THOR CT3 Meeting – Torshavn 2009 – Fischer/Visbeck/Zantopp/Nunes

In the Labrador Sea, overflow water from the Denmark Strait and from the Iceland-Scotland region, which has been modified and augmented by entrainment, joins with newly formed deep water masses, produced by deep convection in the Labrador and Irminger Seas. This water is exported from the Labrador Sea mainly by the boundary current and subsequently out of the subpolar region. As all deep water mass components are included, the total deep water export and variability is one possible indicator of THC-strength and variability. This will be monitored by a moored array across the boundary current. In combination with the current measurements at key depths, both the salinity and temperature field will be measured to derive a transport index for all water masses in the Deep Water range.

In THOR, the results will be combined with high resolution modeling and assimilation efforts to both interpret the observations and for adjusting the observational design if necessary. Moored records will besupplemented by repeat hydrography and velocity sections along the former WOCE lines:AR7W and AR7E.

Task 3.1.3 aims at interpreting decadal time series of currents and water mass variability to: • Determine inter-annual to decadal boundary current variability• Develop and continue an index of Deep Water Export (THC-Index) in combinationwith other available data for validation of model and assimilation products (Core Theme 2).• Evaluate the penetration of newly formed water masses into the boundary current incombination with other observations (e.g., those derived from the ARGO program)and THOR model results.

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

THOR CT3 Meeting – Torshavn 2009 – Fischer/Visbeck/Zantopp/Nunes

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

Herein the focus is on the interpretation of the data and the generation of products for the THOR modelling and assimilation community --

Plans for the first 18 months:

• Moored Array is designed for next deployment period: May 2009 to Summer 2010 -------moorings recovered and deployed-------• • Update of existing hydrographic (including Argo) and mooring data for estimating interannual to decadal variability of DWBC export from Labrador Sea (underway)• index of deep water export in the DWBC (THC-index) will be developed and continued• make data and products available to other WP‘s (Modeling and observational groups)

Fieldprogram:1)Joint (Quadfasel/Fischer) cruise with RV Merian in May/June 2009 finished2)Proposed (Quadfasel/Fischer) cruise with RV Meteor in summer 2010 -- approved

D06: Reference data set for pre-existing data on properties and fluxes through key sections: Greenland-Scotland Ridge, Deep Western Boundary Current and across 26.5° N. (Month 12)

LS Boundary Current Data1.Two years with full array2.10 years reduced coverage

Full Array

Reduced Array

Structure by EOF‘s Expl. Variances

Det. of uncertainties

regress to EOF‘s

Integrate 12 y BC

Transport + Errortime series

Science THOR

Current meter array at 53°N in 1997-1999

and actual array 2009-2010

Complete array from June 1997- July 1997 only

1999-2001: 2 moorings

2001-2003: 3 moorings

2003-2005: 1 mooring

2005-2007: 1 mooring

2007-2009: 3 moorings

2009-2011: 5 moorings planned (funded elsewhere)

53°N

Present -- DSOW part will be strengthened

THOR CT3 Meeting – Torshavn 2009 – Fischer/Visbeck/Zantopp/Nunes

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

• temperature at 1500m increases by about 0.05° per year between 1997 and 2007

•Are there any trends for the other water masses? Igor Y. works

on DSOW variability --- collaboration envisioned

53°N

Average 1997-1999 2000-2005K2: 0.14m/s 0.17m/sk9: 0.12m/s 0.15m/s

THOR CT3 Meeting – Torshavn 2009 – Fischer/Visbeck/Zantopp/Nunes

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

THOR CT3 Meeting – Torshavn 2009 – Fischer/Visbeck/Nunes

• Overview of available current data at single station K9; rotated to 310° for mean outflow direction

• annual mean profiles and decadal mean

•Variations O 20% of decadal mean

K7 K8 K9 K10 K16

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

THOR CT3 Meeting – Torshavn 2009 – Fischer/Visbeck/Nunes

Short term variability

Intraseasonal Variability:Max near surfacePeak 10-20 day periodsMin at DSOW Level, but mean is highestNo sytematic variations of energy level

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

THOR CT3 Meeting – Torshavn 2009 – Fischer/Visbeck/Nunes

EOF‘s 1997-1999•Mode 0 – mean flow field•Mode 1 – LC-mode with annual cycle•Mode 2 – meander mode •Mode 3 – BC pulsation

Modes for annual subsets very similar!

Modes 1-3 explain 80% of the variance

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

Spectra of EOF time series (above) – max high frequencyenergy in meander mode (2) at 10 to 30d periodsExplained variance plot (rhs) – K10 and K9 are most important for variance

Mean

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

THOR CT3 Meeting – Torshavn 2009 – Fischer/Visbeck/Zantopp/Nunes

Reference-Dataset (40 hlp data)southward Transports onlytime period 1997 – 1999from 500m to the bottom 12h resolutionLow-passed at 10d includes meandersLow-passed at 60d no meanders

Green line is for two year meantransport (32.4Sv) below sth=27.68

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

Special Focus on DSOW core --- LADCP and Moorings

Task 3.1.3 Monitoring the export of deep waters at the exit of the Labrador Sea

Time series of DSOW at various places in the SPNA (I. Yashayaev, pers. com.)

CTD measurements aided by moored instruments(microcats)