IX. Upper Ocean Circulation

Transcription

1 IX. Upper Ocean Circulation

2 World Ocean Covers 71% of Earth s surface Contains 97% of surface water Arctic Ocean NH: 61% ocean, 39% land Pacific Ocean Atlantic Ocean Southern Ocean Indian Ocean SH: 81% ocean, 19% land global satellite image (NOAA/NGDC) false colors = elevation (blue is below sea level, avg. dep. ~4000m)

3 generalized vertical structure upper ocean is less salty upper ocean is warmer upper 1-200m is mixed seasonally by winds

4 generalized vertical structure upper ocean is thus partly separated from deep ocean by a large density gradient (pycnocline) more next class on the role of density (neg. buoyancy) forcing in the global circulation

5 upper ocean circulation upper ocean circulation is largely a wind-driven circulation i.e wind pushes on ocean surface water (this is called wind stress) which way will the water move?

6 review differential heating leads to variations of pressure at surface air moves from areas of high to low pressure Coriolis effect causes moving air to deflect to the right in NH (left in SH) this leads to clockwise movement of air around areas of high pressure in the NH (anti-clockwise in the SH)

7 surface winds

8 wind-driven circulation which way will the water move? there are 3 main factors Coriolis effect Ekman transport geostrophic balance westward intensification

9 Ekman transport Ekman transport occurs in response to the balance between wind stress Coriolis effect friction

10 Ekman layer and Ekman spiral ~100 m imagine wind pushing on surface surface water moves off to right due to Coriolis, but only by about 20-45

11 Ekman layer and Ekman spiral ~100 m imagine wind pushing on surface surface water moves off to right this water now pushes the water below, which now moves off to the right now THIS layer pushes the water below, which moves off to the right and so on and so on...

12 Ekman layer and Ekman spiral ~100 m averaged over the top 100m or so (the Ekman layer) the water moves EXACTLY 90 to the right of the wind direction

13 Ekman layer and Ekman spiral NET here you can see the velocity and amount of friction diminish downward- by ~100m depth friction has greatly diminished the velocity and reversed the direction of flow - NET flow is 90 RT of the wind

14 consider winds over N. Atlantic...

15 clicker question: water seems to be moving in from all around????? What must happen? a) convergence b) makes a pile c) divergence d) makes a hole e) both a) & b)

16 Ekman transport leads to convergence convergence leads to pile!

17 convergence creates a pile water Sea surface height (North Atlantic Subtropical Gyre) X X is ~80 cm higher than surroundings

18 convergence creates a pile water Sea surface height (North Atlantic Subtropical Gyre) H i.e. an area of HIGH pressure

19 geostrophic flow geostrophic flow occurs when the movement due to pressure gradient balances that from Coriolis effect geostrophic flow is thus balanced flow (geostrophic = earth turning )

20 geostrophic flow top of pile initial movement is down the pressure gradient (i.e. down hill) but turns until pressure gradient force and Coriolis balance, the balanced flow follows lines of equal pressure in a circular pattern

21 sea surface height (Feb 95) IGOS TOPEX UTCSR

22 clicker question: c b e d c a e d clockwise flows occur at: a), b), c), d), or e)

23 wind-driven surface gyres c c e e large scale circular balanced flows or currents

24 wind-driven surface gyres large scale circular flows or currents mostly <400 m, i.e. 10% of ocean volume dominated by the great subtropical gyres (up to 1000 m) suptropical gyres clockwise in NH (i.e. high pressure) anti-clockwise in SH (i.e. high pressure)

25 Western Boundary Currents fast (~2 m/s, 7 km/hr), deep (~1000 m), narrow (~70 km wide) warm, nutrient-poor, clear N Atl: Gulf Stream S Atl: Brazil Current N Pac: Kuroshio Current S Pac: East Australian Current Ind: Agulhas Current

26 meanders Gulf Stream

27 Eastern Boundary Currents slow (~2 km/hr), shallow (~100 m), wide (to ~1000 km) cold, nutrient-rich N Atl: Canary Current S Atl: Benguela Current N Pac: California Current S Pac: Peru Current Ind: West Australian Current

28 Antarctic Circumpolar Current (West Wind Drift) Southern Ocean, south of ~40 S flows completely around globe not very fast (~3 km/hr) but largest volume flux ~2-4 km deep, up to 2000 km wide driven by westerlies (geostrophic)

29 subtropical gyres c c e e e the western boundary currents are warm the eastern boundary currents are cold

30 clicker question: c c e e e the subtropical gyres produce a net heat transport that is: a) equatorward, b) poleward, c) E-W d) in all directions, e) negligible

31 subtropical gyres pick up and transport heat from tropical ocean toward the poles

32 global heat transport southward northward great subtropical gyres reach and carry heat to ~40 lat. (exception is N. Atl as we will see next class)

33 up- and down- welling so far talked about lateral motions arising from wind stress what about vertical motions these are required by mass balance

34 convergence v. divergence + subtropical gyre centers convergence of mass mass balance requires sinking subpolar gyre centers, equator divergence of mass mass balance requires upwelling

35 upwelling = biological productivity upwelled water is rich in nutrients when nutrients enter sunlit upper 1-200m, photosynthesis can occur (in the so-called photic zone) upwelling regions are therefore biologically rich and diverse very active marine ecosystems good fishing

36 visible light in seawater blue penetrates farthest ~1% reaches 250 m photic zone

37 upwelling and cholorophyll SeaWiFS Sep Aug. 98 equatorial upwelling

38 normal equatorial upwelling easterly trades straddle equator Ekman divergence to N and S divergence balanced by upwelling direction of coriolis force changes at equator upwelled water is cold water sea surface temp. C

39 normal equatorial upwelling easterly trades straddle equator Ekman divergence to N and S divergence balanced by upwelling direction of coriolis force changes at equator and rich in nutrients chlorophyl (mg/m 3 ) El Nino or La Nina?

40 coastal upwelling equatorward winds parallel eastern boundaries Ekman transport away from shore surface waters replaced by subsurface waters

41 upwelling and cholorophyll SeaWiFS Sep Aug. 98 coastal upwelling

42 CA summer coastal upwelling Winds blowing south along coast move water in the Ekman layer to the right. Upwelling occurs along the coast to replace water moved offshore..

43 CA summer coastal upwelling

44 convergence and downwelling dynamic height recall: subtropical gyre centers convergence of mass mass balance requires sinking +

45 convergence and downwelling dynamic height chlorophyll nutrient starved openocean desert, why? coastal upwelling, why?

46 sea surface temperature (Sun.) divergence and upwelling vs. convergence (and downwelling) upwelling of cold deep water produces cooling: downwelling permits heating

47 El Niño the leading source of natural, year-to year variations in weather and climate around the world driven by changes in atmosphere-ocean circulation over equatorial Pacific

48 Equatorial Pacific mean state along equator: water flows in direction of wind (Coriolis = 0) warm waters pile up in west cold waters upwell in east (just south of equator) warm pool cold tongue sea surface temperatures

49 La Niña El Niño Strong easterlies Strong eastern upwelling (cold SSTs) Heavy rainfall in western warm pool Weak easterlies Weak eastern upwelling (warm SSTs) Eastward shift in rainfall both states recur every 2-7 yr usually develop April-June reach max Dec-Feb last ~9-12 months (or more) coupled with widespread changes in atmospheric circulation

50 Walker Circulation (tropical E-W linkages) la Nina ( normal ) low surface pressure cold PERU el Nino area of convection moves east weak monsoon low surface pressure warm

51 Walker Circulation (tropical E-W linkages) la Nina ( normal ) warm low surface pressure cold PERU el Nino area of convection moves east what might be the atmospheric response to warm surface water here warm

52 Walker Circulation (tropical E-W linkages) la Nina ( normal ) low surface pressure cold PERU el Nino area of convection moves east weak monsoon warm low surface pressure

53 El Niño precipitation anomalies (vs. normal ) (La Niña pattern approx. opposite) NOV-APR DRIER cm/yr WETTER notice anomaly pattern alternates E-W in association with changes in the Walker Circulation

54 1997/8 El Niño drought, rainforest burning (Amazon, Indonesia), and monsoon weakening, California storms are major impacts Wet Dry

55 1999 La Niña flooding in South America, Indonesia, SE Asia and Africa, drying of central Pacific islands and American SW : extreme tropical weather Wet Dry

56 El Nino is the leading source of natural, year-to-year variation in weather and climate around the world How will it change in response to global warming? Different models give different results, none are particularly reliable (this is a good example of physics that is difficult to represent in models subject to changing conditions)

57 summary points upper ocean circulation is largely a wind driven circulation, i.e. winds push on surface (providing surface wind stress) pushing occurs layer-by-layer in upper ocean leading to Ekman spiral Ekman transport is exactly 90 to the right of the wind in the NH (or, to the left in SH) This allows water to pile up near middle of the ocean basins Higher height provides pressure gradient and permits balanced geostrophic flow in large circular gyres more...

58 summary points wind pushing on surface of ocean can cause divergence (and upwelling) or convergence (and downwelling) upwelling can occur along coasts (esp. eastern margins w/ equatorward winds) and along the equator upwelled water is nutrient-rich, downwelled water is nutrient poor (nutrients having been consumed in the sunlit surface) thus, upwelling links the biology, marine ecosystems (and carbon use) to climate El Nino is a complex ocean-atmospheric interaction involving large changes in upwelling regime that affects climate of much of the Earth on year-to-year timescales

59 learning goals explain how the properties of the ocean vary above and below meters depth and why be able to predict which way water will move in response to prevailing wind direction describe how the net flow of the upper ~100 m responds to winds, friction and the Coriolis effect be able predict where waters might pile up at the ocean surface and explain how the near-circular flows of the subtropical gyres arise explain the role of the subtropical gyres in moving heat be able to predict the relationship between winds along the coast line, divergence or convergence, and up- or down- welling be able to predict the relationship between vertical motions of water (up- and down- welling) and biological productivity more

60 more learning goals be able to predict the relationship between the state of upwelling in the E. Pacific (El Nino vs. La Nina) the where atmospheric convection and uplift will occur and where atmospheric sinking will occur