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1 an Isopycnal View of the Nordic Seas Hydrography with Focus On An Isopycnal View of the Nordic Seas Hydrography with focus on Properties of the Lofoten Basin. T. Rossbya*, Vladimir Ozhiginb, Victor Ivshinb, and Sheldon Baconc aGraduate School of Oceanography, University of Rhode Island, Kingston, RI, 02881 bPolar Research Institute (PINRO), Knipovich St. 6, 183763, Murmansk, Russia cNational Oceanography Centre, European Way, Southhampton, SO14 3ZH, U.K. Submitted to: Deep Sea Research: December 2, 2008 *Corresponding author. Tel: +1 401 874 6521; fax: +1 401 874 6728. E-mail address: [email protected] 1 Abstract Few basins in the world exhibit such a wide range of water properties as do the waters of the Nordic Seas with cold fresh waters from the Arctic in the western basins and warm saline waters from the Atlantic in the eastern basins. In this study we present a 50-year hydrographic climatology of the Nordic Seas in terms of depth and temperature on four different specific volume anomaly surfaces. This approach allows us to better distinguish between change due to variations along such surfaces and change due to depth variations of the stratified water column. Depth variations indicate changes in the mass field while property variations along isopycnals give insight into isopycnal advection and mixing, as well as diapycnal processes. We find that the warmest waters on each surface are found in the north, close to where the isopycnal outcrops, a clear indication of downward mixing of the warmer, more saline waters on shallower isopycnals due to convective cooling at the surface. Using a specific volume anomaly surface that exists all year except in the coldest regions we show i) the role of topography in isolating water masses to either side of the mid- ocean ridges, ii) the great depth of the pycnocline in the center of the Lofoten Basin, deeper than almost anywhere else in the Nordic Seas, and iii) the increase in temperature (and hence salinity) near where it outcrops in the northeast. Close inspection indicates that the deepening results from the expulsion of anticyclonic eddies from the continental escarpment just offshore of the Lofoten Islands and their pooling in the center of the basin. This pooling of warm waters, which leads to an upper ocean anticyclonic density structure, is a key factor to the large heat losses in the Lofoten Basin. Time series analysis of isopycnal depth in the Lofoten Basin shows it to be rather stable over time with a small but distinct annual cycle superimposed. However, in 1968-1969 it shoaled over 400 m. Almost certainly this resulted from excessive heat loss to the atmosphere during those two very cold winters. This excess loss also shows up as the greatest temperature anomaly in the entire 50-year record of this analysis. Interannual variations in pycnocline depth correlate with the NAO index. 2 Keywords: thermohaline circulation; water properties; isopycnal; Nordic Seas; Lofoten Basin; heat loss; interannual variability 3 Introduction The Nordic Seas have perhaps been studied longer and more thoroughly than any other ocean. Already in 1887 Mohn published a chart of the circulation of the Norwegian Sea clearly indicating the inflow of warm North Atlantic waters on the eastern side and flow south of Arctic waters in the west. This study was followed a couple of decades later by the pioneering Helland-Hansen and Nansen (1909) monograph on the hydrography of these northern waters. Using water mass analysis (reversing thermometers and accurate salinity titrations) and the dynamic method, the circulation patterns they deduced have stood the test of time impressively well. Even today their figure of salinity in the southern Norwegian Sea and across the Iceland-Faroe Ridge provides a remarkably accurate synthesis of the regional circulation. They detailed the route by which warm North Atlantic waters flowed north through the Norwegian Sea and beyond towards the Barents Sea and Svalbard. A striking aspect about the Helland-Hansen and Nansen study was its emphasis on horizontal distributions. They could do this thanks to the systematic hydrographic surveys throughout the Norwegian and Greenland Seas. Helland-Hansen and Nansen set the standard for subsequent studies including the major atlases of water properties at selected standard depths by Dietrich (1969) and Koltermann and Lüthje (1989). In recent decades, with increasing awareness that in stratified oceans fluid exchange takes place along isopycnal surfaces, and with increasing computer power to interpolate and plot observations, it has become increasingly common to examine properties as a function of density in order to better resolve where along a section or a surface changes in water properties take place. Various measures of ‘isopycnality’ can be used; these are (with increasing accuracy) surfaces of constant sigma-t, sigmat-θ, specific volume anomaly, and gamma (neutral surface). When plotted as a function of isopycnal one can see precisely where and how changes in water properties take place along a section (e.g. Arhan, 1990; McCartney and Mauritzen, 2001). Parallel sections can be used to construct maps of properties on isopycnal surfaces. Thus, Bower et al. (1985), using temperature and oxygen distributions on two different sigma-theta surfaces, could show how the Gulf Stream acts as a ‘barrier’ to cross-stream exchange on shallow isopycnals and as a 4 ‘blender’ on deeper surfaces. Although such studies require 2-dimensional coverage in the horizontal, charting distributions on isopycnal surfaces can give valuable insight into pathways of spreading and mixing processes. Our interest in developing an isopycnal surface view of the Nordic Seas was stimulated by the availability of a huge hydrographic database. Well over 300,000 stations have been archived over the last half-century. This allows one examine in considerable detail isopycnal surfaces directly, their depths and their physical properties, and how these vary over time. A change in depth of an isopycnal (surface) implies a change in the density profile and hence pressure field, a change of dynamical consequence, whereas a change in temperature/salinity composition on an isopycnal implies a change in water type. It does not impact the pressure field, but contains valuable information on advection, mixing (or lack thereof), and diapycnal processes. Knowing that the Nordic Seas exhibit strong contrasts both spatially and seasonally, it seemed appropriate to explore whether and how an isopycnal approach might help us to better understand the nature of these patterns and their eventual change over long time. The Nordic Seas have a basic west-east organization with waters from the Arctic and the North Atlantic flowing south and north along the respective margins. But additional fluid pathways through the Nordic Seas exist thanks to the set of ridges that define the various sub-basins. Thus, the inflow of Atlantic water through the Shetland Channel comprises two branches, one (aka the inner branch of the Norwegian Atlantic Current) that flows along the continental margin north towards the Lofoten Basin, and the outer branch that flows north just west of the Vøring Plateau, and along the Mohn and Knipovich Ridges towards Fram St (Orvik and Niiler, 2002). See Figure 1 for the names and locations of the major ridges and basins. As we will see, the Jan Mayen Ridge, the Mohn Ridge and the Knipovich Ridge play an overarching role in the west-east organization of the Nordic Seas. To the west the Greenland and Iceland Seas are both very cold and fresh. To the east, the Norwegian Sea, which comprises the Norwegian Basin in the south and the Lofoten Basin to its northeast, exhibits quite counterintuitive properties with a deep thermocline in the Lofoten Basin and the warmest, saltiest waters even farther north between the Mohn/Knipovich ridge and the Barents Sea escarpment. In this isopycnal 5 study, we use specific volume anomaly (abbreviated as δ) to examine these properties in some detail. We begin with a survey of several δ-surfaces in order to establish the mean state of the Nordic Seas from near the surface down across the main pycnocline. We then focus on one of these, the δ =2.1x10-7 m3kg-1 surface, which is the shallowest surface that on average does not outcrop in winter except along the margin of the Greenland Sea and thus remains well-defined throughout most of the year (almost all surfaces will outcrop in the center of the Greenland Sea). We use this surface as the stage to examine the annual cycle and variations on longer time scales. While this means foregoing a detailed discussion of corresponding developments other surfaces, both shallower and deeper, we intend that this isopycnal approach provide a complementary perspective to present-day hydrographic descriptions of the Nordic Seas as well as provide a starting point for more detailed studies in the future. In the next section we describe the database, the procedures for quality control and preparation of the isopycnal fields used in this study. The following section describes the mean state of four δ-surfaces across the Nordic Seas for the 1951-2000 period. We then use one of these surfaces to document the annual cycle and RMS variability. The following section shows how - given the mean field description - one can determine the condition or state a region at any given time. The examples include Nordic Seas-wide variations, and an example of a very large anomaly or departure from the mean state that took place in the late 1960s. As a further illustration of the methodology we show that the state of the Nordic Seas at the time of the Helland-Hansen and Nansen (1909) study was quite typical of the last half-century. We then discuss some of the more striking findings such as the causes of the deep pycnocline in the Lofoten Basin, which has enormous implications for the heat balance in the Nordic Seas.
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