Essentials of Oceanography Ocean Currents Essentials of Oceanography

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

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

Surface Ocean Currents

Why do surface currents only flow around the outside of ocean basins? Why doesn't the Coriolis Effect turn the currents into the center of the gyres? The observed surface current is called geostrophic flow balance b/w Coriolis and gravity keeps circulation around the periphery. Types of geostrophic currents Transverse currents Western boundary Eastern boundary

Transverse Currents Driven toward east or west by Trades and Westerlies. Trades strongest, most consistent, so equatorial currents strong. Equatorial currents ITCZ displaced to north of the equator = caloric equator Due to land distribution. Equatorial Counter Current Returns flow at the doldrums No wind to prevent back flow ITCZ = Intertropical Convergence Zone

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

Cause of Strong Western Boundary Current Convergence of trade winds all water along the equator is being driven to the west Water piles up on western margin Westerlies don't converge Rotation of the earth "hill" offset to west, so flow confined to narrower channel.

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

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

Divergence and Convergence Where currents or current and land split apart or come together. 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

Divergence and Convergence

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

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. ENSO = combined effects of atmosphere and oceans

Normal El Nino

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

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 1990-1998 -- prolonged ENSO conditions Natural variability vs. global warming effects

Effects of ENSO Largest effect is on global precipitaion patterns

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

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

Deep Water Formation Northern hemisphere deep water formation Winter cooling and evaporation Produces cold, moderately saline waters Primarily in Greenland and Norwegian Seas Evaporation in the Mediterranean Sea creates very saline intermediate water Combine to form North Atlantic Deep Water (NADW) Southern hemisphere bottom water formation Formation of sea ice Produces of cold, saline waters Densest water in the global ocean!! Created off the shores of Antarctica Called Antarctic Bottom Water (AABW)

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

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