Kjetil Våge, UiB Bob Pickart, WHOI Mike Spall, WHOI Kent Moore, UoT H éðinn Valdimarsson , MRI

Kjetil Våge, UiBBob Pickart, WHOIMike Spall, WHOIKent Moore, UoTHéðinn Valdimarsson, MRISteingrimur Jónsson, MRI/UNAKDan Torres, WHOISvetlana Erofeeva, OSUSvein Østerhus, UNI BCCRTor Eldevik, UiBJan Even Ø. Nilsen, NERSC

Revised circulation scheme north of the Denmark Strait

Blosseville coast, north of the Denmark StraitView from R/V Knorr, October 2008

Nordic seas exchange Crucial part of climate system Reasonably well quantified

Greenland-Scotland Ridge

Transformation of warm

inflow

into dense overflow waters

north of the ridge

from Hansen et al.(2010)

1 Sv = 106 m3/s

Revised circulation scheme north of the Denmark Strait- background and motivation

from www.whoi.edu

Revised circulation scheme north of the Denmark Strait- origin of Denmark Strait Overflow Water

Transformation within the boundary current loop (Mauritzen, 1996)

Revised circulation scheme north of the Denmark Strait- origin of Denmark Strait Overflow Water

Transformation within the boundary current loop (Mauritzen, 1996) Approach along the Iceland slope in the North Icelandic Jet (Jónsson and Valdimarsson, 2004)

North Icelandic Jet

Circulation north of the Denmark Strait

The North Icelandic Jet The East Greenland Current Revised circulation scheme

Revised circulation scheme north of the Denmark Strait- outline


The North Icelandic Jet overflow water masses and pathways formation of the North Icelandic Jet overturning circulation schemes

The East Greenland Current Revised circulation scheme


High-resolution surveys off northwest IcelandHydrographic and direct velocity measurements

R/V Knorr KN194 – October 2008 vessel-mounted ADCP

R/V Bjarni Sæmundsson BS010 – August 2009 lowered ADCP


Maximum density above sill depth (650 m)KN194 – October 2008

KN194 - October 2008

Dense water high on the Iceland slope Recirculation of dense EGC waters?


Dense water high on the Iceland slope Hydrography suggests that this does not originate from the EGC


Maximum density above sill depth (650 m)KN194 – October 2008


Dense water high on the Iceland slope Hydrography suggests that this does not originate from the EGC Consistent flow of overflow water toward the Denmark Strait the NIJ

Mean flow of overflow waterOverflow range: σθ > 27.8 and depth < 650 m



The North Icelandic Jet originates east of the

Kolbeinsey Ridge


Absolute geostrophic velocityKN194 – October 2008

Mean flow of overflow waterOverflow range: σθ > 27.8 and depth < 650 m

BS010 - August 2009


Absolute geostrophic velocity

The October 2008 and August 2009 surveys are fully consistent


Path of the North Icelandic Jet

The NIJ core was typically found above the 650 m

isobath – the same depth as the Denmark Strait sill.

A coincidence?


Transport of the North Icelandic JetThe North Icelandic Jet The East Greenland Current Revised circulation scheme

Transport of the North Icelandic JetTransport of overflow water upstream of the sill (σθ > 27.8 and depth < 650 m)

For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 Sv



For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 SvFor σθ > 28.03 kg/m3: T = 0.6 ± 0.1 Sv (densest component)




Sill


Transport of the North Icelandic JetTransport of overflow water in the NIJ compared to transport at the sill (σθ > 27.8 and depth < 650 m)

For σθ > 27.8 kg/m3: T = 1.5 ± 0.2 Sv

For σθ > 27.8 kg/m3: T = 2.9 ± 0.5 Sv





The NIJ accounts for about half of the total overflow and nearly all of the densest component


Formation of the North Icelandic JetIdealized model simulations

Key elements A sill An island Bouancy loss Cyclonic wind stress curl

Key features Warm inflow Interior convection

MITgcm, idealized configuration, 5 km horizontal resolution

Mean surface temperature and bottom topography


Formation of the North Icelandic JetModel circulation and hydrography

a) Mean surface temperature and bottom topographyb) Mean surface temperature and velocity at 650 mc) Vertical section of meridional velocityd) Vertical section of temperature

Model processes NIIC disintegrates Lateral exchange Interior convection Densified water returned Along-boundary sinking Feeds the NIJ

C

Water mass transformation in the central Iceland Sea and the NIIC/NIJ current system implicated in the deep limb of the AMOC


Transformation within boundary current loop (Mauritzen, 1996)


Overturning circulation schemes

Transformation within boundary current loop (Mauritzen, 1996) Transformation within interior loop (Våge et al., 2011) Roughly equal contribution from either source


Overturning circulation schemes

BathymetryThe Blosseville Basin


East Greenland Current (EGC)

North Icelandic Jet (NIJ)

Two currents advecting overflow water into the Denmark Strait

Two currents merging to form the DSOW plume?

Overturning circulation schemesThe North Icelandic Jet The East Greenland Current Revised circulation scheme

EGC NIJ???

Greenland Iceland

Mean absolute geostrophic velocity at Kögur computed from 4 realizations

Observed circulation at the Kögur transectAn unknown current in the interior Blosseville Basin


shelf break

EGC NIJ

Greenland Iceland

Mean absolute geostrophic velocity at Kögur computed from 4 realizations

separated

EGC

Observed circulation at the Kögur transectThe separated East Greenland Current


East Greenland Current (EGC)

North Icelandic Jet (NIJ)

Overturning circulation schemesRevised circulation scheme



The North Icelandic Jet The East Greenland Current

presence of separated EGC a permanent feature of the circulation hypotheses to explain the separated EGC

gyre scenario eddy scenario

Revised circulation scheme


Mean conditions at the Kögur transectThe North Icelandic Jet The East Greenland Current Revised circulation scheme

Gyre scenario

Eddy scenario

Synoptic realizations of absolute geostrophic velocityTwo scenarios for the formation of the separated East Greenland Current


Nearly ¼ of the EGC system FW transport takes place in the interior

Reference salinity = 34.8

TransportsFreshwater


Nearly ¼ of the EGC system FW transport takes place in the interior

The majority of the OW approaches the strait along the Iceland slope

Reference salinity = 34.8

TransportsOverflow water


Dynamic height of sea surface relative to 500 db

Iceland Sea Gyre

EGC

shelf break EGC

separated EGC

Historical circulationThe North Icelandic Jet The East Greenland Current Revised circulation scheme

Potential temperature Salinity

Vertically averaged between 50-100 m (potential temperature) and 10-30 m (salinity)

Historical hydrographyNear-surface layer


Potential temperature Salinity

Maximum value between 27.9 and 28.0 kg/m3

Historical hydrographyOverflow water layer


Annual mean sea level pressure and 10 m wind speed/vectors

Icelandic LowBlosseville Basin

ERA-Interim

barrier winds

Atmospheric forcingThe North Icelandic Jet The East Greenland Current Revised circulation scheme

anticyclonic wind stress curl

Annual mean wind stress curl and 10 m wind vectors

ERA-Interim



North American Regional Reanalysis




North American Regional Reanalysis

anticyclonic wind stress curl closed bathymetry contours the separated EGC is part of an anticyclonic gyre


Atmospheric forcingGyre scenario


MITgcm channel oriented along the east coast of Greenland southern outflow becomes northern inflow salinity restored to initial conditions in the northern end (32 at the surface on the shelf, 35 in the deep interior), temperature is constant 1 km horizontal resolution, 30 vertical levels forced by steady annual mean wind stress

Numerical simulationsThe North Icelandic Jet The East Greenland Current Revised circulation scheme

sharp gradient near the shelf break at high latitudes supporting a shelf break jet offshore diversion of freshwater near y = 500 km

Numerical simulationsMean sea surface salinity over final 2 years


southward flow near the Greenland shelf break → shelf break EGC anticyclonic circulation over the deep Iceland slope → a gyre?

Meridional velocity

Salinity

Anticyclonic ring

Numerical simulationsSynoptic section at y = 320 km on day 360


eddies and filaments dominate the Blosseville Basin freshwater diversion from the shelf break is highly time dependent

Numerical simulationsSea surface salinity on day 770


Numerical simulationsTemporal evolution of near-surface layer at y = 320 km


southward flow inshore of x = 50 km offshore salinity front near x = 120 km, coincident with southward flow the separated EGC arises from eddies that coalesce when encountering the Iceland slope gyre scenario not supported by the model

Numerical simulationsEddy scenario


Numerical simulationsEddy generating mechanism


mean winds are generally parallel to the coast onshore Ekman transport maintains the EGC’s baroclinicity frontal instabilities are inhibited

Difference in angle between the mean wind direction and the orientation of the shelf break

Not the case at the northern end of the Blosseville Basin

→ baroclinic instabilities

generate eddies

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


http://www.naclim.eu/

Kjetil Våge, UiB Bob Pickart, WHOI Mike Spall, WHOI Kent Moore, UoT H éðinn Valdimarsson , MRI

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