Model LSW formation rate (2 yr averages) estimated from: (red) CFC-12 inventories, (black) mixed layer depth and (green) volume transport residual. Also shown are selected published estimates based on in-situ measurements (cyan, gray and blue) and on a model experiment (magenta). Smethie & Fine (2000) Rhein et al. (2002) Kieke et al. (2007) Böning et al. (2003) using T 45°N – T 65°N below   =27.68 kg m -3 using LS   =27.68 kg m -3 outcrop area and m.l.d. using CFC-12 inventories

Deep water formation, the strength of the subpolar gyre, and the AMOC in the subpolar NA Monika Rhein, IUP, Universität Bremen

Does the observed changes affect the MOC ? What is the meridional heat and volume transport in the subpolar North Atlantic ? What role plays the interior transport versus boundary current transport?

Moorings (red) and PIES (yellow), PIES B12-B15 deployed Aug. 2006, data received August 2008, continued at least to 2012 B12 B13 B14 B15

Meridional heat and volume transport across 47°N What signal is to be expected ?

- Positive MOC anomalies follow periods of intensified LSW formation -- Amplitude of decadal MOC variability: ~ 2 Sv at 45°N NAO+ 8 Sv 0 Effect of variable thermohaline forcing in isolation ORCA-HEAT+FW: climatological wind stress; interannual heat and fw fluxes Labrador Sea Water formation (Sv) MOC anomalies (C.I.= 0.5 Sv) Böning and Biastoch, 2008

ORCA cases: HEAT+FW REF WIND 3 Sv 0 -3 Superposition of wind-driven MOC anomalies Böning and Biastoch, 2008

Strength of MOC dependent on density of Overflows Latif et al., 2006 Observed density decrease during the last 4 decades (due to freshening)

black: based on monthly, red: based on 2-yr-filtered time series Correlation volume and heat transport (ORCA 0.5°) Böning and Biastoch, 2008

MOC strength at 43°N between 13 and 21 Sv Results strongly dependent on constraints used in inverse models Relation between MOC, deep water formation and subpolar gyre strength ? Time series of mass AND heat transport needed ! Expected annual variability : Sv ? in subpolar North Atlantic, mass and heat flow not correlated ? Measure annual mean volume and heat transport with uncertainty < +/- 10 %.

Planned Bremen array at 47°N, proposal submitted Nov 2008

Boundary current array,. Background: meridional velocity from LADCP (August 2008). Black lines: isopycnals Will be deployed in

Time series of the meridional mass and heat transport: Separate velocity field V in V rel : geostrophic velocity relative to reference level V fluc : geostrophic velocity fluctuations at the reference level V mean : mean velocity in the reference level Objective: measure directly the mean velocity at the reference level

Geostrophic velocity relative to reference level from travel time of C-PIES Reference level: depth of moored C-PIES The geostrophic fluctuations at the reference level: calculate from pressure fluctuations p‘ measured by C- PIES -- mean pressure cannot be used: unknown drift,... Geostrophic velocity relative to reference level from travel time of C-PIES Reference level: depth of moored C-PIES The geostrophic fluctuations at the reference level: calculate from pressure fluctuations p‘ measured by C- PIES -- mean pressure cannot be used: unknown drift,...

Mean velocity at the reference level directly measured by current meter attached to the C-PIES (located above the bottom boundary layer) Horizontal resolution must be sufficient to resolve mean velocity field Mean velocity at the reference level directly measured by current meter attached to the C-PIES (located above the bottom boundary layer) Horizontal resolution must be sufficient to resolve mean velocity field

Meridional velocity at 47°N from sc-ADCP data, 2005 N/O THALASSA C. Mertens

S. Hüttl, C. Böning Meridional velocity at 47°N annual mean /12° FLAME model

Rossby Radius of deformation : 20 km Observed horizontal scales of NAC eddies and meanders : 100 km Mean horizontal distance between the C-PIES: km 30 C-PIES are needed