Hydrographic variability in the Norwegian Sea during 1995-2010
Heat content in the Norwegian Sea during 1995-2010
Kjell Arne Mork and Øystein SkagsethInstitute of Marine Research, Norway
The Norwegian Sea
Main surface currents in the Nordic SeasSST from NODC (4km), Kilpatrick et al., 2001
Nor
weg
ian
Sea
Hydrographic data
• ICES and Argo data• From April 15th – June 15th, 1995-2010. A period
with international coordinated cruises (WPGNAPES).
• Data control (range check, density stability, etc.)• Seasonal correction of temperature using World
Ocean Atlas 2009 (WOA09)• Gridding: statistical interpolation (objective
analysis) using background field (all obs.+WOA09)
Data coverage varies with year
1998
2001
All years: 1995-2010
Heat content
1) h = constant (500 m): Ocean Heat Content (OHC)
2) h = the depth of the density surface (st=27.9); h = h (x,y,t): Relative Heat Content (RHC)
sea surface
z = -h, T=Tref
T(z)
Integrate temperature, relative to a reference temperature (Tref), from the sea surface to a reference surface (z=-h),
2) will reduce the effect of eddies and internal waves (e.g. Bindoff and McDougall, 1994; Palmer and Haines, 2009)
Time-averaged depth of the density surface (st=27.9) and temperature at the density surface
Warmest water in the north because of downward mixing in the north (Rossby et al., 2009)
Temperature (Tref)Depth
Relative Heat Content (RHC) anomaly
Jm-2
1995-2002
2003-2010
Heat content anomaly (J m-2)
OHC :0 - 500 m
RHC :0 – h (st=27.9)
1995-2002 2003-2010
-1.5x109
0
1.5x109
1995-2010: a warming period
Temperature anomaly in the Svinøy-NV and Gimsøy-NV section. Data are 5 years moving averages
Increase in temperature and salinity due to SPG changes (Hatun et al., 2005; Holliday et al., 2008)
Divide into two areas: North and South
• North: Lofoten Basin
• South: Norwegian Basin
Time series of heat content anomaly
Lofoten Basin
Norwegian Basin
Solid lines: h = density surface (st= 27.9)Dashed lines: h = 500 m
Relative contribution from changes in temperature and depth
of the density surface
sea surface
z = -h
T(z)
Using h = density surface
-> contribution from changes in temperature and depth of st
Changes in depth may indicate changes in the ocean circulation (e.g. Palmer and Haines, 2009; Rossby et al., 2009).
Relative contribution to heat content from temperature and depth of st=27.9
North: Lofoten BasinSouth: Norwegian Basin
Mainly temperature contribution except for some years
2004 2004
Anomaly in the depth of st:
2004
Reflect changes in the ocean circulation?
Memory of the ocean: autocorrelation
0 1 2 3 4 5
-0.5
0
0.5
1
Time lag (year)
Au
toco
rre
latio
n
Heat content anomaly
NorthSouth
0 1 2 3 4 5
-0.5
0
0.5
1
Time lag (year)
Au
toco
rre
latio
n
Heat content anomaly - detrended
NorthSouth
Autocorrelation (one year lag) in heat content from detrended data.
Interannual changes (heat flux)
South (Norwegian Basin) North (Lofoten Basin)
RHC : Yearly changes in the heat content
Interannual changes (heat flux)
South (Norwegian Basin) North (Lofoten Basin)
RHC : Yearly changes in the heat contentQ – North: One year averaged air-sea heat flux over the Lofoten BasinQ – South: One year averaged air-sea heat flux over the Norwegian BasinAir-sea heat fluxes from NCEP reanalysis data
Air-sea heat flux Air-sea heat flux
Summary• Heat content in the Norwegian Sea increased
during 1995-2010 with maximum in 2004.• Changes in heat content depend mainly on
temperature changes• Significant autocorrelation (one year time lag)
in the Lofoten Basin• Changes in the heat content: linked to the air-
sea heat flux, but other processes (e.g. advection) are also important