3Wind-driven Ocean Circulation Surface ocean circulationmixed layer above top 100 mcontrolled by winds + coriolisOverhead wind patternsWind blows against surface- friction sets water into motionContinents interfere with the winds and redirect airflowResult- circulation cells within each ocean basin
4GyresGyre -closed, circular flow of water around an ocean basin5 gyres:North AtlanticSouth AtlanticNorth PacificSouth PacificIndian OceanPlus circulation around Antarctica- closed circuitwind and water can freely flow around Antarctica
5Surface 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
6Western Boundary Currents Flow from equator to pole along western margin of basinsStrong, fast, narrow, focused flowTransports heat to higher latitudes
7Gulf 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 FranklinMajor mechanism for transport of heat to North.Climate in England vs. NewfoundlandThe 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.
8Eastern Boundary Currents Eastern flow more diffuse, wider, slowerCold water currents
9Divergence and Convergence Where currents or current and land come together or split apartConvergence leads to downwellingDivergence leads to upwelling- brings cold, nutrient-rich water up from about 500 m2 important areas of upwellingPacific equatorial regionNear shoreAlong shore winds force water off the coast - creates low water pressureEastern margins of ocean basins - Calif. Coast, PeruUpwelling 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).
10El Niño- Southern Oscillation (ENSO) Represents interactions between:Atmospheric circulationOcean circulationClimateBegins in equatorial Pacific, but has global effectsCause is not well understoodEl Nino refers to changes in ocean circulationNamed for anomalous warm current off Peru that occurs at Christmas timeNormally - cold current off of Peru due to upwelling
11El Niño- Southern Oscillation (ENSO) Refers to changes in atmospheric conditionsOscillation in the distribution of high and low pressure systems across the equatorial Pacific“Affect wind patterns, which affects surface ocean circulation.
12El 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)
13Southern Oscillation Index (SOI) Pressure values in the eastern Pacific (Tahiti) and western Pacific (Darwin, Australia) are monitoredSOI = pressure departure at Tahiti - pressure departure at DarwinSOI < 0 = NormalSOI > 0 = ENSO
15ENSO Comparison Normal years El Nino years Lower pressure over IndonesiaHigher pressure over eastern equatorial PacificDriven by strong trade windsWeak equatorial counter currentStrong upwelling near Peru (and Calif)W. Pac ~ 8º warmer than E Pac.Rain in western Pacific, dry in eastern PacificEl Nino yearsHigher pressure over IndonesiaLower pressure over eastern PacificDecreased pressure gradient across the equatorial Pacific weakens trade windsStronger countercurrent transports warm water to the eastReduced upwellingShift in rainfall to the eastLa 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”
16Periodicity ENSO periodicity 2-7 years ~1 event every 4 years for past century and 1 strong event per decadeBut duration and extent variable (each unique)Appear to be becoming more frequent over past few decadesprolonged ENSO conditionsNatural variability vs. Global warming effects
17Effects of ENSOLargest effect is on global precipitaion patterns
18Oceanic Deep-water Circulation Subsurface currents arise from the density differences between water massesProduced by the variations in water temperature (thermal effect) and salinity (haline effect)Collectively referred to as thermohaline circulationWinds 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.
19Thermohaline Circulation Evaporation and lower temperatures cool surface waters from ~ 45º N and ~ 45º S latitude to the polesCold (and therefore dense) polar water sinks and then drifts equatorward, below warmer, less dense surface waterCold water descends to a depth of corresponding density, 'sliding' under less dense water and over more dense waterDeep waters slowly return to the surface (after ~1000 years) through upwelling along the equator and in coastal regions
20Global CirculationNADW sinks and flows southward along the western side of the Atlantic OceanNADW and AABW mix in the Antarctic Circumpolar CurrentMixed water mass of NADW and AABW flows northward into the Indian and Pacific OceansUpwells in the N. Pacific and Indian Oceans and returns to the south as warm shallow watersThis 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.
21Summary 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 precipitationDeep water circulation is driven by gravity through density changes caused by temperature and salinity