MAR 110: Introductory Oceanography

1. MAR 110: Introductory Oceanography 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 Ocean currents

3. 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. Ocean currents

4. The Gulf Stream, part 2 • 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. Ocean currents

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7. The Gulf Stream, part 3 • 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 currents

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9. 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. Ocean currents

10. 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. Ocean currents

11. 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. Ocean currents

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13. 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. Ocean currents

14. 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 Ocean currents

15. 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. Ocean currents

16. Ekman transport, part 2 • 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. Ocean currents

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18. Ekman transport, part 3 • 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. Ocean currents

19. 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. Ocean currents

20. 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. Ocean currents

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22. 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. Ocean currents

23. Wind-driven currents, part 2 • 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. Ocean currents

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25. Wind-driven currents, part 3 • 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. Ocean currents

26. Wind-driven currents, part 4 • 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. Ocean currents

27. Wind-driven currents, part 5 • 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. Ocean currents

28. Wind-driven currents, part 6 • 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. Ocean currents

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30. Wind-driven currents, part 7 • 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. Ocean currents

31. Wind-driven currents, part 8 • 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. Ocean currents

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33. Wind-driven currents, part 9 • 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. Ocean currents

34. Wind-driven currents, part 10 • 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. Ocean currents

35. 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. Ocean currents

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38. Upwelling, downwelling, part 2 • 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. Ocean currents

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40. Upwelling, downwelling, part 3 • 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. Ocean currents

41. 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. Ocean currents

42. 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. Ocean currents

43. 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. Ocean currents

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48. Monitoring deepwater, part 2 • Acoustic tomography provides information on temperature patterns, among other characteristics. • Soundings provide depth data. Ocean currents

49. 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. Ocean currents

50. Water masses, part 2 • 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. Ocean currents