Hydrography in the Greenland Sea Gyre (2010)

K.Latarius and D.Quadfasel (1)

Corresponding author : Katrin Latarius

(1) : University of Hamburg, Germany

Full paper: Latarius. K. and Quadfasel, D. 2010, Seasonal to interannual variability of temperature and salinity in the Greenland Sea Gyre : heat and freshwater budgets, Tellus, 62A, 497-515, doi:10.1111/j.1600-0870.2010.00453.x.

Among the four cyclonical gyres associated to the four sub-basins of the Nordic Seas (the Greenland Sea, Lofoten and Norwegian basins and the Iceland Plateau), the Greenland Sea Gyre is known for open ocean convection triggered by extreme heat loss to the atmosphere in winter. In this study (Latarius et al.,2010), after the analysis of the Greenland Sea hydrographic variability at the seasonal and interannual time scales, heat and freshwater budgets are estimated to quantify the relative importance of the Greenland Sea Gyre for the overall transformation of the Atlantic Water into Overflow Water. Results swhow that the net effect of the gyre on the water mass conversion is small and the gyre does not re-enforce the Nordic Seas overturning circulation.

Data and Method

Data from 13 Argo floats deployed in the Greenland Sea Gyre from 2001 to 2007 are used to describe the typical seasonal cycle and associated interannual variability of temperature and salinity for the last decade. All floats were operated with a 10-day working cycle, 1000 dbar parking depth and 2000 dbar profiling depth. 500 profiles in 6 years, giving a mean of 10 profiles per month, are analysed.

Figure 1. Argo-float profiles deployed in the Greenland Sea Gyre: orange, Argo-Denmark 2001–2003; red, Argo-Germany 2004–2007. Arrows illustrate the Atlantic water edge current circulation and the bifurcating branches linked to topography (after Blindheim and Østerhus, 2005). The four sub basins of the Nordic Seas are the Greenland Sea Basin (GS), the Lofoten Basin (LB), the Norwegian Basin (NB) and the Iceland Plateau (IP).

Time series of temperature, salinity and sigma are obtained for the Greenland Sea for the period 2001-2007, on the basis of monthly mean profiles of float data (figure 2). Then, the heat and freshwater budgets of the Greenland Sea Gyre are quantified on the seasonal time scale to be analysed with respect to the atmospheric forcing and lateral exchange.

Surface heat/freshwater flux data are taken from seven different meteorological models and satellite observations : NCEP/NCAR (Kalney et al., 1996) ; ECMWF-ERA (Molteni et al., 1996), REMO (Jacob and Podzun, 1997), NOC (Berry and Kent, 2009), OAflux (Yu and Weller, 2007), J-OFURO (Tomita et al.,2010), HOAPS (Jost et al., 2002). The development of the heat and freshwater content in the ocean is derived from Argo-float profile data, and the difference between observed and estimated heat content gives the order of lateral exchange and vertical convective mixing between gyre and boundary.

Main Results

A general trend appears clearly on the hydrographic pattern of temperature values throughout the upper 2000m. On longer time scales, a mean warming trend of 0.05°C/year is seen over the top 1500m of the water column during the observation period. There is no similar trend in the heat fluxes between ocean and atmosphere, therefore heat must be transported into the region by lateral exchange with the boundary current.

In contrast, the salinity time series is dominated by inter-annual changes not showing a clear trend. Inter-annual changes are most pronounced in the depth range from 50 to 400m with relatively low values in 2001 and 2005 and high values during 2002-2003 and from 2006 onward.

A strong seasonal signal is visible both in temperature and salinity in the upper 400m. The seasonal cycle of temperature is in phase with the atmospheric forcing, with a warming phase during summer months (from April to September) and a cooling phase during winter (from October to March).

Figure 2. Time series of temperature (a), salinity (b) and sigma (density minus 1000) (c), on the basis of monthly mean profiles from the float data. (d) Number of independent measurements per month, which are available to calculate the monthly mean values. Only float data within the Greenland Sea Gyre are used (area specified in Fig. 1). Dots in (c) mark the maximum winter convection depth, as derived with a delta = 0.005 kg m−3 criterion for density. The upper 500 m of the water column are stretched; the colour scales are non-linear to include low temperatures, salinities and densities.

Large differences appear in the heat and freshwater fluxes at the air-sea interface derived from atmospheric models and satellite observations. Mean heat fluxes from the six data sets vary by more than 50%, the freshwater fluxes are even ambiguous in there direction. However, using fluxes averaged over all atmospheric data sets give stable results, and budget calculations show that the gyre imports freshwater from the atmosphere and in the upper 50m during winter, and exports it laterally across its boundary throughout the water column. Heat is imported laterally between 50-1500m during the whole year, and from the atmosphere during summer, and exported to the atmosphere and in the upper 50m during winter.

Figure 3. The integral seasonal (summer and winter) heat and freshwater balances between horizontal input, exchange between atmosphere and ocean, vertical fluxes and development within the Greenland Sea Gyre for three merged layers (surface layer 0–50 m, Atlantic layer 50–500 m, deeper layer 500–1500 m) from 2001 to 2007. (a) Summer balances, when vertical mixing is neglected and exchange of heat and freshwater between atmosphere and ocean is therefore acting only on the upper 50 m of the water column (May to October); (b) winter balances, when vertical mixing during convection is transporting heat upward and freshwater downward (November to April). (c) Balances of heat and freshwater fluxes for the whole year. Red numbers mark heat or freshwater gain, blue numbers heat or freshwater loss in a layer. Values altered by the different models are given as mean plus standard deviation from all models .

Consequently, the surrounding of the gyre between 50-1500m looses heat and salt by exchange with the Greenland Sea Gyre. But the net effect of the Greenland Sea Gyre to the water mass conversion in the Nordic Seas is small. The Atlantic Water entering the Nordic Seas has a temperature of 12°C with a salinity of about 35.25. Looking at figure 4, which is a zonal temperature and salinity section across the Greenland Sea (dashed line mentionned figure 1), it appears that, by the time the water, carried in the Arctic Circumpolar Boundary Current, has reached the western Greenland Sea, temperatures have dropped to values between 0 and 0.5°C, while salinities are in the range between 34.9 and 34.95. When leaving the Nordic Seas via the overflows, temperatures are down to -0.5°C and salinities are 34.89. The heat and freshwater exchange between Greenland Sea Gyre and the boundary current leads to a drop in temperature of about 0.4K (about 3% of total temperature transformation) and to a freshening corresponding to a salinity reduction of about 0.004 (about 1% of total salinity transformation). The Greenland Sea Gyre thus affects the edge currents along its boundaries only after most of the water mass transformation has already taken place on its loop around the Artic Mediterranean.

Figure 4. Potential temperature (left) and salinity (right) section along 75° N across the East Greenland Current, Greenland Sea and West Spitsbergen Current (from left to right) in summer 2003 (reproduced from Budéus and Ronski, 2009). The location of the section is shown in figure 1 by the dashed line; red arrows mark the western and eastern edges of the Greenland Sea Gyre at this latitude, as defined in figure 1 by the yellow line.

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