Measuring the eastern boundary inflow to the Labrador Sea Outline 1. Insights on the overturning...

Measuring the eastern boundary inflow Measuring the eastern boundary inflow to the Labrador Sea to the Labrador Sea

Measuring the eastern boundary inflow Measuring the eastern boundary inflow to the Labrador Sea to the Labrador Sea

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 Greenland coast viewed from R/V Knorr , Oct 2008

(Photo by Ben Harden)(Photo by Ben Harden)

Robert S. Pickart (WHOI)

Labrador Sea general circulation

Wintertime storm tracks (from 45 years of ECMWF re-analysis data) Wintertime storm tracks (from 45 years of ECMWF re-analysis data)

VVååge et al. (2009) ge et al. (2009)

Mean storm track

Heat loss from wintertime cold-air outbreaks Heat loss from wintertime cold-air outbreaks

Color = heat loss in Watts/mColor = heat loss in Watts/m22 (from NCEP reanalysis) (from NCEP reanalysis) Ice

Heat loss from wintertime cold-air outbreaks Heat loss from wintertime cold-air outbreaks

Color = heat loss in Watts/mColor = heat loss in Watts/m22 (from NCEP reanalysis) (from NCEP reanalysis)

Contours = Absolute geostrophic Contours = Absolute geostrophic pressure (Lavender et al. (2000) pressure (Lavender et al. (2000)

Ice

Surface eddy speed Surface eddy speed

After Lilly et al. (2003) After Lilly et al. (2003)

Color = surface eddy Color = surface eddy speed (cm/s) speed (cm/s)


Surface eddy speed and regions of deep convection Surface eddy speed and regions of deep convection

Color = surface eddy Color = surface eddy speed (cm/s) speed (cm/s)


Mixed-layer depth in winter 1997 Mixed-layer depth in winter 1997

Color = mixed layer depth (m) Color = mixed layer depth (m)


WOCE AR7W lineInverse calculation

AR7W AR7W

Time period of high NAO in the early to mid 1990s

Float data

Mean circulation at 700m 1995Mean circulation at 700m 1995–19–1999 99

Lavendar et al. (2000) Lavendar et al. (2000)

AR7WAR7W

Referencing the geostrophic velocity section

Mean velocity at 700m from PALACE Mean velocity at 700m from PALACE



Mean temperature/density section Mean temperature/density section

X

Robert S. Pickart

--adjustment across the entire section is 0.3 cm/s--across eastern boundary current is 1.75 cm/s



Mean temperature/density section Mean temperature/density section

X

Total transport in: 35.4 SvTotal transport in: 35.4 SvTotal transport out: 35.5 SvTotal transport out: 35.5 Sv

Robert S. Pickart

--adjustment across the entire section is 0.3 cm/s--across eastern boundary current is 1.75 cm/s

Absolute geostrophic velocity

AR7W AR7W


Total Boundary Current transport = 28.5 Sv Total Boundary Current transport = 28.5 Sv

Recirculation transport = 2.5 Sv Recirculation transport = 2.5 Sv

Overflow water transport = 12.4 Sv Overflow water transport = 12.4 Sv Throughput


Total Boundary Current transport = 28.5 Sv Total Boundary Current transport = 28.5 Sv

Recirculation transport = 2.5 Sv Recirculation transport = 2.5 Sv

Overflow water transport = 12.4 Sv Overflow water transport = 12.4 Sv Throughput

Near zeroNear zerovelocity! velocity!

Depth space: Overturning and horizontal components of the flow

Decompose the full velocity: Decompose the full velocity:

Then decompose the deviation velocity: Then decompose the deviation velocity:

),(')(),( zxvzvzxv x

),(~)('),(' zxvxvzxv z

Overturning Overturning Deviation Deviation

Horizontal gyre

Horizontal gyre

Baroclinic gyre

Baroclinic gyre

Transport components

Overturning Cell Overturning Cell

Horizontal cell Horizontal cell

Transport components

95% of sinking happens 95% of sinking happens

in boundary current in boundary current

Density space: Overturning transport

Density space: Overturning transport

> 95% of the transport imbalance > 95% of the transport imbalance occurs in the boundary current occurs in the boundary current

What are the important points for OSNAP? What are the important points for OSNAP?

Seaward of the 700m isobath there is mass balance across Seaward of the 700m isobath there is mass balance across the section (i.e. excluding the shallow shelf-edge flows). the section (i.e. excluding the shallow shelf-edge flows).

The horizontal cell is significantly larger than the overturning cell. The horizontal cell is significantly larger than the overturning cell.

Nearly all of the overturning, in depth space and density space,Nearly all of the overturning, in depth space and density space,occurs along the rim of the basin. Flows in the middle of theoccurs along the rim of the basin. Flows in the middle of theBasin are weak.Basin are weak.

There is substantial mesoscale variability, so low-passing will be There is substantial mesoscale variability, so low-passing will be necessary. necessary.

Ongoing Timeseries in the Labrador Sea Ongoing Timeseries in the Labrador Sea

OSNAP-east moorings

Considerations for OSNAP eastern boundary array Considerations for OSNAP eastern boundary array


1997 hydrographicSections

Two eastern boundary Two eastern boundary crossings in Feb 1997crossings in Feb 1997

Salinity (color) overlain Salinity (color) overlain by density (contours) by density (contours)

North North

South South

16 Feb 16 Feb

26 Feb 26 Feb


2001 hydrographic Section

Eastern boundary crossing Eastern boundary crossing in Aug 2001 in Aug 2001

Salinity (color) overlain bySalinity (color) overlain bydensity (contours) density (contours)

Absolute geostrophic velocity (color) Absolute geostrophic velocity (color) overlain by density (contours) overlain by density (contours)

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 Strawman OSNAP-West Boundary Arrays

Remaining issues: shelf, near-surface, recirculation Remaining issues: shelf, near-surface, recirculation

Positive aspects of eastern array location Positive aspects of eastern array location

Bottom topography is gentle enough to effectively resolve Bottom topography is gentle enough to effectively resolve the boundary current. the boundary current.

Array will coincide with an altimeter line (providing additional Array will coincide with an altimeter line (providing additional surface velocity information). surface velocity information).

By moving south, the array is a true input boundary condition forBy moving south, the array is a true input boundary condition forthe Labrador Sea (to compare to the western export array). the Labrador Sea (to compare to the western export array).

The offshore recirculation is geographically confined, which The offshore recirculation is geographically confined, which means it can be effectively sampled with floats. means it can be effectively sampled with floats.

Negative aspects of eastern array location Negative aspects of eastern array location

It is not the AR7W line! (which provides a wonderful context). It is not the AR7W line! (which provides a wonderful context).

Logistically more difficult to service the array.Logistically more difficult to service the array.

Closer to OSNAP Cape Farewell array (less contrast). Closer to OSNAP Cape Farewell array (less contrast).

Measuring the eastern boundary inflow to the Labrador Sea Outline 1. Insights on the overturning...

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