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Wind-Driven Circulation in a Stratified Ocean
Consider the ocean in several isopycnal layers that can be separated into two groups:
Layers that outcrop at the sea surface and layers that are shielded from the sea surface
When a region is connected to the sea surface, it is ventilated
Beyond the circulation theories of ocean with uniform density
In each isopycnal layer, there can be three regions:
1). That filled with subducted water (ventilated region)
2). A western unventilated pool with streamlines connecting from the western boundary and back into the western boundary without entering the surface layer
3). An eastern quite (shadow) zone between the eastern most subducting streamline and the eastern boundary
Water mass formation by subduction occurs mainly in the subtropics.
Water from the bottom of the mixed layer is pumped downward through a convergence in the Ekman transport
Water “sinks” slowly along surfaces of constant density. This “downward” advection into the ocean, along
isopycnals and therefore without significant diapycnal mixing after leaving the surface, is call subduction
The opposite of subduction is obduction, i.e. water from subsurface isopycnals enters the surface layer
Subduction
Sketch of water mass formation by subduction
First diagram: Convergence in the Ekman layer (surface mixed layer) forces water downward, where it moves along surfaces of constant density. The 27.04 t surface, given by the TS-combination 8°C and 34.7 salinity, is identified. Second diagram: A TS- diagram along the surface through stations A ->D is identical to a TS-diagram taken vertically along depths A´ - D´.
Over the T-S range of the thermocline, the vertical profile at 20oS are nearly identical with the T-S diagram of the surface temperature and salinity between 30oS and 45oS in late winter (August-October) but different during all other seasons
Transport:Florida current 30Sv (15-38Sv)Cape Hatteras 85Sv65oW 150Sv(contributions by Antilles CurrentGulf Stream rings 40Sv)
Florida current (flowing over continental shelf) is characterized by salinity minimum due to the Antarctic Intermediate Water brought in by the South Equatorial Current
Antilles Current is composed mostly of North Atlantic Water
Southwestward countercurrent in deep ocean (9-18cm/s)
Transport 15Sv (2-50Sv)
Width: 120kmCold wall: 1/200
ThermostadBetween 16o-20oC
18o waterIts properties are very consistent within a few tenth of a degree on either side of 18oC and with a salinity of 36.4 to 36.6
It is possibly winter formed. Its rate of annual formation is not well known
Gulf Stream Ring and Ocean Eddies
150-300 km in diameterVertical dimension 3000mLife time up to 2 years
Cold ring, cyclonicThermocline 500m higher than surrounding waterSSH 95cm deeper (obs)
Warm ring, anticyclonicSSH 75cm higher (obs)
At any time, there may be 3-4 warm rings north of the stream and 8-15 cold rings south of it
Move south or southwestward a few km/dayMerge with the stream again
Equatorial Under Current
Core close to the equator, ~1m/s
Below mixed layer
Thickness ~ 100 m
Half-width 1-2 degrees (Rossby radius at the equator)
Forced by zonal pressure gradient established by equatorial easterlies
Surface slope 5 x 10-8