Volume Transports

The geostrophic transports are shown in Fig. 13.2, central panel. The total in- and outflows range from 5 Sv to almost 15 Sv with an average inflow of ~6 Sv and an outflow close to 9 Sv. This is smaller than the transports obtained from the direct current measurements, but not alarmingly so. The net outflow, 2.5 Sv, is, however, larger than that reported from the current meter array (e.g. Schauer et al. 2004; ASOF-N 2nd annual report 2005; ASOF-N final report 2006). The total in- and outflows estimated here include everything that is moving north and south and do not discriminate between eddies and more organized exchanges. The slight increase in total transports that is noticed in recent years might then be due to the closer station spacing on the later sections.

Fig. 13.2 Centre frame: Total in (red), out (blue), and net transports (black) in Sv obtained from the geostrophic computations. Upper frames: Mean inflow temperature (red) and mean outflow (reference) temperature (blue) and heat transport into the Arctic Ocean (red), the heat export (blue) is zero. Lower frames: Mean inflow (reference) salinity (red) and mean outflow salinity (blue) and the liquid freshwater export (blue). The freshwater import (red) is zero

Fig. 13.2 Centre frame: Total in (red), out (blue), and net transports (black) in Sv obtained from the geostrophic computations. Upper frames: Mean inflow temperature (red) and mean outflow (reference) temperature (blue) and heat transport into the Arctic Ocean (red), the heat export (blue) is zero. Lower frames: Mean inflow (reference) salinity (red) and mean outflow salinity (blue) and the liquid freshwater export (blue). The freshwater import (red) is zero

Fram Strait probably contributes more than 60% of the inflow and 80-90% of the outflow volumes, if the deep exchanges are included. Although the Barents Sea inflow supplies intermediate and deep water to the Arctic Ocean, these dense waters are created, by cooling and also by freezing, in the Barents Sea. Similarly a small amount of Pacific water is made dense enough on the Chukchi Sea to enter the Canada Basin deep water. However, Fram Strait is the only passage that allows deep water to enter, and perhaps more important, the only passage that permits an outflow of deep water.

13.3 Reference Temperatures and Reference Salinities

To properly assess the Fram Strait contribution to the heat and freshwater balances of the Arctic Ocean all in- and outflows have to be accounted for, and a mass balance must first be established. Although this is one of the ultimate aims of ASOF, it has, as yet, not been accomplished. Without mass balance the heat and freshwater transports will depend upon the choice of reference temperature and reference salinity. Often these have been set as -0.1 °C and 34.80, taken as representing the mean temperature and the mean salinity of the Arctic Ocean (e.g. Aagaard and Greisman 1975; Aagaard and Carmack 1989; Simonsen and Haugan 1996; Schauer et al. 2004; Serreze et al. 2006). These values were determined in the 1970s, if not earlier, when the observational basis for forming such averages was very slim, and the variability in space and time of the Arctic Ocean water masses that has become evident during the last 10-15 years (e.g. Quadfasel et al. 1991; Polyakov et al. 2005) makes it doubtful that values determined 30 years ago can still be used without qualification.

Acknowledging the fact that we do not have, at present, sufficient observations from the other passages to formulate a mass balance of the Arctic Ocean, and taking into consideration the temporal variations of the Arctic Ocean mean temperature and salinity, we here choose a different approach. In view of the overreaching importance of the exchanges through Fram Strait we deem it sensible to estimate the inflow of heat to the Arctic Ocean and the outflow of freshwater from the Arctic Ocean through each section in Fram Strait relative to the mean outflow temperature and the mean inflow salinity determined on that section. This implies that no heat is transported by the outflowing water and no freshwater is transported by the inflowing water through the sections in Fram Strait. It should be noted that since the outflow is larger than the inflow, these choices give the largest transports of heat and freshwater through Fram Strait, unless reference temperatures, higher than the mean outflow temperature, and reference salinities, higher than the mean inflow salinity, are used. To compare the results obtained here with other estimates using different reference values, the differences in reference values should be multiplied with the net volume transport.

This does not eliminate the necessity to close the mass (volume) budget for the Arctic Ocean to really determine the fate of the heat entering the Arctic Ocean through Fram Strait and to estimate the relative contribution of the momentary export of liquid freshwater through Fram Strait in the Arctic Ocean freshwater budget. To use these varying reference salinities and temperatures might therefore appear a futile exercise. However, they bring the balances down to simple inflow/ outflow terms, which makes it possible to discuss the mass imbalance, its origin and what it can reveal about the redistribution of the heat carried by the entering Atlantic water.

Furthermore, by comparing the time series of the heat transport, the reference temperature and the inflow and outflow volumes different factors contributing to the variability of the heat transport can be assessed. In a similar manner the variability of the freshwater export can be related to the variability of the reference salinity and the exchanged volumes (Fig. 13.2). These tasks have not been attempted here. Before we turn our attention to the heat and freshwater fluxes, we shall further discuss the exchange of different water masses through Fram Strait and how the transports are distributed in different parts of the strait.

13.4 Exchanges of Different Water Masses

The obtained estimates do not, so far, say anything about the exchanges of different water masses, nor where in the strait the main transports take place. A detailed water mass definition for the Arctic Mediterranean Sea has been formulated elsewhere (Rudels et al. 2005), but for the transports here we introduce a simplified water mass classification of 6 water masses, Surface water (SW), Atlantic water (AW), dense Atlantic water (dAW), Intermediate water (IW), Deep water I (DWI) and Deep water II (DWII) separated mainly by isopycnals but in the case of dAW and IW by the 0 °C isotherm (Table 13.2 and the 0-S diagrams in Fig. 13.5).

The net outflow occurs as surface water and in the dense Atlantic water and the intermediate water ranges. It appears reasonable that waters from other passages that leave the Arctic Ocean through Fram Strait create net outflows with properties that at least partially reflect their initial characteristics. The low salinity of the less dense surface outflow (see Fig. 13.5a) reveals that it originates from the part of the Barents Sea inflow, mainly comprising Norwegian Coastal Current water, that stays on the shelves and incorporates most of the Siberian river runoff. Some ice melt might also be present as well as low salinity Pacific water from Bering Strait,

Table 13.2 Simplified water mass classification

Surface water (SW) Atlantic water (AW)

Dense Atlantic water (dAW) Intermediate water (IW)

Deep water I (DWI) Deep water II (DWII)

Fig. 13.3 Transports in Sv of different water masses based on geostrophic calculations. Inflow (red), outflow (blue) and net transport (black)

although the Pacific water mainly leaves the Arctic Ocean through the Canadian Arctic Archipelago (Jones et al. 2003). The net outflow in the denser, intermediate water range largely derives from the part of the Barents Sea inflow that enters the deeper Arctic Ocean water column via the St Anna Trough (Fig. 13.3).

The Fram Strait sections are subdivided into five different areas. Four of them, the eastern slope, the eastern deep part, the western deep part and the western slope shelf western slope

Fig. 13.4 Transports in Sv in different parts of Fram Strait between 6° W and 9° E based on geostrophic calculations. The western and eastern slopes extend down to 2,200 m and the eastern and western basins are separated by the Greenwich meridian. Inflow (red), outflow (blue) and net transport (black). The shelf transports are determined from geostrophic calculations with the velocity set to zero at the bottom

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