Labrador Sea

Freshwater fluxes across the Labrador Shelf

The formation of deep dense water during convection in the Labrador Sea is one of the driving mechanisms of the meridional overturning circulation (MOC). In the winter large air-sea fluxes remove surface buoyancy and allow the onset of the deep mixing. Recent increases in freshwater input in high latitudes through accelerated rates of Greenland and Arctic ice melt has the potential to add buoyancy to the region of convection and consequently slow or suppress dense water formation. Such a slow down would in turn result in a decrease of the MOC. It is therefore more important than ever to understand pathways of freshwater into the region of deep convection to predict how future climate change will impact ocean circulation. Using a Lagrangian approach, by tracking particles in a NEMO 1/12 degree ocean model, we examine where and when freshwater in the top 30 m enters the Labrador Sea basin (missing reference).

Role of Greenland in the AMOC

Paleoclimate records have shown that at the end of the last ice age, an enormous glacial lake over North America released freshwater and ice into the North Atlantic. This resulted in an abrupt climate change observable in temperature proxies which was associated with the shutdown of the ocean meridional overturning circulation (MOC). Recent observations of the MOC in the subtropical North Atlantic have shown it to be highly variable, on timescales of days to years, with an overall reducing trend over a decade of observations. Over the same time period, melting from Greenland (and the Arctic) has been increasing, resulting in freshwater input to the northern North Atlantic at rates not seen for several decades. Here, we examine the evidence for present day melting from the Greenland ice sheet influencing the large scale ocean circulation [Frajka-Williams et al., 2016].

Seaglider observations during active convection

Four of the five Seagliders deployed in the Labrador Sea observed deep convection, with well-mixed water columns down to the depth of observation (1000 m). This offers an unprecedented look at deep convection with horizontal resolution of 10 km [Frajka-Williams et al., 2014].

Seaglider method to extract vertical velocities

Vertical water velocity is notoriously difficult to measure, in part because it is so small. Deep convection offered a unique opportunity to investigate the velocities as estimated using a Seaglider and its hydrodynamic flight model [Frajka-Williams et al., 2011]. The method is estimated to be accurate to 0.6 cm/s.

Spring bloom: from space

The Labrador Sea spring bloom has a distinct pattern: a triangle with centroid over where the 3000m isobath turns westward away from the Greenland shelf. We explain the bloom pattern in terms of haline stratification, and sources of that stratification (mean background flow and eddy transport from the West Greenland Shelf) and analyze interannual relationships between the bloom, runoff from Greenland and eddies [Frajka-Williams and Rhines, 2010].

Spring bloom: from Seagliders

Phytoplankton blooms in the Labrador Sea occur under a variety of distinct physical conditions: in terms of the sources of stratification and the structure of the bloom and hydrography (eddies, fronts and thin layers) [Frajka-Williams et al., 2009].

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References

1. Frajka-Williams, E., J. Bamber, and K. Vage (2016), Greenland Melt and the Atlantic Meridional Overturning Circulation, Oceanogr., doi:10.5670/oceanog.2016.96.
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2. Frajka-Williams, E., C. C. Eriksen, P. B. Rhines, and R. R. Harcourt (2011), Determining Vertical Velocities from Seaglider, J. Atmos. Ocean. Tech., 28(12), 1641–1656, doi:10.1175/2011JTECHO830.1.
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3. Frajka-Williams, E., P. Rhines, and C. Eriksen (2009), Physical controls and mesoscale variability in the Labrador Sea spring phytoplankton bloom observed by Seaglider, Deep Sea Res. I, 56, 2144–2161, doi:10.1016/j.dsr.2009.07.008.
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4. Frajka-Williams, E., and P. B. Rhines (2010), Physical controls and interannual variability of the Labrador Sea spring phytoplankton bloom in distinct regions, Deep Sea Res. I, 57(4), 541–552, doi:10.1016/j.dsr.2010.01.003.
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5. Frajka-Williams, E., P. B. Rhines, and C. C. Eriksen (2014), Horizontal stratification during Deep convection in the Labrader Sea, J. Phys. Ocean., 44(1), 220–228, doi:10.1175/JPO-D-13-069.1.
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