7.4 What are the main surface circulation patterns in each ocean basin?

  • specific pattern of surface currents varies from ocean to ocean, depending on the geometry of the ocean basin, the pattern of major wind belts, seasonal factors, and other periodic changes

Antarctic Circulation

  • dominated by the movement of water masses in the southern Atlantic, Indian, and Pacific Oceans south of about 50 degrees south lat

Antarctic Circumpolar Current (ACC)

  • main current = antarctic circumpolar current aka west wind drift

  • encircles antarctica and flows from west to east @ approx. 50 deg S lat

  • approx 40 deg S lat = subtropical convergence, forms northernmost boundary of antarctic circumpolar current

  • ACC driven by powerful prevailing westerly wind belt, which creates winds so strong that these southern hemisphere latitudes have bene called the “Roaring Forties”, “Furious Fifties”, and “Screaming Sixties”

  • east wind drift driven by polar easterlies, flows around antarctica from east

  • antarctic convergence and divergence are caused by interactions at the boundaries of the east and west wind drifts

  • ACC is the ONLY current that completely circumscribes earth, allowed to do so bc of lack of land barriers at high southern lats

    • greatest restriction through Drake Passage between antarctic peninsula and S islands of South America, approx. 1000 km wide

    • current is not speedy, but transports avg 130 Sverdrups (130 million cubic meters/second), more than any other surface current

Antarctic Convergence and Divergence

  • antarctic convergence (antarctic polar front) = colder, denser, antarctic waters converge w/ and sink below warmer, less dense sub0arctic waters @ about 50 deg S lat

    • marks northernmost boundary of southern ocean

  • east wind drift = current propelled by polar easterlies

    • moves from an easterly direction around the margin of antarctica

    • most extensively developed to the east of the antarctic peninsula in the weddell sea and ross sea regions

  • as west wind drift and east wind drift flow around antarctica in opposite directions, they create a surface divergence

    • coriolis effect: deflects masses to the left in southern hemisphere —> east wind drift is deflected toward continent, west wind is deflected away from continent

    • = antarctic convergence = abundant marine life in the summer bc of the mixing of these two currents, which supplies nutrient-rich water to the surface through upwelling

Antarctic Ocean Circulation

  • surface circulation consists of two large subtropical gyres: North Atlantic and South Atlantic

The North and South Atlantic Subtropical Gyres

  • N atlantic: rotates clockwise

  • S atlantic: rotates counterclockwise

    • both due to effects of trade winds, prevailing westerlies, and coriolis effect

  • each gyre consists of a poleward-moving warm current, and an equator-moving cold “return” current

  • two gyres are partially offset by shapes of surround continents and the Atlantic Equatorial countercurrent moves between them

  • S equatorial current: reaches greatest strength just below the equator, where it encounters the coast of brazil and splits in two

    • part of it moves along northeastern coast of s america toward caribbean sea and N atlantic

    • rest is turned southward = brazil current

      • merges w/ west wind drift

      • brazil current = much smaller than northern counterpart (Gulf Stream) due to splitting of south equatorial current

    • benguela current = slow moving and cold, flows toward equator along africa’s western coast

  • outside gye: Falkland current (Malvinas current), moves a lot of cold water along coast of argentina

Gulf Stream

  • world’s best studied ocean current

  • moves northward along east coast of US, warming coastal states and moderating winters in these and northern european regions

  • network of currents in n atlantic ocean contribute to flow of gulf stream

    • north eqautorial current: moves parallel to equator in northern hemisphere, splits into:

      • antilles current = passes along atlantic side of west indies

      • caribbean current = passes through yucatan channel into guld of mexico

        • both of these converge = florida current

  • 4 major currents in N Atlantic gyre

    • gulf stream, N atlantic current, canary current, N equatorial current

  • florida current: flows close to shroe over continental rate, moves in northeasterly direction = gulf stream, which is a western boundary current —> subject to western intensification

    • only 50-70 km wide, but reaches depths of 1.5 km, and speed from 3-10 km/hr = world’s fastest ocean current

Sargasso Sea

  • gulf stream merges eastward w/ water of sargasso sea

  • sargasso sea = water that circulates around N atlantic gyre’s center of rotation, the stagnant eddy on the western side of the north atlantic gyre

  • name derived from a type of floating marine alga called sargassum that appears on its surface

Warm and Cold Core Rings

  • meanders = snake-like bends in the current that often disconnect from the gulf stream and form large rotating masses of water called vortices, aka eddies or rings

  • meanders near the north boundary of the gulf stream pinch off and trap warm sargasso sea water in eddies that rotate clockwise —> warm-core rings

  • warm-core rings: shallow, bowl-shaped masses of warm water approx. 1km deep, diameters of 100km

    • remove large volumes of water as they disconnect from the gulf stream

  • cold-core rings: cold nearshore water spins off to the south of the gulf stream as counterclockwise rotating rings surrounded by warmer water

    • cold wrings consist of spinning cone-shaped masses of cold water that extend over 3.5 km deep, may exceed 500km in diamater at surface

      • diameter of cone increases with depth and sometimes reaches all the way to the sea floor, where cones have a tremendous impact on sea floor sediment.

  • both warm and cold core rings maintain unique temp characteristics —> contain distinct populations of marine life

    • studies of rings have found they’re isolated habitats for either warm water organisms in a cold ocean, or cold water organisms in a warmer ocean

    • organisms survive as long as the ring does —> up to 2 years

    • cold-core rings associated w/ high nutrient levels and an abundance of marine life; warm core rings are zones of downwelling that lack nutrients and are deficient in marine life

Other North Atlantic Currents

  • gulf stream breaks into numerous branches, many of which become cold and dense enough to sink beneath the surface

    • Labrador Current

    • Irminger Current

    • Norwegian Current

    • North Atlantic Current

    • Canary Current

Climate Effects of North Atlantic Currents

  • gulf stream moderates temps along east coast of US and in northern europe

    • temps across atlantic at diff lats are much higher in europe than in north america bc effects of heat transfer from gulf stream to europe

Indian Ocean Circulation

  • india mostly in southern hemisphere

  • nov-mar, equatorial circulation in indian ocean is similar to that in atlantic ocean, w/ two westward-flowing equatorial currents separated by an eastward-flowing equatorial countercurrent

  • compared to circulation in atlantic, equatorial countercurrent in indian ocean lies in a more southerly position

  • shape of indian ocean basin and proximity to high mountains of asia cause it to experience strong seasonal changes

Monsoons

  • winds of northern indian ocean have a seasonal pattern called monsoon winds

  • during winter, air over asian mainland rapidly cools, creating high atmospheric pressure, causing the wind to blow from southwest asia off the continent and out over the ocean

  • northeast trade winds = northeast monsoon

    • during this season, little precipitation because air associated with high pressure over land is so dry

  • during summer, winds reverse - bc of low heat capacity of rocks and soil compared w/ water, asian mainland warms faster than the adjacent ocean, creating low atmospheric pressure over the continent

    • result: winds blow strongly from indian ocean onto asian landmass = southwest monsoon

      • during this season, there’s heavy precipitation on land bc air brought in from idnian ocean is warm and full of moisture

  • northern indian ocean = only place in world where reversing seasonal winds actually cause major ocean surface currents to switch direction

  • winter: northeast monsoon causes north equatorial current to flor from east to west and its extension, somali current, flows south along coast of africa

  • movement of winds during summertime monsoon affects sea surface temps, which cool near arabian peninsula bc of upwelling as water is drawn away from shore —> large pops of phytoplankton during summer southwest monsoon

Indian Ocean Subtropical Gyre

  • when northeast trade winds blow, south equatorial current provides water for equatorial countercurrent and agulhas current

  • agulhas retroflection = created when agulhas current makes an abrupt turn as it meets ACC

Leeuwin Current

  • eastern boundary currents in other subtropical gyres = cold drifts toward equator producing arid coastal climates

  • Leeuwin current: displaces western australian current (WAC) offshore

    • southerly current

    • produces mild climate

Pacific Ocean Circulation

  • two large subtropical gyres

Normal Conditions

  • “normal” conditions = misnomer bc experienced so infrequently

  • various atmospheric and oceanic disturbances dominate conditions in the pacific

North Pacific Subtropical Gyre

  • NPSG includes north equatorial current, flowing estward into western intensified Kuroshio Current near Asia

    • Kuroshio current —> Japan’s climate warmer than expected for its latitude

  • kuroshio flows —> N Pacific Current —> California current

  • california current flows south along coast of california to complete the loop

South Pacific Subtropical Gyre

  • south equatorial current —> east australian current —> westwind drift —> peru current

Fisheries and the Peru Current

  • some of earth’s richest fishing grounds - why?

    • coastal winds —> Ekman transport moving water away from shore = upwelling of cool, nutrient-rich water

      • upwelling increases productivity and results in an abundance of marine life including anchovies

  • anchovies provide a food source for many larger marine organisms and also supply peru’s commercial fishing industry

Walker Circulation

  • high pressure and sinking air dominate coastal region of S America —> clear, fair, dry weather

  • west side of pacific, low-pressure region and rising air create cloudy conditions w/ plentiful precipitation in indonesia, new guinea, and northern australia

  • pressure difference causes strong southeast trade winds to blow across the equatorial south pacific —> atmospheric circulation cell

    • Walker Circulation Cell

Pacific Warm Pool

  • southeast trade winds move water across the pacific from east to west

  • water warms as it flows in equatorial region, creating a wedge of warm water on the western side of the pacific ocean = pacific warm pool

  • bc of equatorial currents to the west, PWP is thicker along the western side of the pacific than along the eastern side

  • thermocline beneath PWP occurs below 100m deep but in eastern pacific, thermocline is within 30m

El Nino-Southern Oscillation (ENSO) Conditions

  • every few years, a current of warm water reduces population of anchovies off coast of Peru —>

    • dramatic decrease in marine life

    • changes in weather: intense rainfall —> dramatically increased plant growth on land

    • ecological and economic disaster

  • pacific warming first described by peruvian navy captain who reported on an unsuaully warm “corriente del nino” (ocean current of the Christ child) named bc it appeared around christmas time —> “El Nino”

ENSO Warm Phase (El Nino)

  • high pressure along coast of S America weakens, reducing the difference between the high and low pressur regions of the walker circulation cell —> southeast trade winds diminish

    • in very strong el nino events, southeast trade winds blow in the opposite direction

  • without trade winds, pacific warm pool built up on western side of pacific begins to flow back across the ocean toward S America = band of warm water stretching across the equatorial pacific ocean

  • warm water begins moving in september, reaches S America by december or january

  • as warm water increases sea surface temps across equatorial pacific, temperature-senstivie corals are decimated in Tahiti, Galapagos, etc.

  • increases average sea level and # of tropical hurricanes formed in eastern pacific

  • sloped thermocline boundary between warm surface waters and cooler waters below flattens out = more horizontal

  • near peru, upwelling brings warmer, nutrient-depleted water to the surface instead of cold, nutrient-rich water

    • downwelling occurs sometimes

    • productivity diminishes and most types of marine life in the area are dramatically reduced

    • low pressure substantially increases precipitation along coastal S America

ENSO Cool Phase (La Nina)

  • similar to normal conditions, intensified bc larger pressure difference across pacific ocean —> stronger walker circulation and stronger trade winds —> more upwelling, shallower thermocline, and a band of cooler than normal water stretching across equatorial S pacific

  • la nina conditions occur following an el nino

  • ENSO Index: shows alternating pattern of el nino or la nina conditions, calculated using a weighted average of atmospheric and oceanic factors i.e. atmospheric pressure, winds, and sea surface temps

    • (+) = el nino

    • (-) = la nina

    • 0 = normal conditions

How often do el nino events occur?

  • avg 2-10 yrs, highly irregular

  • typically lasts 12-18 months, then followed by la nina which occur for similar length of time

  • some el nino or la nina conditions can last for several years

  • el nino events may occur more frequently bc of global warming

  • PDO Pacific Decadal Oscillatoin: 20-30 years, inclufneces pacific sea surface temps

Effects of El Ninos and La Ninas

  • mild el ninos influence only equatorial S pacific ocean

  • strong el nino events can influence worldwide weather patterns

    • alter atmospheric jet stream —> unusual weather around the world

    • flooding, erosion, droughts, fire, tropical storms, effects on marine life

  • can be beneficial in some areas:

    • tropical hurricane formation —> rain in some much needed desert areas and warm water organisms thrive in pacific

CONCEPT CHECK 7.4

  1. Explain why Gulf Stream eddies that develop northeast of the Gulf Stream rotate clockwise and have warm-water cores, whereas those that develop to the southwest rotate counterclockwise and have cold-water cores.

Eddies rotate clockwise or counterclockwise depending on nature of their origin.

  • warm water cores = trap warm water from sargasso sea

  • cold core = product of warm water currents wrapping around a colder water mass as it breaks away from its current

  1. Describe changes in atmospheric pressure, precipitation, winds, and ocean surface currents during the two monsoon seasons of the Indian Ocean.

Winter (northeast) = asian mainland rapidly cools —> high atmospehric pressure, causing wind to blow from southwest asia off the continent and out over the ocean, little precipitatoin, dry air

Summer (southwest) = lower heat capacity of rocks than water, asian mainland warms faster than adjacent ocean, = low atmospheric pressure over ocean —> winds blow strongly from indian ocean onto asian landmass

  • heavy precipitation on land bc air brought in from indian ocean is warm and full of moisture

  1. Describe changes in atmospheric and oceanographic phenomena that occur during El Niño/La Niña events, including changes in atmospheric pressure, winds, Walker Circulation, weather, equatorial surface currents, coastal upwelling/downwelling and the abundance of marine life, sea surface temperature and the Pacific Warm Pool, sea surface elevation, and position of the thermocline.

El Nino:

  • atmospheric pressure: high pressure weakens —> low pressure

  • winds: diminish, sometimes reverse

  • walker circulation:

  • weather: warm weather

  • equatorial surface currents and PWP: flows back across ocean toward S America —> warm band of water

  • sea surface temps: much much higher

  • sea surface elevation: increases

  • position of thermocline: becomes flatter

La Nina

  • atmospheric pressure: high pressure —> high pressure difference

  • winds: stronger —> more upwelling

  • walker circulation: stronger

  • weather: cold

  • equatorial surface currents:

  • sea surface temps: lower

  • sea surface elevation: lower

  • position of thermocline: shallow

  1. How often do El Nino events occur? Has the pattern of el nino events occurred at regular intervals?

every 2-10 years, but very irregular intervals.

  1. how is la nina different from el nino? Describe the pattern of la nina events in relation to el ninos.

La nina = cooling of surface ocean in eastern pacific following an el nino, strong trade winds

  1. describe the global effects of severe el ninos.

  • alter atmospheric jet strea —> unusual weather in most parts of the globe

    • weather is drier than normal, or wetter

    • flooding, erosion, droughts, fires, tropical storms, effects on marine life

    • some areas get much needed rain