Offshore transport of dense shelf water to the Arctic basin across a shelf break region
Takashi Kikuchi*
Japan Marine Science and Technology Center, Yokosuka, 237-0061, JAPAN
1. INTRODUCTION
Dense shelf water is formed and modified in the continental shelf accompanied with sea ice formation. The dense shelf water has relatively high salinity. Therefore, it has been recognized as one of the source of Arctic intermediate and deep water masses. Several researchers have investigated the dynamical process of offshore transport of dense shelf water. Some results show that an eddy development process contributes to the offshore transport of dense shelf water (e.g., Gawarkiewicz and Chapman (1995),, and others show that bottom dense plume carry the dense water over a continental shelf, mixing with the ambient water (e.g., Jiang and Garwood (1995)). On the other hand, it has reported that small scale eddies, which have been measured in the Canadian basin, might be generated on the periphery of the basin and might transport the intermediate water from the shelf break region (e.g., D'asaro(1988),. The importance of shelf break process is common to such phenomena.
The purpose of the present study is to clarify the offshore transport process of dense shelf water. In particular, I focus on an importance of a change in bottom topography to the process. At first, I examine an effact of changes in a depth and a bottom slope. The response and behavior of dense shelf water in a shelf break region are shown for this purpose. Next, I examine effects of a submarine canyon to the transport of shelf water. This result shows one possible mechanism that the outflow of the dense shelf water from the canyon might generate the small scale eddies.
For these purpose, I use a three-dimensional primitive equation model (POM), which uses a sigma coordinate system as a vertical coordinate and a second-order turbulent closure scheme to provide vertical mixing parameters. The numerical domain is idealized for a shelf break topography. The surface salt flux is applied in a continental shelf region as the only driving force to the model ocean.
2. EFFECTS ON CHANGES OF THE DEPTH AND BOTTOM SLOPE
Three numerical experiments, Basic Case (BC), Steep Slope Case (SS) and Gentle slope case (GS), which have different depth and bottom slope, were carried out to clarify the effects of changes in the depth and bottom slope. The transport process is common among all of three experiments. First, Dense shelf water which is produced over a continental shelf is transported through an eddy development process, i.e., eddy flux, over a continental shelf. As the dense shelf water reaches at the region that the depth becomes deep and the bottom slope steepens, the dense water transport through the eddy flux weakens. A salinity front between the dense shelf water and the offshore surface water is developed at a shelf break, accompanied with development of bottom dense plumes which carry the dense shelf water over the slope. Investigating balances of the salinity conservation, it is found that the dense water transport through the eddy flux is limited in the onshore side of the shelf break front. On the other hand, beyond the shelf break front, bottom dense plumes are prominent to the transport process. The bottom dense plumes is mainly driven by a pressure gradient force. The force balances with Colioris force and bottom stress at the bottom boundary layer, so the bottom dense plumes flow offshore. The pressure gradient force also drives a surface current with a left-bounded direction on the shelf break front. In the bottom boundary layer, the balances of salinity conservation become more complicated. The vertical diffusion term of the equation becomes more important over a continental slope than over a continental shelf.
To clarify the effects of changes in the depth and bottom slope, comparison among the results of three experiments was carried out. I focus on the position of shelf break front which means where the transport process of dense shelf water changes from the eddy flux to the bottom dense plume. The shelf break front is developed at y〜55.0 km for SS and at y〜85.0 km for GS, whereas at y〜70.0 km for BC. Note that the location of the shelf break front is nearer to the coast and its depth becomes shallower, as the bottom slope of numerical experiment steepens. To clarify a relation between eddy development and bottom topography (the depth and bottom slope), I use a two-layer quasi-geostrophic model over a sloping bottom. It is found that both the deepening and steepening stabilize the surface mean current which was flowing over the slope with the same sense as the interface slope. I apply the analytical model to investigate the stability of the shelf break fronts shown in the numerical experiments. Each neutral point of the stability of the mean surface current corresponds to those of a shelf break front shown in the numerical experiment.
Figure 1 shows a conclusion of a transport process across a shelf break region with no alongshore variation. The dense shelf water is transported by eddy flux in a continental shelf and by bottom dense plumes
* Corresponding author address. Takashi Kikuchi, Japan Marine Science and Technology Center, 2-15 Natsushima Yokosuka 237-0061 JAPAN; e-mall address: takashik@jamstec.go.jp
