The Circulation of the Atmosphere:

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1 The Circulation of the Atmosphere: Laboratory Experiments (see next slide) Fluid held in an annular container is at rest and is subjected to a temperature gradient. The less dense fluid near the warm wall rises, moves radially toward the cold wall where it sinks. The flow returns to the warm wall at lower levels in the tank. Low rotation speeds about the central axis: an axially symmetrically flow is seen with most of the heat transport confined to the boundary layers. The symmetrical motion from a balance between the effects of the Coriolis force in the rotating frame of reference and the radial pressure gradients that result from the variations of the fluid density with temperature. High rotation speeds about the central axis: The boundary layers become thin and transport less heat. The symmetric motion is also inefficient at transporting heat. Heat is transported by non-axisymmetric motions are produced. These wavelike motions have a meandering region of high velocity that is very effective at transferring heat. As rotation speed increases waves become less regular, heat is transported in these waves by sloping convection, c.f. upright convection in the absence of rotation. Climate and Energy P607 lecture 6

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3 The Circulation of the Atmosphere: Both symmetric and wavelike flows are observed in the atmosphere Near Equator the effects of rotation are small at higher latitudes wavelike motions with sloping convection are found. The Hadley Circulation: The symmetrical flow regime. Air rises due heating near the Equator. The Coriolis Force turns the poleward moving air eastwards and increases with latitude. The CF also turns the low level returning low level flow westwards. These are the so called Easterly Trade Winds. Were the Earth to not be rotating the atmosphere would have one cell. However, the CF directs the poleward air westward and so prevents the Hadley circulation from transferring heat to the pole directly. NOTE: Symmetric flow at all latitudes would imply an easterly surface flow at all latitudes. This would decelerate the rotation of the Earth due to frictional drag. Essentially altering the length of the day as the total angular momentum of the Earth- Atmosphere system must remain constant Climate and Energy P607 lecture 6

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6 Inter Tropical Convergence Zone Climate and Energy P607 lecture 6

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8 The Circulation of the Atmosphere: The Ferrel Cell: Wavelike motions transport warm air upwards and polewards, and cool air equator-wards and downwards in different parts of the wave. The motion can be thought of as interchanging air parcels along a slant path of average slope χ. If the slope of constant density (essentially constant entropy) surfaces is greater than χ the exchange leads to a reduction of PE and the KE of the wavelike motion increases Lines of constant density z χ latitude In the wavelike flows the mean zonal flow is westerly even though equator-ward motion leads to a locally easterly flow near the surface. Climate and Energy P607 lecture 6

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10 Conservation of Angular Momentum in the Earth s Atmosphere Since the angular momentum of the Earth constant the angular momentum of the atmosphere must also remain constant. Thus angular momentum must be transferred between the tropical regions with Easterly winds at the surface to the higher latitudes where the winds are westerly (when zonally averaged). In the waves this required positive correlation between u and v, the eastward and northward components of the flow (I.e. the velocity components after subtraction of the zonal means). This implies that the axes of the wavelike motions must be tilted: symmetrical waves cannot transfer angular momentum.. Climate and Energy P607 lecture 6

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