Geography and Oceanography - Chapter 13 pps

Ocean Surface CirculationMotion 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-

Ocean Surface Circulation

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

why do they move in circles?

Jack Barth (barth@coas.oregonstate.edu)

NASA web site: http://oceanmotion.org

Two types of Ocean Circulation:

➨Surface Circulation Wind-driven

➨Deep Circulation – Density-driven

Density of water is influenced by

Temperature and Salinity, so density-

driven circulation is often called the

“Thermohaline” Circulation

Friday’s lecture

Atmospheric Circulation

Temperature and Pressure

✦ As the Earth’s surface is heated, air is

warmed, expands and rises (Low Pressure)

✦ Warm air carries water vapor

✦ In the upper atmosphere the air cools and sinks (High Pressure)

✦ Surface winds blow from High Pressure to Low Pressure

✦ 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

So, in the frame rotating CCW (like

northern hemisphere), unforced particle

in motion is deflected to the right

If frame rotates CW, motion of particle is

to the left (reverse film)

velocity

Coriolis Force (northern

hemisphere)

velocity

Coriolis Force (southern

hemisphere)

Coriolis Deflection

“During the naval engagement near the Falkland Islands which occurred early in World War I, the British gunners were surprised to see their accurately aimed salvos falling

100 yards to the left of the German ships The designers

of the sighting mechanisms were well aware of the

Coriolis deflection and had carefully taken this into

account, but they apparently were under the impression that all sea battles took place near 50°N latitude, and

never near 50°S latitude The British shots, therefore, fell

at a distance from the targets equal to twice the Coriolis

deflection.”

Jerry B Marion, “Classical Dynamics of Particles and

Systems”, 2 nd edition, 1971.

Global Wind Circulation

westerlies

trades

trades

westerlies

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

Ocean Surface Current Speed

cm/second

How fast is a cm/second?

100 centimeters in a meter; 1000 meters in a kilometer

so 100,000 centimeters per kilometer

24 hrs x 3600 sec/hr = 86,400 sec~100,000 seconds per day

1 cm/second = 1 km/day R Lumpkin (NOAA/AOML)

106 m3/sec (Sverdrup) = all the rivers

Gulf Stream - Benjamin Franklin

1760sSailing 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

westerlies

trades

trades

westerlies

Ekman Transport moves

water 90°to the winds

Ekman (1905)

Geostrophic Currents

Coriolis deflection plus the Pressure Gradient

steers the currents around the gyres

Northern Hemisphere Gyres

westward intensification

~1000

meters

Surface Circulation

Upwelling and Oregon’s Ocean

downwelling

upwelling

Winter Summer

Oregon’s Summer

Thanks to Alan Dennis (COAS/OSU)

Cold, nutrient-rich water near the

Oregon coast: leads to phytoplankton blooms

Barth (2007)

T(ºC)chl

(mg/m3)

Equatorial Divergence

Antarctic Circulation

How do we track ocean

Beach Swap Meets!

Marine Debris: Pacific Trash

What about the debris from

the recent Japanese tsunami?

US Navy photo AFP-Getty Images

How long before debris might

reach the US west coast?

North Pacific Current

~ 10 cm/s ~ 10 km/day

~7300 km

Courtesy of N Maximenko & J Hafner(UH)

about 2 years for the first of it … but much will sink

and enter the North Pacific Garbage Patch

Ocean Surface Circulation

• surface currents driven by winds

• Coriolis and pressure forces result in

oceanic gyres

• wind-driven currents reach down

several 100s of meters up to 1km

• speeds of 10-100 cm/s (0.1-1.0 m/s ~

0.2-2 knots); strongest on western

sides of ocean basins

• Ekman flow away from coast leads to

coastal upwelling and plankton blooms

NASA web site: http://oceanmotion.org