Ocean Processes I Oceanography Department University of Cape Town South Africa

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1 Ocean Processes I Isabelle.Ansorge@uct.ac.za Oceanography Department University of Cape Town South Africa 1

2 9/17/2009 Lecturer in Oceanography at UCT About me! BSc in England University of Plymouth MSc and PhD University of Cape Town Sea-going Oceanographer Southern Ocean projects Currently interested in ocean eddy heat fluxes from the Antarctic to the Subantarctic 2

3 9/17/2009 Train our postgraduate students at sea 3

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5 ..Ocean Physics outline Wind driven ocean circulation Connecting to the deep and the importance of the oceans and small scale processes in a changing planet II I Two types of Ocean Circulation: 5

6 The Shallow, Swift Wind-driven Circulation The Slow, Deep Thermohaline Circulation Antarctic Circumpolar Wind driven surface circulation Learning Objectives The role of solar heating How does the wind drive surface currents? Consider this! Theories from Ekman, Stommel West vs East why the difference Coastal and open ocean upwelling 6

7 What regulates the motion of fluid particles in the ocean and atmosphere? Forces generating currents Internal Forces External Forces Forces exerted by wind, thermohaline (TSp) Pressure gradient solar radiation. Tidal forces Forces retarding currents Friction..just to recap.. what drives ocean currents? Two external forces influence the World Ocean generating ocean currents - gravitation and the energy flux from the sun. Gravitation includes tidal forces resulting from the interaction of water mass with the moon and the sun, and rotation of the Earth. IoE The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography 7

8 The radiation flux from the sun results in wind stress, heating and cooling of the ocean surface. A complex process of interaction between these forces results in a complex and variable pattern of ocean circulation. IoE The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography 1. What drives ocean currents? Solar heating is different at different latitudes, because equal amounts of sunlight are spread over a greater surface area near the poles than in the tropics. 8

9 Simplified!! Warm air rises and cool air sinks; a convection current forms in a room resulting from uneven heating and cooling. Equator South Pole Simplified!! Warm air rises and cool air sinks; a convection current forms in a room resulting from uneven heating and cooling. 9

10 surface Equator/Tropics Higher Latitudes surface Equator/Tropics Higher Latitudes 10

11 Vertical view of the atmosphere Polar Cell Ferrel Cell Equator Pole v 11

12 ..so that s the role of solar heating covered BUT 12

13 How do we get to this?.importance of coriolis force.. Two important reasons: (1) the Earth rotates eastwards (2) the velocity of a point on the Earth is a function of latitude Coriolis parameter f=2ωsin(ø) ω = rotation of earth = 7.292E -5 s -1 The Coriolis deflection is therefore related to the latitude. 13

14 Circumference of the Earth at the Equator = 40,000 kilometers Time to complete one Rotation = 24 hours Speed of Rotation = Distance/Time = 40,000 km / 24 hr = 1670 km/hr Circumference of the Earth at the Equator = 40,000 kilometers Time to complete one Rotation = 24 hours Speed of Rotation = Distance/Time = 40,000 km / 24 hr = 1670 km/hr Circumference of the Earth at 40 N = 30,600 kilometers Time to complete one Rotation = 24 hours Speed of Rotation at 40 North = Distance/Time = 30,600 km / 24 hr = 1280 km/hr 14

15 So we know Changes in latitude affect solar radiation..affects pressure gradients Pressure gradient = winds High to Low Now throw in earths rotation and we have a deflection correlating to the hemisphere. but how do we get the ocean currents? 15

16 Fridtjof Nansen noticed that wind tended to blow ice at an angle of to the right of the wind in the Arctic. Three forces must be important: 1. Wind Stress W; 2. Friction F; 3. Coriolis Force C. and that W + F + C = 0 16

17 Wind stress - the transfer of horizontal momentum. Thus momentum is transferred from the atmosphere to the ocean by wind stress. Wind stress Τ (kg m -1 s -2 or Newton per m 2 ) is an important quantity in the process of wind driving ocean currents. Τ = C d ρ a U 2, where C d is the dimensionless "drag coefficient" of the wind on the surface (about ), ρ a is air density (about 1.3 kg m -2 ), U is wind speed at 10 m above sea level (m s -1 ). Wind stress is a square function of wind speed because the wind forcing depends on wind speed and sea roughness, which in turn accelerates wind speed. 17

18 surface balance between friction and rotation LEFT IN S. HEMISPHERE RIGHT IN N. HEMISPHERE ~100 meter depth But how deep does this extend too?? 18

19 Ekman Layer Depth (D E ) Ekman proposed that the thickness of the Ekman layer depth (at which the current velocity is opposite to the velocity at the surface), occurs at a depth DE Ekman Layer Depth (D E ) Ekman proposed that the thickness of the Ekman layer depth (at which the current velocity is opposite to the velocity at the surface), occurs at a depth DE (m) 19

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21 Horizontal circulation - Ekman drift The wind-driven component of water transport is directed perpendicular to the mean wind stress. The magnitude (kg m -1 s -1 ) is M e = τ / f, where τ is wind stress and f is Coriolis force. 21

22 Effect of Ekman Currents In a nutshell: Wind-driven currents are produced by the interaction between the wind and the water. As wind moves across the water, energy is transferred from the air to the water. - Water moves at about ~3% of the wind speed. Westerly-driven ocean currents in the trade winds, easterly-driven ocean currents in the Westerlies and deflection of the ocean currents by the continents results in a circular current, called an ocean gyre, which occupies most of the ocean basin in each hemisphere. 22

23 So, do the gyres just follow the winds? Not exactly! But the winds get the motion in the ocean started The oceans respond by flowing and turning Water piles up in the center of gyres Productivity varies depends on the physical processes.. 23

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25 Pressure gradients develop in the ocean because the sea surface is warped into broad mounds and depressions with a relief of about 1m. Depressions are caused by divergences, places from where water rises to the surface and flows outward. 25

26 Mounds are caused by convergences, places where water flows together and sinks. 26

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28 Winds at the sea surface drive Ekman transports to the right of the wind in this northern hemisphere example. The converging Ekman transports driven by the trades and westerlies drives a downward geostrophic flow just below the Ekman layer (bold vertical arrows), leading to downward bowing constant density surfaces. 28

29 Application of Ekman Theory coastal Horizontal divergence of the Ekman transports leads to a vertical velocity in the upper boundary layer of the ocean, a process called Ekman Pumping. Levels of nutrients brought to the surface Ekman pumping Wind stress Ekman vertical velocity drives a vertical geostrophic current 29

30 Daily sea surface temperature off Cape Town in summer Coastal Upwelling 30

31 Application of Ekman Theory open ocean 3 cases Application of Ekman Theory open ocean 3 cases 31

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33 Application of Ekman Theory open ocean 3 cases 33

34 Areas of convergence.. here water is moving together and sinking Basically cold dense antarctic waters sink below warm subtropical waters. These convergence zones allow for the NORTHWARD movement of water masses Application of Ekman Theory open ocean 3 cases 34

35 In a non-rotating world this would be our circulation! After Stommel 1948 made the first assumption that coriolis force needs to be included! But the earth rotates! 35

36 Sea surface height 36

37 In an idealised world this mound would be central. BUT rotation of the earth means that the flow is LOP-SIDED!!! 37

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40 The current flow pattern in gyres is asymmetrical with narrow, deep and swift currents along the basin s western edge and broad, shallow slower currents along the basin s eastern edge. This deflection is called WESTERN INTENSIFICATION The geostrophic mound is deflected to the western part of the ocean basin because of the eastward rotation of the Earth on its axis THIS IS CALLED WESTERN INTENSIFICATION. 40

41 Sea Surface Height see the effects of western boundary currents as high areas!! 41

42 So we know Wind effects first observed by Nansen The Ekman layer at the sea surface has the following characteristics: (a) Direction: 45 to the right of the wind looking downwind in the Northern Hemisphere. (b) Surface Speed: 1 3% of wind speed depending on latitude. (c) Depth: approximately m depending on latitude and wind velocity. Ekman pumping, which is driven by spatial variability of winds, drives a vertical current, which drives the interior geostrophic circulation of the ocean. Stommel showed that western boundary currents are required for flow to circulate around an ocean basin when the Coriolis parameter varies with latitude. role of eddies and frontal systems? How do we measure surface? CTD ADCP ARGO floats Drifters Satellite 42

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45 Horizontal circulation Geostrophic flow Mean dynamic height (m 2 s -2 ), or steric height multiplied by gravity, for the World Ocean at 0 m relative to 2000 m. Arrows indicate the direction of the implied geostrophic movement of water. IoE The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography Water at station A is denser than water at station B. As the weight of the water above z = z 0 is the same, the water column must be longer at B than at A. Distribution of isobars and isopycnals at any depth level above z = z 0. In geostrophic flow, water moves along isobars, with the higher pressure on its right in the Northern Hemisphere (away from the equator). IoE The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography 45

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51 Ocean currents can be determined from the slope of the sea surface measured by the satellite altimeter 51

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