Project

Data Analysis:

Numerical Modeling:

 

 

NansenIntroduction

Cascading is a specific type of buoyancy driven current, in which dense water formed by cooling, evaporation or freezing in the surface layer over the continental shelf descends down the continental slope to a greater depth.

The general concept of cascading was formulated by Fritjof Nansen (1906). He was also the first to make direct measurements of cascading over the Rockall Bank in the North Atlantic Ocean (Nansen, 1913).

The term ‘cascading’ was introduced later by Cooper and Vaux (1949). Now it is generally used by oceanographers (e.g. Lane-Serff, Encyclopedia of Ocean Science, 2001; Aquatic Sciences and Fisheries Thesaurus, web-site), but sometimes the same phenomenon is also referred as ‘shelf/slope convection’.

f3

The process of cascading evolution is outlined in Fig. 1.

In the Arctic Ocean cascading is normally fed by salty/denser water, which forms over the shallow shelf areas inside the openings in the ice cover in winter. These openings, known as polynyas, are created by favorable wind which cracks fast ice and moves it offshore. Vertical heat flux from the ocean to the atmosphere in polynyas has the scale of hundreds of watts per square meter, which is about two orders of magnitude higher than the vertical flux through the surrounding fast and pack ice (e.g. Marqueda et al., 2004).

As a result of this huge vertical heat flux water in polynyas rapidly freezes up forming frazil and surface ice. The brine, which is released during this process admixes to the water making the water column inside polynya saltier/denser than the surrounding water (e.g. Aagaard et al., 1981).

This denser water starts moving off shelf, cascading down slope under the influence of gravity force and then is deflected to the right (along the slope) by the Coriolis forcing. Various types of instability affect the dense water flow. The major one is baroclinic instability, which causes meandering and eddy formation (e.g. Chapman and Gawarkiewitz, 1995).

In the stratified Arctic Ocean descending dense water does not normally reach the base of the slope. At the depth, where the density in the water ‘plume’ equalizes the density of the ambient water, the plume detaches of slope intruding into the water column. This depth is often referred as the ‘equilibrium density level’ (e.g. Shapiro et al., 2003).

At the equilibrium density level, cascading plume is transported away from the region of its origin by local currents, gradually dispersing and releasing heat and salt into the ambient water.

a1 The development of cascading is illustrated in the following animation

f4

Fig. 2

 

How wide is cascading spread around the Arctic Ocean?

In winter the water filling various shallow areas is denser than the water at the same depth over the adjoining slopes (Fig.2). However, whether all this dense water finally reaches the slope producing cascades?

f5

Fig. 3

The map in Fig. 3 shows 10 locations in the Arctic Ocean where dense water cascading was ever detected in hydrographic data (Ivanov et al., 2004) :

1. Bear Island Channel
2. Storfjord
3. Central Bank
4. West Novaya Zemlya
5. Franz-Victoria Channel
6. St. Anna Trough
7. Severnaya Zemlya
8. Chukchi Sea shelf
9. Barrow Canyon
10. Beaufort Sea shelf

f6

Fig. 4

Here we present some results related to cascading from the northwestern Laptev Sea shelf (region 7, Severnaya Zemlya). This region, is often mentioned in the context of cascading (e.g. Aagaard et al., 1981; Martin and Cavalieri, 1989; Rudels et al., 2000; Golovin, 2005). Formation of dense water is expected to occur inside the shallow area marked by brown dots (Fig. 4). It is anticipated that descending dense water may pass through the intermediate Atlantic Water, marked by red arrows, and reach deep layers of the Nansen Basin (Rudels et al., 2000).