RETREAT OF THE COLD HALOCLINE LAYER IN THE ARCTIC OCEAN
Michael Steele*
University of Washington, Seattle, WA, 98105 USA
Timothy Boyd
Oregon State University, Corvallis, OR, 97330 USA
1. INTRODUCTION
How much of the Arctic Ocean is covered by a Cold Halocline Layer (CHL)? The answer is critical to studies of the sea ice mass balance, since this layer insulates the ice pack from the heat that lies at depth throughout the Arctic Ocean [Aagaard et al., 1981 ; Steele et al., 1995].
The CHL represents a transition between two core water masses: cold, fresh surface waters and cold, salty Lower Halocline Water (LHW). The low salinity surface layer derives from river and Bering Strait inflows. The origins of LHW are still uncertain. Its formation has been ascribed to ice growth on the continental shelves of the Amundsen and Nansen Basins (collectively, the “Eurasian Basin”) [e.g., Aagaard et al., 1981] as well as to air-ice-ocean exchange processes at the Marginal Ice Zone of the North Atlantic [Steele et al, 1995]. The concept in these schemes is that the CHL forms as cold, salty LHW from the periphery of the Arctic Ocean advects towards the deep basins, interleaving at the appropriate depth (about 100 m) into a “pre-existing condition” in which a fresh, cold surface layer overlies a relatively warm, salty Atlantic Water (AW) layer. This is illustrated in Figure 1a, where “upstream” and “downstream” are relative to the location along the AW inflow path where a CHL first appears (i.e., where both a fresh surface layer and saltier LHW first appear together). A very different mechanism was recently proposed by Rudels et al. [1996], in which the “pre-existing condition” is a cold, salty, and deep (〜100 m) convective winter mixed layer with LHW properties, and it is relatively fresh shelf waters that advect into the surface and near-surface layers (Figure 1b). In regions where these shelf waters have intruded, the deep winter layer becomes isolated from the surface; subsequent convection is presumed to be much shallower. Thus the main difference between the two CHL mechanisms is the origin of LHW waters at around 100 m depth; one is advective (Figure 1a) while the other is convective (Figure 1). Both are winter processes.
The 1990's have marked a watershed in the acquisition of Arctic Ocean hydrographic data. Recent work has indicated that large-scale changes have occurred in the extent and strength of AW and certain halocline types during the 1990's relative to previous years [McLaughlin et al., 1996; Morison et al., 1998]. Here, we consider what changes have occurred in the CHL using data collected during SCICEX'95, SCICEX'93, and Oden'91. We also examine the CHL, in the new Russian-American Arctic climatology published by the Environmental Working Group (EWG) [Arctic Ocean Atlas, 1997].
2. METHODS
One climatology (EWG) and three data sets (Oden'91, SCICEX'93, and SCICEX'95) are used here.
The EWG climatology is described in Arctic Ocean Atlas
*Corresponding author address: Michael Steele, APL, 1013 NE 40th St., Seattle, WA, 98105, USA; email: mas@apl.washington.edu
