Observed variability of hydrography and transport at 53°N in the Labrador Sea Johannes Karstensen GEOMAR Helmholtz Centre for Ocean Research Kiel With input from: Jürgen Fischer, Rainer Zantopp, Robert Kopte, Sebastian Milinski, Sunke Schmidtko CT 2: Monitoring of North Atlantic Parameters

The Atlantic meridional overturning circulation consists of a poleward net transport of warm water at/near the surface and a southward net flow of cold deep water (Deep Western Boundary Current) The conversion from upper to lower as well as the strength, characteristic, and pathways of deep flow are key component of the Earth’ s climate system and therefore must be fully understood

Southward Deep Water return flow Water in “Deep Western Boundary current” (DWBC) is composed of  Denmark Strait Overflow Water  Modified Iceland/Scotland Overflow Water – Northeast Atlantic Deep Water  Labrador Sea water (eventually re- ventilated in Irminger Sea) Interaction of the dense and surface water in the Overflow regions

Southward Deep Water return flow Water in “Deep Western Boundary current” (DWBC) is composed of  Denmark Strait Overflow Water  Modified Iceland/Scotland Overflow Water – Northeast Atlantic Deep Water  Labrador Sea water (eventually re- ventilated in Irminger Sea) Interaction of the dense and surface water in the Overflow regions

Observing the Deep Water flow at key locations: 53°N array Observations of DWBC transport and characteristic at the southern exit of the Labrador Sea Up to 7 moorings with current meters and T/S sensors 53°N

53°N array Operational since 1997 Different number of moorings and and sensor coverage Optimized for DWBC since 2009 Most continues time series K9, some years only 1 mooring Ship occupations provide full depth picture but only at selected time 53°N K9

53°N: 15 yrs. average (ship sections) SalinityTemperature

53°N: 15 yrs. average (ship sections) SalinityTemperature DSOW NEADW cLSW

53°N: 15 yrs. average (ship sections) 2005 to 2012 MINUS 1996 to 2003  uLSW/cLSW warming & salinification  Separation: Density 27.8 kg/m 3  Efficient cooling of interior water through convection:~1400m Maximum DSOW NEADW cLSW Diff Temperature Diff Salinity

Ship versus high resolution moored observations Ship observations DSOW temperature variability from 1996 to 2012 (data below 3200m) Year

Ship versus high resolution moored observations Trend ? Ship observations DSOW temperature variability from 1996 to 2012 (data below 3200m) Year

53°N: Ship versus high resolution moored observations DSOW - Moored instruments Trend – No But: More pattern of multiannual/decadal variability Year

53°N current structure Average current from 12 ship occupations

53°N current structure Average current from 12 ship occupations Labrador Current (LC – LSW & NEADW) Deep Western Boundary Current (DWBC -DSOW) Recirculation LC DWBC Recir- culatio n NEDAW

Transport 2 periods with good instrumental coverage

Transport 2 periods with good instrumental coverage Strong transport variability in water mass classes, but:

Transport 2 periods with good instrumental coverage Strong transport variability in water mass classes, but: Change in transport or/and change in water mass characteristic? What is it we are really interested in? Transport in a layer (e.g. 200m above sea floor)? Transport in a density class (that may change due to changes in hydrography)? …

General question: What do we want to compare? “Pattern match” hydrography? (implications for heat/freshwater fluxes) Spectra of variability Warming/cooling/freshening/… trends? Integrated transport? In density classes? Depth ranges?

Example: Energy of Variability Comparison of DWBC spectra Most energy is at about 10days – Topographic Waves Is this important?

CMIP5 models and observations Experiment 3.2: March 2005 No DWBC Model is warmer than observations 6°C 1°C T_observation T_model

CMIP5 models and observations March 1968 (coldest winter in Model) Widespread Deep convection in central gyre! 6°C 1°C T_observation T_model

Summary Moored instrumentation provide data that important for long term monitoring as well as for process studies Deep Western Boundary Current variability is most intense in the core of the deep flow - with periods in the range of weeks rather than months - It is unclear if the variability has any consequences for the correct representation of the interior ocean in models - or if it is just wave like motion… First “pattern match” analysis of a CMIP5 model with 53°N array is encouraging – but model miss many “details” (model is too warm, no DWBC, …) Further discussion on suitable indices for “observation/model comparison” is required

Embedded in national/international programs OSNAP VITALS RAPID BMBF, RACE Line W ICES

The research leading to these results has received funding from the European Union 7th Framework Programme (FP ), under grant agreement n NACLIM