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Equatorial Currents and Counter Currents

Kurt House

3/25/2004

Equitorial_Currents1

The major ocean currents of the tropics (I.e., the equatorial currents and counter currents) are driven by the global wind system

•Exchange of kinetic energy between water parcels

•Wind stress (flat ocean)

•Transfer of kinetic energy from the ocean into heat

•If 100% of the ocean’s kinetic energy were transferred into heat, the ocean temp would rise ~.01C

Again, the coriolis force confounds our intuition by creating currents at right angles to the wind

0

1( )

U PU DEL U fv friction

t x

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Tfv

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If we assume that energy transfer into the interior of the fluid is accomplished by an eddy viscosity process, then it can be shown that:

A = eddy viscosity coeff.

The solutions for u and v indicate that velocity structure is a spiral in z and the surface water moves at 45º angle to the surface (Ekman Motion, after V.W. Ekman)

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T utA

Z zT v

tAZ z

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uA fv

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Four major currents dominate near the equatorial region

Northern Equatorial Current

Southern Equatorial Current

Northern Equatorial Counter Current

20º N

10º S

Equatorial undercurrent (Cromwell Current)

Northern Tradewinds

Southern Tradewinds

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The Northern and Southern equatorial currents range from ~10ºN - 20 ºN and ~0 º - 10 º S

Northern Equatorial Current

Southern Equatorial Current

•Mostly confined to the well mixed surface layer (~200)

•Velocity of the ocean current varies between .3m/s and 1.0m/s

•Below the thermocline, the current speeds are much weaker

•Each current transports ~15 sv

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The geostrophic balance resulting from the coriolus force deflection and the pressure force causes the sea-surface to slop up away from the equator

5º

Sea Surface

Thermocline

Thus, as one looks west (I.e., downstream of the equatorial currents) one will see water accumulating to your right in the

northern hemisphere and to your left in the southern hemisphere

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Between the wind driven northern and southern equatorial currents, there is the relatively narrow (300-500 km wide) eastward-flowing equatorial counter-current •Intertropical convergence (ITC)

•An area of weak and variable winds between the two trade-wind systems

•The counter current exists along the equator were winds are very weak. Thus, allowing water pushed westward by the north and south equatorial currents to flow back eastward.

•The northern and southern equatorial currents pushed eastward by the trade winds pile up water in the western portion of each ocean basin

•The surface ocean is ~.5m higher in the western region of the pacific (by Indonesia) then in the eastern region (by S.A.) In the absence of wind and Coriolis force, gravity pushes the water east.

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The equatorial undercurrent (AKA, The Cromwell Current), is a powerful eastward-flowing subsurface current

Coriolis Force

Coriolis Force Coriolis Force

Coriolis Force

Equator

N

E

•The Cromwell Current can move at speeds of up to 1.5 m/s

•The core of the generally coincides with the thermocline and is generally centered on the equator

•Gravity is one driver of the undercurrent

•The other driver is the stability provided by the Coriolis force

•If an easterly flow is perturbed either north or south, then the Coriolis force tends to drive the current back to the equator

•On the other hand, if a westerly current is perturbed, then the Coriolis force tends to drive the current away from the equator

100

200

100

10~50

Velocity (cm/s)5º5º 0º

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A recent paper by Gordon et al used a complex multi-depth measurement system in the straight of Makassar and determined that the mean ITF water is closer to 15c°

•The surprisingly cool ITF results from a restricted contribution of warm surface water

•The surface layer transport varies strongly with season

•Low southerward speeds are interspersed with periods of northward flow during the northwest monsoon season

•The paper purposes a buoyancy boundary generated by the monsoon season that restricts flow

•During the northern monsoon, a northward salinity gradient is created

As a result, the atmosphere warmed surface water flows less during the northern monsoon and the deeper-cooler waters dominate the ITF flow

Source: Cool Indonesian Throughflow as a Consequence of Restricted Surface Layer Flow, 2003 Nature 824-826