Jacob Opher Alex Brearley Stephen Dye Ian Renfrew Robert Pickart Michael Meredith Outline Background Angmagssalik mooring data Characterising DSOW seasonality Origin of freshening events Concurrent mooring observations in 201112 ID: 776114
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Slide1
Investigating the hydrographic seasonality of Denmark Strait Overflow Water
Jacob Opher
Alex Brearley
Stephen Dye
Ian Renfrew
Robert
Pickart
Michael Meredith
Slide2Outline
Background
Angmagssalik mooring data
Characterising
DSOW seasonality
Origin of freshening events: Concurrent mooring observations in 2011-12
Discussion, conclusions
Slide3Formation of North Atlantic Deep Water
Dickson and Brown (1994)
Slide4North 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
Slide5The data
Angmagssalik array
Mooring occupations from 1987-2015
Slide6The data
Angmagssalik arrayMooring occupations from 1987-2015Rotor Current Meter observations + MicroCATs since 199816 years of MC data from multiple moorings across DSOW plume
Slide7The data
Slide8Characterising DSOW seasonality
Slide9Annual 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
)
Slide10Characterising DSOW seasonality
Freshening phase
Slide11Characterising DSOW seasonality
Slide12Temporal and spatial variability
Slide13Phase lag of fresh events
**should say F2, not F1
Slide14Origin of freshening events
source water masses
entrainment/entrained water masses
Slide15Source water - EGC
EGC
advects
dense Atlantic origin water, alongside polar origin water
Supplies 2/3 (~2.5
Sv
) of the overflow water
Greenland
Norway
EGC* - East Greenland Current
Slide16Source water – NIJ
Greenland
Norway
Dense water mass formed through deep convection
Advected by the NIJ
*
, which accounts for ~1
Sv
of the overflow
NIJ* - North Icelandic Jet
Slide17Source water
Kögur mooring array
Harden et al. 2016 – figure 2
Kögur
mooring data captures variability of sources
Boundary between the two sources located over Iceland slope
Slide18Origin of freshening
Distinguishing sources
Source water boundary identified using algorithm of Harden et al. 2016
Slide19Origin of freshening
Distinguishing sources
Source water boundary identified using algorithm of Harden et al. 2016
Northward flowing grid cells removed
Slide20Origin of freshening
Distinguishing sources
Source water boundary identified using algorithm of Harden et al. 2016
Northward flowing grid cells removedHorizontal averaging within boundaries
Slide21Origin of freshening
Distinguishing sources
Freshening events in November and February/March
Both events are associated with heaving of 27.8
isopycnal
Slide22Origin of freshening
2011-12 case study
Harden et al. 2016
Slide23Origin of freshening
Denmark Strait sill salinity
2011-12
Slide24Lagged correlations
NIJ salinity can be tracked to the trough at Denmark Strait sill
Advection time between 20 and 100 days
Slide25Origin of freshening
Downstream salinity
2011-12
Slide26Origin of freshening
Tracking salinity variability
Positive correlations
Negative correlations
Slide27Origin 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
Slide28Discussion
Downwelling favourable winds associated with low density (fresh) anomaly on Greenland slope ----------->N/NE winds are strong between oct-marchRegion of forcing is uncertainIs there a link between NAO and DSOW salinity?
Håvik
and
Våge
(2018)
Slide29Future 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
Slide30Summary
DSOW plume freshens in winter/springFresh events are weakened and delayed downslopeTwo freshening events are identified in the EGC north of DS in 2011-12The freshening events are detected 700km downstream ~10w laterSeasonal Ekman downwelling of coastal water proposed as mechanism
Slide31Thanks!
Slide32References
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.