CHAPTER 7 Ocean Circulation Fig. CO7

Ocean currents Large-scale moving seawater Surface ocean currents Transfer heat from warmer to cooler areas Similar to pattern of major wind belts Affect coastal climates Deep ocean currents Provide oxygen to deep sea Affect marine life

Types of ocean currents Surface currents Wind-driven Primarily horizontal motion Deep currents Driven by differences in density caused by differences in temperature and salinity Vertical and horizontal motions http://www.global-greenhouse-warming.com/images/thermohaline.jpg

Measuring surface currents Direct methods Floating device tracked through time Fixed current meter Indirect methods Pressure gradients Radar altimeters Satellites measuring bulges which are due to shape of ocean flow and currents Doppler flow meter (Acoustic Doppler Current Profiler) Measures shift in frequency of sound waves to determine current movement Fig. 7.1a http://www.pmel.noaa.gov/tao/images/nxcur.gif

Measuring deep currents Floating devices tracked through time Chemical tracers (radioactive isotopes) Tritium Chlorofluorocarbons Characteristic temperature and salinity Arrays of bottom monitors and cables

Surface currents Frictional drag between wind and ocean Wind plus other factors such as… Distribution of continents Gravity Friction Coriolis effect cause Gyres or large circular loops of moving water http://static.howstuffworks.com/gif/ocean-current-4.jpg

Ocean Gyres World’s 5 subtropical gyres: North Atlantic Gyre South Atlantic Gyre North Pacific Gyre South Pacific Gyre Indian Ocean Gyre

When talking about location of currents, we refer to the ocean basin – not the land! For instance, the Gulf Stream is a Western Boundary current of the North Atlantic Even though it runs along side the eastern US

Ocean gyres 3 2 4 1 4 2 Subtropical gyres Center of each is about 30o N or S Major parts Equatorial currents flow west Western Boundary currents flow north or south Northern or Southern Boundary currents – push water east Eastern Boundary currents flow north or south 3

Other surface currents Equatorial countercurrents flow east, counter to equatorial currents in the subtropical gyres As trade winds push equatorial current of subtropical gyre west, water builds up in western part of ocean basin Since coriolis effect is minimal at equator, that build-up of water then flows east at equator Subpolar gyres flow eastward Fig. 7.5

Other factors affecting surface currents Ekman transport Geostrophic currents Western intensification of subtropical gyres All of these are connected Fig. 7.5

Ocean Circulation To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Ekman spiral and transport Surface currents move at angle to wind Ekman spiral describes speed and direction of seawater flow at different depths Each successive layer moves increasingly to right (N hemisphere) Initial push of water and then Coriolis effect comes into play

Ekman spiral and transport Average movement of seawater under influence of wind affected by Coriolis effect 90o to right of wind in Northern hemisphere 90o to left of wind in Southern hemisphere

Ekman Spiral and Coastal Upwelling/Downwelling To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Geostrophic flow http://maritime.haifa.ac.il/departm/lessons/ocean/ Ekman transport piles up water within subtropical gyres Surface water flows downhill (gravity) and Also to the right (Coriolis effect) Balance of downhill and to the right causes net geostrophic flow around the “hill” Fig. 7.8

Western intensification Top of hill of water displaced toward west due to Earth’s rotation  water piles up on western side of gyre Western boundary currents intensified Faster Narrower Deeper Warm http://maritime.haifa.ac.il/departm/lessons/ocean

Eastern Boundary Currents Eastern side of ocean basins Tend to have the opposite properties of Western Currents Slow Wide Shallow Cold http://maritime.haifa.ac.il/departm/lessons/ocean

Ocean currents and climate Warm ocean currents warm air at coast Creates warm, humid air on coast Humid climate on adjoining landmass Cool ocean currents cool air at coast Cool, dry air Dry climate on adjoining landmass August temperatures February temperatures

Ocean currents and climate August temperatures February temperatures Fig. 7.9

Diverging surface seawater Divergence winds move surface seawater away from geographical equator due to Coriolis effect Deeper seawater (cooler, nutrient-rich) flows up to replace surface water Equatorial Upwelling High biological productivity Fig. 7.10

Converging surface seawater Convergence surface seawater moves towards an area Surface seawater piles up Nutrient depleted surface water moves downward Downwelling Low biological productivity Occurs at center of gyres Fig. 7.11

Coastal upwelling and downwelling Winds blowing down coasts Ekman transport moves surface seawater… onshore (downwelling) or… offshore (upwelling) Fig. 7.12

Upwelling at higher latitudes Remember there is no pycnocline at the poles Therefore, water moving towards the poles cools and becomes more dense, like all of the other water at the poles This allows there to be significant vertical mixing Very productive waters – this is where large animals (such as whales) go to feed

Antarctic circulation Antarctic Circumpolar Current (West Wind Drift) Encircles Earth Transports more water than any other current East Wind Drift from polar easterlies Antarctic Divergence from opposite directions of west and east wind drifts Antarctic Convergence piles up and sinks below warmer sub-Antarctic waters Fig. 7.14

Atlantic Ocean circulation North Atlantic Subtropical Gyre Gulf Stream (GS) North Atlantic Current Canary Current (C) North Equatorial Current (NE) Merges with S. Equatorial current to form Antilles and Caribbean currents  Converge to form Florida Current (F) thru Florida Strait May form Loop current into Gulf of Mexico Atlantic Equatorial Counter Current (EC) – between North and South Equatorial currents No. Atlantic current F

North Atlantic Subtropical Gyre http://www.amnh.org/exhibitions/permanent/ocean/images/01_dioramas/features/05_sargasso/overview_df.jpg North Atlantic Subtropical Gyre Sargasso Sea Center of gyre – eddy Sargassum “weed” accumulates in convergence Provides habitat for many marine animals http://www.safmc.net/Portals/0/Sargassum/Sargassum_Ross1.jpg http://upload.wikimedia.org/wikipedia/commons/b/b4/Sargasso.png

Fig. 7.16

Atlantic Ocean circulation South Atlantic Subtropical Gyre South Equatorial Current (SE) Brazil Current (Br) Antarctic Circumpolar Current (WW) Benguela Current (Bg) Fig. 7.14

Gulf Stream Best studied Meander is a bend in current  may pinch off into a loop Warm-core rings form to north Cold-core rings form to south Unique biological populations Fig. 7.17b

Warm core ring Warm core ring forming Cold core rings http://sam.ucsd.edu/sio210/gifimages http://ocw.mit.edu/NR/rdonlyres/Global

Core ring vertical profiles http://kingfish.coastal.edu/marine/gulfstream

Warm core ring off “Loop Current” in the Gulf of Mexico http://storm.rsmas.miami.edu/~nick/

In 2005, winds of Hurricanes Katrina and Rita increase as they pass over warm loop current in Gulf of Mexico http://en.wikipedia.org/wiki/ http://ccar.colorado.edu/~leben/

Other North Atlantic currents Labrador Current Irminger Current Norwegian Current North Atlantic Current

Climate effects of North Atlantic currents Gulf Stream warms East coast of U.S. and Northern Europe North Atlantic and Norwegian Currents warm northwestern Europe Labrador Current cools eastern Canada Canary Current cools North Africa coast

Pacific Ocean circulation North Pacific subtropical gyre Kuroshio North Pacific Current California Current North Equatorial Current Alaskan Current Fig. 7.19

Figure 7.19

Pacific Ocean circulation South Pacific subtropical gyre East Australian Current Antarctic Circumpolar Current Peru Current South Equatorial Current Equatorial Counter Current

Relationship of sea surface temp and high altitude pressure Atmospheric and oceanic disturbances in Pacific Ocean – El Niño-Southern Oscillation (ENSO) Relationship of sea surface temp and high altitude pressure Normal conditions Air pressure across equatorial Pacific is higher in eastern Pacific Strong southeast trade winds Pacific warm pool on western side Thermocline deeper on western side Upwelling off the coast of Peru bring nutrient-rich waters to surface As, you are looking at these figures pay attention to where thermocline is (remember that raising thermocline leads to upwelling which brings nutrients and oxygen-rich waters up to surface for organisms

Normal conditions Fig. 7.20a

El Niño-Southern Oscillation (ENSO) Warm (El Niño) High pressure in eastern Pacific weakens Weaker trade winds In strong El Nino events, trade winds can actually reverse Warm pool migrates eastward Thermocline deeper in eastern Pacific Downwelling  lowers biological productivity in East Pacific Corals particularly sensitive to warmer seawater Fish kills

El Niño and La Niña To view this animation, click “View” and then “Slide Show” on the top navigation bar.

El Niño-Southern Oscillation (ENSO) Warm (El Niño) Produces heavy rains and flooding in equatorial western South America Droughts in western Pacific Droughts in Indonesia and Northern Australia

Cool phase (La Niña) Increased pressure difference across equatorial Pacific – overshoots return to normal from El Niño Stronger trade winds Stronger upwelling in eastern Pacific Shallower thermocline Higher biological productivity Cooler than normal seawater Higher than normal hurricane season in Atlantic

El Niño-Southern Oscillation (ENSO) Cool phase (La Niña) Fig. 7.20c

ENSO events El Niño warm phase about every 2 to 10 years Highly irregular Phases usually last 12 to 18 months Fig. 7.22

ENSO events Fig. 7.21 Strong events effect global weather 1982-83, 1997-98 Mild events only effect weather in equatorial South Pacific 2002-2003, 2004-2005, 2006-2007 Effects of strong event can vary Can be drier than normal or wetter than normal Can be warmer than normal or cooler Can produce thunderstorms and tornados in midwest Can produce droughts in west Pushes jet stream eastward, pushing Atlantic hurricanes east, away from U.S. Less hurricanes reach US during El Nino, more during La Nina Flooding, drought, erosion, fires, tropical storms, harmful effects on marine life Examples: coral reef death in Pacific, crop failure in Philippines, increased cyclones in Pacific, drought in Sri Lanka Fig. 7.21

“Severe” (1997) and “Mild” (2002) El Nino http://science.nasa.gov/headlines/

Freeze probability lower during La Nina Freeze probability slightly higher during El Nino

Thermohaline circulation Caused by density differences due to temp and salinity in different areas Below the pycnocline 90% of all ocean water Slow velocity http://www.john-daly.com/polar

Thermohaline circulation Bottom water formation Movement caused by differences in density (temperature and salinity) Formed by freezing saltwater  dense brine sinks Cooler seawater denser Saltier seawater denser http://www.geology.um.maine.edu/ges121/lectures/19-ocean-conveyor

Thermohaline circulation Originates in high latitude surface ocean Near Greenland & Iceland in North Atlantic Weddell Sea  Antarctic Bottom Water (coldest) Forms densest oceanic water  flows on bottom Moves north in all three ocean basins Once surface water sinks (high density) it changes little Deep-water masses identified on T-S diagram

Thermohaline circulation Fig. 7.26

Thermohaline circulation Selected deep-water masses Antarctic Bottom Water North Atlantic Deep Water Antarctic Intermediate Water Oceanic Common Water Cold surface seawater sinks at polar regions and moves equatorward

Conveyor-belt circulation Combination deep ocean currents and surface currents Fig. 7.27

Ocean Circulation To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Deep ocean currents Cold, oxygen-rich surface water to deep ocean Dissolved O2 important for life and mineral processes Deep ocean currents do bring water across the equator Changes in thermohaline circulation can cause global climate change Example: warmer surface waters with increased melting of ice caps  less dense surface waters Bottom water will not form and sink  Gulfstream may be altered  long-term cooling, particularly in northern Europe less oxygen deep ocean

Misconceptions Earth events taking place within the global environment are not interconnected, such as El Nino is not important to people living in the midwest. Humans are the only cause of global warming. The greenhouse effect will cause all living things to die.

Ocean Literacy Principles 1c - Throughout the ocean there is one interconnected circulation system powered by wind, tides, the force of the Earth’s rotation (Coriolis effect), the Sun, and water density differences. The shape of ocean basins and adjacent land masses influence the path of circulation. 1d - Sea level is the average height of the ocean relative to the land, taking into account the differences caused by tides. Sea level changes as plate tectonics cause the volume of ocean basins and the height of the land to change. It changes as ice caps on land melt or grow. It also changes as sea water expands and contracts when ocean water warms and cools. 3a - The ocean controls weather and climate by dominating the Earth’s energy, water and carbon systems. 3b - The ocean absorbs much of the solar radiation reaching Earth. The ocean loses heat by evaporation. This heat loss drives atmospheric circulation when, after it is released into the atmosphere as water vapor, it condenses and forms rain. Condensation of water evaporated from warm seas provides the energy for hurricanes and cyclones. 3c - The El Niño Southern Oscillation causes important changes in global weather patterns because it changes the way heat is released to the atmosphere in the Pacific. 3d - Most rain that falls on land originally evaporated from the tropical ocean. 3f - The ocean has had, and will continue to have, a significant influence on climate change by absorbing, storing, and moving heat, carbon and water. 3g - Changes in the ocean’s circulation have produced large, abrupt changes in climate during the last 50,000 years.

Sunshine State Standards SC.8.P.8.4 Classify and compare substances on the basis of characteristic physical properties that can be demonstrated or measured; for example, density, thermal or electrical conductivity, solubility, magnetic properties, melting and boiling points, and know that these properties are independent of the amount of the sample. SC.912.E.7.2 Analyze the causes of the various kinds of surface and deep water motion within the oceans and their impacts on the transfer of energy between the poles and the equator. SC.912.P.10.4 Describe heat as the energy transferred by convection, conduction, and radiation, and explain the connection of heat to change in temperature or states of matter