Oceans and the Global Environment: Lec 2 taking physics and chemistry outdoors the flowing, waving ocean Peter Rhines 1 Eric Lindahl 2 Bob Koon 2, Julie Wright 3 www.ocean.washington.edu/courses/has221a-08
The ocean circulation is largely driven from the top the sea surface. There the atmosphere exerts both forces (from winds and pressure variations) and thermal forcing (heating, cooling, evaporation, precipitation, freezing, melting of ice). These two primary mechanisms interact strongly. About the only other external agent is the tidal forcing by moon and sun, and some very weak heating from below, from the warmth of the solid Earth.
winds the colors represent low (blue) pressure and high (red) pressure centers. The lows are storms generated over the oceans.
Snapshot of wind stress in the northern Atlantic. The right panel shows the analysis from the grid of meterological observations, while the left panel shows a more accurate picture from the satellite scatterometer radar instrument known as QuikScat.. This is an intense low- pressure storm developing south of Greenland
the average winds blowing on the sea surface, over a year, look like this Josey et al. J. Physical Oceanography 2002
And.. the result is this circulation of the upper few hundred meters of the oceans, more or less in the direction of the winds: gyres that look like the wind yet are concentrated on the western sides of the oceans in currents like the Gulf Stream and Kuroshio ( Black Current ) off Japan. This is the pressure field (simply related to the average height of the sea surface, by the hydrostatic pressure rule). Think of these as flow lines, with the currents forming clockwise gyres in the northern subtropics, and anti-clockwise gyres at high northern latitude. The sense reverses in the southern hemisphere, since the Earth s rotation takes the opposite sense down-under. Because the flow between to adjacent flow lines involves the same volume of fluid flow, the speed of the currents is greater where the lines bunch together. The Gulf Stream and Kuroshio are boundary currents, very rapid (~ 2 to 4 knots nautical miles per hour) as a result
the Southern Ocean connects the Atlantic, Pacific and Indian oceans, o and the Antarctic Circumpolar Current circles round Antarctica in response to the very strong westerly ( eastward ) winds
Since 1992 it has been possible to see this circulation from space, using radar altimeters on the Topex/Poseidon and JASON satellites of NASA and the European Space Agency. This a view of the global distribution of sea surface height. The gyres of circulation are not so clear because this instrument shows only the time-varying part of the pressure field. NASA is working to determine the background mean circulation missing here, using a remarkable technology of the GRACE satellite mission. What you do see are thousands of swirling eddies which propagate ate slowly westward and sometimes originate in the intensest currents the Gulf Stream, Kuroshio,, and Antarctic Circumpolar Current figure from Dudley Chelton, Oregon State Univ.
The layers of ocean are not horizontal: so they must be moving. The forces acting on them are few: gravity and the stress of the wind overhead. Pressure variations within the ocean act to redistribute these external forces (like people elbowing one another in a crowd). So, how would you expect the pressure and velocity of the ocean to be related? atmosphere ocean? lower pressure higher pressure
The peculiar laws of Newtonian physics, when applied to a rotating planet, say that a body tends to move perpendicular to the force exerted on it.not exactly but t largely so. Here we have a ring of air (or water) centered on the North Pole. A northward force f moves it toward the Pole. The angular momentum, H, of the ring is conserved: the product of distance from the rotation axis, and total east-west velocity. Total velocity means the sum of the wind speed relative to the Earth, call it u, plus the Earth s eastward velocity, Ωr. So H = ru + Ωr 2 does not change, as r changes: as r decreases, u increases, corresponding to an acceleration of the eastward winds. This can be called the figure-skater effect. The Earth spins at a rate Ω meaning that the length of the day is 2π/Ω. So Ω = 2 π/24 hours or 7.27 x 10-5 sec -1 Ω is called the angular velocity
upwelling
As a result of this Earth-rotation rotation effect, the very surface of the ocean (say, the top 25 meters) flows perpendicular to the winds blowing on it. i This is called the Ekman flow. Along the US west coast, in winter the winds blow to the north, causing flow into the coast, and downwelling at the coast. In summer, the winds largely come from the northwest, blowing the top of the ocean offshore, and sucking up deep, cold waters from below.
Thus the picture we had of the ocean circulation applies below this t Ekman layer in the top 25m of ocean. That thin layer is moving at right angles to the wind s force (purple arrows). This horizontal flow at the very top of the ocean forces vertical flow up from the deep at high latitude, and down into the deep in subtropical latitudes
This image from the SeaWiFS satellite of NASA shows the primary productivity of the world s oceans and land surface in late summer of 1997. The satellite looks at the color of the ocean surface using u several spectral bands and turns this into an estimate of chlorophyll plant life invovling phytoplankton. The greens and then reds are high chlorophyll and a the blues are low ocean deserts. Notice how closely the blue regions correspond to downwelling regions forced by the winds, and the regions of growth occur where the water upwells to the surface, bring nutrients up into the sunlight. The Ekman flow is horizontal yet it forces these vertical currents where it is divergent or convergent. The Equator is visible: think about the reason for upwelling g there where the winds are dominantly westward. El Niňo events in the Pacific occur when these westward (that is easterly ) winds die down, and the upwelling fails. Notice also the upwelling driven biology along the coasts and a in shallow seas. Animations of these images show the seasons passing, with the region of plankton growth sweeping poleward in springtime. The desert regions of Earth s land surface also are strongly y related to vertical velocity yet for very different reasons; try to imagine where the atmosphere is rising and hence moisture rains out, and where it is sinking, and clouds and rain disappear. Visit NASA at http://oceancolor.gsfc.nasa.gov oceancolor.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 robotic Seagliders, 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 upwelling zones. At sea in the Labrador Sea the first thing you notice (other than the severe weather) w is the bird life. They are there because there is food. And large animals are there e too.