model lsw formation rate (2 yr averages) estimated from: (red) cfc-12 inventories, (black) mixed...

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 T45°N – T65°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

www.ocean.uni-bremen.demrhein@physik.uni-bremen.de

•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°NWhat 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+ 8Sv

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 Strength of MOC dependent on density of Overflows

Latif et al., 2006

Observed density decreaseduring 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 : 2 - 3 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 2009 - 2010

Time series of the meridional mass and heat transport:

Separate velocity field V in

Vrel : geostrophic velocity relative to reference level

Vfluc : geostrophic velocity fluctuations at the reference level

Vmean : 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 1996-2004

1/12° 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: 70-90 km 30 C-PIES are needed