ocean circulation

Download Ocean circulation

Post on 03-Dec-2014




0 download

Embed Size (px)


Geologist Indonesia


  • 1. Ocean circulation

2. What are ocean currents? Mass movement or flow of ocean water Two types of major currents Surface currents Deep ocean currents / thermohaline circulation. 3. Surface currents Surface currents move water horizontally parallel to theEarths surface extend to approximately 100-150m depth(depending on strength of winds) Two external forces influence the World Ocean generatingocean currents - gravitation and the energy flux from thesun. Gravitation includes tidal forces resulting from theinteraction of water mass with the moon and the sun, androtation of the Earth.. The radiation flux from the sun results in wind stress,heating and cooling of the ocean surface, and evaporationand precipitation of water. A complex process of interaction between these forcesresults in a complex and variable pattern of oceancirculation 4. 1. What drives ocean currents? The amount of heat radiation is of maximum at the equator. The cold air at the poles is denser than the warm air at the equator; hence, air pressure at sea level is higher at the poles than at the equator. The high air pressure in the poles moves towards the equator.IoE 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography 5. Current Generation Wind acting on the surface of the water, causes a partialtransfer of kinetic energy from the wind to the water. Wind-driven currents decline with depth, and aregenerally limited by the permanent pycnocline 100 to200 m but in some cases they may go as deep as1000 m. The southeast trades and the northeast trades cause ageneral westward current. The westerlies cause a general eastward current. Because of the continents, these currents areinterrupted. A westward current becomes a southwardcurrent in the southern hemisphere, and a northwardcurrent in the northern hemisphere. Eastward currents are forced to turn north in thesouthern hemisphere, and south in the northernhemisphere. 6. Winds on a NON-rotating earth flow north-southIn fluid and gases pressuregradients produce flow fromregions of high pressure toregions of low pressure. Ifthe earth were not rotating,the response to thesepressure gradients wouldbe direct and simple.IoE 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography 7. 1. What drives ocean currents?The rotation of theEarth modifies thepattern of atmosphericcirculation. As airmoves toward theequator, the rotationof the earth shiftsocean and landeastward under it. Theresult is "easterly"winds (PolarEasterlies and Tradewinds).Traveling from the equator to the pole air in the upper atmospheric layer cools. About30N and 30S the air becomes dense enough to fall back to earth surface, formingtwo Hadley cells of atmospheric circulation. Similar cells are created between thepoles and 60 latitude. The zones between 30 and 60 are called Ferrel cells, where"westerly" winds dominate.IoE 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography 8. Global wind patternThe global pattern of winds cause the majorocean currents in the surface layer. 9. The movement of water as influenced by the CoriolisSurface Currents effect and gravity. Controlled by three factors Global winds Coriolis Effect Continental Deflections Global Winds cause surface currents to flow in the direction the wind is blowing.The Coriolis effect is the shifting of winds and surfacecurrents caused by Earths rotation. The Coriolis effectcreates clock-wise gyre in northern hemisphere and anti-clockwise in southern hemisphere.Continental DeflectionsShape of continents change the direction of current flow 10. Wind driven circulation About 10% of the water is moved by surface currents Surface Currents occur at upper 400 m. The currents move above the pycnocline. Surface currents are primarily driven by the wind andwind friction Move fast relative to thermohaline circulation (1 to 2 m/s) Reflect global wind patterns and Coriolis effect! The pycnocline separates the surface layer from thedeep thermohaline circulation. 11. Gyre Formation Two great circular currents(gyres) are formed, one thenorthern hemisphere andone on the southernhemisphere, in both theAtlantic and the Pacific. winds When the wind acts onwater in the open ocean fora long time, the resultantcurrents are about 1-3% ofthe wind speed. Ocean currents aremassive. They can persistcurrentsfor long times, even if thewinds are light or evenagainst them. 12. Current gyres Gyres are large circular-moving loops of water Subtropical gyres Five main gyres (one in each ocean basin): North Pacific South Pacific North Atlantic South Atlantic Indian Generally 4 currents in each gyre Centered at about 30 north or south latitude 13. Subpolar gyres Smaller and fewer than subtropical gyres Generally 2 currents in each gyre Centered at about 60 north or south latitude Rotate in the opposite direction of adjoining subtropical gyres 14. Western boundary currents These are narrow,deep, fast currents found at the western boundaries of oceanbasins. The Gulf Stream The Japan Current The Brazil Current The Agulhas Current The Eastern Australian Current 15. Eastern boundary currents These currents arecold,shallow and broad, and their boundaries are notwell defined The Canary Current The Benguela Current The California Current The West Australian Current The Peru Current 16. Major Currents of the World Ocean 17. Ekman spiral Wind flows over surface and creates drag on water Wind driven flow is deflected to right in N hemisphere byCoriolis effect Water flows at only about 3% of the speed of the drivingwind. Current flows at 45o to the right of the wind direction inthe northern hemisphere But, only the surface feels the wind Each layer down only feels the layer above so isdeflected based on the layer above Each layer down moves more slowly than the layerabove 18. Wind creates a drag on surface waters and successive layersexert drag on each successive layer below.Each layer is subject to Coriolis deflection 19. Ekman flow Wind exerts frictional drag on water causing a thinlayer of water to move Transfer of momentum is not efficient; induced current is about 2% of wind speed Coriolis force causes water to veer right or left of wind As the surface layer of water begins to move, itexerts frictional drag on the layer below And so on, each layer moving slower and deflectedrelative to the layer above Produces a pattern of decreased speed with depthand increased angle between flow and winddirection with depth (Ekman spiral) 20. Flow in Ekman layer Surface current typically 20-40o towind direction Average or net flow of water inEkman layer is 90o to wind Average or net flow in Ekmanlayer is the drift current Thickness of Ekman layer isapproximately 100m 21. Fig. 5-1 22. Coastal upwelling Northern hemisphere 23. Coastal downwellingNorthern hemisphere 24. Southern hemisphere 25. Pressure gradients develop in the oceanbecause the sea surface is warped into broadmounds and depressions with a relief ofabout one meter. Mounds on the oceans surface are caused byconverging currents, places where water sinks. Depressions on the ocean;s surface are caused bydiverging currents, places from where water rises. Water flowing down pressure gradients on theoceans irregular surface are deflected by theCoriolis effect. The amount of deflection is afunction of latitude and current speed. 26. In the center of gyreswater piles up (converges)upper ~100 m Fig. 5-3 (a) Ekman spiral Fig. 5-3 (b) Ekman transport 27. Downwelling of waterCreation of geostrophic currents asa result of the pressure gradientUpwelling of deep water to replacesurface water in areas of divergence- e.g., along the equator 28. Consequences of Ekman transport At the center of the gyres: Convergence Water tends to pile up Convergence is associated withdownwelling At the edge of continents, divergenceoccurs Divergence is associated with upwelling 29. Consequences of Ekman transport,Coriolis, and gravity Ekman transport causes water to pile up in themiddle of gyres Gravity then acts to force it down Coriolis act in the opposite direction as gravity The forces balance, and flow tends to occurparallel to the topographic contours This is called geostrophic flow 30. Geostrophy a frictionless balancebetween the pressure gradientAnd the Coriolis acceleration generates currents that moveAround the hill 31. Measured average topography of the North Atlantic (red-high) 32. Permanent convergences and divergencesConvergences - downwelling 5 major permanent zones of convergence tropical convergence at equator N. subtropical convergence 30 to 40 N and S S. subtropical convergence mark the center of the gyres Antarctic convergence at 50 S Arctic convergence at 50 NDivergences - upwelling 3 major permanent zones of divergence N. tropical divergence S. tropical divergence Antarctic divergence 33. Upwelling and downwelling Vertical movement of water Upwelling = movement of deep water to surface Brings cold, nutrient-rich water to surface Produces high productivities and abundant marine life Downwelling = movement of surface waterdown Moves warm, nutrient-depleted surface water down 34. Review of horizontal wind driven circulation upwellingdownwelling downwellingupwelling downwellingdownwelling upwelling 35. AntarcticCircumpolarCirculation 36. Areas of upwelling coastal upwelling has seasonality (as do winds) 37. May global wind induced upwelling 38. Deep-Ocean Circulation Throughout most of the oceans, the layers form three principlezones: the surface zone ( the mixed layer), the pycnocline zone,and the deep zone. Deep ocean currents are known as thermohaline circulation Thermohaline circulation is a density driven flow of watergenerated by differences in salinity or temperature. Water at the surface is exposed to changes in salinitythrough evaporation or precipitation and in temperatureth