1. Introduction to Oceanography Dynamic Oceanography: Currents and Water masses

2. Geostrophic flow allows currents to flow long distances with no apparent Coriolis deflection. Coriolis deflects water into the center of the gyres, forming a low mound. As height of the mound increases, the pressure gradient steepens pushing the water outward in an attempt to level the mound. When the pressure gradient equals coriolis deflection, the current flows parallel to the wind around the mound as a geostrophic current and this is called geostrophic flow. Gyres in the northern hemisphere rotate clockwise and in the southern hemispheres counterclockwise. 6-2 Surface Ocean Currents

5. The current flow pattern in gyres is asymmetrical with narrow, deep and swift currents along the basin’s western edge and broad, shallow slower currents along the basin’s eastern edge. The geostrophic mound is deflected to the western part of the ocean basin because of the eastward rotation of the Earth on its axis. The Sargasso Sea is a large lens of warm water encircled by the North Atlantic gyre and separated from cold waters below and laterally by a strong thermocline. Western boundary currents, such as the Gulf Stream, form a meandering boundary separating coastal waters from warmer waters in the gyre’s center. 6-2 Surface Ocean Currents

6. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Water flow in the Gulf Stream and the Canary Current, parts of the North Atlantic gyre. Currents Within Gyres

7. The Gulf Stream

8. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Geostrophic Gyres Geostrophic gyres are gyres in balance between the pressure gradient and the Coriolis effect. Of the six great currents in the world’s ocean, five are geostrophic gyres.

9. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. What currents are found within gyres? Western boundary currents -These are narrow, deep, fast currents found at the western boundaries of ocean basins.  the Gulf Stream  the Japan Current  the Brazil Current  the Agulhas Current  the Eastern Australian Current Currents Within Gyres

10. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. What currents are found within gyres? Eastern boundary currents –These currents are cold, shallow and broad, and their boundaries are not well defined.  the Canary Current  the Benguela Current  the California Current  the West Australian Current  the Peru Current Currents Within Gyres

11. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Currents Within Gyres

12. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Countercurrents and Undercurrents Countercurrents flow on the surface in the opposite direction from the main currents. Undercurrents are countercurrents that flow beneath the surface current.

13. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Wind induced vertical circulation is vertical movement induced by wind- driven horizontal movement of water. Upwelling is the upward motion of water. This motion brings cold, nutrient rich water towards the surface. Downwelling is downward motion of water. It supplies the deeper ocean with dissolved gases. Upwelling And Downwelling

16. Langmuir circulation is a complex horizontal helical (spiral) motion that extends parallel to the wind. Adjacent helices rotate in opposite directions creating alternating zones of convergence and divergence. Material floating on the surface becomes concentrated in the zones of convergence and form sea stripes which parallel the wind direction. 6-2 Surface Ocean Currents

18. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Surface currents distribute tropical heat. The warm ocean currents are shown in red and cold currents in blue. Effects of Surface Currents on Climate

19. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. El Niño In an El Niño year, the trade winds diminish and reverse. Storms over land may increase in these years. An El Niño YearA Non-El Niño Year

20. Thermohaline circulation is a density driven flow of water generated by differences in salinity or temperature. Water at the surface is exposed to more rapid changes in salinity through evaporation or precipitation and in temperature through cooling or heating. Once water is isolated from the atmospheric influences, salinity and temperature are largely set for an extended period of time. Based upon depth, surface water masses can be broadly classified as Central waters (from 0 to 1 km), Intermediate waters (from 1 to 2 km), and Deep and bottom waters (greater than 2 km). Most deep and bottom water originated at the surface where cooling and increased salinity raised their density until they sank. Ocean basins interconnect and exchange water with each other and with the surface. Inter-ocean basin circulation and exchange between surface and deep water appears largely driven by waters of the North Atlantic. 6-3 Deep-Ocean Circulation

21. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. A general temperature-salinity (T-S) diagram. The Temperature-Salinity Diagram

22. 6-3

23. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. The movement of water due to different densities is thermohaline circulation. Remember that the ocean is density stratified, with the densest water at the bottom. There are five common water masses:  Surface water  Central water  Intermediate water  Deep water  Bottom water Thermocline Circulation

24. Water Masses: AABWAntarctic Bottom Water AADWAntarctic Deep Water AAIWAntarctic Intermediate Water CoWCommon Water ECWEquatorial Central Water MIWMediterranean Intermediate Water NABWNorth Atlantic Bottom Water NACWNorth Atlantic Central Water NADWNorth Atlantic Deep Water NPCWNorth Pacific Central Water NPIWNorth Pacific Intermediate Water PSWPacific Subarctic Water RSIWRed Sea Intermediate Water SACWSouth Atlantic Central Water SICWSouth Indian Central Water SPCWSouth Pacific Central Water

25. = height of Angel Falls, Venezuela 5x10 6 m 3.sec -1 of water flow over the Denmark Strait (cf. Guairá Falls, 0.013x10 6 m 3.sec -1 ) Underwater waterfalls!

26. The major thermohaline currents appear to flow mainly equatorward, but this is because they originate in the polar regions and their outward flow is confined between the continents. Warmer water (>10 o C) is confined between 45 o north and south latitude. Poleward of 45 o, density of water increases because of declining temperature and increased salinity because of evaporation or ice formation. The water sinks to a density-appropriate level and then slowly flows outward in all directions across the basin until they are blocked by a continent. 6-3 Deep-Ocean Circulation

27. Deep water gradually mixes with other water masses and eventually rises to the surface. The Atlantic Ocean has the most complex ocean stratification containing the following layers: Antarctic Bottom Water, Antarctic Deep Water, North Atlantic Deep Water, Arctic Intermediate Water, and Mediterranean Intermediate Water The Pacific Ocean has a less complex stratification, is weakly layered, displays sluggish circulation and is remarkably uniform below 2000m. The Indian Ocean has the simplest stratification consisting of Common Water, Antarctic Intermediate Water, and Red Sea Intermediate Water. 6-3 Deep-Ocean Circulation

28. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. A model of thermocline circulation caused by heating in lower latitudes and cooling in higher latitudes. Thermohaline Circulation Patterns

31. Most seas are indentations into continents, partially isolated from the ocean and strongly influenced by continental climate and river drainage. As Atlantic Ocean water flows through the Straits of Gibraltar into the Mediterranean Sea at the surface, warm, highly saline Mediterranean Sea water flows out through the Straits at the bottom. In the Black Sea the surface water is brackish because of excess precipitation and river inflow. 6-4 Water Flow in Semi-enclosed Seaways

34. © 2002 Brooks/Cole, a division of Thomson Learning, Inc. How can scientists study currents? Float methods depend on the movement of a free-floating object. Flow methods measure the current as it flows past a fixed object CODAR is a groundwave radar detection system that fixes on small wave movements. The Gulf of Maine is one of the first experimental sites for a large scale test of CODAR Studying Currents