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Chapter 4

Ch4 Variation in the Environment.docx

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University of Toronto St. George
Spencer Barrett

chapter 4: VARIATION IN THE ENVIRONMENT  the physical environment varies widely over the surface of the earth  due to differences in temperature, light, substratum, moisture, salinity, soil nutrients  climate zones are determined by solar radiation patterns and the redistribution of heat and moisture by wind and water currents  within climate zones, topography and bedrock composition further differentiate the environment  climatic patterns are determined by the intensity of sunlight falling on the earth’s surface, and the different surfaces (which absorb sunlight differently)  this creates different heating and cooling Effect of Solar Radiation  although many variations occur, there are also patterns in climate (due to the intensity of sunlight)  cold and dry at high latitudes  hot and wet near the equator  the sun warms the atmosphere, ocean, and land most when it lies directly overhead  sunlight is spread over a greater area when the sun approaches the horizon (i.e. at higher altitudes)  it also travels through a longer path (where most of its energy is reflected or absorbed)  strikes the earth at a lower angle  at the equator, the sun shines at a higher angle (close to perpendicular) over a small area  directly at the earth’s surface  zenith is the sun’s highest position each day  directly overhead in the tropics  near the horizon in polar regions  the warming effect of the sun diminishes from the equator to the poles  periodic climatic cycles follow astronomical cycles  rotation of the earth causes daily cycles of light and dark and changes in temperature  revolution of moon around earth creates lunar cycles (28 days) which changes tides  revolution of earth around the sun creates seasonal change Distribution of Solar Energy with Respect to Latitude  the equator is tilted 23.5° with respect to the earth’s orbit around the sun  Northern Hemisphere receives more solar energy than the Southern during the northern summer and vice versa  variation in temperature increases with distance from the equator (esp. in the Northern Hemisphere where there is less ocean to moderate temperature)  the tilt of the earth also creates a seasonal shift in the latitudinal belt near the equator  this area moves north and south seasonally with the solar equator  solar equator is the parallel of latitude lying directly under the sun’s zenith  the variation from 23.5°N to 23.5°S throughout the year causes complex patterns of precipitation in the tropics Water Vapour in the Atmosphere  equilibrium water vapour pressure is the amount of water vapour in the atmosphere when the tendency of liquid water to evaporate and the tendency of water vapour to condense is balanced  increases with temperature  thus, warm air can hold more water vapour than cold  water vapour is measured as how much it contributes to the total pressure of the atmosphere (approx. 100kPa or 10 at sea level)  any air mass can contain less than the EWVP, in which case water will continue to evaporate  if the water vapour pressure exceeds the equilibrium value (ex. when temperature of air decreases rapidly), excess gas will condense and leave the atmosphere as precipitation  this relationship btn temp. and the EWVP controls patterns of evaporation and precipitation  with air currents, they establish the distributions of wet and dry envmts Hadley Circulation  warming air tends to rise  the EWVP increases as air heats up, evaporation quickens (double per 10°C rise)  warming effect of sun in greatest near the equator  air in the tropics begins to rise in an upward-moving convection current  when it reaches the upper layer of the atmosphere (10-15 km), it spreads to higher latitudes north and south  the air is replaced by surface-level air moving in from subtropical latitudes  forms trade winds  the tropical air mass cools as it expands under the lower pressure of the upper atm  radiates heat into space  by the time this air gets to approx. 30°N and S, it is dense enough to sink to earth’s surface, thus completing a cycle called Hadley circulation  Hadley circulation is the vertical and latitudinal circulation pattern of air in the atmosphere driven by the warming effect of the sun (includes Hadley cells, Ferrel cells, and Polar cells)  Hadley cell is the circulation pattern of rising and falling air within the tropics  warm, moist air rises in the tropics, and cool, dry air moves toward the tropics from subtropical latitudes to replace it  there are two Hadley cells one on each side of the equator  their sinking air drives the Ferrel cells (in temperate regions 30°-60°N and S) which circulate in the opposite direction  Ferrel cells cause air to rise at 60°N and S, causing Polar cells to form  all this air circulation is driven by differential solar heating of atm at diff. latitudes Coriolis Effect and Jet Streams  in 1700s, George Hadley used the Coriolis effect to explain why trade winds blow from NE to SW (instead of directly from N to S)  earth’s rotation causes winds to veer right of their direction of travel in the NH and left in the SH to conserve momentum  the air rising at the equator is travelling faster than the air descending at 30°N  thus, as it moves north, it gets far ahead to the east relative to the earth’s surface creating westerly winds  opposite in the south  the trade winds fall behind the rotation of the earth as it moves south, veering west  thus, temperate weather tends to move W to E  the subtropical jjet streamis a rapidly moving west-to-east air current (10 km above earth’s surface) that forms in as a warm air mass moves from the equator (Hadley cell) and converges at high altitudes with cooler air moving towards the equator (Ferrel cell)  a more powerful one forms where the Ferrel and Polar cells meet  have tremendous, but unpredictable effect on the weather Intertropical Convergence and Subtropical High-Pressure Belt  intertropical convergence is the region where surface currents of air from the northern and southern subtropics meet near the equator and begin to rise under the warming influence of the sun  the tropics are more humid not because there is more water there, but because water cycles more rapidly in the atm there (due to the heating effect of the sun)  when air is cooled as it rises and expands, it causes precipitation because colder air has a lower EWVP  subtropical high-pressure belts are regions of high atmospheric pressure caused by descending masses of heavy air north and south of the equator  come from the high altitude air masses moving north and south away from the intertropical convergence  these masses have already lost much of their water to precipitation in the tropics  because this air has cooled, it becomes denser and begins to sink  the air begins to warm again at the subtropics and its EWVP rises  descends to ground level drawing moisture from the land  creates zones of arid climate at approx. 30°N and S Ocean Currents Redistribute Heat  physical conditions of the oceans have huge variations  caused by winds (currents) and the topography of ocean basins  deep-water currents caused by differing densities from variations in temperature and salinity  Coriolis effect affects the direction of ocean circulation  cold surface water circulates toward the tropics along western coasts and warm water circulates poleward along eastern coasts  clockwise ocean currents in the NH and counterclockwise in the SH  surface currents have huge affects on the climate of nearby land  ex. Peru Current (moves north) in eastern Pacific Ocean creates cool, dry envmts  ex. Gulf Stream creates mild climate into western Europe  upwelling is any upward movement of ocean water  occurs wherever surface currents diverge  as surface currents move apart, they tend to draw water upward from deeper layers  ex. in the western tropical Pacific Ocean  ex. on western coasts of continents where surface currents move toward the equator and then veer away from the continental margins  deep water is rich in nutrients, thus upwelling zones have high biological productivity  ex. fisheries of the Benguela Currents along the western coast of Southern Aftica and the Peru Current Thermohaline Circulation  thermohaline circulation causes currents by changes in the density of water caused by variations in temperature and salinity  as wind-driven surface currents (i.e. Gulf Stream) move towards higher latitudes, the water cools, becomes dense, and sinks  in the far north (towards Iceland and Greenland) ocean surfaces freeze in the winter  salts are excluded from sea ice, thus the underlying water increases in salinity  this cold water becomes more dense and sinks, forming the North Atlantic Deep Water current (similar near Antarctica)  then flow through abyssal depths of the ocean basins back into equatorial regions o eventually surfaces as upwelling currents  causes extensive mixing of oceans  distributes heat energy from tropics to higher latitudes  the southward movement of the NADW is crucial to the northward movement of the Gulf Stream on the surface  the global thermohaline circulation pattern is like the ocean conveyor belt Shutdown of Thermohaline Circulation and the Younger Dryas  melting Greenland ice sheets and Arctic Ocean sea ice will flood the North Atlantic with low-salinity surface waters, preventing the formation of NADW  would shut down the Gulf Stream  drastically affect Europe’s climate (no Gulf Stream = no heat from the tropics)  this event occurred about 12 700 year ago and resulted in a period of cold weather in the region called the Younger Dryas period (lasted 1300 years)  over tens of millions of years, climate has been influenced by continental drift  opened and closed connections between water basins, altering the flow of currents  changed the distribution of heat over the earth’s surface Latitudinal Shifting of the Sun’s Zenith  north/south mvment of solar equator determines time of rainy season in the tropics  intertropical convergence follows  produces a moving belt of rainfall  at the equator, Bogota, Colombia has 2 rainy and 2 moderately dry seasons  20°N and S (Merida, Mexico and Rio de Janeiro, Brazil resp.), has 1 rainy and 1 dry season (but at alternating times of the year)  rainy season occurs over the intertropical convergence  dry season occurs under the influence of the subtropical high-pressure belt  westerlies influence climates at middle latitudes  Mediterranean climate is a pattern found at middle latitudes on the western side of continents, characterized by cool, wet winters and warm, dry summers Seasonal Cycles in Temperate Lakes  water gains and loses heat slowly (reduces temp. fluctuations in large bodies of water)  but small midcontinental lakes in temperate zone respond quickly to changing seasons  changes in temp. causes changes in water density  causes a pattern mixing of the water  in temperate areas (cold winters, warm summers), a lake undergoes 2 periods of vertical mixing, and 2 with little vertical mixing (water is layered)  in winter, it exhibits an inverted ttemperature profile  coldest water at surface, warmer water (max. 4°C) at bottom  b/c density of water increases btn the freezing point and 4°C  in early spring, lake surface warmed gradually by the sun  warmed surface water sinks below until temp. exceeds 4°C  this vertical mixing distributes heat throughout the water  results in an uniform temperature profile  at the same time, winds drive surface currents that can cause deep water to rise (similarly to upwelling currents)  this spring overturn brings nutrients from the bottom sediments to the surface and oxygen to the depths  in late spring/early summer (sun rises higher and air above lake warms up), surface layers gain heat faster than the deeper layers  creates a tthermocline a zone of abrupt temperature change at intermediate depth  once established, water doesn’t move across it b/c of stratification  stratification is a condition where warmer, less dense surface water literally floats on the cooler, denser water below  depth of thermocline depends on winds and depth & turbidity of lake o around 5m-20m below surface o lakes less than 5m usu. lack stratification  epilimnion is the upper layer of warm water above the thermocline o where most of the production occurs (sunlight most intense here) o photosynthesis supplies oxygen here and supports animal life o but the abundance of plants and algae often deplete the mineral nutrients here, thus limiting their own production  hypolimnion is the deeper layer of cold water below the thermocline o the thermocline isolates the hypolimnion from the surface (where there is little or no photosynthesis), so animals and bacteria there may deplete the water of oxygen (creating anaerobic conditions)  lakes abundant in organic matter in the epilimnion have depths that are particularly in short supply of oxygen o bacteria in the depths use up any available oxygen while decomposing organic material drifting down from the surface  in late summer, temperate lakes become severely unproductive  nutrients needed to support plant life are depleted on the surface  oxygen needed to support animal life is depleted in the depths  in autumn, surface layers cool more rapidly than deeper layers (becomes denser, and sinks below)  this vertical mixing is the fall overturn  persists into late fall until temp. of surface drops below 4°C (winter stratification results)  fall overturn speeds mvment of oxygen to depths and pushes nutrients to surface  in lakes where hypolimnion becomes warm by midsummer, deep vertical mixing may take place in late summer while temp. remain favourable for
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