective state took place abruptly, without going through a phase of preconditioning. Changes in hemispheric air temperature, storm tracks, the flux of fresh water to the Labrador Sea and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air, making the surface water sufficiently cold and dense to initiate deep convection. Given this complexity, we conclude that it will be difficult to predict when deep mixing may occur again