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