Features of dense water cascades off the Arctic shelves

Transcription

1 V.V.Ivanov,3, G.I.Shapiro, Features of dense water cascades off the Arctic shelves. School of Earth Ocean and Environmental Science, University of Plymouth, UK. P.P. Shirshov Institute of Oceanology RAS, Moscow Russia 3. Arctic and Antarctic Research institute, St.Petersburg, Russia Introduction Cascading (or shelf convection) is a specific type of buoyancy driven current, in which dense water is formed over the continental shelf and then descends down the slope to a greater depth. We present analysis of observed cases of dense water cascades around the Arctic Ocean, and discuss their specific features in connection with associated physical processes. We use numerical parameters, both dimensional and non-dimensional for quantitative comparison and contrasting of single cases.

2 Physical processes Horizontal density contrast can be produced by temperature, salinity, or by the combination of both. Over the most part of the Arctic Ocean ice cover is persistent all year round and the surface temperature is close to the freezing point (T frez ). The source water acquires excessive density mainly due to freezing and subsequent brine ejection. Z Q Q brine S S S S S D S D T frez T frez H S Shelf H P Topographic control A thinner homogeneous layer of water over the shelf (H S ) provides a greater salinity response to the equal amount of ejected brine a thicker layer over the deep sea (H P ). Unequal rate of brine ejection is caused by inhomogenity in ice cover and is observed inside the flaw polynyas and marginal ice zones, MIZ. Initial salinity distribution can hamper cascading if by the end of summer the shallow water was essentially fresher than the deep water (S S << S D ).

3 Data analysis Possible range of salinity variation in the shelf water is illustrated by two sections occupied by Russian research vessels in 98s west off Novaya Zemlya Archipelago 7 'N / 5 'E 'N / 53 55'E Depth, m Salinity, PSU West Novaya Zemlya shelf September, Distance, km 'N / 5 'E 'N / 53 5'E Depth, m Salinity, PSU West Novaya Zemlya shelf May, 98 8 Distance, km

4 Broad shelves, encircling the Arctic Ocean interior possess high potential for the dense water formation. In winter the water filling various shallow areas is denser than the water at the same depth over the adjoining slopes: 8 7 Winter salinity in -5m layer Arctic Ocean Atlas (EWG, 998) W 9E However, whether all this dense water finally reaches the slope producing cascades?

5 The map below shows confirmed occurrences of dense water cascades off the Arctic shelves, related to varied sites: 8 USA 7 Canada Russia Canadian Basin 8 Laptev Sea W Nansen Basin Kara Sea 9E Greenland Svalbard 3 Barents Sea N Location Number of Data source cases Bear Island Channel Bar Kode, 999 Storfjord BarKode, Central Bank Quadfasel et al., 99 Westeren Novaya Zemlya shelf 7 AARI archive + Barents Sea Atlas (Matishov et al.,998) 5 Franz-Victoria Channel BarKode, 999 St.Anna Trough BarKode, Severnaya Zemlya shelf AARI archive 8 Chukchi Sea shelf NODC Atlas (Levitus et al., 998) 9 Barrow Canyon NODC Atlas (Levitus et al., 998) Beaufort Sea shelf Melling and Moor, 993,995

6 What information about cascade can we get from a single -D section? Z Z = A Z D A D Z E Z C C E Z B B A = B H C = E D X Z=Z( ρ A) X A X D X C X B X We identify 5 key points on the density section: (A, B, C, D and E). Each of these points is associated with a set of five basic parameters, i.e. T, S, ρ, X, and Z.

7 Key points and corresponding basic parameters can be determined directly from the cross-section displaying the dense water cascade: 73 'N / 3'E 7 'N / 5 5'E Example Depth,m E B C D A T-scale Region: West Novaya Zemlya shelf, Barents Sea Time: August 97 Data source: Barents Sea Atlas, (Matishov et al.,998) Note: there is no neutral density level at this section. Hence, B-point is taken in the deepest depression Distance,km Example 7 5'N / 59 5'W 7 'N / 59 'W Region: Barrow Canyon, Chukchi Sea Time: March 98 Data source: NODC Atlas, (Levitus et al., 998) Note: there is no density minimum at this section. Hence, C-point is taken in the deepest depression (no B and E points). A Depth,m S-scale D 3. C Distance,km

8 In 3 cases, available observational data were full enough to determine the complete set of numerical parameters. ρ Τ, ρ S [kg/m3] ρ β(s max -S amb, ) -ρ α(t max -T amb, ) Z A (m) The forcing of Arctic cascades is depthdependent. In the upper layer temperature contribution to the density is commonly much smaller than that of salinity. In the deeper layers the salinity contrast decreases and changes sign, allowing temperature to prevail and solely drive the cascade. The scale (nondimensional) analysis gives the following relationship between reduced gravity (g'), the Coriolis parameter (f), depth of the shelf (H s =- Z A ), the steepness of the seabed slope (s), the thickness of the dense water pool (H c ), and the age parameter, r. 5 log(g'/f H c ) 3 Corr= log[8s -.5 r - h] 5

9 Analysis of the Density ratio shows that cascades deliver colder and fresher water to the deep ocean in cases. Three cascades (all in the Barents Sea) initially driven by temperature contrast, appeared to be warmer and saltier than the ambient water at Z E depth and experienced strong mixing. Conclusions Average thermohaline contrasts between the cascade and the ambient water, calculated for all cases are as follows: -.73 C,.3 PSU and. kg/m 3. In the areas of quasisteady polynyas horizontal density contrasts may exceed kg/m 3. The average velocity of dense water down slope leakage varies in the limits.3 3 cm/s, depending on the forcing of cascade and its stage. In the majority of the considered cases, cascades delivered colder and fresher water to the deep Arctic Ocean. Acknowledgement This study was co-funded by the EU INTAS grant 99- References Shapiro, G.I., Huthnance, J.M. and Ivanov, V.V. (3). Dense water overflow off continental shelves. Journal of Geophysical Research, VOL 8; C, art. no. 339 Ivanov, V.V., Shapiro, G.I., Huthnance, J.M., Aleynik, D.L., Golovin, P.N., 3. Dense water cascades around the World Ocean, Progress in Oceanography,,, 7-98.