Observed variability of hydrography and transport at 53°N in the Labrador Sea

1. CT 2: Monitoring of North Atlantic Parameters 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, SunkeSchmidtko

2. 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

3. 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

4. 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

5. 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

6. 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 K9 53°N

7. 53°N: 15 yrs. average(ship sections) • Mean hydrography suggesting 4 layers: • uLSW: Upper Labrador Sea Water • cLSW: Classic/lower Labrador Sea Water • NEADW: Northeast Atlantic Deep Water • DSOW: Denmark Strait Overflow Water Salinity Temperature

8. uLSW 53°N: 15 yrs. average(ship sections) cLSW NEADW • Mean hydrography suggesting 4 layers: • uLSW: Upper Labrador Sea Water • cLSW: Classic/lower Labrador Sea Water • NEADW: Northeast Atlantic Deep Water • DSOW: Denmark Strait Overflow Water DSOW Salinity Temperature

9. uLSW 53°N: 15 yrs. average(ship sections) cLSW NEADW • 2005 to 2012 MINUS 1996 to 2003 • uLSW/cLSW warming & salinification • Separation: Density 27.8 kg/m3 • Efficient cooling of interior water through convection:~1400m Maximum DSOW 0.03 0.3 0.4 0.04 Diff Salinity Diff Temperature

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

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

12. 53°N:Ship versus high resolution moored observations • DSOW - Moored instruments Trend – No But: More pattern of multiannual/decadal variability Year

13. 53°N current structure • Average current from 12 ship occupations

14. 53°N current structure • Average current from 12 ship occupations • Labrador Current (LC – LSW & NEADW) • Deep Western Boundary Current (DWBC -DSOW) • Recirculation LC Recir-culation NEDAW DWBC

15. Transport • 2 periods with good instrumental coverage

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

17. 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)? • …

18. 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?

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

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

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

22. 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

23. 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