mar 110: introductory oceanography ocean currents

MAR 110: Introductory Oceanography

Ocean currents

Rev. 26 October 2006 Ocean currents 2

There is a river in the ocean. In the severest droughts it never fails, and in the mightiest floods it never overflows. Its banks and its bottom are of cold water, while its current is of warm. The Gulf of Mexico is its fountain, and its mouth is in the Arctic Seas. It is the Gulf Stream.

– From Physical Geography of the Sea


The Gulf Stream, part 1

• Evidence for the existence of the Gulf Stream was known as far back as 1519.

• Hans Resen of Copenhagen drew the first map of the Gulf Stream in 1605, which was based largely on the observations of explorer Martin Frobisher.– The current was subsequently mapped by Athanasius

Kircher in 1678 and by Happelius in 1685.



• Benjamin Franklin, as colonial deputy postmaster general, noticed that ships coming to the colonies from England took longer to cross the Atlantic than those going to England from the colonies.– He consulted a cousin, Timothy Folger, who was a whaling

captain, who had known of the current, and published a map showing the location of the current.

– Franklin, in his own crossings of the Atlantic, measured sea surface temperatures and noticed that the Gulf Stream was warmer than surrounding ocean water.



• Matthew Fontaine Maury made extensive studies of winds and currents in the world’s oceans.– Maury estimated the speed and direction of the Gulf

Stream throughout the year; This information was vital to efficient navigation.

– Knowledge of currents is still important.


Ocean-atmosphere interactions

• Energy and matter are constantly being exchanged between the atmosphere and oceans.– Kinetic energy exchanges in the form of momentum create

horizontal currents as winds drag surface waters along.

– Winds also create vertical currents in the surface layer.

• Evaporation, precipitation, heating, and cooling alter temperature, salinity, and density of ocean waters.– The density changes create deeper currents in the oceans;

these deeper currents form part of the thermohaline circulation.


Vertical structure of oceans, part 1

• The ocean is divided into three horizontal depth zones based on density (except at high latitudes):– A mixed layer, usually down to about 100 m, where wind-

driven surface currents that are subject to the influence of wind, precipitation, evaporation, heating, and cooling; the mixed layer has a relatively uniform density.

– A pycnocline, where water density increases rapidly with depth, usually between 500 and 1,000 m in depth.

• In areas where temperature changes are most responsible for density changes, the pycnocline also serves as a thermocline.

• In areas where salinity changes are most responsible for density changes, the pycnocline also serves as a halocline.


Vertical structure of oceans, part 2

• Three horizontal depth zones (continued):– The deep layer is dark and cold; density increases gradually

with depth.• Water moves sluggishly through the deep layer; only rarely does

the water move fast enough to count as a current.

• The deep layer accounts for most of the mass of ocean water.


Stability

• The pycnocline is relatively stable, that is, it tends to persist in its original state – little vertical motion of the water – except when disturbed by a major event, such as a strong storm.– Unstable systems will shift toward more stable systems.

– Neutrally stable systems will not return to the original state following a disturbance, thus remain easily mixed.


Forces

• The wind sets surface waters in motion.• Once in motion, the following modify the speed and

direction of the currents:– Coriolis effect

– Ekman transport

– Configuration of the ocean basins


Ekman transport, part 1

• Ripples or waves generate the surface roughness needed for wind to interact with water.

• The frictional drag of wind against the ocean surface generates currents.

• On a non-rotating Earth, a thin layer of surface water would travel in the same direction as the wind.– The effect would propagate downward, but diminish at

depth.



• One a rotating Earth, the Coriolis effect deflects the motion of the water (to the right of the wind direction in the Northern Hemisphere and to the left of the wind direction in the Southern Hemisphere).

• This effect propagates downward, and diminishes at depth, but creates a spiraling motion of water.



• Water motion is theoretically deflected about 45 degrees relative to wind direction at the surface, but net transport of water is 90 degrees relative to wind direction; actual deflection/transport is usually less than theoretical amounts.– This transport of water is called Ekman transport; it piles

up water in some places and removes it from others.


Gyres, part 1

• Movement of ocean surface currents generally mirrors the general circulation of the atmosphere.– The bulk of ocean currents are driven by the tropical

easterlies and midlatitude westerlies.

• The interactions of the wind belts with ocean surface waters sets into motion circular currents called gyres.– The Coriolis effect contributes to the circular motion.

– Subtropical gyres rotate in a clockwise manner in the Northern Hemisphere and in a counterclockwise manner in the Southern Hemisphere.


Gyres, part 2

• Ekman transport in the gyres displaces water toward the center of rotation; this essentially piles water in the centers, creating a mound of water – with high pressure in the center.– Water in the center flows downhill due to gravity

(essentially a pressure gradient force), thus outward from the gyre.

– When the pressure gradient force and Coriolis effect are balanced, geostrophic flow results.


Wind-driven currents, part 1

• There are five subtropical gyres: in the North Pacific, South Pacific, North Atlantic, South Atlantic, and Indian oceans.– The northeastward flowing portions of the North Atlantic

and North Pacific gyres, the Gulf Stream and Kuroshio currents, respectively, are among the swiftest surface currents with velocities ranging from 3 to 4 km/hr.

• The southward flowing arms, the California and Canary currents, respectively, are relatively slugging with velocities as low as 1 km/hr.



• Subtropical gyres (continued):– The South Atlantic and South Pacific gyres behave much

as their Northern Hemisphere counterparts.

– The Indian Ocean gyre varies more than the others because of the effect of the monsoons.

– The surface waters in the center of the gyres (centers of the ocean basins) are dominated by light winds or calm conditions.

• Fair weather and high temperatures lead to high evaporation rates, which leads to increasing salinity of surface waters.

• The Sargasso Sea is an example.



• Prevailing winds drive the Antarctic Circumpolar Current, which flows toward the east.– The Antarctic Circumpolar Current marks the boundary

between the Southern Ocean and the Atlantic, Indian, and Pacific oceans to the North.

• Subpolar gyres are smaller than their subtropical counterparts.– The Alaska gyre in the North Pacific is an example.



• Equatorial currents are driven westward by the trade winds.– Some of the water is returned to the eastern part of ocean

basins by equatorial counter currents and equatorial under currents.

– The currents lie in the neighborhood of the Intertropical Convergence Zone, which generally remains north of the equator.

– Equatorial counter currents are at the surface, while equatorial under currents flow beneath the surface.



• Equatorial currents (continued):– Atlantic and Pacific equatorial currents cross the equator

from south to north, thus transporting Southern Hemisphere water to the Northern Hemisphere.

– The Brazil Current is an example.

– The Indonesia Archipelago marks the boundary between the Indian and Pacific Oceans, but waters are exchanged between the islands from the Pacific to the Indian Ocean.

• The Pacific water replaces Indian Ocean water lost through evaporation.



• Western boundary currents move faster than their eastern counterparts.– Part of the reason lies in the intensification of the Coriolis

effect with increasing latitude.• Western boundary currents flow toward the poles.

– Transport of water toward the western margin of the oceans piles water up along western shorelines, thus steeping the elevation gradient of the water surface and increasing the pressure gradient force, which in turn increases the strength of geostrophic flow.



• Western boundary currents (currents):– Western boundary currents are stronger in the Northern

Hemisphere because of more extensive land barriers to westward flow.

– Western boundary currents separate warm tropical open-ocean waters from cool coastal waters.

• Rings are cold- or warm-core eddies that spawn in a turbulent fashion off western boundary currents or the Antarctic Circumpolar Current.– Marine organisms can be isolated within rings.



• Rings (continued):– Rings originate as meaners that are cut off from the main

current.

– They extend to some depth.

– Warm-core rings spawn on the cold side of a current, off the northern side of the Gulf Stream, for example.

• Warm-core rings are typically 100 to 200 km in diameter.

• Warm-core rings may reach as deep as 1,500 m, but may be shallower over continental shelves.

• Ekman transport piles water up in the center of the ring.

• Under certain circumstance, they may bring unusual organisms to the coast.



• Rings (continued):– Cold-core rings spawn on the warm side of a current, off

the southern side of the Gulf Stream, for example.• Cold-core rings may reach 300 km in diameter.

• Cold-core rings may reach as deep as 4,000 m.

• Ekman transport spreads water away from the center of the ring; this may result in upwelling of nutrient-rich water at the center, thus resulting in a more diverse array of marine life inside the ring than outside it.

– Rings move more slowly than their parent currents.• Some may be resorbed by the parent current, especially in confined

waters.



• Rings (continued):– Some of the largest rings form in the Gulf of Mexico Loop

Current.

– The Indian Ocean’s Agulhas Current also spawns large rings.


Upwelling, downwelling, part 1

• In coastal areas of some oceans and inland seas, winds, the Coriolis effect, Ekman transport, and constrained flow caused by shorelines and shallow bottoms cause net vertical movement of water to or from the surface.– Coastal upwelling occurs when Ekman transport moves

surface waters away from the coast.

– Coastal downwelling occurs when Ekman tranport moves surface waters toward the coast.

– Upwelling and downwelling illustrate mass continuity.



• Upwelling is most common along the west coast of continents, such as along California, Northwest Africa, Chile, Peru, or Southwest Africa.

• Upwelling and downwelling also occur in the open ocean where winds cause waters to diverge (causing upwelling) or converge (causing downwelling).

• Upwelling and downwelling affect sea surface temperatures and biological productivity.– Nutrients from the depths are brought to the surface as a

result of upwelling, thus boosting productivity.



• Weakening and strengthening of upwelling off the coast of Ecuador and Peru during the waxing and waning of El Niño events can have drastic effects on fisheries.

• Such weakening and strengthening of upwelling can affect storm development off coastlines.


Thermohaline circulation, part 1

• The thermohaline circulation is a deep-ocean circulation driven by changes in temperature and salinity, hence density, of ocean water.

• Deep-ocean and near-bottom currents are strongest near the western sides of ocean basins – the strongest overall is in the western North Atlantic.– The waters originate in the polar regions.

• Sea-floor topography influences the flow of near-bottom water.– Mid-ocean ridges obstruct flow.


Thermohaline circulation, part 2

• Deep-ocean waters move slowly to the surface, and vice versa.– Bottom water in the Pacific is about 1,500 years old.

– The time required for the overall conveyor circulation is about 1,000 years.


Monitoring deepwater, part 1

• Casts that bring up water and sediment samples, as well as measurements of water characteristics, have yielded most of what we know about deep water.

• ARGO floats can measure the characteristics and movements of currents at depth.– They are fitted with instruments to measure the

characteristics of the water, and with transmitters to send the data to a satellite while on the surface.

– They descend to a specified depth (as deep as 2,000 m) and drift with the current for a specified time before surfacing to transmit data to satellites.


Monitoring deepwater, part 2

• Acoustic tomography provides information on temperature patterns, among other characteristics.

• Soundings provide depth data.


Water masses, part 1

• A water mass is a large, homogenous volume of water having characteristic values of temperature and salinity.

• Most deep-ocean water masses form at high latitudes as cold, dense polar water sinks.

• Water masses sink to the point where their density equals that of surrounding water, then they move horizontally.

• Water masses are defined on the basis of their region of origin as well as their relative depth.



• Source regions: Atlantic, Pacific, Indian oceans• Depth

– Central waters: warm, wind-driven surface waters down to the pycnocline.

– Intermediate waters: between 1 km and 2 km in depth.

– Deep and bottom waters: deeper than 2 km.



• The Atlantic subsurface circulation is best understood.– Deep and intermediate waters form in high latitudes.

– Warm intermediate, salty water comes from the Mediterranean Sea.

– Antarctic bottom water (ABW) forms primarily in the Weddell Sea, sinks to the bottom, and flows north as far as 45 degrees N.

– Antarctic deep water is slightly warmer and less dense, and is sandwiched between the ABW and the North American Deep Water (NADW).



• Atlantic subsurface circulation (continued):– Intermediate waters typically form between the polar deep

waters and waters associated with subtropical gyres.

– Mediterranean Intermediate Water (MIW) forms in the Western Mediterranean as dry Mistral winds cool the surface waters and enhance evaporation.

• Deep-water movements in the Pacific are less well understood.

• Indian ocean deep circulation is similar to that of the Pacific.


Oceanic conveyor belt, part 1

• The global oceanic conveyor belt connects the surface and thermohaline regimes, transporting heat and salt on a planetary scale and regulating both long- and short-term climatic cycles.

• Starting from Greenland and Iceland, where cold, dry winds blow form northern Canada chill surface waters. This, along with evaporation and sea-ice formation increases density of the water, which produces North Atlantic Deep Water.



• NADW sinks and flows southward along the continental slopes of the Americas toward Antarctica.

• The NADW flows eastward around Antarctica and mixes with AABW and AADW to form Common Water (Antarctic Circumpolar Water).

• Common Water flows northward into the three ocean basins to the north (mostly the Indian and Pacific), where it warms and mixes with overlying waters.

• This mixed water slowly rises to the surface.



• At the surface, the waters in the Pacific flow through the Indonesian Archipelago into the Indian Ocean, eventually entering the Agulhas Current.

• The Agulhas Current flows around Africa into the South Atlantic Ocean.

• In the Atlantic, the water becomes saltier by evaporation. Trade winds transport some of the the water vapor across the Isthmus of Panama into the Pacific.



• The Atlantic surface waters eventually return north to the Greenland and Labrador seas.

• If the thermohaline circulation shuts down or weakens, climatic chaos may result. The Little Ice Age (1400 to 1850 C.E.) may have been triggered by a weakening of the thermohaline circulation.

mar 110: introductory oceanography ocean currents

Documents

density of ocean waters

water density

density changes

surrounding ocean water

mass of ocean water

horizontal currents

cold density

surface layer