The Surface Currents OCEA 101

Transcription

1 The Surface Currents OCEA 101

2 Why should you care? - the surface ocean circulation controls the major ocean biomes - variations in ocean circulation control the supply of nutrients for marine organisms - a combination of the ocean circulation and natural selection determines spawning cycles

3 Overview The Ekman spiral Ekman layers and transport Wind-driven gyres Western boundary currents Eddies Upwelling Circulation influences on marine organisms

4 The Wind-Driven Circulation Recall the Coriolis force: - fluid motion is deflected to the right of initial motion in the northern hemisphere - fluid motion is deflected to the left of initial motion in the southern hemisphere Consider what happens when a wind blows over the surface of the ocean. The small ripples on the ocean surface that result from surface tension, allow the surface winds to more effectively grip the ocean.

5 A Simple Thought Experiment Now let s do a simple thought experiment. Imagine that the ocean is made up of lots of thin layers. WIND Layer 1 Ocean surface Layer 2 Layer 3 Current Depth Layer 4 Friction Layer 5 etc What will happen? What about the Coriolis Force?

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7 The Ekman Spiral Clockwise spiral in NH Counter clockwise spiral in SH Ekman layer depth depends on latitude and viscosity. Ekman Layer ~100m What is the AVERAGE direction of motion?

8 Ekman Transport Ekman transport is the net transport (velocity x depth) over the Ekman layer Ekman transport is perpendicular to the wind direction: - to the RIGHT of wind direction in NH Ekman - to the LEFT of wind direction in SH

9 Ekman Transport

10 Ocean Gyres Tropical trade winds drive Ekman transport AWAY from equator in both hemispheres. Midlatitude westerlies drive Ekman transport towards equator both hemispheres. The result is convergence and elevated sea level at subtropical latitudes.

11 ~ 2m PGF PGF F g F g LOW P HIGH P LOW P Consider pressure at some reference depth

12 Geostrophic Flow F g = Pressure gradient force F c = Coriolis force V = surface current F g = F c GEOSTROPHIC BALANCE water flows along isobars

13 Wind Pattern Development

14 Wind-Driven Ocean Currents

15 The Major Wind-Driven Ocean Gyres N. Pacific Subpolar Gyre N. Atlantic Subpolar Gyre N. Atlantic Subtropical Gyre N. Pacific Subtropical Gyre S. Pacific Subtropical Gyre S. Atlantic Subtropical Gyre S. Indian Subtropical Gyre

16 Western Boundary Currents The currents in all ocean gyres are typically much stronger along the western margins (i.e. along eastern coastal boundaries) than in the interior parts of the ocean. This is due to a process called western intsensification. How does it work?

17 Western Intensification Western intensification is due to the combined effects of: - Latitudinal variations in the strength of the Coriolis effect (varies as sine of latitude) - Latitudinal variations of the wind direction - Friction along the coast

18 Vorticity Vorticity=angular momentum or spin.

19 Planetary Vorticity due to rotation of the earth

20 Relative Vorticity due to Shear

21 Absolute Vorticity Absolute Vorticity= Planetary vorticity + Relative vorticity Absolute vorticity is CONSERVED by fluid motion.

22 Gain of +ve vorticity from boundary by friction requires large velocity gradients Gain of -ve vorticity from wind

23 Rossby Waves Vorticity is carried by Rossby waves.

24 Rossby Waves Rossby waves have some very special properties: (1) Long wavelength Rossby waves propagate rapidly westwards (2) Short wavelength Rossby waves propagate very slowly eastward (3) At a western ocean boundary (continental east coast) long RWs reflect as short RWs which means that short RW energy accumulates along the western boundary. This leads to so-called western intensification and the formation of a western boundary current.

25 Western Boundary Reflection Long Rossby Wave (Fast) Western Boundary Eastern Boundary Short Rossby Wave (Very slow)

26 Eastern Boundary Reflection Short Rossby Wave (Very slow) Western Boundary Eastern Boundary Long Rossby Wave (Fast)

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28 Rossby Waves The dynamics of the ocean circulation along the eastern and western continental margins is fundamentally different because of Rossby wave dynamics.

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30 The North Atlantic Subtropical Gyre

31 Major Western Boundary Currents N. Atlantic Gulf Stream S. Atlantic Brazil Current N. Pacific Kuroshio Current S. Pacific East Australia Current N. Indian Somali Current S. Indian Agulhas Current

32 Ocean Eddies Eddies are ubiquitous in the ocean! They are very important because: - they can transport heat - nutrients - marine organisms - they can mix up the ocean

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34 California Coast Temperature Chlorophyll

35 Satellite Measured Surface Irradiance (AVHRR)

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38 GOM SSH and CHL Movies ~Andy/OCEA80B/SSH_1993to2006.mov

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40 Convergence and Divergence Z Convergence Divergence Downwelling Upwelling Significantly influences marine organisms in open ocean Significantly influences marine organisms in some coastal and equatorial regions

41 Langmuir Circulations An example of small-scale convergence and divergence

42 Large-Scale Convergences and Divergences

43 Convergence Zones Although NADW and AABW are most important, convergence zones form intermediate water

44 Ventilation and Subduction Z Subtropical Convergence Westerly (Eastward) Wind ρ 1 Ekman Transport ρ 2 ρ 2 > ρ 1 Poleward

45 The Global Ocean Conveyor Belt Circulation

46 Intermediate Waters AAIW, NPIW and Nordic Sea Waters

47 Bottom Waters - AABW

48 Shallow Mode Waters Associated with Shallow Overturning Cells

49 Heat transport (PW) (Talley, 2003, JPO)

50 Most of the basin is filled by the gyres nutrient poor, warm waters deep nutricline due to Ekman convergence and downwelling

51 Thermocline and Nutricline Generally closely related Thermocline is a very effective barrier against vertical motion. Small vertical motions of sea surface are manifest as large displacements of the thermocline (and nutricline) in the opposite direction.

52 Pacific Distribution of Euphausia Brevis Note the remarkable correlation of distribution with subtropical gyre circulation. Biantitropical distribution.

53 Central Subtropical Gyres Satellite obs show low surface Chlorophyll. Surveys suggest significant zooplankton communities in central gyre regions. How come? Several possibilities

54 1. All phytoplankton is consumed by zooplankton immediately 2. Phytoplankton are below the surface and not visible from satellite 3. Phytoplankton and zooplankton obs are not coincidental (satellite obs from 1990 s, zooplankton net data from 1950s and 60s) perhaps there have been decadal changes? It appears that 1 and 2 are both true

55 Observations show that the daily production of phytoplankton stock is consumed by zooplankton every day

56 Subsurface Chlorophyll Maximum Vertical section of Chlorophyll from Alaska to Hawaii South of 45N maximum in Chlorophyll is ~100m deep.

57 Several factors: Nutricline close enough to surface that there is an intermittent supply of nutrients from below. Cyanobacteria are important for recycling DOM via the microbial loop Some phytoplankton species have evolved to utilize the blue light wavelengths at depth efficiently because of certain pigments.

58 Coastal upwelling and downwelling is promoted by Ekman transport associated with alongshore winds

59 High The California Current (CC)

60 California Upwelling Temperature Coastal upwelling is often enhanced by the presence of coastal capes and headlands eg Point Sur and Point Reyes. Chlorophyll

61 California Current Euphausid Distributions North Pacific distribution of Nyctiphanes simplex, a euphausid.

62 The California Current System

63 Eastern Boundary Currents Equatorward Eastern boundary current Geostrophic balance Equatorward Wind CF F c PGF F g Reference depth Eastern boundary currents: California Current (N Pac) Humboldt Current (S Pac) Canary Current (N Atl) Benguela Current (S Atl) Leeuwin Current (S Ind) High Pressure PGF Low Pressure Northern Hemisphere Example

64 Other Forms of Upwelling Topographic Upwelling Capes and Points Differential Currents Langmuir Cells

65 Equatorial Upwelling

66 Seasonal Variations in Upwelling and Downwelling

67 Map of the equatorial Pacific with isopleths of surface nitrate concentration.

68 Ocean Chlorophyll and Major Coastal Upwelling Regions

69 Distributions of tropical species of Copepods and Euphausids