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Unformatted text preview: Atmospheric and Ocean Circulations • Atmospheric Composition • Atmospheric Pressure • LandSea Breezes • Forces Controlling Global Winds • Air Circulation in the Tropics • Air Circulation in High Latitudes • Ocean Currents Constituent Gases of the Lower Atmosphere Table 3.2 Wate r vap ou r Vari able, up to 3% Earth’s at mosphere consists of a mechanical mixture of gases surrounding the Earth and held by gravitational attraction. 1 Properties of Principal Atmospheric Gases • Nitrogen (N ): a largely inert “filler” gas, 78% by volume 2 • Oxygen (O ): highly active chemically; critical for 2 combustion and respiration, 21% by volume • Argon (Ar): inert gas of little importance in natural processes • Carbon Dioxide (CO ): makes up 0.035% of atmosphere; 2 emitted by volcanoes and combustion processes. Taken up by plants. An important Greenhouse gas • Water Vapour: 0.5% 3% by volume, depending on humidity levels. Another important Greenhouse gas Atmospheric Pressure • At mospheric pressure exists because air mo lecules have mass, m, and are pulled downwards by Earth’s gravitational accelerat ion, g = 9.8 m/s2. Mass x gravit y causes weight. • At mospheric pressure is the force produced by the weight of a column o f air above a unit area of surface: Weight or air column, W = mg (Newtons) 2 Pressure at surface = W/area = Newtons/m 2 One Newton/m = 1 Pascal (in honour of Blaise Pascal) 2 Thus, 1N/m = 1Pa Mean sea level air pressure = 101,300 Pa = 101.3 kPa In old millibar units, this is 1013 mb Thus, 1kPa = 10mb 2 Measurement of Atmospheric Pressure • General term for a device to measure air pressure is a barometer. We thus speak of ‘barometric pressure’ = air pressure. th • One of the earliest devices (18 century) was the mercury barometer, which demonstrates the concept of pressure very clearly. Mercury was chosen because it was the densest liquid readily available (13.6 times the densit y of water). • All ships at sea would carry a tall barometer fro m which changes in air pressure could be read and recorded. Fig 6.2) Atmospheric pressure decreases with altitude Note nonlinear decrease of pressure with height. Why is it nonlinear? Stratosphere Troposphere Fig 3.3 3 Total Gas Pressure and Partial Gas Pressures th Air is a mechanical mixture of gases. In the early 19 century, John Dalton determined that the total pressure exerted by a mixture of gases was equal to the sum of the pressures exerted by each o f the gases taken separately (the socalled ‘partial pressures’): Thus, total gas pressure = sum of the partial gas pressures. For Earth’s at mosphere: Total pressure, P = P(N ) + P(O ) + P(Ar) + P(H O) + P(CO )…. T 2 2 2 2 and the list could be extended further to include ‘trace’ gases. In what way is the above equat ion just another statement of the Principle of Energy Conservat ion? The term P(H O) is termed ‘water vapour pressure’ (a.k.a. ‘vapour 2 pressure’) and is very important in atmospheric sciences, as we will see in next week’s notes. Atmospheric density Densit y is a property of all materials, and is measured by determining the mass of material, m, contained in a unit vo lume, V. Thus densit y, ρ (Greek rho) = mass/vo lume = m/V. In commo n parlance, densit y is o ften confused with weight. We talk about Vancouver snow as being ‘heavier’ than that in, say, Alberta. What we really mean is that Vancouver snow is denser. 3 One m of Vancouver snow will therefore weigh more than the same vo lume of snow in Alberta, in general. Some typical densit y values are: 3 Densit y o f air (at sea level) = 1 kg/m , but much lower alo ft 3 Densit y o f water = 1,000 kg/m 3 Densit y o f sea water = 1,012 kg/m 3 depend ing on wat er co nt ent Densit y o f snow = 50 to 500 kg/m , 3 Densit y o f glacier ice ( max. possible) = 917 kg/m 4 Introduction to Atmospheric Motion • Wind is simply horizontal air motion, a.k.a. advection • Wind is caused by a pressure gradient, i.e. a difference in surface atmospheric pressure from place to place. Air moves from higher to lower pressure in an attempt to equalize the surface pressures. • Wind direction is expressed in terms of the direction from which the wind is coming: e.g., 090° = easterly, 180° = southerly, 270° = westerly, etc • Air motions that are mainly vertical are not really winds, but updrafts or downdrafts, a.k.a. convection Winds and Pressure Gradients Lines joining places ha ving the sa me surface air pressure are ter med isobars. Isobars allow pressure gradients to be deter mined, by deter mining the change in pressure (mb) with respect to map distance between two places. The strength of the pr essure gradient is deter mined from the spacing of the isobars. If the distance between points X and Y below is 100 km, then P.G. = 8 mb per 100 km, = .08 mb/km .X .Y
Fig. 6.7 5 Smallscale air motions: the LandSea Breeze • One of the simplest air motions to comprehend is the LandSea Breeze. This is a local wind which blows from the ocean to the land by day (Sea Breeze), and then from the land to the sea by night (Land Breeze). • The basic cause of the landsea breeze is a temperature difference between the air over the land and the sea. By day the land is warmer than the sea. By night the sea is warmer than the land. • The reasons for the temperature differences were discussed earlier in the course, and relate to the differing thermal properties of land and water. Thermal Differences between Land and Water: Revisited Fig 5.7 6 Air temperature differences and atmospheric motion: 1 • The fundamental cause of atmospheric motion is the variation in air temperature from place to place, and the effect that this has on air density (mass per unit volume). • It is a well established fact that heating a gas causes it to expand, and as it expands the volume increases but the mass stays constant. Therefore the density must decrease. • Cooling a gas causes it to contract to occupy a smaller volume, but again with mass unchanged. Therefore cold air must have a higher density than warm air. Air temperature differences and atmospheric motion: 2 Consider two adjacent ‘columns’ of air, both containing the sa me mass of air, and ther efor e both having the same initial surface pressure: One is cold, the other warm. In cold air, surface chilling causes the air to contract and densify near the surface. In war m air, the air expands upwards because it is heated. So there is less mass near the base of the warm air column. The result is a slower decrease of pressure with height in the warm column. pressure gradient higher pressure lower pressure 850 850 cooler air 900 950 surface 1000 mb warmer air 900 950 surface 1000 mb 7 Heating and Convection Convection is caused by heating and cooling of air. Note that pressure decreases more slowly wit h height in the warmer air, which sets up a pressure gradient aloft, which causes outflow alo ft into the cooler air columns. This flow causes mass transfer fro m warm column to the cold column, which then changes the surface pressures. Air ascends over the land and descends over the sea, to complete a convection loop. Sea and Land Breezes • Sea breeze: afternoon wind that brings cool air off the water towards the land, because a surface low pressure has developed over the land by day due to greater heating. • Land breeze: a nighttime wind that brings cooler air, chilled over the land by nighttime radiant cooling, towards the water 8 Largescale Circulation of Earth’s Atmosphere The preceding sea breeze is an example of a ‘local’ air circulat ion. It is of smallscale (just a few km), is driven primarily by local heating differences and is relatively easily understood. When we transfer to the large scale, and consider regional and so called planetaryscale winds, things get more complicated because the air is now moving for considerable distances (100s or 1,000s of km) across the surface of a rotating Earth. To visualize what is invo lved consider the next slide, which depicts the easterly mot ion of an airborne object. Throughout its motion, the object maintains a constant direct ion with respect to a fixed reference frame external to the Earth. But to an Earthbound observer, rotating with the Earth, the object appears to move to the right of the expected path. Deflection of Motion on a Rotating Earth A flying object seen at two differ ent times, t 0 and t , maintaining 1 a constant absolute direction, yet having a deflection to the right with respect to the expected path for an Earthbound observer. This deflection is due to the Coriolis effect. t 0 t 0
Direction at t is 0 due east t 1 Direction at t 1 is southeast Fig. 7.12 9 Coriolis Effect • Corio lis effect is due to Earth’s rotation. An object in motion on the Earth’s surface appears to be deflected away from its expected course. – Deflect ion is to the right in the northern hemisphere – Deflect ion is to the left in the southern hemisphere • Deflect ion is strongest at the poles and zero at the equator. • The Corio lis effect is important only where frict ion is small, such as largescale atmospheric motions and ocean current circulat ion, plus aircraft movements. For a car or train, because of frict ion between the tires/road or wheels/rails, the Coriolis effect can be ignored. But what about very, very high speed rail?? • The Corio lis effect is certainly very important in longrange artillery and (gulp!) intercontinental ballist ic missile trajectories. Actual versus expected air/plane movements Fig 6.9 10 Coriolis Effect and Wind Patterns A parcel o f air in motion near the surface is subjected to 3 forces: 1. Corio lis force (F ) c – always acts at 90° to the right of the wind direction in the Northern Hemisphere and increases with wind speed 2. Frict ional force (F ) (exerted by the ground surface on the f wind) – this force is proportional to the wind speed and always acts in the direct ion opposite to the direction o f motion 3. Pressure gradient force (F ) p – Always acts at 90° to the isobars and pushes the parcel toward lower pressure Forces involved in air movement The (vector) sum of the three forces produces a resultant direct ion of wind motion that is toward low pressure but at an angle to the pressure gradient line. Fig. 6.8 equiv. F c F p F f
Northern Hemisphere case Air is expected to move at 90 degrees to the isobars towards the low pressure, but instead it mo ves oblique to the isobars but still towards the lower pressure. 11 Nearsurface wind movements: cyclones • Cyclone: centre of low air pressure where air spirals inward toward a low then rises up. Inward spiraling motion is called convergence Cyclones are associated with cloudy and rainy weather Northern Hemisphere Southern Hemisphere Nearsurface wind movements: anticyclones Anticyclone: a centre of high air pressure where air spirals outward and descends towards the surface. Outward spiraling motion is called divergence. Anticyclones are associated with clear skies and fair weather Northern Hemisphere Southern Hemisphere 12 Surface Wind Flows in the Tropics (Fig 6.12) Surface Trade Winds converge towards the Equator from each hemi spher e. They meet at the Intertropical Conver gence Zone (ITCZ). Note the rising air at the ITCZ and subsiding air in the subtropical highs. This vertical circulation system is known as a Hadley Cell – one occurs in each hemispher e. Vertical crosssection through atmosphere to illustrate Hadley Cell circulation Note the airstreams (Trades) converging and rising at Equatorial Low and subsiding and diverging at subTropical Highs. Fig 6.12(b) cooling subsidence Hadley Cell ascent 13 Hadley Cell Circulation: Causes Recall fro m the seabreeze example that when air is heated, a vertical convect ion loop forms. Similarly, the surface and atmosphere at the Equator are heated more strongly yearround than the subtropical areas (at about 30° N & S). A huge convection cell known as the Hadley Cell forms in each hemisphere because of differential heating o f the Equator relative to the subtropics. Above the Equator, warmer air means a slower decrease of pressure wit h height, and thus the development of a pressure gradient aloft towards the subtropics. This means that air is diverging aloft above the Equator, and as in the seabreeze case this leads to the development of an area of lower pressure at the surface known as the Equatorial Trough. Trade winds then converge toward this low pressure area and then rise at the ITCZ. Subsidence over the subtropics completes the Hadley Cell circulat ion. Implications for weather and climate in the Tropics • Convergence and ascent of air at the ITCZ means that mo ist air is lifted, cools and mo isture condenses to form clouds. • This means that clouds are constantly forming at or near the ITCZ, and that rainfall is an almost daily occurrence. Welco me to the humid tropics and the great Equatorial rain forests of the planet, located in the Amazon and Congo basins and southeast Asia! • Subsidence and divergence of air at the subtropical ant icyclo nes means compressio n and warming o f air, the evaporation of clouds and therefore very little precipitation. Welco me tothe great tropical deserts of the world, located in North Africa, Arabian peninsula and Australia! 14 Major surface pressure features of the Earth: January average Note the prominence of the subtropical ant icyclo nes (Fig 6.10a) Major surface pressure features of the Earth: July average Note the lat itude shift of the subtropical ant icyclo nes (Fig 6.10b) t 15 At mospheric pressure and winds: January average At mospheric pressure and winds: July average 16 Monsoon circulations: the great Asian Monsoon • The term monsoon refers to a type of tropical climate in which there exist very distinct wet and dry seasons together with a seasonal reversal o f wind directions. • Rainfall is very heavy and concentrated in the warmest summer months. Very warm, rainbearing winds blow onshore (fro m sea to land) during the wet season. • The cooler season is very dry by co mparison, with almost no rainfall for periods of months. Much cooler winds blow offshore (from land to sea) during the dr y season. • The monsoon climate regions are ho me to most of the world’s populat ion and rely on seasonal rains to support intensive agriculture, especially rice cult ivat ion. Fundamental cause of the Asian Monsoon: Summer Intense heat ing of the African and south Asian landmasses in the period May – July causes centres of low pressure to develop – like the sea breeze effect, only on a gigant ic scale. This causes the ITCZ to migrate well north of the Equator, and the normal northeast Trade wind flo w is replaced by the southwest Monsoon. As the ITCZ migrates it brings a broad band of clo uds from which are released copious amounts of rainfall. Note how the winds change direction when the Equator is crossed, due to a change in the Coriolis force direction. The southeast Trades in the southern hemispher e become the southwest Monsoon in norther n hemispher e (marked by arrows for clarity). warm, moist inflow ITCZ S.E. trades 17 Fundamental cause of the Asian Monsoon: Winter Prolonged cooling of the N. Hemis. landmasses in the period October – March causes centr es of higher pressure to develop – like the land breeze effect, only on a gigantic scale. The ITCZ meanwhile has migrated south of the Equator, because ther ma l lows are now developing over Australia and southern Africa. Offshore flow now predominates over south/east Asia, bringing much drier air from interior Asia. Dryness prevails. cool, dry outflow ITCZ Note how the winds change direction when they cross the Equator because of a change in direct ion of the Corio lis force (marked by arrows for clarit y) At mospheric circulation in the extraTropical areas To understand the predominant wind dir ections in the areas polewards of the tropics, we need to return to the average world air pressure maps. Note the belts of westerly flow in midlatitudes in both hemispher es, ter med The Westerlies. This contrasts with the easterly (Trade Wind flows) in the intertropical belt. The subtropical highs are seen to be regulating wind flows over a huge area of the globe. w e s t e r l i e s e a s t e r l i e s w e s t e r l i e s 18 Westerly Flow in MidLatitudes • Wit hin the westerly belt in the zone 40 60° N and S there are no major reversals of wind direct ion with the seasons, unlike in the Monsoon belt. Winds, especially well above the surface, consistent ly blow fro m a westerly direction and swirl rapidly around the planet at mid to high latitudes. • Where there is a fairly abrupt change in air temperature, a Jet Stream tends to develop. This occurs because of the ver y rapid change in pressure with height in the colder air column relative to in warmer air. • In the midtroposphere, at 56 km height, a typical pressure recorded is 500 mb. If we now map the elevation of this constantpressure surface, we see where the high points (warm air) and low points (cold air) occur. Winds Aloft in the Upper Troposphere (810 km) • Air at higher levels in the troposphere, air moves in response to pressure gradients and of course is influenced by the Corio lis effect • Recall: Earth’s inso lation is greater in the tropics relat ive to the poles • Recall: pressure decreases less rapidly wit h height in warmer air than in co lder air. • Because the atmosphere is warmer in the tropics, pressure decreases slower in the warmer air and a strong PGF acts aloft towards the colder polar regions. • PGF increases wit h alt itude, so winds are stronger at higher altitudes 19 cold air warm air Largescale oscillat ion in the westerly flow over North America. Note that the 500mb surface is higher (a ridge) in the warmer air and much lower (a trough) in the colder air. Note the rapid downward slope of the surface over Hudson’s Bay. Zones of rapid change in the height of the surface are zones of steep pressure gradients aloft, and this is where jet streams will tend to occur. Fig 6.15 Geostrophic Winds • Air movement in the upper atmospher e has very low friction acting on it, so that only the pressure gradient force (PGF) and the Coriolis force (CF) significantly affect its movement. • As an air parcel moves in response to the PGF, it is turned progr essively to the right until the PGF and the Coriolis force just balance, producing the geostrophic wind. • Geostrophic winds occur at upper levels of the atmosphere and flow roughly parallel to the isobars. The Jet Stream is simply the most rapidly moving cor e of the westerly geostrophic flow. PGF CF 500 510 520 530 540 550 mb 20 Rossby Waves • Rossby waves: are horizontal undulations in the flow path of the upperair westerlies – waves arise in a zone of contact between cold polar air and warm tropical air, called the Polar Front • Rossby waves cause advection of warm air poleward and cold air equatorward, and thus are a primary mechanism of for N/S sensible heat transport • Rossby wave circulation is also the reason why weather in the midlatitudes is so variable from day to day. – pools of warm, moist air and cold, dry air alternately invade midlatitude areas Rossby Waves Cont. 21 Ocean Currents • Ocean current: any persistent, dominantly horizontal flow of ocean water • Ocean currents act to exchange heat between low and high latitudes and are essential in sustaining the global energy balance • Two types of ocean currents: 1. Surface currents: driven by prevailing winds 2. Deep currents: currents of cold, dense water originating from subarctic and arctic latitudes Major Surface Ocean Currents • The general direct ion ofocean currents is determined by the dominant wind direct ions above particular areas of the oceans. E.g., areas dominated by the subtropical ant icyclo nes produce a net clockwise rotation (gyre) of surface water in the Northern Hemisphere and a counterclockwise gyre in the S. Hemisphere. • Warm currents occur where warm tropical water is pushed either northwards by the westerly flow, or eastwards roughly parallel to the Equator by the Trade Winds. • Notable co ld currents occur in Arct ic and Antarctic areas, as expected, but also in tropical areas where offshore wind flow occurs, causing a wellingup of cold water near the coast (e.g., along the west coasts of S. America and southern Africa). 22 Major Ocean Currents Note the dominant influence of the subtropical anticyclo nes in determining the direction o f rotation of the major current systems, known as gyres. Fig 6.21 Conclusions: Atmosphere and Ocean Circulations • The fundamental cause of atmospheric motion on Earth is the variation in surface heat ing with latitude, which itself is caused by lat itudinal differences in inso lat ion. • Largescale differences in air temperature cause both horizontal and vertical air mo vements to develop, creating distinctive zones of convergence and divergence of air. • Persistent wind movements over large ocean areas drive the surface ocean currents. Patterns of warm or cold currents thus develop in response to winds. But…hang on! these differences in ocean surface temperature then influence air temperatures! • Thus, it is evident that a complex interchange exists between the atmospheric and ocean current circulations. 23 ...
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- Spring '09