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Essentials of Oceanography

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1 Essentials of Oceanography
Ocean Currents Essentials of Oceanography


3 Wind-driven Ocean Circulation
Surface ocean circulation mixed layer above top 100 m controlled by winds + coriolis Overhead wind patterns Wind blows against surface- friction sets water into motion Continents interfere with the winds and redirect airflow Result- circulation cells within each ocean basin

4 Gyres Gyre - closed, circular flow of water around an ocean basin 5 gyres: North Atlantic South Atlantic North Pacific South Pacific Indian Ocean Plus circulation around Antarctica- closed circuit wind and water can freely flow around Antarctica

5 Surface Ocean Currents
Global winds drag on the water’s surface, causing it to move and build up in the direction that the wind is blowing. And just as the Coriolis effect deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, it also results in the deflection of major surface ocean currents to the right in the Northern Hemisphere (in a clockwise spiral) and to the left in the Southern Hemisphere (in a counter-clockwise spiral). These major spirals of ocean-circling currents are called “gyres” and occur north and south of the equator. They do not occur at the equator, where the Coriolis effect is not present

6 Western Boundary Currents
Flow from equator to pole along western margin of basins Strong, fast, narrow, focused flow Transports heat to higher latitudes

7 Gulf Stream Good example of a Western Boundary
Current that flows like a river- amount of water carried = 100x discharge from all rivers! First mapped by Ben Franklin Major mechanism for transport of heat to North. Climate in England vs. Newfoundland The Gulf Stream is a powerful western boundary current in the North Atlantic Ocean that strongly influences the climate of the East Coast of the United States and many Western European countries. About 50 miles wide, and travels about 5 miles per hour, relatively fast for an ocean current. It is about degrees celsius.

8 Eastern Boundary Currents
Eastern flow more diffuse, wider, slower Cold water currents

9 Divergence and Convergence
Where currents or current and land come together or split apart Convergence leads to downwelling Divergence leads to upwelling- brings cold, nutrient-rich water up from about 500 m 2 important areas of upwelling Pacific equatorial region Near shore Along shore winds force water off the coast - creates low water pressure Eastern margins of ocean basins - Calif. Coast, Peru Upwelling and downwelling also occur in the open ocean where winds cause surface waters to diverge (move away) from a region (causing upwelling) or to converge toward some region (causing downwelling).

10 El Niño- Southern Oscillation (ENSO)
Represents interactions between: Atmospheric circulation Ocean circulation Climate Begins in equatorial Pacific, but has global effects Cause is not well understood El Nino refers to changes in ocean circulation Named for anomalous warm current off Peru that occurs at Christmas time Normally - cold current off of Peru due to upwelling

11 El Niño- Southern Oscillation (ENSO)
Refers to changes in atmospheric conditions Oscillation in the distribution of high and low pressure systems across the equatorial Pacific “Affect wind patterns, which affects surface ocean circulation.

12 El Niño- Southern Oscillation (ENSO)
Develops when: * Sea surface temperatures (SST) in tropical eastern Pacific Ocean are warmer than normal * pressure patterns weaken (and may reverse) * trade winds weaken (and may reverse)

13 Southern Oscillation Index (SOI)
Pressure values in the eastern Pacific (Tahiti) and western Pacific (Darwin, Australia) are monitored SOI = pressure departure at Tahiti - pressure departure at Darwin SOI < 0 = Normal SOI > 0 = ENSO

14 Normal El Nino

15 ENSO Comparison Normal years El Nino years
Lower pressure over Indonesia Higher pressure over eastern equatorial Pacific Driven by strong trade winds Weak equatorial counter current Strong upwelling near Peru (and Calif) W. Pac ~ 8º warmer than E Pac. Rain in western Pacific, dry in eastern Pacific El Nino years Higher pressure over Indonesia Lower pressure over eastern Pacific Decreased pressure gradient across the equatorial Pacific weakens trade winds Stronger countercurrent transports warm water to the east Reduced upwelling Shift in rainfall to the east La Niña is defined as cooler than normal sea-surface temperatures in the central and eastern tropical Pacific ocean that impact global weather patterns. La Niña conditions recur every few years and can persist for as long as two years. El Niño and La Niña are extreme phases of a naturally occurring climate cycle referred to as El Niño/Southern Oscillation. Both terms refer to large-scale changes in sea-surface temperature across the eastern tropical Pacific. Usually, sea-surface readings off South America's west coast range from the 60s to 70s F, while they exceed 80 degrees F in the "warm pool" located in the central and western Pacific. This warm pool expands to cover the tropics during El Niño, but during La Niña, the easterly trade winds strengthen and cold upwelling along the equator and the West coast of South America intensifies. Sea-surface temperatures along the equator can fall as much as 7 degrees F below normal. La Nina is when conditions are more intensely “normal”

16 Periodicity ENSO periodicity 2-7 years
~1 event every 4 years for past century and 1 strong event per decade But duration and extent variable (each unique) Appear to be becoming more frequent over past few decades prolonged ENSO conditions Natural variability vs. Global warming effects

17 Effects of ENSO Largest effect is on global precipitaion patterns

18 Oceanic Deep-water Circulation
Subsurface currents arise from the density differences between water masses Produced by the variations in water temperature (thermal effect) and salinity (haline effect) Collectively referred to as thermohaline circulation Winds drive ocean currents in the upper 100 meters of the ocean’s surface. However, ocean currents also flow thousands of meters below the surface. These deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation.

19 Thermohaline Circulation
Evaporation and lower temperatures cool surface waters from ~ 45º N and ~ 45º S latitude to the poles Cold (and therefore dense) polar water sinks and then drifts equatorward, below warmer, less dense surface water Cold water descends to a depth of corresponding density, 'sliding' under less dense water and over more dense water Deep waters slowly return to the surface (after ~1000 years) through upwelling along the equator and in coastal regions

20 Global Circulation NADW sinks and flows southward along the western side of the Atlantic Ocean NADW and AABW mix in the Antarctic Circumpolar Current Mixed water mass of NADW and AABW flows northward into the Indian and Pacific Oceans Upwells in the N. Pacific and Indian Oceans and returns to the south as warm shallow waters This animation shows the path of the global conveyer belt. Cold, salty, dense water sinks at the Earth's northern polar region and heads south along the western Atlantic basin. The current is "recharged" as it travels along the coast of Antarctica and picks up more cold, salty, dense water. The main current splits into two sections, one traveling northward into the Indian Ocean, while the other heads up into the western Pacific. The two branches of the current warm and rise as they travel northward, then loop back around southward and westward. The now warmed surface waters continue circulating around the globe and eventually end up back at the north Atlantic where the cycle begins again.

21 Summary Surface circulation is driven by global wind patterns
El Nino is a warming of the west coast of South America and causes a disruption of global precipitation Deep water circulation is driven by gravity through density changes caused by temperature and salinity

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