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