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)

Ocean Surface Circulation

Ocean Surface Circulation

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

why do they move in circles?

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:

Two types of Ocean Circulation:

Surface Circulation -- Wind-drivenDeep Circulation – Density-driven

Density of water is influenced by Temperature and Salinity, so density- driven circulation is often called the“Thermohaline” Circulation

Surface Circulation -- Wind-drivenDeep 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 CirculationAtmospheric Circulation

Temperature and PressureTemperature and Pressure

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

Warm air carries water vaporIn the upper atmosphere the air cools and

sinks (High Pressure)Surface winds blow from High Pressure to

Low PressureThis round-trip is called a “cell”

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

Warm air carries water vaporIn the upper atmosphere the air cools and

sinks (High Pressure)Surface winds blow from High Pressure to

Low PressureThis round-trip is called a “cell”

Things get interesting!Things get interesting!

On a rotating planet, moving objects appear to be deflected

Why is this?

On a rotating planet, moving objects appear to be deflected

Why is this?

Coriolis Deflection 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

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

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 DeflectionCoriolis 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”, 2nd edition, 1971.

Consequences of Coriolis

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

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 CirculationGlobal Wind Circulation

westerlies

trades

trades

westerlies

Wind-Driven Ocean 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

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 CirculationSurface Ocean Circulation

Main FeaturesMain Features

Five large gyresAntarctic Circumpolar CurrentEquatorial CountercurrentVelocities vary -- fastest are

meters/sec

Five large gyresAntarctic Circumpolar CurrentEquatorial CountercurrentVelocities vary -- fastest are

meters/sec

Ocean Surface Current Speed

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 kilometer24 hrs x 3600 sec/hr = 86,400 sec~100,000 seconds per day

1 cm/second = 1 km/dayR. Lumpkin (NOAA/AOML)

106 m3/sec (Sverdrup) = all the rivers

106 m3/sec (Sverdrup) = all the rivers

Gulf Stream - Benjamin FranklinGulf Stream - Benjamin Franklin

1760sSailing times

to and from Europe

1760sSailing times

to and from Europe

Gulf Stream from satelliteGulf Stream from satellite

So, do the gyres just follow the winds?

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

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 CirculationGlobal Wind Circulation

westerlies

trades

trades

westerlies

Ekman Transport -- moves water 90°to the winds

Ekman Transport -- moves water 90°to the winds

Ekman (1905)

Geostrophic CurrentsGeostrophic Currents

Coriolis deflection plus the Pressure Gradient steers the currents around the

gyres

Coriolis deflection plus the Pressure Gradient steers the currents around the

gyres

Northern Hemisphere Gyres

westward intensification

Northern Hemisphere Gyres

westward intensification

~1000meters

Surface CirculationSurface Circulation

Upwelling and Oregon’s Ocean

Upwelling and Oregon’s Ocean

Winter winds from the south -- downwelling

Summer winds from the north -- upwelling

Winter winds from the south -- downwelling

Summer winds from the north -- upwelling

Winter SummerWinter Summer

Oregon’s Summer Oregon’s Summer

Thanks to Alan Dennis (COAS/OSU)

Cold, nutrient-rich water near the Oregon coast: leads to

phytoplankton blooms

Cold, nutrient-rich water near the Oregon coast: leads to

phytoplankton blooms

Barth (2007)

T(ºC)

chl (mg/m3)

Equatorial Divergence Equatorial Divergence

Equatorial Divergence Equatorial Divergence

Antarctic Circulation Antarctic Circulation

How do we track ocean circulation?

How do we track ocean circulation?

Fixed Buoys -- measure current speed and direction

Drifters -- travel with the currents and transmit their location

Fixed Buoys -- measure current speed and direction

Drifters -- travel with the currents and transmit their location

Beach Swap Meets!Beach Swap Meets!

Tracking Currents:The Story of the Lost Nikes

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

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

1: 60,000 shoes spilled, May 1990

2-8: 1990-’91 9: 1993 10: 1994

Marine Debris: Pacific Trash

Marine Debris: Pacific Trash

What about the debris from the recent Japanese tsunami?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?

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 CirculationOcean 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

• 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 bloomsNASA web site: http://oceanmotion.org