Reino Unido

The flux of freshwater, influenced by the processes of ice formation and melt, evaporation and precipitation and river runoff, has a central role in ocean circulation and climate change. At high latitudes the density structure is controlled mainly by salinity, and the freshwater flux is of particular significance to the vertical stratification. From the equation of state for seawater at low temperatures, it follows that temperature stratification has very little effect on the density structure (since the thermal expansion coefficient is so small). The introduction of fresh water in the high latitudes can therefore prevent convective overturning even in the case of substantial surface cooling (Aagaard and Carmack, 1989).

The surface layers of the Arctic Ocean receive considerable input of freshwater both from oceanic sources of sea ice melt and continental sources of rivers and glacial melt. Both sources are seasonal, but there is a zero net freshwater contribution from sea ice (due to the melt/freeze cycle) and a positive net contribution from the continental sources. They provide an average freshwater input of 4650!km3_!yr-1 _{although there is significant variability on both annual and interannual time scales. Of}

this, an estimated 3300!km3_!yr-1 _{is from river runoff (Aagaard and Carmack, 1989; Treshnikov,}

1985). There is significant annual and interannual variations in the flows from these Arctic rivers (Cattle, 1985), with the interannual flow variability in individual rivers typically 5!-!20!% of the annual mean. In addition to these inputs, 1500!-!2000 km3_!yr-1 _{enters as the freshwater fraction of}

the Bering Strait inflow (Coachman and Aagaard, 1974). A number of studies have used hydrographic and tracer data (salinity, nutrients, dissolved oxygen and d18!O), to quantify the components of Arctic freshwater: river runoff, sea ice meltwater and Pacific water (Bauch et al., 1995, Ekwurzel et al., 2001). The remaining component of the Arctic freshwater balance is the estimated input from precipitation less evaporation (P!–!E). There is considerable uncertainty in estimates of this flux, but is suggested to be in the region of 900!km3_!yr-1 _{(Aagaard and Carmack,}

Chapter 2: Overview 22 The export of freshwater from the Arctic Ocean through Fram Strait provides the largest contribution of freshwater to the Nordic Seas. It is linked to the northern hemisphere thermohaline circulation particularly in its influence on, and potential control of, deep water formation. This contribution (through both ice and liquid water) is estimated to be in the region of 4000!km3_!yr-1

(Aagaard and Carmack, 1989). Meredith et al., (2001) used hydrographic and d18_{O sections across}

Fram Strait (August - September 1997, 1998) to examine the freshwater contributions to the EGC. They derived meteoric water fluxes of ~3680 km3_yr-1 _{in 1997, and ~2000!km}3_yr-1 _{in 1998. They}

found the ratio for the mean summer fluxes through Fram Strait of meteoric water to sea ice to be ~!2:1. In this, they differ from the Aagaard and Carmack (1989) annual budget of the Arctic and Nordic seas which included sea ice as the largest contribution to the freshwater export through Fram Strait. Interannual variability and apparent correlation with processes such as the NAO and changing cyclonicity of the Arctic circulation could be a factor in this discrepancy (Proshutinsky and Johnson, 1997; Vinje et al., 1998). Meredith et al. (2001) also found a large volume of meteoric water on the East Greenland shelf which may prove to be a significant contributor to the overall freshwater flux through Fram Strait. Given the importance of the Fram Strait freshwater flux to the regional and global climates, it is important to understand the variability in both the solid and liquid phases of export (ice and water) and to determine the relative contributions of sea ice and meteoric water to the overall freshwater flux.

The land masses of Norway, Greenland and Iceland contribute a total runoff to the Nordic Seas of ~545!km3_!yr-1_{. This estimate is based on ~350!km}3_!yr-1 _{runoff along the Norwegian coast (Aagaard}

and Carmack, 1989), ~133!km3_!yr-1 _{from iceberg calving along the east Greenland coast (Hardy et}

al., 2000), and ~62!km3_!yr-1 _{runoff from the north coast of Iceland (Stefansson, 1962). The}

contribution to the annual fresh water flux from P!–!E is not well quantified but is estimated to be in the region of 790!km3 _{from the Gorshkov (1983) atlas. Although not a dominant factor, it does}

represent ~13!% of the total freshwater input to the Nordic Seas. The final contribution is the input of freshwater from the Baltic Seas through the Skagerrak via the Norwegian Coastal Current. This is estimated to be ~950!km3_!yr-1 _{(Aagaard and Carmack, 1989).}

Although the exchanges of freshwater in the Nordic Seas are known to be important processes, large-scale estimates of oceanic transports of freshwater north of the Greenland-Scotland Ridge are rare. Estimates have been limited mainly to budget calculations (e.g. Aagaard and Carmack, 1989), although Oliver and Heywood (2003) estimated a summer freshwater transport between Norway and Greenland of 0.10 ±!0.05!Sv away from the Arctic, and Bacon (1997) derived an estimate of the freshwater flux between Greenland and the European continental shelf which calculates an estimated net freshwater gain by the Arctic of 0.17!±!0.06!Sv.

Chapter 2: Overview 23

2.4

Circulation Hypotheses

Various scenarios have been proposed for the circulation of the Nordic Seas and Arctic Ocean, and for the consequent production of deep water and formation of overflow waters, but the precise pathways and mechanisms remain largely undetermined.

To begin with, in order to adequately describe the circulation of the Nordic Seas and to consider the formation mechanisms of the overflows, it is important to differentiate between the different water masses. As Mauritzen (1999a) discussed, this can be problematic in the Nordic Seas. In particular, high interannual variability means that waters from the same source will not necessarily have the same characteristics each year. Swift and Kolterman (1988) observed that Norwegian Sea Deep Water (NSDW), not all of which actually enters the Norwegian Sea, was “named by virtue of characteristics rather than location”. The situation is further complicated by the lack of a consensus in the conventions used to name water masses within the Nordic Seas, with different terms being used to describe waters with the same hydrographic characteristics. Mauritzen (1999a) based her water mass definitions on persistent extrema apparent in hydrographic profiles. These extrema are observed within the same density range in different data sets and years and the existence of a core implies circulation (an advective supply). To make a synthesis of terms found in the literature, initial divisions into Atlantic Water (AW), Polar Water (PW), Intermediate Water (IW) and Deep Water (DW) are followed here. A full description of these water masses is given in Table!2.1, and the conventions followed in thesis are specified in Chapter 5 (Table 5.1).