Kerstin Jochumsen 1 , Detlef Quadfasel 1 , Manuela Köllner 1,2 , and Stephen Dye 3

1. On the modifications of the Denmark Strait overflow plume during its descent into the North Atlantic Kerstin Jochumsen1, Detlef Quadfasel1, Manuela Köllner1,2, and Stephen Dye3 1 University of Hamburg, ZMAW, Germany 2 now at GEOMAR, Kiel, Germany 3 CEFAS, Lowestoft, UK

2. Motivation Denmark Strait Overflow Water (DSOW) forms the densest part of North Atlantic Deep Water (~6 Sv after entrainment) Dickson et al., 2002 Fischer et al., 2010 van Akenand de Jong, 2012

3. Open questions Will Arctic temperature/salinity variability be seen in downstream DSOW measurements? Which water masses are responsible for the variability observed in the Deep Western Boundary Current? EG Spill Jet • Is the influence of the East Greenland Spill Jet detectable in the downstream mooring time series? from Vage et al. (2011), modified

4. Observations Mooring arrays DenmarkStraitsill (NORTH) – DSOW entrainmentregion (CENTRAL) – DSOW atAmmassalik (SOUTH)

5. Mooring designs Denmark Strait sill array (NORTH)NORTH σθ=27.8 σθ=27.85 σθ=27.8 Ammassalik array (SOUTH)

6. Array coherence Pot. temperature fluctuations are similar within the mooring arrays NORTH mean: -0.08°C ± 0.13°C r=0.8 CENTRAL pot. temperature [°C] r>0.9 mean: 1.10°C ± 0.21°C r>0.7 mean: 1.45°C ± 0.16°C SOUTH

7. Array coherence Salinity fluctuations are small at DS sill and increase downstream NORTH mean: 34.900 ± 0.004 r=0.6 CENTRAL r>0.8 salinity mean: 34.897 ± 0.008 mean: 34.895 ± 0.010 r>0.9 SOUTH

8. Array coherence Velocity fluctuations are dependent on the plume position NORTH r=0.4 r=-0.4 SOUTH

9. Signal propagation pot. temperature salinity velocity NORTH NORTH NORTH r=0.8, 2d lag r<0.3 CENTRAL CENTRAL r<0.4 16d lag r<0.3 16d lag r>0.7 10-14d lag r>0.6 8-12 d lag r>0.5 13-16d lag SOUTH SOUTH SOUTH Pot. temperature signals are advected (mean speed: 45 cm/s), salinity and velocity signals are strongly modified by entrainment

10. Mixing of Water Masses DeepAmmassalik (SOUTH) measurementscloseto DS 5-7(CENTRAL) → entrainmentoccursmainlybetweenDS silland DS 5-7 (180 km downstream)

11. Mixing of Water Masses Modified Atlantic Water (from CTD sections) East Greenland CurrentWater (e.g. Rudels et al., 2002) DSOW at Ammassalik DSOW atthesill

12. Conclusions • good correlation of temperature time series → temp. signals are advected from the sill • low/no significant correlation in salinity and velocity • → entrainment processes strongly modify salinity/velocity • East Greenland current water is needed to obtain low salinities • entrainment is dominant between DS sill and DS 5-7 EGC spill jet plays a minor role for DSOW south of 64°N intermittent spill events likely combine to long term effects • downstream DSOW properties do not necessarily reflect Nordic Seas conditions (especially in salinity)

13. Conclusions Will Arctic temperature/salinity variability be seen in downstream DSOW measurements? Probably not, only if the signals are very pronounced. Which water masses are responsible for the variability observed in the Deep Western Boundary Current? The original overflow crossing the GSR, but also the Atlantic Water in the Irminger Sea, as well as the EGC spill jet waters. • Is the influence of the East Greenland Spill Jet detectable in the downstream mooring time series? Yes, in low salinity signals.

14. Longtermperspective DSOW transports, updated from Jochumsen et al. (2012) and Dickson et al. (2007) no trends in transports, warming from 1998 - 2006, since then t is rather stable

15. This work was supported by the Co-Operative Project ‘‘RACE - Regional Atlantic Circulation and Global Change’’ funded by the German Federal Ministry for Education and Research (BMBF), 03f0651a RACE • http://race.zmaw.de 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 http://www.naclim.eu