Ocean Circulation I

I. Ekman Spiral/Transport Revisited

• recall that when winds blow over water, they create drag on the surface; this surface drag sets a thin layer of water into motion which in turn drags on the thin layer beneath it, setting it into motion too. ETC. ETC.
• such transfers of momentum are inefficient and energy is lost in the process; thus current speed decreases with increasing depth (surface currents move at ~2% of the speed of the wind that caused them)
• also because water is more dense than air, surface waters are deflected to the right of wind






• movements of surface water are represented by a series of vectors (denoting speed), where each layer is further deflected right of the layer above it
• strength of deflection will be based on both wind strength and depth of the water
• in shallow waters, wind-generated currents are deflected less; also winds are generally not constant enough to set up a full Ekman spiral
• thus, net movement of waters is 90° to the right of the wind in the Northern Hemisphere (or to the left in the Southern Hemisphere) -- this is called Ekman transport

II. Geostrophic Currents

 

prevailing winds move surface waters toward subtropical regions both from tropical areas and from high-latitude areas  these subtropical gyres are zones of convergence where surface waters accumulate here water piles up, forming slopes and surface layers thicken
	where winds blow surface waters away from an area, get divergence  here subsurface waters move upward to replace waters that have moved away from the region, causing the surface layer to be thinner than normal upwellings are special cases of divergences

• get convergences and divergences in polar, tropical, and subtropical regions
• as a result of convergences and divergences, ocean surface has a subtle topography and the slopes respond very slowly to wind shifts
• thus, ocean currents are equally slow to change which means that ocean topography responds to winds over many days and essentially winds up "storing" pulses of energy from wind and storms
• water responds to the slopes of the ocean surface as it would on land -- it runs downhill, but because of earth's rotation it is deflect and most of it runs parallel to the side of the hill
• balance between gravitational forces, making water run down the hill, and the Coriolis effect, making it deflect, gives rise to geostrophic currents which are strongest on the steepest slopes and weakest on the gentle slopes






• remember there are boundary currents (those that flow close and parallel to the continental margins and usually run north-south)
• western boundary currents are strongest; they extend to depths of around 1 kilometer and are too deep to come up onto a continental shelf -- thus they flow along continental margins
• include the Gulf Stream (N. Atlantic); Kuroshio Current (N. Pacific); Brazil Current (S. Atlantic); Agulhas Current (Indian Ocean); East Australian Current (S. Pacific)
• western boundary currents become intensified because earth's rotation displays gyres towards the west and presses the gyres against continents (which makes the sea surface slopes more steep on western sides of ocean basins); additionally, trade winds blow generally westward along the equator and this piles up water on the western sides of equatorial oceans
• also because western boundary currents generally move northward or southward up/down from the equator, they have higher-than-average speeds (remember the relative speed increases as you go towards the poles even though the equator is moving faster around the earth than either of the poles)

• eastern boundary currents are much weaker and more diffuse; they carry cold water equatorward
• include: Canary Current (N. Atlantic); Benguela Current (S. Atlantic); California Current (N. Pacific); West Australian Current (Indian Ocean); Peru/Humbolt Current (S. Pacific)
• they are also shallower and can therefore run over continental shelves
• eastern boundary currents will have slower than normal speeds because they move from the poles toward the equator
• also remember that there are transverse currents -- those that flow from east to west or west to east and link the western and eastern boundary currents
• include the North and South Equatorial Currents (in Atlantic and Pacific, respectively); the North Pacific Current; the North Atlantic Current; the West Wind Drift (aka Antarctic Circumpolar Current)

III. Meanders and Rings

• meanders are bends in a current's flow path; generally occur in response to coastal configurations or bottom topography
• rings are large, whirl-like eddies formed by a cut off meander in a strong current; they are associated with western boundary currents and extend to 2 km depth (so can't intrude upon a continental shelf); they trap warm or cold water within their centers
• get both warm-core rings (anticyclonic/ clockwise) and cold-core rings (cyclonic/ counterclockwise)
• transport heat, momentum, dissolved constituents, and weakly swimming organisms
• BUT they do NOT transport water

 

small current rings are called eddies and they are common in the western portions of the Atlantic and Pacific; currents in eddies are weaker than those in rings, but are 100 times more energetic than average deep-ocean currents

IV. Undercurrents/Countercurrents

• countercurrents are surface currents flowing in the opposite direction from the main current and are commonly found in equatorial currents (e.g. in North Pacific see westward flow of the equatorial currents but also have a separate countercurrent sandwiched in between the two equatorial currents)
• if countercurrents exist under the surface, they are called undercurrents; these are found under most major currents and are not well understood

V. Other Currents

• monsoon currents are found primarily in the Indian Ocean when winds over the ocean change direction to blow from the south and southwest -- this causes water to flow clockwise (instead of counterclockwise) and the normally westward flowing North Equatorial Current reverse flow to become the eastward-moving Southwest Monsoon Current
• is temporary but has dramatic consequences for climate in the region (get alternations of droughts and then heavy monsoon, flooding rains)
• high-latitude currents flow in apparent contradiction to the Coriolis force
• form when larger currents are split by land masses and the polar easterlies keep the water moving westward
• flow rates are primarily influenced by the tendency of water to flow around obstacles
• include the Alaska and Oyashio Currents in the Pacific and the Greenland, Labrador and Norwegian Currents in the Atlantic

VI. Langmuir Cells/Circulation

• winds blowing steadily across the ocean cause small waves which then result in long sets of counter-rotating cells in the surface water
• these cells form rows aligned in the direction of the wind
• where the rows result in convergence of adjacent vortices, you get streaks of foam, seaweed, or debris -- these are called windrows
• areas of divergence of adjacent vortices remain clear (so get windrows, clear areas, and then windrows again)
• these were observed and described by Irving Langmuir in 1938 and were named Langmuir circulation in his honor; they only operate in the upper 20 m of the surface waters

VII. El Niño/La Niña

• during "normal" years, the intense trade winds cause warm surface waters to pile up in the western tropical Pacific, while cold water is exposed to the surface in the east
• thus surface currents at the equator are also intense and westward in their movements
• also waters warmer than 28°C normally cover one-third to one-half of the tropical oceans, primarily in equatorial eastern Indian and western Pacific oceans, while cooler surface waters predominate the Pacific coast of South America and the equatorial ocean

• but during an El Niño year, the trade winds relax and with this relaxation of the winds, comes a relaxation of the pile up of warm water to the west
• this warm water flows downhill (geostrophically) and surges eastward -- this surge often causes the westward equatorial currents to reverse direction, redistributing warm surface waters eastward
• because this shift occurs around Christmas, local fisherman named it El Niño -- Spanish for the Christ Child

• what causes the relaxation of the trade winds?
• at the beginning of the 20th century, Sir Gilbert Walker initiated research on monsoons in India, particularly years when monsoons failed (didn't come -- this caused severe famines in India)
• he knew that interannual pressure fluctuations over Indian Ocean and eastern tropical Pacific were out of phase so when pressure was high in the Pacific, it was low in the Indian Ocean from Africa to Australia
• he called this the Southern Oscillation and correlated it with major changes in rainfall patterns and wind fields over the tropical Pacific and Indian Oceans and with temperature fluctuations in southeastern Africa, southwestern Canada, and the southern U.S.
• it wasn't until sea-surface temperature data became widely available that people were able to determine whether the ocean was involved in this oscillation -- that confirmation didn't come until 1957-58
• the oscillation consists of two phases: El Niño and La Niña; for this reason it is often called an ENSO event (El Niño-Southern Oscillation)
• over regions of high sea-surface temperatures (and high land temperatures) air tends to rise, so that low-level winds converge onto these areas -- winds carry moisture, evaporated from the ocean and during the rising phase, condensation of that moisture occurs, and clouds and heavy rains result
• during a La Niña, the region of rising air is confined to the western tropical Pacific where sea-surface temperatures are high; Ekman transport in the Southern Hemisphere causes water to move to the left, away from the coast, which causes coastal upwellings and high productivity
• during an El Niño (lasts 1-2 years) the warm surface waters spread eastward and so does the region of heavy rainfall; this pushes water back onto the eastern coasts disrupting the upwelling and causing a thermal stratification of the ocean




 
 
• the oscillation is the result of a positive feedback loop: during a period of intense trade winds, a small disturbance (which is not well understood) causes the winds to relax somewhat and reverse -- these wind reversals only last for a few days to a few weeks usually in November through April and are sometimes accompanied by strong typhoons; the reversals are weak every year, but strong every 7 to 10 years
• some of the warm surface waters that are piled up in the west will tend to surge eastward and the associated change in the sea-surface temperature pattern will cause a further relaxation of the winds so that even more warm water moves eastward
• the atmosphere responds rapidly to changes in sea-surface temperatures, BUT the ocean takes far longer (many months) to adjust to a change in the winds -- this is what sets up the oscillation because the oceanic conditions are not simply determined by the winds present at that time, but on the winds previously
• thus, El Niños are terminated by the "memory" of the wind patterns that prevailed during a La Niña and vice versa
• the consequences of El Niños are global:
• areas of heavy rainfall shift from Indonesia eastward to the normally dry islands of central equatorial Pacific -- this causes sea level changes (because the warm water is surging eastward) combined with heavy rains and thus flooding of the low lying islands
• there are fewer hurricanes in the Atlantic so areas that depend upon the hurricanes for summer rainfall experience droughts
• because rainfall leaves the Indonesian area, get droughts in Indonesia, Southeast Asia, and India (monsoon failure)
• thick layers of warm waters at the surface that flow eastward displacing the cold waters disrupt the process that causes abundant growth of microscopic marine organisms
• since many fish populations depend on these organisms, their populations are reduced and fisheries crash
• loss of the fish populations negatively impact bird populations, such that nest failures occur and chicks are abandoned; also seabird nests on the ground often are flooded and destroyed (results in the death of all the chicks in one year)
• at the end of an El Niño, surface currents return to their normal conditions, but they often first overshoot that condition, resulting in unusually cold waters along the Peruvian coast and in the eastern equatorial Pacific -- this unusually cold water along the equator causes the intertropical convergence zone (the doldrums) to shift northward, which then displaces the jet stream and leads to droughts in the U.S.