Ocean Processes I

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Ocean Processes I

[email protected] DepartmentUniversity of Cape Town

South Africa

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� Lecturer in Oceanography at UCT� BSc in England – University of Plymouth� MSc and PhD University of Cape Town

About me!

� Sea-going Oceanographer – Southern Ocean projects� Currently interested in ocean eddy heat fluxes from the

Antarctic to the Subantarctic

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� Train our postgraduate students at sea

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..Ocean Physics outline☺Wind driven ocean circulation I☺Connecting to the deep and the importance of ☺Connecting to the deep and the importance of

the oceans and small scale processes in a changing planet II

Two types of Ocean Circulation:

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The Shallow, Swift Wind-driven Circulation

The Slow, Deep Thermohaline Circulation

Antarctic Circumpolar

Learning Objectives

The role of solar heating

Wind driven surface circulation

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

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What regulates the motion of fluid particles in the ocean and atmosphere?

Forces generating currents

Internal Forces Forces exerted by wind, Internal Forces Forces exerted by wind, thermohaline (TSp)

External Forces 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 184 - The Basics of Satellite Oceanography. 1. The Basic Concepts of Oceanography

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The radiation flux from the sun results in wind stress, heating and cooling of the ocean surface.and cooling of the ocean surface.

A complex process of interaction between these forces results in a complex and variable pattern of ocean circulation.


Solar heating is different at different latitudes, because equal amounts of

1. What drives ocean currents?

equal amounts of sunlight are spread over a greater surface area near the poles than in the tropics.

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Simplified!! Warm air rises and cool air sinks; a c onvection current forms in a room resulting from uneven heating and c ooling.

Simplified!! Warm air rises and cool air sinks; a c onvection current forms in a room resulting from uneven heating and c ooling.

Equator South Pole

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Equator/Tropics Higher Latitudes

surface

Equator/Tropics Higher Latitudes

surface

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Vertical view of the atmosphere

Equator

Polar CellFerrel Cell

PolePole

v

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..so that’s the role of solar heating covered… BUT…

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How do we get to this?

�….importance of coriolis force..

� Two important reasons:� (1) the Earth rotates eastwards� (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.

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– 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

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So we know

�Changes in latitude affect solar radiation �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?

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Fridtjof Nansen noticed� that wind tended to blow ice at an angle of

20◦–40◦ 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

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Wind stress - the transfer of horizontal momentum. Thus momentum is transferred from the atmosphere to the ocean by wind stress.

Wind stress ΤΤΤΤ (kg m -1 s-2 or Newton per m 2) is an important quantity in the process of wind driving ocean currents.

|Τ|Τ|Τ|Τ| = C ρρρρ U2|Τ|Τ|Τ|Τ| = Cd ρρρρaU2, where

Cd is the dimensionless "drag coefficient" of the wind on the surface (about 0.0013),

ρρρρa 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 beca use the wind forcing depends on wind speed and sea roughness, which in t urn accelerates wind speed.

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surface

balance between friction and rotation

LEFT IN S. HEMISPHERE

~100 meter depth

LEFT IN S. HEMISPHERE

RIGHT IN N. HEMISPHERE

But how deep does this extend too??

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� Ekman Layer Depth (DE)

� 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 (DE)

� 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)

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Horizontal circulation - Ekman drift

The wind-driven component of water transport is dir ected perpendicular to the mean wind stress.

The magnitude (kg m -1 s-1) is

Me = ττττ / f, where

ττττ is wind stress and f is Coriolis force.

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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 transferredfrom 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, by the continents results in a circular current, called an ocean gyre, which occupies most of the ocean basin in each hemisphere.

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So, do the gyres just follow the

winds?

� Not exactly! But the winds get the motion in the ocean startedstarted

� The oceans respond by flowing and turning

� Water piles up in the center of gyres

� Productivity varies depends on the physical processes..

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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.

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

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� 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.

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Application of Ekman Theory – coastal

� Horizontal divergence of the Ekman transports leads to a

� 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 surfaceto the surface

Ekman pumping Wind stress

Ekman vertical velocity

drives a vertical geostrophic current

Ekman vertical velocity

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•Daily sea surface surface temperature off Cape Town in summer

•Coastal • Coastal Upwelling

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Application of Ekman Theory – open ocean – 3 cases


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Areas of convergence..here water is moving together and sinking

Basically cold dense Basically cold dense Basically cold dense Basically cold dense antarctic waters sink antarctic waters sink antarctic waters sink antarctic waters sink below warm below warm below warm below warm subtropical waters.subtropical waters.subtropical waters.subtropical waters.

These convergence zonesThese convergence zonesThese convergence zonesThese convergence zonesThese convergence zonesThese convergence zonesThese convergence zonesThese convergence zonesallow for the allow for the allow for the allow for the NORTHWARD NORTHWARD NORTHWARD NORTHWARD movement ofmovement ofmovement ofmovement ofwater masseswater masseswater masseswater masses


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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!�But the earth rotates!

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Sea surface height

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In an idealised worldthis mound would be central.

BUT rotation of the earthmeans that the flow isLOP-SIDED!!!

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•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 –the Earth on its axis –

� THIS IS CALLED WESTERN INTENSIFICATION.

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Sea Surface Height – see the effects of western bou ndary currents as high areas!!

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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 theNorthern Hemisphere.Northern Hemisphere.(b) Surface Speed: 1–3% of wind speed depending on latitude.(c) Depth: approximately 40–300 m depending on latitude and windvelocity. � 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?� role of eddies and frontal systems?

How do we measure surface?

�CTD�CTD

�ADCP

�ARGO floats

�Drifters

�Satellite�Satellite

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Horizontal circulationGeostrophic 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. Arro ws indicate the direction of the implied geostrophic movement of wa ter.


Water at station A is denser than water at station B. As than water at station B. As the weight of the water above z = z0 is the same, the water column must be longer at B than at A.

In geostrophic flow, water moves along isobars, with the higher pressure on its

Distribution of isobars and isopycnals at any depth level

above z = z0.

the higher pressure on its right in the Northern Hemisphere (away from the equator).


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Ocean currents canbe determinedfrom the slope of thefrom the slope of thesea surface measured by the satellitealtimeter

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Documents

Ocean Processes I