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Circolazione Oceanica “Wind Driven” R. Mosetti: Corso di Oceanografia A.A. 2011-2012 Stewart: Capitolo 9: paragrafi: 9.2; 9.3; 9.4 Le principali correnti oceaniche indotte dal vento How do winds drive the ocean circulation? Ekman dynamics: equations f is Coriolis parameter, u and v are the zonal and meridional ocean currents, a ‘viscosity’ parameter. What is is saying is that at each level, the Coriolis force is balanced by the momentum flux convergence (i.e. friction from within the mixed layer) Boundary conditions: at surface, the vertical shear of the current is tied to the wind stress; at depth, currents approach zero at infinite depth. x and y are the wind stress in N/m2, and the 0 the seawater density. is the momentum diffusion coefficient The solution of the above equations with these boundary conditions give the Ekman spiral. They can be integrated over depth to give the depth-integrated transport: The above relationships thus gives the total Ekman transport over the mixed layer given the wind stress, density of seawater 0, and the Coriolis parameter f. Note that •Meridional wind stress gives a zonal transport, and zonal wind stress a meridional transport (as we talked about before) •the transport goes as the inverse of the Coriolis parameter f - the closer it is to the equator, the more ‘effective’ the Ekman transports. •Relationship breaks down at the equator (since f=0 there) Example: suppose the zonal wind stress in the subtropics at 30 degrees N is 0.15 N/m2 to the west. What is the Ekman transport associated with this wind stress? Ans: y = 0 N/m2 so no zonal volume transport. x = -0.15 N/m2 (note sign). Note that f = 2 sin (pi/6) = , and 0 ~ 1035 kg/m3, so the meridional volume transport (applying the equation in the previous slide) is VE = - (-0.15 N/m2) / (1035 kg/m3 x 7.3x10-5s-1) = 1.99 m2/s - its to the North Note that if you multiply VE by the density of seawater, you get 2.05x103 kg/(m.s) which is the mass transport (i.e. 2.05 x 103 kg/s across a unit meter of zonal distance) How do winds drive the ocean circulation? Ekman currents: comes through surface wind on the mixed-layer ocean; the stress is transmitted into the mixed layer through turbulent motions, and is balanced by Coriolis effects. The ocean velocity is at 45 degrees to the right of the wind at the surface, and ‘spirals’ with depth (see figure). The mean motion averaged over the mixed layer is perpendicular (right in the NH, left in the SH) of the wind direction. Wind stress is the stress of the wind on the water surface. Units are N/m2. Typically denoted as x and y for the zonal and meridional components. “The Ekman spiral is one of the oldest results in dynamical oceanography. It was first proposed (conceptually) by the great Norwegian explorer Fridtjof Nansen. As part of a polar expedition in the late 1890s, Nansen froze his ship Fram into the ice north of Spitzbergen Island and allowed it to drift for more than two years. During the expedition he noticed that the drift of the boat was generally to the right of the wind. Nansen proposed that this motion was the result of the Coriolis force, which causes objects to veer to the right in the northern hemisphere and to the left in the southern hemisphere. He supposed further that as the ice pushed on the water immediately below it, that water would move still further to the right of the wind, though a little more slowly. Extended down through the water column, the result would be a spiral structure. “ (from Anand Gnandesikan’s website:http://www.gfdl.gov/~a1g/) Ekman layer depth • Depth: depends on eddy viscosity AV (why?) E-Depth = (2AV/f)^1/2 • Eddy viscosity AV is about 0.05 m2/sec in turbulent surface layer, so Ekman layer depth is 20 to 60 m for latitudes 80° to 10°. How to create a subtropical gyre circulation I Recall that surface winds in the 15-45 degree range consists of easterly trades and midlatitude westerlies….. How to create a subtropical gyre circulation III Ocean surface ‘domes’ up, and a geostrophic ocean current is produced that balances the resulting pressure gradient Blue: Ekman flow Red: Geostrophic flow Ekman Transport Westerlies The red box... Southward transport Trades Ekman transport is proportional to wind stress greater transport for greater wind stress Northward transport At the convergence, water piles up and sinks, thus depressing the thermocline and deepening the nutricline! The convergence of waters by the Ekman transports cause downwelling The opposite - caused by divergence of surface waters - is called upwelling Important points to note • The sea level pile-up is a result of the convergence of the Ekman transport • The Ekman layer is ONLY 50-100 meters thick • The resulting pressure gradient is felt throughout the water column • Thus the geostrophic current occurs over a MUCH greater depth than the depth of the wind-driven layer, as much as the top 200 to 500 meters Equatorial upwellling. Ekman flow upwelling X EQ wind Equatorial upwellling. Recall that the winds on the equator are easterly - from the east. Now, consider what the Ekman flow would do north and south of the equator. Because of the reversal in the Coriolis parameter, it turns out that the Ekman flow is polewards in both hemispheres - in other words, the ocean circulation is diverging at the equator. In order to compensate for this divergence, water has to be brought up from below the mixed layer - hence equatorial upwelling. This mechanism is responsible for the cold tongues over the eastern equatorial Pacific and Atlantic. Wind-driven ocean circulations (cont) Equatorial current and countercurrents Equatorial undercurrent QUELLO CHE CI SI ASPETTA Quello che succede nella realtà…… Coastal upwelling: suppose this is the northern hemisphere, and the wind is parallel to the coastline as shown below. The Ekman flow as a result will be to the right of the wind - in other words, away from the coastline. But, in order to obey mass continuity, this has to be compensated for by bringing in water from below the mixed layer - i.e. upwelling. Since the upwelled water is cold, the upwelling regions have cold sea surface temperature. Cosa succede nell’ “Interior”? Stewart: Capitolo 11 Sverdrup Transport (Interior) Tutte e due (a e b) sono consistenti con il wind stress! Qual’è quella reale?