Chapter 6c Geostrophic currents

Redistribution of heat on the Earth surface

Thermal gradients – redistributing heat across the surface of the Earth

Two main processes:

atmospheric transport – relatively fast, but the heat capacity of air is not great

oceanic transport – relatively slow, but moves a tremendous amount of heat

On average over the course of a year (or several years), the atmosphere and the ocean transport about the same amount of heat from the equator to the poles

Southern Hemisphere

Large, fairly stationary atmospheric high-pressure system in the eastern South Pacific

The northeastern side of this high-pressure system produces Trade Winds blowing parallel to the Peru coast – this will be important for coastal upwelling

Video clip: water vapor transport

http://svs.gsfc.nasa.gov/vis/a000000/a000100/a000172/a000172.mpg

The 1995 Hurricane season May-October

Things to look for:

equatorial convection – Northern Hemisphere summer

low-pressure systems on Polar Front

both north and south

stable high-pressure system off Peru

spawning Hurricanes in eastern Atlantic

Today’s weather

Showing the effect of the jet stream along the Polar Front, and a subtropical jet coming from the eastern equatorial Pacific

12-hour loop for continental US weather

http://www.weather.unisys.com/satellite/sat_ir_enh_us_loop-12.html

Distribution of low- and high-pressure systems over North America

http://www.weathertap.com/unprotected/static/services_about_surfanal.html

Distribution of water vapor, and transport from equatorial region by subtropical jet stream

http://www.weather.unisys.com/satellite/sat_wv_us_loop-12.html

Satellite view of Western Hemisphere – shows series of lows along Polar Front, tropical convection cells

http://www.weather.unisys.com/satellite/sat_ir_enh_hem_loop-12.html

Jet stream map – large waves in the Jet Stream propagate along the Polar Front, outbreaks of cold air from Canada into US

http://virga.sfsu.edu/crws/jetstream.html choose different views

Geostrophic currents

Surface ocean:

Geostrophic – geo “Earth” strophic “turned”

surface currents driven by a balance

between a pressure gradient and Coriolis acceleration

Also acts on moving air masses in the atmosphere

Ekman transport

-- example in Northern Hemisphere

wind blowing across the water moves the first layer of water 45° to right of the wind

the first layer of water moving sets the next layer down moving,

at a slight angle to the right

3-4% of wind velocity transferred to the water

net transport of the upper ~100 m of the water is 90° to right of the wind

this was discovered by watching the movement of icebergs

Ekman transport – open ocean

strong, persistent wind from the Trade Winds and Westerlies sets up large-scale Ekman transport in the open ocean

equatorial divergence – Ekman transport driven by Trade Winds moves surface water away from the equator both north and south

convergence in the central gyre – example of the North Atlantic gyre

Trade winds & westerlies

Westerlies set up Ekman transport to south, Trades set up Ekman transport to north, the transport converges in the center of the gyre, producing a “mound” of water

Creating a geostrophic current – Pressure offset by Coriolis

Starting with a mound of water, the pressure gradient is “downhill”

Once the water starts moving down the pressure gradient, Coriolis bends it to the right

The Coriolis force balances the pressure gradient, and the net movement of the water is around the mound

Dynamic topography of the North Atlantic

Map of the sea-surface topography

The highest part of the North Atlantic is in the Sargasso Sea, near the Bahamas – the mound is offset to the west because of the rotation of the Earth

Geostrophic currents in the North Atlantic

Western Boundary Current – the Gulf Stream

steep slope to sea surface

current is narrow, deep, and fast

Eastern Boundary Current – the Canary Current

gentle slope

current is wide, shallow, and slow

Formation of Gulf Stream rings

Big, looping meanders of the Gulf Stream will pinch off, producing “spinners”

Think of these as high-pressure and low-pressure cells

The physics works in reverse: spinning water will induce vertical flow

Cold-core ring – cyclonic spin (counter-clockwise, N. Hemis.)

acts as a low-pressure system, draws cold water up to surface from mid depths

draws nutrients to the surface, commonly have phytoplankton blooms inside cold-core rings

Warm-core ring – anticyclonic spin (clockwise, N. Hemis.)

acts as a high-pressure system, pushes warm surface water down

nutrient-depleted surface water, little phytoplankton productivity within warm-core ring

For some further explanation, and example satellite images: http://dcz.gso.uri.edu/amy/avhrr.html

Equatorial Pacific currents

*** Since your textbook no longer includes a discussion of these equatorial currents, you won’t be responsible for them on the exam ***

Four main currents:

NEC – North Equatorial Current

SEC – South Equatorial Current

These two are geostrophic currents produced by large-scale transport of surface water driven by the Trade Winds, from east to west on either side of the equator

ECC – Equatorial Counter Current

A geostrophic current that flows from west to east at the surface about 5° north of the equator

Relatively weak compared to the other three

EUC – Equatorial Under Current

A high-velocity jet flowing from west to east right at the equator, between about 100 and 300 m depth

This type of current can only exist at the equator, for fluid moving west to east

Driven by flow down the pressure gradient caused by the pile of water in the western equatorial Pacific

Once the water is moving eastward, Coriolis “pinches” the flow toward the equator (bends north in S. Hemis., bends south in N. Hemis.)

The strongest, fastest sustained current in the ocean