Water mass transformation in the Iceland Sea

Irminger Sea, R/V Knorr, October 2008

Kjetil Våge Kent Moore Steingrímur JónssonHéðinn Valdimarsson

Water mass transformation in the Iceland Sea- the Denmark Strait Overflow Water

Denmark Strait Largest overflow plume Source of densest water to the lower limb of the AMOC

Iceland Sea Wintertime convection First definitive scenario for the source of DSOW (Swift et al., 1980)

Water mass transformation in the Iceland Sea - overturning circulation schemes

Formed in the

Iceland Sea

(Swift et al., 1980)

Transformation

within boundary

current loop

(Mauritzen, 1996)

Water mass transformation in the Iceland Sea - the North Icelandic Jet – another source of overflow water?

from Jónsson and Valdimarsson (2004)

Water mass transformation in the Iceland Sea - overturning circulation schemes

Formed in the

Iceland Sea

(Swift et al., 1980)

Transformation

within boundary

current loop

(Mauritzen, 1996)

Transformation

within interior loop

(Våge et al., 2011)

Water mass transformation in the Iceland Sea - climatological winter total turbulent heat flux

from Moore et al. (2012)

Winter (DJFM) climatological mean total turbulent heat flux from ERA-Interim

from Jónsson (2007)

Cyclonic circulation in the

central Iceland Sea

Typical wintertime mixed

layer depths about 150-200 m

Surface densities exceeding

27.8 kg\m3 common in winter

Surface circulation

Water mass transformation in the Iceland Sea - circulation in the Iceland Sea

Water mass transformation in the Iceland Sea - historical hydrographic measurements in the Iceland Sea

Collection of historical hydrographic measurements (1980 - present)

Determination of mixed-

layer depth and properties visual inspection of all profiles automated detection routines

employed manually determined when

those failed

→ robust data set

Water mass transformation in the Iceland Sea - February-April mixed-layer depths

Map of mixed-layer depths

Water mass transformation in the Iceland Sea - February-April mixed-layer depths

Map of mixed-layer depths

Contours of dynamic height

Water mass transformation in the Iceland Sea - February-April mixed-layer densities

Map of mixed-layer potential densities

Water mass transformation in the Iceland Sea - convection in the north-central Iceland Sea

Profiles located within the north-central Iceland Sea

Water mass transformation in the Iceland Sea - the annual cycle

Mixed-layer depths

Water mass transformation in the Iceland Sea - the annual cycle

Mixed-layer depths Mixed-layer potential densities

Water mass transformation in the Iceland Sea - convective activity in the north-central Iceland Sea

Potential density in the central Iceland Sea (time vs. depth)

Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet

σθ > 28.03 kg/m3

Potential density in the central Iceland Sea (time vs. depth)

Transport of σθ > 28.03 kg/m3 in the NIJ: 0.6 ± 0.1 Sv

Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet

Mixed-layer depths Mixed-layer potential densities

Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet

Mixed layers denser than σθ = 28.03 kg/m3

5 profiles from 2013

Important caveats sparse data set huge spatial and temporal variability

Temporal evolution of potential vorticity along Argo float trajectory

Water mass transformation in the Iceland Sea - convective activity as recorded by Argo float winter 2008

Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet

Profiles at the outer end of the Langanes section

Langanes repeat hydrographic section

Langanes 6

Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet

Depth of the 28.03 kg/m3 isopycnal at Langanes 6

Water mass transformation in the Iceland Sea - the densest component of the North Icelandic Jet

Difference: ~60 m

Depth of the 28.03 kg/m3 isopycnal at Langanes 6

→ Reduced production of dense water?

→ Different circulation regime?

Water mass transformation in the Iceland Sea - atmospheric forcing

Decrease in the total turbulent heat flux, discontinuity around 1995

Water mass transformation in the Iceland Sea - atmospheric forcing

Decrease in the total turbulent heat flux, discontinuity around 1995

Decrease in the wind stress curl, discontinuity around 1995

Water mass transformation in the Iceland Sea - change in wintertime atmospheric circulation

1980-1995

1996-2013

Difference between the periods

1980-1995 and 1996-2013 Increased pressure Reduced northerly winds Anti-cyclonic circulation anomaly

Water mass transformation in the Iceland Sea - change in wintertime atmospheric circulation

Difference between the periods

1980-1995 and 1996-2013 Increased pressure Reduced northerly winds Anti-cyclonic circulation anomaly

1980-1995

1996-2013 Difference between the periods

Water mass transformation in the Iceland Sea - frequency of high heat flux events

Frequency of high heat flux events Decreasing occurrence of heat flux

events exceeding the 90th percentile value

Consistent with a weakening of the northerly winds

Water mass transformation in the Iceland Sea - composite means of high heat flux events

Nature of high heat flux events Retreat of sea ice Northward shift of the highest

fluxes Narrowing of marginal ice zone Reduced number of events

(75 during first period, 65 during last)

1980-1989

2004-2013

Water mass transformation in the Iceland Sea - ramifications of reduced forcing

November profiles from the Iceland Sea – initial conditions

from Moore et al. (2014)

Water mass transformation in the Iceland Sea - ramifications of reduced forcing

1D mixed-layer model in the Iceland Sea

from Moore et al. (2014)

Ramifications of reduced forcing Gradual reduction in depth and density of

convection If this continues, it may weaken the

overturning loop that feeds the NIJ and reduce the supply of the densest water to the AMOC

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

Water mass transformation in the Iceland Sea

Water mass transformation in the Iceland Sea - NAO and ILD indices

North Atlantic Oscillation (NAO) index

Icelandic Lofoten Dipole (ILD) index

Water mass transformation in the Iceland Sea - ramifications of reduced forcing

from Moore et al. (2014)

Model-data comparisons suggest that the 1D mixed-layer model is reasonable

Water mass transformation in the Iceland Sea - Summertime stratification

Difference in potential density between 10 and 250 m

Water mass transformation in the Iceland Sea - June-August mixed-layer densities

Map of mixed-layer potential densities

Polar inflow

Arctic domain

Surface salinity, from Swift and Aagaard (1981)

Local modification

leads to formation of

Arctic Intermediate Water

Contributes to

overflows east and west

of Iceland

Atlantic inflow

Water mass transformation in the Iceland Sea - the Arctic domain

•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