Oceans and the Global Environment: Lec 2

taking physics and chemistry outdoors

the flowing, waving ocean

Peter Rhines1 Eric Lindahl2

Bob Koon2, Julie Wright3

www.ocean.washington.edu/courses/has221a-08

The ocean circulation is largely driven from the top…the sea The ocean circulation is largely driven from the top…the sea surface. There the atmosphere exerts both forces (from winds surface. There the atmosphere exerts both forces (from winds and pressure variations) and thermal forcing (heating, cooling, and pressure variations) and thermal forcing (heating, cooling, evaporation, precipitation, freezing, melting of ice). These twevaporation, precipitation, freezing, melting of ice). These two o primary mechanisms interact strongly. About the only other primary mechanisms interact strongly. About the only other external agent is the tidal forcing by moon and sun, and some external agent is the tidal forcing by moon and sun, and some very weak heating from below, from the warmth of the solid very weak heating from below, from the warmth of the solid Earth.Earth.

windswinds

the colors represent low (blue) pressure and high (red) pressurecenters. The ‘lows’ are storms generated over the oceans.

Snapshot of wind stress in the northern Atlantic.Snapshot of wind stress in the northern Atlantic.The right panel shows the analysis from the grid of The right panel shows the analysis from the grid of meterologicalmeterological observations, observations, while the left panel shows a more accurate picture from the satewhile the left panel shows a more accurate picture from the satellite llite ‘‘scatterometerscatterometer’ radar instrument known as ’ radar instrument known as QuikScatQuikScat. This is an intense low. This is an intense low--pressure storm developing south of Greenland pressure storm developing south of Greenland

the average winds blowing on the sea surface, over a year, the average winds blowing on the sea surface, over a year, look like thislook like this

Josey et al. J. Physical Oceanography 2002

And..And.. the result is this circulation of the upper few hundred meters the result is this circulation of the upper few hundred meters of the oceans, more or less in the of the oceans, more or less in the direction of the winds: ‘gyres’ that look like the wind yet are direction of the winds: ‘gyres’ that look like the wind yet are concentrated on the western sides of the concentrated on the western sides of the oceans in currents like the Gulf Stream and oceans in currents like the Gulf Stream and KuroshioKuroshio (‘Black Current’) off Japan. This is the pressure (‘Black Current’) off Japan. This is the pressure field (simply related to the average height of the sea surface, field (simply related to the average height of the sea surface, by the hydrostatic pressure rule). Think by the hydrostatic pressure rule). Think of these as flow lines, with the currents forming clockwiseof these as flow lines, with the currents forming clockwisegyres in the northern subtropics, and antigyres in the northern subtropics, and anti--clockwise gyres at high northern latitude. The sense reversesclockwise gyres at high northern latitude. The sense reversesin the southern hemisphere, since the Earth’s rotation takes thein the southern hemisphere, since the Earth’s rotation takes the opposite sense ‘downopposite sense ‘down--under’.under’.

Because the flow between to adjacent flow lines involves the same volume of fluid flow, thespeed of the currents is greater where the lines bunch together. The Gulf Stream and Kuroshioare ‘boundary currents’, very rapid (~ 2 to 4 knots…nautical miles per hour) as a result

the Southern Ocean connects the Atlantic, Pacific and Indian othe Southern Ocean connects the Atlantic, Pacific and Indian oceans, and ceans, and the Antarctic Circumpolar Current circles round Antarctica in rethe Antarctic Circumpolar Current circles round Antarctica in response to sponse to

the very strong westerly (‘eastward’) windsthe very strong westerly (‘eastward’) winds

Since 1992 it has been possible to see this circulation from spaSince 1992 it has been possible to see this circulation from space, using radar altimeters on the ce, using radar altimeters on the TopexTopex/Poseidon /Poseidon and JASON satellites of NASA and the European Space Agency. Thisand JASON satellites of NASA and the European Space Agency. This a view of the global distribution of sea a view of the global distribution of sea surface height. The gyres of circulation are not so clear becausurface height. The gyres of circulation are not so clear because this instrument shows only the timese this instrument shows only the time--varying part varying part of the pressure field. NASA is working to determine the backgroof the pressure field. NASA is working to determine the background ‘mean circulation’ missing here, using a und ‘mean circulation’ missing here, using a remarkable technology of the GRACE satellite mission.remarkable technology of the GRACE satellite mission.

What you do see are thousands of swirling eddies which propagWhat you do see are thousands of swirling eddies which propagate slowly westward and sometimes originate inate slowly westward and sometimes originate inthe the intensestintensest currents…the Gulf Stream, currents…the Gulf Stream, KuroshioKuroshio, and Antarctic Circumpolar Current, and Antarctic Circumpolar Current

figure from Dudley Chelton, Oregon State Univ.

The layers of ocean are not horizontal: so they must be moving. The layers of ocean are not horizontal: so they must be moving. The forces acting on them are few: gravity and the stress of theThe forces acting on them are few: gravity and the stress of thewind overhead. Pressure variations within the ocean act towind overhead. Pressure variations within the ocean act toredistribute these external forces (like people elbowing oneredistribute these external forces (like people elbowing oneanother in a crowd). So, how would you expect the pressure and another in a crowd). So, how would you expect the pressure and velocity of the ocean to be related?velocity of the ocean to be related?

ocean

atmosphere

higher pressurelower pressure

?

The peculiar laws of Newtonian physics, when applied to a rotatiThe peculiar laws of Newtonian physics, when applied to a rotating planet, say that a body tends ng planet, say that a body tends to move perpendicular to the force exerted on it….not exactly buto move perpendicular to the force exerted on it….not exactly but largely so. Here we have a t largely so. Here we have a ring of air (or water) centered on the North Pole. A northward fring of air (or water) centered on the North Pole. A northward force moves it toward the Pole. orce moves it toward the Pole. The angular momentum, H, of the ring is conserved: the product The angular momentum, H, of the ring is conserved: the product of distance from the rotation of distance from the rotation axis, and total eastaxis, and total east--west velocity. ‘Total velocity’ means the sum of the wind spwest velocity. ‘Total velocity’ means the sum of the wind speed relative to eed relative to the Earth, call it u, plus the Earth’s eastward velocity, the Earth, call it u, plus the Earth’s eastward velocity, Ωr. Ωr.

So So H = H = ruru + Ωr+ Ωr22 does not change, as r changes: as r decreases,does not change, as r changes: as r decreases,u increases, corresponding to an acceleration of the eastward wiu increases, corresponding to an acceleration of the eastward winds. This can be callednds. This can be calledthe ‘figurethe ‘figure--skater effect.’skater effect.’

The Earth spins at a rate Ω meaning that the length of the day is2π/Ω. So Ω = 2 π/24 hours

or 7.27 x 10-5 sec-1

Ω is called the angularvelocity

upwellingupwelling

As a result of this EarthAs a result of this Earth--rotation effect, the very surface of the ocean (say, rotation effect, the very surface of the ocean (say, the top 25 meters) flows perpendicular to the winds blowing on ithe top 25 meters) flows perpendicular to the winds blowing on it. This is t. This is called the ‘called the ‘EkmanEkman flow’. Along the US west coast, in winter the winds flow’. Along the US west coast, in winter the winds blow to the north, causing flow into the coast, and blow to the north, causing flow into the coast, and downwellingdownwelling at the at the

coast. In summer, the winds largely come from the northwest, blcoast. In summer, the winds largely come from the northwest, blowing the owing the top of the ocean offshore, and sucking up deep, cold waters fromtop of the ocean offshore, and sucking up deep, cold waters from below.below.

Thus the picture we had of the ocean circulation applies below tThus the picture we had of the ocean circulation applies below this ‘his ‘EkmanEkmanlayer’ in the top 25m of ocean. That thin layer is moving at riglayer’ in the top 25m of ocean. That thin layer is moving at right angles to the ht angles to the wind’s force (purple arrows). This horizontal flow at the very wind’s force (purple arrows). This horizontal flow at the very top of the ocean top of the ocean forces forces vertical vertical flowflow up from the deep at high latitude, and down into the deep up from the deep at high latitude, and down into the deep in subtropical latitudesin subtropical latitudes

This image from the This image from the SeaWiFSSeaWiFS satellite of NASA shows the primary productivity of the world’ssatellite of NASA shows the primary productivity of the world’s oceans and land surface in late summer oceans and land surface in late summer of 1997. The satellite looks at the color of the ocean surface uof 1997. The satellite looks at the color of the ocean surface using several spectral bands and turns this into an estimate of sing several spectral bands and turns this into an estimate of chlorophyll…plant life chlorophyll…plant life invovlinginvovling phytoplankton. The greens and then reds are high chlorophyll aphytoplankton. The greens and then reds are high chlorophyll and the blues are low…’ocean deserts’.nd the blues are low…’ocean deserts’.

Notice how closely the blue regions correspond to Notice how closely the blue regions correspond to downwellingdownwelling regions forced by the winds, and the regions of growth occur regions forced by the winds, and the regions of growth occur where the water where the water upwellsupwells to the surface, bring nutrients up into the sunlight. The to the surface, bring nutrients up into the sunlight. The EkmanEkman flow is horizontal yet it forces these vertical flow is horizontal yet it forces these vertical currents where it is ‘divergent’ or ‘convergent’. currents where it is ‘divergent’ or ‘convergent’.

The Equator is visible: think about the reason for upwellinThe Equator is visible: think about the reason for upwelling there where the winds are dominantly westward. El g there where the winds are dominantly westward. El NiňoNiňo events in the events in the Pacific occur when these westward (that is ‘easterly’) winds diePacific occur when these westward (that is ‘easterly’) winds die down, and the upwelling fails.down, and the upwelling fails.

Notice also the upwelling driven biology along the coasts aNotice also the upwelling driven biology along the coasts and in shallow seas. Animations of these images show the seasonnd in shallow seas. Animations of these images show the seasonsspassing, with the region of plankton growth sweeping passing, with the region of plankton growth sweeping polewardpoleward in springtime. in springtime.

The desert regions of Earth’s land surface also are stronglThe desert regions of Earth’s land surface also are strongly related to vertical velocity yet for very different reasons; y related to vertical velocity yet for very different reasons; try to imagine try to imagine where the atmosphere is rising and hence moisture rains out, andwhere the atmosphere is rising and hence moisture rains out, and where it is sinking, and clouds and rain disappear. where it is sinking, and clouds and rain disappear.

Visit NASA at Visit NASA at http://http://oceancolor.gsfc.nasa.govoceancolor.gsfc.nasa.gov//

The Greenland waters reaching out over the Labrador Sea also carry strong primary productivity with them…as seen in SeaWIFS ocean color (May 2004). We have been working in this region with roboticSeagliders, and have identified the physical circulation that enables the dominant plankton bloom of spring, seen here from the SeaWiFS satellite (again, yellow to red is high biological activity, blues are low activity).

2005 days 91-120

It is not just plankton, but the entire food chain lights up in It is not just plankton, but the entire food chain lights up in upwelling zones. At sea in the upwelling zones. At sea in the Labrador Sea the first thing you notice (other than the severe wLabrador Sea the first thing you notice (other than the severe weather) is the bird life. eather) is the bird life.

They are there because there is food. And large animals are therThey are there because there is food. And large animals are there too.e too.