Download Lecture 1. Course Introduction

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-driven
Deep Circulation -- Density (T,S) driven
Atmospheric Circulation
Temperature and Pressure
As the Earth’s surface is heated, air is
warmed, expands and rises (Low P)
Warm air carries water vapor
In the upper atmosphere the air cools and
sinks (High P)
Surface winds blow from High P to Low P
This round-trip is called a “cell”
Things get interesting!
On a rotating planet, moving objects
appear to be deflected
Why is this?
Coriolis Deflection
Apparent force due to Earth’s rotation
Deflection in path of motion when viewed from
a rotating reference frame
Gustave-Gaspard Coriolis (1835)
Familiar from merry-go-rounds
Significant 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 Features
Five large gyres
Antarctic Circumpolar Current
Equatorial Countercurrent
Velocities vary -- fastest are meters/sec
106 m3/sec (Sverdrup) = all the rivers
Gulf Stream - Benj Franklin
1760s
Sailing 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