I. Ocean Layers and circulation types

Transcription

1 OCEAN CIRCULATION

2 I. Ocean Layers and circulation types

3 1) Ocean Layers Ocean is strongly Stratified Consists of distinct LAYERS controlled by density takes huge amounts of energy to mix up the stable layers!

4 Temperature slice in the central Atlantic North South Thermocline a sharp transition in temperature and hence DENSITY (marked by many contour intervals) The main thermocline divides WARM SURFACE from COLD DEEP water.

5 Ocean Layers SURFACE OCEAN THERMOCLINE DEEP OCEAN

6 What forces can move layers of water? Water above the thermocline can be moved by wind (via friction with the atmosphere) Water below thermocline- much larger barrier, due to density layers.

7 1) Circulation types two very separate types of ocean circulation 1) surface -driven by wind 2) deep driven by density

8 II. Wind Fields and Main Ocean Gyres

9 Recall: Main WINDBELTS these drive Surface circulation! Rev: Main-wind belts Prevailing Westerlies Recall again.. Trade Easterlies

10 If wind drives the circulation. What major ocean currents would you predict these wind bands would create?

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12 W Subtropical gyres and their Main Current Types 60 N Prevailing Westerlies High latitude E currentl 30 N 0 Prevailing Westerlies Equatorial/tropical current H Eastern Boundary Easterly current Trade L Easterly Trade 30 S H 60 S High L latitude current

13 5 Major subtropical gyres 30 N H 30 S H Notice smaller Sub-polar Gyres as well!

14 More realistic Global Surface Currents Kaufman Fig. 4.12

15 How do we know they exist? Ben Franklin s map of the Atlantic deduced the Atl. Gyre from the way ships would drift along their route to and from the New World Had Just started: The strange story of Nansen s Franklin icebergs map

16 Summary: Ocean Gyres Large ~ circular current flows Set in motion by main wind patterns Bounded by continents on E and W subtropical gyres most pronounced

17 But What is wrong with this picture (so far..)?

18 II. ECKMAN TRANSPORT (CORIOLIS strikes again)

19 Nansen contemplating ice

20 Nansen noticed icebergs moving to right of wind direction Nansen s funky icebergs

21 Ekman Spiral Model of surface response to wind forcing - When wind blows over the ocean, surface water moves 45 o to the right of the wind in the northern hemisphere - Current rotates and weakens deeper in water column

22 90 o Ekman Transport Net flow of surface layer when forced by wind Most important point: Add up all the arrows, the AVERAGE direction of flow is 90 o to the right of the wind!

23 Ekman Transport Net flow of surface layer when forced by wind Most important point: Add up all the arrows, the AVERAGE direction of flow is 90 o to the right of the wind!

24 Ekman Transport Causes Convergence of Water in Middle of Gyres Prevailing Westerlies 30 N CONVERGENCE H Northeasterly Trades

25 Para ver esta película, debe disponer de QuickTime y de un descompresor. H

26 Para ver esta película, debe disponer de QuickTime y de Convergence Zone un descompresor. H Gyre Flow into screen H Gyre Flow out of screen X Ekman Trans. Surface Layer CONVERGENCE Thickens Ekman Trans.

27 Para ver esta película, debe disponer de QuickTime y de Geostrophic Flow un descompresor. H PRESSURE GRADIENT FORCE PRESSURE GRADIENT FORCE X CORIOLIS CORIOLIS PGF = High to Low Pressure Flow Plus Coriolis Results in Flow Clockwise around Gyre (NH) Strength of Flow related to PG

28 Pressure gradients in surface ocean Wind fields create areas of lower and higher water: humps and valleys of water. Result: just like in atm = Pressure gradients And just like in atm, water tries to flow from Hight to Low pressure zones

29 Dynamic Height Maps Maps of relative height of surface ocean from the measurements of the ocean density Can be used to calculate current strengths. From Regional Oceanography M. Tomczak and J. S. Godfrey

30 SUMMARY: WHAT DRIVES SURFACE CURRENTS? 1) Direct Effect of Wind Confined to upper m Frictional surface layer flow is ~90 to right of wind direction in NH, to left in SH -> Ekman Transport 2) Indirect Effects of Coriolis Eckman Transport sets up Pressure gradients drive focused currents (jets) Can be 1-2 km below surface

31 Summary of Main Gyre Circulation. Wind stress drives surface convergences Sea surface rises Balance between coriolis effect and PG at the gyres: Geostrophic current moving in a circular path around the gyre hill

32 One more thing: Notice anything funny about this gyre?

33 WESTERN INTENSIFICATION & W. Boundary Currents

34 Gyre Circulation & W. Boundaries. Geostrophic current is around the sea-surface high BUT Gyres are squeezed up against western boundaries of ocean basins Notice asymmetry in circulation

35 Reasons for western intensification (asymmetric gyres) 1) Earth s rotationcoriolis effect asymmetries with latitude: Westerlies more effect than Easterlies => higher speed along the western margin of the hill compared to the eastern side. Notice asymmetry in circulation

36 Western boundary currents Found in all the ocean basins: NH and SH Fast (~1 m/s) and narrow (~100 km) Figure 7-4

37 Gulf stream Classic W. Boundary current. Starts as the Florida current between Florida and the Bahamas Leaves coast at Cape Hatteras, NC, cross the Atlantic. Transports HUGE amounts of water and HEAT keeps Europe warm..

38 Water transport Water flow rates, units are in Sverdrups (= 1 million cubic meters/second) 100 ~ 6

39 Moves so fast, that creates: Gulf Stream rings Temperature June 11, 1997 cold water surrounded by warm rings:unique physical characteristics and biological habitats

40 Warm core rings

41 Currents shape climate! Warm current - warms air (L) = high water vapor= humid coast East Coast USA, also E. Coast Asia (Japan in Aug..) Cool current cools air (H) = low water vapor dry coast W. Coast USA (Santa Cruz)

42 Finally: Back to the coasts- How would Eckman transport affect coastal surface circulation?

43 UPWELLING (revisited)

44 W Sea level middle of gyre Coastal Upwelling Example from Northern Hemisphere Ekman Transport Typical Northerly wind E 1000 m

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46 wind

47 Coastal Upwelling Example from Southern Hemisphere W Sea level middle of gyre Ekman Transport X southerly wind X E 1000 m

48 Peru and CA- Amazingly productive Upwelling-driven Environments! -due to the upwelling of nutrients to the surface

49 Global chl map Chlorophyl a: productive upwelling areas vs oceanic desserts

50 Equatorial Divergent Upwelling map view 2 N EQ 2 S DIVERGENCE UPWELLING Ekman Transport {to right in NH} Trade Winds Ekman Transport {to left in SH} West East

51 summary cartoon

52 Equatorial Divergent Upwelling cross-section Trade Winds 2 S X X X 2 N Ekman Trans. {to left in SH} Surface DIVERGENCE Layer Thins West UPWELLING East Ekman Trans. {to right in NH}

53 What about in the deep? Does it circulate? How?

54 Next time: DEEP OCEAN CIRCULATION