· Climatic patterns originate as the earth absorbs the energy in sunlight.
· The ability to absorb sunlight varies across the earth's surfaces and this creates differential heating and cooling.
· The earth's climate is cold and dry towards the poles and hot and wet towards the equator.
· The primary cause of global variation in climate is the greater intensity of sunlight at the equator.
· The light that does strike the poles, does so at a lower angle and it is spread over a greater area whereas the light that strikes the equator does so at a higher angle and a smaller area therefore the warming effects of the sun clearly diminish from the tropics to the poles.
· There are daily changes in climate result from the rotation of the earth around its axis.
· The revolution of the moon around the earth influences the tides on a monthly basis, and
· The rotation of the earth around the sun brings seasonal changes to part of the world.
· The earth is tilted 23.5 degrees with respect to the path it follows on its rotation around the sun.
· The northern hemisphere receives more sun during our summer than the southern hemisphere and they have summer while we have winter.
· The seasonal range in temperature in the northern hemisphere is greater than that for the southern hemisphere because there is less water in the north (more land masses).
· The solar equator is directly under the suns zenith. The location of the solar equator varies during the year. It is at 23.5 deg. N on 21 June and at 23.5 deg S on 21 Dec.
· Heated air in the tropics rises and spreads to the north and south in upper layers of the atmosphere.
· The air is replaced from below by surface-level air from the subtropics.
· The rising tropical air mass cools as it radiates heat back into space,
· When the air has moved to about 30 deg. N and S it has become dense enough to sink back to the earth's surface and this is how the Hadley cells are formed
· Around the equator there are two Hadley cells that are like waistbands encircling the earth.
· The sinking air of tropical Hadley cells drives secondary cells in temperate regions which circulate in the opposite direction.
· The circulation of temperate Hadley cells (30-60 degrees) cause air to rise at 60 degrees north and south forming polar Hadley cells.
· The intertropical convergence is where the tropical Hadley cells meet. As moisture laden tropical air rises and cools in the convergence zone, the moisture condenses to form clouds and precipitation.
· The regions north and south of the equator are known as subtropical high pressure belts because the descending mass of heavy air creates high atmospheric pressure.
· As air sinks and spreads to the north and south, it draws moisture from land into the air creating deserts at 30 degrees north and south. All the great deserts of the world are found within subtropical high pressure belts
· The rotation of the earth deflects surface flow in Hadley cells and surface flows are shifted. The tradewinds blow from east to west from 30 degrees N or S towards the equator and westerlies blow from the west to the east from 30 degrees N or S to 60 degrees N or S
· The positions of the continental landmasses exert a secondary effect on the global pattern of rainfall.
· More rain falls in the Southern Hemisphere because oceans and lakes make up more of the surface areas (81% of the surface as opposed to 61% in the Northern Hemisphere).
· Interior parts of the continents receive less rain than coasts.
· Coastal (maritime) climates vary less than interior (continental climates) because the heat storage capacity of water reduces temperature fluctuations.
· Global wind patterns interact with other features of the landscape to create precipitation.
· Mountains force air upward causing it to cool and lose moisture as precipitation on the windward side of a mountain range. As air descends on the leeward slope and travels across lowlands, it picks up moisture creating arid environments in rain shadows
· The physical conditions of the oceans are also complex.
· Winds propel major surface currents and the underlying topography of ocean basins also affects currents.
· Deep currents are established because of differences in density caused by variation in temperature and salinity.
· In large ocean basins, cold water circulates towards the tropics along the western edges of continents and warm water circulates towards temperate latitudes along the eastern coasts of continents.
· This figure shows the location of the major ocean currents. The cold Peru Current off the eastern Pacific moves northward from the Antarctic Ocean along the coasts of Chile and Peru. This creates cool, dry environments along the west coast of South America.
· Upwelling is the upward movement of ocean water, and it occurs when surface currents diverge.
· As surface currents move apart during upwelling, they draw water upward from deeper layers.
· Strong upwelling zones are established on the western sides of continents where surface currents move towards the equator.
· Within the tropics, the movement north and south of the solar equator determines rainfall seasonality.
· The intertropical convergence zone follows the solar equator producing a moving belt of rainfall.
· Seasonality is most pronounced at the higher tropical latitudes.
· At middle latitudes, climates are influenced by westerly winds.
· Many aspects of climate are unpredictable, and it is difficult to forecast far into the future. In southern Florida, we can absolutely understand this.
· The Peru Current flows north along the western boundary of the continent then veers offshore near Ecuador and heads out to the Galapagos.
· Each year, a warm countercurrent known as El Niño moves southward along the coast.
· In some years, the El Niño current forces the Peru Current offshore and this has a negative effect on fishes.
· During El Niño events, there is an oscillation of high pressure areas (center moves from Tahiti to Darwin, Australia). This switch is known as the Southern Oscillation and together these phenomena are known as the El Niño Southern Oscillation Events or ENSO events for short.
· The change in the location of the high pressure system causes winds to shift.
· Warm water piles up off the coast of South America, the westward flowing equatorial currents stop or reverse and upwelling weakens or ceases.
· This cartoon shows what happens during El Niños. There is a high pressure over Australia and a low over Tahiti.
· Weak easterly winds allow warm water to move east.
· The subtropical jet stream carries moisture east and there are fewer Atlantic hurricanes.
· El Niños are followed by La Niña events.
· During the La Niñas, strong trade winds accentuate normal currents and the high and low pressure areas switch to the "normal configuration".
· The cloud bank moves north, the jet stream is weakened, and Atlantic hurricanes develop at an increased rate.
· ENSO events are felt globally -- some areas are unusually wet and some are unusually dry.