Ocean Surface Ocean Surface CirculationCirculation

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

why do they move in why do they move in circles?circles?

Two types of Ocean Two types of Ocean Circulation:Circulation:

Surface Circulation -- Wind-Surface Circulation -- Wind-drivendriven

Deep Circulation -- Density Deep Circulation -- Density (T,S) driven(T,S) driven

Atmospheric Atmospheric CirculationCirculation

Temperature and Temperature and PressurePressure

As the Earth’s surface is As the Earth’s surface is heated, air is warmed, expands heated, air is warmed, expands and rises (Low P)and rises (Low P)

Warm air carries water vaporWarm air carries water vapor In the upper atmosphere the air In the upper atmosphere the air cools and sinks (High P)cools and sinks (High P)

This round-trip is called a This round-trip is called a “cell”“cell”

Things get Things get interesting!interesting!

On a rotating planet, moving On a rotating planet, moving objects appear to be objects appear to be deflecteddeflected

Why is this?Why is this?

Coriolis Deflection Coriolis Deflection (aka “I Could Care Less” (aka “I Could Care Less”

force)force) Apparent force due to Earth’s Apparent force due to Earth’s rotationrotation

Deflection in path of motion when Deflection in path of motion when viewed from a rotating reference viewed from a rotating reference frameframe

Gustave-Gaspard Coriolis (1835)Gustave-Gaspard Coriolis (1835) Familiar from merry-go-roundsFamiliar from merry-go-rounds Significant only for large distances Significant only for large distances (not toilets and billiards!)(not toilets and billiards!)

Coriolis “movie”Coriolis “movie”

Coriolis DeflectionCoriolis Deflection

Consequences of Consequences of CoriolisCoriolis

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

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

Global Wind Global Wind CirculationCirculation

Wind-Driven Ocean Wind-Driven Ocean CirculationCirculation

Steady winds produce waves and Steady winds produce waves and set the surface water in motionset the surface water in motion

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

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

Surface Ocean Surface Ocean CirculationCirculation

Main FeaturesMain Features Five large gyresFive large gyres Antarctic Circumpolar CurrentAntarctic Circumpolar Current Equatorial CountercurrentEquatorial Countercurrent Velocities vary -- fastest are Velocities vary -- fastest are meters/secmeters/sec

101066 m m33/sec (Sverdrup) = all /sec (Sverdrup) = all the riversthe rivers

Gulf Stream - Benj Gulf Stream - Benj FranklinFranklin

Gulf StreamGulf Stream

So, do the gyres So, do the gyres just follow the just follow the

winds?winds? Not exactly! But the winds get Not exactly! But the winds get the motion in the ocean the motion in the ocean startedstarted

The oceans respond by flowing The oceans respond by flowing and turningand turning

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

Ekman Transport -- Ekman Transport -- moves water 90° to the moves water 90° to the

windswinds

Upwelling and Oregon’s Upwelling and Oregon’s OceanOcean

Winter winds from the south -- Winter winds from the south -- downwellingdownwelling

Summer winds from the north -- Summer winds from the north -- upwellingupwelling

Winter SummerWinter Summer

Global Wind Global Wind CirculationCirculation

Geostrophic CurrentsGeostrophic Currents

CoriolisCoriolis deflection plus the deflection plus the Pressure Pressure GradientGradient steers the currents around steers the currents around

the gyresthe gyres

Northern Hemisphere Northern Hemisphere GyresGyres

Surface CirculationSurface Circulation

How do we track ocean How do we track ocean circulation?circulation?

Fixed Buoys -- measure Fixed Buoys -- measure current speed and directioncurrent speed and direction

Drifters -- travel with the Drifters -- travel with the currents and transmit their currents and transmit their locationlocation

Beach Swap Meets!Beach Swap Meets!

Tracking Currents:Tracking Currents:The Story of the Lost The Story of the Lost

NikesNikes 1:1: 60,000 shoes 60,000 shoes spilled, May 1990spilled, May 1990

2-8:2-8: 1990-’91 1990-’91 9:9: 1993 1993 10:10: 1994 1994