Labrador Sea Export  -- the DWBC at 53°N as a Fingerprint of the AMOC?

J. Fischer, J. Karstensen, M. Visbeck, R. Zantopp, R. Kopte

Annual Conference, BerlinOctober, 2014

The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299 NACLIM www.naclim.eu

Mid-depth circulation in the SPNA

Mean circulation at 1500 m depth derived from Argo deep displacements (ANDRO atlas, Ollitrault and Rannou., 2013)

OSNAP west

53°N array

Configuration of the 53°N array

▶ All 3 branches of NADW pass the array on their way south (LSW, NEADW, DSOW)

▶ Currently 17 years of ocean top-to-bottom observations

▶ New array (2014-2016) was supplemented with additional instrumentation due to its role in the international OSNAP program

▶ Length of the time series allows analysis on a number of time scales

• Intra-seasonal• Seasonal• Interannual• Multiannual

Mean alongshore flow at 53°N

Three flow regimes

▶ Shallow Labrador Current

▶ Deep Western Boundary Current with deep velocity core

▶ Offshore recirculation

[km] [km]

ShipboardMoorings

Intra-seasonal variability

Fischer et al., 2014

▶ Dominant variability is found in the week-to-month period range

▶ Caused by topographic waves trapped by steep topography all around the western basins

Va

rian

ce [

(cm

/s)2 ]

A joint NACLIM / RACE collaboration paper

Seasonal variability

Kopte, 2013 (MSc. Thesis)

Shallow LC:• Varying seasonal amplitudes up to 6 cm/s• Phase lock to winter monthsDWBC:• Seasonal amplitudes below 2 cm/s• No phase lockDWBC tail:• individual years of enhanced seasonal amplitudes• Phase in May/June

Indicating:▶ No ‘intervening’ frequency between

intra-seasonal variability and possible long-term fluctuations/trends

▶ Negligible summer-bias in LADCP mean section

Long-term warming

Update from Fischer et al., 2010

Potential temperature evolution at LSW level

K9, K8 – at 53°N

▶ Uniform warming trend in the entire Labrador Sea above 2000 m due to weakened or absent formation and deep convection of LSW until 2009

▶ Does the warming cause a change in the intensity of circulation and outflow of the DWBC at 53°N?

K1 – in the central LS?

DWBC evolution

▶ Summer-to-summer averages of the flow field

• White dots: Moored records• Green dots: Best estimate data to

terminate Deep Labrador Current properly

• Red: Mean position of isopycnals representing water mass boundaries

Indicating:▶ Gross structure of the flow field is

persistent throughout the observational period

▶ Strength of deep core appears to vary interannually

NADW transport time series

400 m

1850 m

2800 m

LSW

NEADW

DSOW

▶ Use isodepths as proxy for isopycnals for transport calculations

▶ During 2003-2007 only central mooring K9 in place: Transport estimate by multiparameter regression

▶ Long-term fluctuations evident in NADW transport

Difference

Which layers cause fluctuations?

2001-03, 2010-12 1998-99, 2007-09

Alongshore flow [cm/s]

▶ The intermediate LSW layer does not participate in the decadal variability

Transports [Sv]

Decadal fluctuations in the North Atlantic

W-Curl

▶ Coherent 9-year fluctuations in DWBC, OVIDE and OLEANDER time series

▶ Phases of shallow fluctuations (OLEANDER and OVIDE) correspond to phase of deep flow (DWBC) within an uncertainty of less than a year

▶ Atmospheric forcing exhibits similar decadal fluctuations in phase with the AMOC components

DWBC

OVIDE

OLEANDER

Outlook --- Fieldwork / Analysis

▶ 17 years of full ocean depth observations make the 53°N DWBC-records increasingly

attractive for joint observation/model studies (envisioned)

▶ Contribution to OSNAP: the 53°N array is an important component of the trans-basin

observational efforts

▶ Extend NACLIM data base

▶ Joint scientific evaluation of the data in the NACLIM consortium (see CT2 Flash Presentation)

▶ Aug. 2014: New array (2014-2016)

installed during Thalassa cruise with

extended instrumentation

▶ Summer 2016: Next turnaround of the

array (ship time already funded)

▶ Optimization of the array

C D

Summary

▶ 15 years (1997-2012) of ocean top-to-bottom observations of the boundary current system

at 53°N are investigated with regard to the variability of the alongshore flow and

transports on various time scales:

Intra-seasonal

▶ Energetic topographic waves

Seasonal

▶ No significant seasonal cycle in DWBC

Interannual - Decadal

▶ 9y-fluctuations in Deep Labrador Current

▶ Coherent with decadal fluctuations across Gulf Stream and shallow AMOC, and atmospheric forcing

▶ Decadal fluctuations of same magnitude as proposed centennial AMOC decline

The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299NACLIM www.naclim.eu