Climate & Earth System Science Introduction to Meteorology & Climate MAPH 10050 Peter Lynch Peter Lynch Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Chapter 07 Global Scale Winds Lecture 14 Simple Introductory Examples: Voyage of Christopher Columbus Transport of heat to equalize temperature Explain on blackboard Bar of steel: one end in the fridge, one in the oven. Tank of water: heated at one end, cooled at the other. 1
Beagle Voyage Composite image (clouds & surface temperature). Note the line of clouds along the ITCZ Fig. 7.10 Fitzroy & Darwin Mean Monthly Cloud Cover (%) in January Mean Monthly Cloud Cover (%) in July 2
GLOBAL CIRCULATION George Hadley first suggested in 1735 the general concept of atmospheric circulation a single cell, to explain the existence of the easterly winds at the surface Cold air at pole - high pressure at surface. Warm air at equator - low pressure at surface. Pressure gradient force at surface will move air from pole to equator at surface. Return path at high altitudes. Coriolis force produces easterlies Hadley Cell GLOBAL CIRCULATION In reality, we have three cells, with boundaries at about 30º and 60º latitude. This results in sinking air at 30ºN and 30ºS. But sinking suppresses cloud development and precipitation. Hence most of the worlds deserts occur along these latitudes. These are the horse latitudes. Three cell model Circulation at the surface moves from 30º to the equator easterly winds. Lecture 15 GLOBAL CIRCULATION Between 30º and 60º the circulation at the surface is from 30º to 60º, giving rise to westerly winds. Between 60º and the Poles, the flow is from the Pole to 60º, leading to easterly winds at the surface. Upwelling at the equator and at 60º precipitation. The Doldrums. 3
Conservation of Angular Momentum Conservation of Angular Momentum: As the parcel of air moves from the equator towards the poles, its velocity increases to conserve angular momentum. Demonstration: Nut-on-a-string At the same time the Coriolis force acts to produce a strong westerly wind the subtropical jet stream. JET STREAMS As the air moves from the equator to 30º at high altitudes, its velocity increases to conserve angular momentum. SUBTROPICAL AND POLAR JET STREAMS IN RELATION TO THE THREE CELLS At the same time the Coriolis force acts to produce a strong westerly wind the subtropical jet stream A similar jet forms at 60º the polar jet, or polar front jet. 4
Cloud band from Pacific to Florida follows Sub-tropical jet Average Global Sea-level Pressure Extra-tropical Westerlies Icelandic Low; Bermuda High Trade winds; ITCZ Doldrums; Horse latitudes Average Global Winds Extra-tropical Westerlies Icelandic Low; Bermuda High Trade winds; ITCZ Doldrums; Horse latitudes 5
Lecture 16 WESTERLIES In the upper troposphere: High pressure over equator, low pressure over poles. Net flow from the equator to the poles. This flow plus Coriolis effect produces westerlies. Winds are (nearly) geostrophic Pressure gradient increases with altitude. So does the wind speed Jet streams are part of the westerlies WAVES IN THE WESTERLIES Dish pan experiment. C. G. Rossby. Waves along the jet streams are known as Rossby waves. Three to six of them around the globe. The air flow along the edge of the waves can be rapid, however the waves move slowly about 15º per day. Higher jet stream speeds in the winter. Jets shifts south in the winter, north in the summer. 6
Index Cycle (A) Zonal flow pattern air flows parallel to latitudes. (B) Meridional flow pattern. (C) Combination of the two flows. Long Waves & Short Waves 7
Rossby waves in a dish-pan WESTERLIES AND THE HEAT BUDGET POLEWARD TRANSPORT OF HEAT BY THE OCEANS AND ATMOSPHERE Major function of atmospheric dynamics is to move heat from the equator to the poles. But how can winds move heat when the predominate wind direction is zonal (E to W, or W to E)? The meanderings of the jet streams continually mix cold and warm air, thus transporting heat. Mean position of the ITCZ in January and July THE SW MONSOON 8
Summer Monsoon: Wet Model Simulation of General Circulation [ /sony/movies/nugam ] Winter Monsoon: Dry Conclusion 9