Pathways, Properties, and Processes of Exchange 1. INTRODUCTION
1.1 Why are the Atlantic Waters of the Arctic important?
Atlantic Waters (AWs) are volumetrically the largest inflow to the Arctic Ocean. They form the major subsurface circum-arctic oceanic transport system and ventilate the interior basins. They are the greatest pan-arctic reservoir of oceanic heat, which may influence upper layers and the sea-ice, for example, through slope upwelling and mixing. Circulation of AW carries tracers and contaminants through the Arctic, and the pan-arctic distribution of AW offers a warm corridor for invasive species. Globally, the AW circulation is a major means of communication between the Arctic and the world ocean: arctic-modification of AW contributes to the North Atlantic overflows and is a high-latitude part of the meridional overturning circulation.
Yet, despite the urgent focus on the Arctic as the canary of changing climate, fundamental questions about the major ocean current in the Arctic remain unanswered. The pathways, flow speeds, transit times, and modification of AW within the Arctic are still only poorly defined or understood. In the western Arctic, there is still disagreement about issues as fundamental as flow direction, and lack of a pan-arctic observationally-based AW analysis is hindering current progress in modeling and theoretical studies. Furthermore, at the conclusion of IPY in 2009, the international community is to deploy an Arctic Observing Network (AON), one portion of which is measurements of the AWs of the Arctic. Yet, to date, the substantial archives of recent arctic data highly relevant to AW circulation remain mostly uncollated and unexploited. To address these pressing needs, we propose an observationally-based synthesis of the fate of the Atlantic Waters within the least-understood region of the Arctic – i.e., the Canadian Basin.
Figure 1: (Left) Schematic arctic bathymetry (contours 500 m). The Lomonosov Ridge divides the Arctic Ocean into two major basins – the Eurasian Basin and the Canadian Basin (the western Arctic). (Right) IBCAO (International Bathymetric Chart of the Arctic Ocean [Jakobsson et al., 2000]) bathymetry of the Canadian Basin (contours 500 m). The Mendeleev and Alpha Ridges divide the Canadian Basin into the Makarov Basin in the north and the Canada Basin in the south, the most southern part of which is also called the Beaufort Sea. NwR=Northwind Ridge. BSl=Beaufort Slope. CSl=Chukchi Slope. BC=Barrow Canyon. Dashed quadrilaterals mark regional focuses discussed below – Chukchi/Beaufort Slopes & Canada Basin, section 2.3a, (green); Chukchi Borderland and Northwind Ridge, section 2.3b, (cyan); Makarov Basin, section 2.3c, (blue).
1.2 What is known and unknown about Atlantic Waters in the western Arctic Ocean?
Atlantic Waters (AWs) enter the Arctic in two branches of comparable volume – the Fram Strait Branch Waters (FSBW) and the Barents Sea Branch Waters (BSBW) [Rudels et al., 1994; Rudels et al., 1999b]. FSBW extend to the surface in Fram Strait, but, due to surface cooling and mixing/capping with cooler fresher shelf waters, in the rest of the Arctic they form a subsurface temperature (T) maximum between 200-600 m with T > 0ºC at salinities (S) greater than 34.5 psu. Advection of warmer FSBW which entered the Arctic in the 1990s is used to trace arctic AW pathways (e.g., Karcher et al., ). BSBW have a large range of T and S, including T < 0ºC at S > 34.5 psu. The BSBW not only mix with the FSBW core, but also intrude below it to depths of ~1300 m [Rudels et al., 1994; Schauer et al., 1997]. The BSBW core, best tracked by radionuclide tracers, is at ~ 800 m [Smith et al., 1999], and the influence of BSBW is often identified by a change of slope in temperature-salinity (T-S) space at a sigma-0 of ~ 28 kg/m3 (e.g., Figure 3).
It is generally thought that (a) the AW flow is strongly steered by seafloor topography [Aagaard, 1989], in contrast to the sea-ice and upper (< 200 m) water layers which are dominantly atmosphere-driven [Rigor et al., 2002; Steele et al., 2004]; and (b) the dominant large-scale AW circulation is a cyclonic boundary current confined over topographic slopes and ridges, with a weak interior flow, significantly populated by eddies [Hunkins et al., 1969; Aagaard, 1989; Rudels et al., 1994; McLaughlin et al., 1996; Carmack et al., 1997; Swift et al., 1997; Rudels et al., 1999b; Jones, 2001; Woodgate et al., 2001]. However, especially in the western Arctic, there is significant disagreement about the detailed route of the boundary current, the importance of various pathways, and the sense (cyclonic or anticyclonic) of the interior flow especially in the Canada Basin. This is evident from different circulation schemes proposed by observational studies (Figure 2).