OCEAN CIRCULATIONOCEAN CIRCULATION

As the world turns.

Ocean currents

• Three kinds– 1. surface currents - Fig 7.5 P 198

Ocean currents cont.

• Three kinds– 2. sub-surface currents - Fig 7.28 p 225

Ocean currents cont.• Three kinds

– 3. boundary currents - (Remember Fig. 6.11)

Ocean currents cont.

• Defined as: nearly closed current patterns in ocean basins -subtropical gyres - centered in the subtropical region north and south of the equator and polar gyres best developed in the Atlantic waters between Greenland and Europe and in the Weddell sea off Antarctica

• Monsoons in Indian Ocean cause seasonally variable currents Fig 7.18 p 214 (book) (remember monsoons from last time)

• A body in motion tends to remain in motion unless acted upon by another force.

• Think continents and ocean waters.

CORIOLIS EFFECT

• Quick review:

• What held for the atmosphere, holds for currents in the oceans.

• At the north pole– rocket goes straight up– then comes straight down one hour later

• Rockets speed to east or west is what ever the land was doing at the time of launch

CORIOLIS EFFECT cont

• Rocket from pole shoot for 30 degrees N (Canary Islands)– Earth rotates ~1400 Km/hr at Canary Islands– The Rocket lands 1400 Km west of Islands one

hour later

CORIOLIS EFFECT cont

• Rocket starts at the Equator– Equator is moving faster than the Canary

Islands at 1600 Km/hr– The rocket lands 200 Km east of Canary

Islands

Ekman spiral Fig 7.7 P 201

• Caused by steady wind blowing across water surface, therefore wind driven

• Currents diminish in strength and rotate to right in northern hemisphere with increasing depth because of Coriolis effect

• USE (L) INDEX FINGER TO INDICATE WIND DIRECTIONAND CUPPED HAND TO INDICATE EARTH ROTATION

Ekman transport

• is perpendicular to wind direction S&A 54 WIND DRIVEN

• Take out time to carefully read the section on ocean gyres Table 7.1 p 199 Fig. 7.5 p 198 (wind driven currents)

Geostrophic currents

• currents where water flow due to gravity is balanced by deflection of Coriolis effect (difficult concept) Fig 7.8 P 203, S&A 55

• 1. Remember Ekamn transport always turns water currents to the right in Northern Hemisphere, and this clock wise rotation tends to produce a convergence of water in the middle of the gyre.

Geostrophic currents cont.

• 2. Water piles up in the center of the gyres - can be > 1 meter above the water level at the margins of the gyres T 85

Geostrophic currents cont.

• 3. As Ekman transport continually pushes water into the hill, gravity also acts to counter this effect moving water down the surface of the slope Fig 7.8 p 203. Coriolis effect deflects the water flowing down the slope to the R in Northern Hemisphere

Geostrophic currents cont.• 4. Sargasso Sea is the classic example of

gyre in geostrophic balance (home work print off a page showing the 1. location of the Sargasso Sea and 2. its biota for next time)

• 5. Ocean surface topography caused by winds (Ekman transport) mapped using data on temperature and salinity remember what a topographic map is.

• 6. Ocean current model SA 55

geostrophic flow• refers to cyclonic fluid motions that are

maintained as a result of a near balance between a gravity-induced horizontal pressure gradient and the Coriolis effect

BOUNDARY CURRENTS

more changeable than the major currents Fig 7.5 p 198 currents

Table 7.2 p 204 (remember what currents I want you to know)

Western boundary currents• strongest in oceans - well developed in the

Northern Hemisphere• Gulf Stream - 20o C salinity around 36%o

• Deep, narrow, swift - can not come up on continental shelf (western intensification)

• Intensified by Earth's rotation (clock wise)

Western boundary currents cont.

• Separated from adjacent slower-moving waters are oceanic fronts, which are marked by changes in water 1. color, 2. temperature, and 3. salinity

Eastern boundary currents

• weaker than western boundary currents

• Broad, shallow, slow-moving - can readily flow over continental margins

• Often associated with up welling areas

• Arctic Ocean currents

• Ocean currents follow the same hydraulics as rivers: wide - slow; narrow - fast

WESTERN INTENSIFICATION OF CURRENTS

• Four processes acting together intensify western boundary currents Fig 7.8 a p 203 S&A 55– 1. earth's rotation

– 2. trade winds

– 3. strong westerlies

– 4. Coriolis effect

WIND INDUCED CIRCULATION• Up welling and down welling, Fig 7.13 p. 207 T

87 Caused by Ekman processes Fig 7.6, 7.7 in the book p 201

• Up welling Fig 7.10 p 206 (book) equatorial upwelling

• Down welling

• Fig 7.11 p 206 (book) Water that piles up in gyres sinks slowly to the bottom

GOOGLE/ You tube

• Click on WALLA!! Many good things and pictures about ocean circulation. I am impressed.

Rings and meanders

• Fig. 7.17 p 213• Pronounced meanders of western boundary

currents• Break off to form isolated rings that move with

surrounding waters• Best known associated with Gulf Stream• cold core rings occur in Sargasso Sea warm core

rings occur in slope waters• Eddies are equivalent to atmospheric storms - are

weaker than rings

Langmuir circulation

Langmuir circulation• Near-surface phenomenon see previous slide• Caused by strong winds - water rotates at a

very low rate• Causes small-scale up welling and down

welling forms straight rows parallel to the direction of the wind trapping plants etc. in zones of convergence between the cells (can be 100 m in length) - little plant material at zones of divergence (you can see Langmuir circulation on Lake Michigan)

• Announcements• 1. You should have your topic chosen for the next abstract

and critique. The paper is due the first class period after break.

• 2. The library is open during break, so you should schedule an appointment for ‘Got Research’, this is worth 20 points quiz grade.

Thermohaline circulation• (temperature salinity) circulation - currents

controlled by density See this on YouTube• http://www.youtube.com/watch?feature=endscree

n&NR=1&v=LkRQjTdTvFE–

– Fig 7.27 p 224

–

–

–

More neat things to watch

• www.youtube.com/watch?v=o8nezU0M7mI

Convergence/divergence circulation

(Fig 7.5 p 198)

– Convergence

– Divergence

Pacific Ocean surface currents• Fig 7.20 p 216 (book)

• Box 7.1 p 206 Read this in the book.

Atlantic Ocean surface currents

• Fig. 7.16 p 211 (Benjamin Franklin)

• Read again at Box 7.2 p 210

Salt lenses

• Subsurface equivalent to rings and eddies. Mediterranean Water eddies, or “meddies”, are large, warm, isolated lenses of highly saline Mediterranean Water that are found in the North Atlantic ocean.

• Studied by acoustic tomography

• Seen in geostrophic circulation Fig 7.8 p 203

Salt lenses

Density Structure

• Fig 7.27 p 224

Water column

• stable - dense below

• unstable - dense above (water tips over)

• We talked about this in the chapter on water.

Depth changes

• pycnocline Fig 5.24 p 153– density increases rapidly with depth

• thermocline Fig 5.25 p 154– rapid temperature change with depth (this can

be several meters thick)

• Thermohaline Fig 5.25 p 154– change in temperature and salinity with depth

Depth changes

• Halocline Fig 5.22 p 151ß– large changes in salinity with depth (read to

class)

• (temperature salinity) circulation –

• Dense water masses form in high latitudes

• (convergence) Fig 5.25 p 154–

Putting it all together

• Put in picture of thermalcline and helocline

Surface Layer Temperatures• see what wind does

• Go to google, type in thermocline changes seasons: click on images

• summer -

• fall and spring –

• Winter:

• Another look at ocean circulation

• CLICK HERE