Ocean Surface Circulation

Motion in the Ocean, Part I, or Why does the ocean have currents, and why do they

move in circles?

Two types of Ocean Circulation:

Surface Circulation -- Wind-drivenDeep Circulation -- Density (T,S)

driven

Atmospheric Circulation

Temperature and PressureAs the Earth’s surface is heated, air is

warmed, expands and rises (Low P)Warm air carries water vaporIn the upper atmosphere the air cools and

sinks (High P)Surface winds blow from High P to Low PThis round-trip is called a “cell”

Things get interesting!On a rotating planet, moving

objects appear to be deflectedWhy is this?

Coriolis Deflection Apparent force due to Earth’s rotationDeflection in path of motion when viewed

from a rotating reference frameGustave-Gaspard Coriolis (1835)Familiar from merry-go-roundsSignificant only for large distances

(not toilets and billiards!)

animation

Coriolis Deflection

Consequences of Coriolis

Moving fluids (atmosphere and ocean) turn to the right in the Northern Hemisphere

Moving fluids (atmosphere and ocean) turn to the left in the Southern Hemisphere

Global Wind Circulation

Wind-Driven Ocean Circulation

Steady winds produce waves and set the surface water in motion

Moving water is deflected to the right (N.Hemisphere) or left (S.Hemisphere)

This starts the main “gyre” motion of the surface ocean

Surface Ocean Circulation

Main FeaturesFive large gyresAntarctic Circumpolar CurrentEquatorial CountercurrentVelocities vary -- fastest are

meters/sec

106 m3/sec (Sverdrup) = all the rivers

Gulf Stream - Benj Franklin1760sSailing times

to and from Europe

Gulf Stream from satellite

So, do the gyres just follow the winds?

Not exactly! But the winds get the motion in the ocean started

The oceans respond by flowing and turning

Water piles up in the center of gyres -- several meters high

Global Wind Circulation

Geostrophic Currents

Coriolis deflection plus the Pressure Gradient steers the currents around the

gyres

Northern Hemisphere Gyres

westward intensification

Surface Circulation

Ekman Transport -- moves water 90° to the winds

Upwelling and Oregon’s Ocean

Winter winds from the south -- downwelling

Summer winds from the north -- upwelling

Winter Summer

Oregon’s Summer

Equatorial Divergence

Equatorial Divergence

Antarctic Circulation

How do we track ocean circulation?

Fixed Buoys -- measure current speed and direction

Drifters -- travel with the currents and transmit their location

Beach Swap Meets!

Tracking Currents:The Story of the Lost Nikes 1: 60,000 shoes

spilled, May 1990 2-8: 1990-’91 9: 1993 10: 1994

Marine Debris: Pacific Trash