Water mass transformation in the Iceland Sea
Kjetil Våge
Kent Moore
Steingrímur Jónsson
Héðinn Valdimarsson
Irminger Sea,
R/V Knorr, October 2008
Water mass transformation in the Iceland Sea
- the Denmark Strait Overflow Water
Denmark Strait
Iceland Sea
 Largest overflow plume
 Source of densest water to
the lower limb of the AMOC
 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
Winter (DJFM) climatological mean total turbulent heat flux from ERA-Interim
from Moore et al. (2012)
Water mass transformation in the Iceland Sea
- circulation in the Iceland Sea
Surface circulation
 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
from Jónsson (2007)
Water mass transformation in the Iceland Sea
- historical hydrographic measurements in the Iceland Sea
Collection of historical hydrographic measurements (1980 - present)
Determination of mixedlayer 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
Potential density in the central Iceland Sea (time vs. depth)
σθ > 28.03 kg/m3
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
Water mass transformation in the Iceland Sea
- convective activity as recorded by Argo float winter 2008
Temporal evolution of potential vorticity along Argo float trajectory
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 6
Langanes repeat
hydrographic section
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
Depth of the 28.03 kg/m3 isopycnal at Langanes 6
Difference: ~60 m
→ 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
Difference between the periods
1980-1995 and 1996-2013
 Increased pressure
 Reduced northerly winds
 Anti-cyclonic circulation anomaly
1996-2013
Water mass transformation in the Iceland Sea
- change in wintertime atmospheric circulation
1980-1995
Difference between the periods
1980-1995 and 1996-2013
 Increased pressure
 Reduced northerly winds
 Anti-cyclonic circulation anomaly
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
1980-1989
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)
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
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
1D mixed-layer
model in the
Iceland Sea
from Moore et al. (2014)
Water mass transformation in the Iceland Sea
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
- 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
Model-data comparisons
suggest that the 1D
mixed-layer model is
reasonable
from Moore et al. (2014)
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
Water mass transformation in the Iceland Sea
- the Arctic domain
Arctic domain
 Local modification leads
to formation of Arctic
Intermediate Water
Polar inflow
 Contributes to
overflows east and west
of Iceland
Atlantic inflow
Surface salinity, from Swift and Aagaard (1981)
•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