Measuring the eastern boundary inflow to the Labrador Sea

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Presentation transcript:

Measuring the eastern boundary inflow to the Labrador Sea Robert S. Pickart (WHOI) Outline 1. Insights on the overturning circulation of the Labrador Sea 2. Challenges associated with measuring the eastern boundary current 3. Strawman array design and remaining questions Greenland coast viewed from R/V Knorr , Oct 2008 (Photo by Ben Harden)

Labrador Sea general circulation

Wintertime storm tracks (from 45 years of ECMWF re-analysis data) Mean storm track Våge et al. (2009)

Heat loss from wintertime cold-air outbreaks Ice Color = heat loss in Watts/m2 (from NCEP reanalysis)

Heat loss from wintertime cold-air outbreaks Ice Color = heat loss in Watts/m2 (from NCEP reanalysis) Contours = Absolute geostrophic pressure (Lavender et al. (2000)

Surface eddy speed Color = surface eddy speed (cm/s) Contours = Absolute geostrophic pressure (Lavender et al. (2000) After Lilly et al. (2003)

Surface eddy speed and regions of deep convection Color = surface eddy speed (cm/s) Contours = Absolute geostrophic pressure (Lavender et al. (2000)

Surface eddy speed and regions of deep convection Color = surface eddy speed (cm/s) Contours = Absolute geostrophic pressure (Lavender et al. (2000)

Mixed-layer depth in winter 1997 Color = mixed layer depth (m) Contours = Absolute geostrophic pressure (Lavender et al. (2000)

WOCE AR7W line Inverse calculation Time period of high NAO in the early to mid 1990s

Float data Mean circulation at 700m 1995–1999 AR7W Lavendar et al. (2000)

Referencing the geostrophic velocity section Mean velocity at 700m from PALACE

Referencing the geostrophic velocity section Mean velocity at 700m from PALACE Mean temperature/density section X

Referencing the geostrophic velocity section Mean velocity at 700m from PALACE Mean temperature/density section X

Referencing the geostrophic velocity section Mean velocity at 700m from PALACE Total transport in: 35.4 Sv Total transport out: 35.5 Sv Mean temperature/density section X

Absolute geostrophic velocity AR7W

Absolute geostrophic velocity Throughput Overflow water transport = 12.4 Sv Recirculation transport = 2.5 Sv Total Boundary Current transport = 28.5 Sv

Absolute geostrophic velocity Near zero velocity! Throughput Overflow water transport = 12.4 Sv Recirculation transport = 2.5 Sv Total Boundary Current transport = 28.5 Sv

Depth space: Overturning and horizontal components of the flow Deviation Decompose the full velocity: Horizontal gyre Baroclinic gyre Then decompose the deviation velocity:

Transport components Overturning Cell Horizontal cell

Transport components 95% of sinking happens in boundary current

Density space: Overturning transport

Density space: Overturning transport > 95% of the transport imbalance occurs in the boundary current

What are the important points for OSNAP? Seaward of the 700m isobath there is mass balance across the section (i.e. excluding the shallow shelf-edge flows). The horizontal cell is significantly larger than the overturning cell. Nearly all of the overturning, in depth space and density space, occurs along the rim of the basin. Flows in the middle of the Basin are weak. There is substantial mesoscale variability, so low-passing will be necessary.

Ongoing Timeseries in the Labrador Sea OSNAP-east moorings

Ongoing Timeseries in the Labrador Sea OSNAP-east moorings

Considerations for OSNAP eastern boundary array

Considerations for OSNAP eastern boundary array 1997 hydrographic Sections

Two eastern boundary crossings in Feb 1997 North Salinity (color) overlain by density (contours) 26 Feb South

Considerations for OSNAP eastern boundary array

Considerations for OSNAP eastern boundary array

Considerations for OSNAP eastern boundary array

Considerations for OSNAP eastern boundary array 2001 hydrographic Section

Eastern boundary crossing in Aug 2001 Salinity (color) overlain by density (contours) Absolute geostrophic velocity (color) overlain by density (contours)

Eastern boundary crossing in Aug 2001 Salinity (color) overlain by density (contours) Absolute geostrophic velocity (color) overlain by density (contours)

Eastern boundary crossing in Aug 2001

Eastern boundary crossing in Aug 2001

Considerations for OSNAP eastern boundary array

Eastern boundary crossing in Aug 2001

Eastern boundary crossing in Aug 2001

Eastern boundary crossing in Aug 2001

Eastern boundary crossing in Aug 2001 Use of current meters and Microcats is safer and allows for 2-year deployments

Strawman OSNAP-West Boundary Arrays

Remaining issues: shelf, near-surface, recirculation

Remaining issues: shelf, near-surface, recirculation

Considerations for OSNAP eastern boundary array

Positive aspects of eastern array location Bottom topography is gentle enough to effectively resolve the boundary current. Array will coincide with an altimeter line (providing additional surface velocity information). By moving south, the array is a true input boundary condition for the Labrador Sea (to compare to the western export array). The offshore recirculation is geographically confined, which means it can be effectively sampled with floats.

Negative aspects of eastern array location It is not the AR7W line! (which provides a wonderful context). Logistically more difficult to service the array. Closer to OSNAP Cape Farewell array (less contrast).