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)

Southwestward countercurrent in deep ocean (9-18cm/s)

Transport 16 Sv (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

Western Boundary current: Gulf Stream

Meso-scale eddies

(Gulf-stream Rings)

Gulf Stream Rings and Ocean Eddies

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

Kuroshio: 40 Sv; Kuroshio Extension: 65 Sv (170oW). Core speed, 75-250cm/s

Width (V > 1m/s): 80km

Equatorial Circulation System

Equatorial Under Current

Geostrophic balance near the equator

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

Geostrophy contributes to the stability of an eastward flow along the equator