Physical ScienceLecture 154
Instructor: John H. Hamilton
Last lecture review
• Driving forces of air motion• Temperature induced pressure gradients• Wind generation• Convective currents• Coriolis affect
Recommended exercises solutions
• Teach somebody how the coriolis affect works• Teach someone, using the 2 air columns and
assumptions, how wind is produced
Lecture review
• Global circulation patterns• Major cells– Hadley – Ferrel– Polar
• Upper atmospheric circulation• Ocean circulation– Surface currents– Deep water currents
Diagram first!
Hadley cells
• Hadley cells are direct. They are convective currents created by solar heating
• The Hadley cell starts on the equator where warm moist air rises straight up and then moves towards the poles.
• The air sinks at about 30 degrees latitude (horse latitudes) and is warmed by compression as it sinks (here are the worlds great deserts)
Polar cells
• Polar cells, like Hadley cells are directly driven by solar heating
• Solar heating at about 60 degrees latitude warms air that rises, moves towards the poles and sinks
Ferrel cells
• Ferrel cells are not directly driven by heating, rather they are indirectly driven by motion of the other cells.
Major winds
• Trade winds• Westerlies• Polar easterlies (easterlies)
Major winds diagram
Upper atmosphere circulation
• In the upper troposphere, at about 9-14 km rivers of air called jet streams move along at 95-190 km/hr
• Each hemisphere has 2 major jet streams, a polar jet stream and a subtropical jet stream
Jet streams, driving force
• At about 60 degrees latitude is the polar front, where cold air from the polar regions encounters warm moist air from the tropics, this temperature gradient causes a pressure gradient that drives the polar jet stream
• At about 30 degrees latitude warm air moving towards the poles encounters the tropical front which also creates a temperature gradient.
Ocean surface currents
• Surface ocean currents are driven and directed by a combination of the prevailing winds and coriolis forces. Called the Ekman transport
Mounding and depressions
• With the prevailing winds being the direction that they are, the Ekman transport is such a that water mounds up at 30 degrees latitude and is depressed at the equator.
Gyres
• Water wants to run “downhill” combining the rises and depressions with the Coriolis affects we get a circular flow of water around the elevated water called a “gyre” all major oceans have a gyre
diagram
Major ocean currents
• Most of the major surface currents are parts of the water flowing arround a gyre
• Example gulf stream
Deep water currents
• Deep water currents are slower than surface currents and are driven by water in th polar regions increasing in salinity because of water freezing out.
• Water freezes at the poles. Since water is leaving and becoming ice, the remaining water has a greater salinity and therefore a greater density and sinks
diagram
Lecture Review
• Global circulation patterns• Major cells– Hadley – Ferrel– Polar
• Upper atmospheric circulation• Ocean circulation– Surface currents– Deep water currents
Recommended exercises
• Teach someone about the 3 major circulatory cells and how they are formed
• Starting with wind and the coriolis affect explain to someone how gyrs are formed and what affect those gyrs have on local climate