Freshwater Storage in the Northern Ocean and the Special Role of the Beaufort Gyre

As part of the global hydrological cycle, freshwater in the form of water vapour inexorably moves from warm regions of evaporation to cold regions of precipitation and freshwater in the form of sea ice and dilute seawater inexorably moves from cold regions of freezing and net precipitation to warm regions of melting and net evaporation. The global plumbing that supports the ocean’s freshwater loop is complicated, and involves land–sea exchanges, geographical and dynamical constraints on flow pathways as well as forcing variability over time (cf. Lagerloef and Schmitt 2006). The Arctic Ocean is a central player in the global hydrological cycle in that it receives, transforms, stores, and exports freshwater, and each of these processes and their rates both affect and are affected by climate variability. And within the Arctic Ocean, the Canada Basin (see Fig. 7.1) is of special interest for three reasons: (1) it processes freshwater from the Pacific, from North American and Eurasian rivers and from ice distillation; (2) it is the largest freshwater storage reservoir in the northern oceans; and (3) it has exhibited changes in halocline structure and freshwater storage in recent years.

In this chapter we examine the distribution of freshwater anomalies (relative to a defined reference salinity) in northern oceans by reviewing criteria that have been used to construct freshwater budgets and then by comparing freshwater disposition in the subarctic Pacific, subarctic Atlantic and Arctic oceans. This comparison provides a useful basis for the interpretation of Arctic Ocean flux measurements and affirms that the Canada Basin is a significant freshwater reservoir (Section 7.2). We next examine various hydrographic data sources within the Canada Basin (a geographical feature) to define the role of the Beaufort Gyre (a wind-forced dynamical feature) in freshwater storage and release (Section 7.3). Due to this latter feature, the upper layer circulation in the Beaufort Gyre is anticyclonic whereas circulation elsewhere in the Arctic Ocean is cyclonic. Then we examine the Canada Basin’s role as a reservoir with respect to sources of its freshwater components (e.g. meteoritic (runoff and precipitation), sea-ice melt and Pacific throughflow), and also to its water mass structure, within which freshwater components are stored (Section 7.4). This distinction among source components and among water mass affiliations is a prerequisite to interpreting downstream freshwater fluxes and to predicting the response of the Arctic system to climate variability. Finally, we combine geochemical data and recent freshwater budget estimates to calculate the relative contributions of freshwater components from the Canada Basin to other Arctic basins (Section 7.5). A summary and outlook is given in Section 7.6.