Investigating the hydrographic seasonality of Denmark Strait Overflow Water Jacob Opher Alex Brearley Stephen Dye Ian Renfrew Robert Pickart Michael Meredith Introduce myself, where I work and who I work with Say title in full ‘Before I dive into the research, I’ll take you through me talk outline’

Outline Background Angmagssalik mooring data Characterising DSOW seasonality Origin of freshening events: Concurrent mooring observations in 2011-12 Discussion, conclusions

Formation of North Atlantic Deep Water Dickson and Brown (1994)

North Atlantic Deep Water at 63.5N Hall et al. (2011) DSOW is fresh and dense, with high transient tracer concentrations (Tanhua et al. 2005) Plume salinity is relatively homogenous (Jochumsen et al. 2015) ISOW and LSW are overlying water masses

The data Angmagssalik array Mooring occupations from 1987-2015

The data Angmagssalik array Mooring occupations from 1987-2015 Rotor Current Meter observations + MicroCATs since 1998 16 years of MC data from multiple moorings across DSOW plume

The data

Characterising DSOW seasonality

Annual variability calculation Use UK1 salinity (+G1 in 2001-2,UK2 in 2006-09) to quantify annual variability: Calculate anomaly by removing the deployment average Calculate average anomaly per calendar month Compute average, standard error and range for all jans (febs,…etc)

Characterising DSOW seasonality Freshening phase

Characterising DSOW seasonality

Temporal and spatial variability

Phase lag of fresh events **should say F2, not F1

Origin of freshening events source water masses entrainment/entrained water masses

Source water - EGC Supplies 2/3 (~2.5 Sv) of the overflow water EGC advects dense Atlantic origin water, alongside polar origin water Supplies 2/3 (~2.5 Sv) of the overflow water Greenland Atlantic water, which derives from the warm and salty North Atlantic Current, flows cyclonically around the Nordic Seas, becoming denser and sinking due to buoyancy loss*click*. It flows alongside fresher water masses advected from the Arctic Ocean by the East Greenland Current on the western side of the basin. The East Greenland Current is estimated to supply 2/3 of the Denmark Strait Overflow…but attention has been drawn to another source recently Norway EGC* - East Greenland Current

Source water – NIJ Dense water mass formed through deep convection Advected by the NIJ*, which accounts for ~1 Sv of the overflow Greenland That is the Iceland Sea. The process of dense water formation is different in this case; deep convection is thought to be important and an research project to uncover the cause and location of deep convection is already underway… This source water is advected by the North Icelandic Jet along the Iceland continental slope to feed the overflow … Variability in the source water properties and transport can affect the salinity of the overflow. And we can analyse this variability with moorings deployed in the pathway of the overflow Norway NIJ* - North Icelandic Jet

Source water Kögur mooring array Kögur mooring data captures variability of sources Boundary between the two sources located over Iceland slope Harden et al. 2016 – figure 2

Distinguishing sources Origin of freshening Distinguishing sources Source water boundary identified using algorithm of Harden et al. 2016

Distinguishing sources Origin of freshening Distinguishing sources Source water boundary identified using algorithm of Harden et al. 2016 Northward flowing grid cells removed

Distinguishing sources Origin of freshening Distinguishing sources Source water boundary identified using algorithm of Harden et al. 2016 Northward flowing grid cells removed Horizontal averaging within boundaries

Distinguishing sources Origin of freshening Distinguishing sources Freshening events in November and February/March Both events are associated with heaving of 27.8 isopycnal

Origin of freshening 2011-12 case study Harden et al. 2016

Denmark Strait sill salinity Origin of freshening Denmark Strait sill salinity 2011-12

Lagged correlations NIJ salinity can be tracked to the trough at Denmark Strait sill Advection time between 20 and 100 days

Origin of freshening Downstream salinity 2011-12

Origin of freshening Tracking salinity variability Positive correlations Negative correlations

Origin of freshening Tracking fresh events Two EGC fresh events detected 50-70d later downstream The upstream and downstream events have similar durations, with intensity dampened downstream

Discussion Downwelling favourable winds associated with low density (fresh) anomaly on Greenland slope -----------> N/NE winds are strong between oct-march Region of forcing is uncertain Is there a link between NAO and DSOW salinity? Håvik and Våge (2018)

Future work Evaluate wind forcing mechanism and determine location of forcing important to setting DSOW properties Examine fresh event pathways using DS sill CTD section data Investigate seasonality of entrainment/entrained water using CTD data around Iceland

Summary DSOW plume freshens in winter/spring Fresh events are weakened and delayed downslope Two freshening events are identified in the EGC north of DS in 2011-12 The freshening events are detected 700km downstream ~10w later Seasonal Ekman downwelling of coastal water proposed as mechanism

Thanks! Introduce myself, where I work and who I work with Say title in full ‘Before I dive into the research, I’ll take you through me talk outline’

References Dickson, R.R. and Brown, J., 1994. The production of North Atlantic Deep Water: sources, rates, and pathways. Journal of Geophysical Research: Oceans, 99(C6), pp.12319-12341. Hall, S., Dye, S.R., Heywood, K.J. and Wadley, M.R., 2011. Wind forcing of salinity anomalies in the Denmark Strait overflow. Ocean Science, 7(6), pp.821-834 Håvik, L. and Våge, K., 2018. Wind‐Driven Coastal Upwelling and Downwelling in the Shelfbreak East Greenlad Current. Journal of Geophysical Research: Oceans, 123(9), pp.6106-6115. Jochumsen, K., Köllner, M., Quadfasel, D., Dye, S., Rudels, B. and Valdimarsson, H., 2015. On the origin and propagation of Denmark Strait overflow water anomalies in the Irminger Basin. Journal of Geophysical Research: Oceans, 120(3), pp.1841-1855 Tanhua, T., Olsson, K.A. and Jeansson, E., 2005. Formation of Denmark Strait overflow water and its hydro-chemical composition. Journal of Marine Systems, 57(3-4), pp.264-288.