Fig. 7.13 Subtropical and Polar jet streams in relation to the three cells
WESTERLIES IN THE UPPER TROPOSPHERE THERE IS HIGH PRESSURE OVER THE EQUATOR, AND A LOW PRESSURE OVER THE POLES. THIS PRODUCES A NET FLOW FROM THE EQUATOR TO THE POLES. THIS FLOW PLUS THE CORIOLIS FORCE PRODUCES WESTERLIES. WINDS ARE GEOSTROPHIC PRESSURE GRADIENT INCREASES WITH ALTITUDE. THUS SO DOES THE WIND SPEED JET STREAMS ARE PART OF THE WESTERLIES
Jet streams on March 11, 1990 The next slide is an image of the total column of ozone measured from a satellite. Ozone can be used to trace the changes in dynamics of the atmosphere. In this case it can be used to locate the jet streams. Note the undulations within the Jet streams (Rossby waves). Also note the cut off low.
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 - 15 DEGREES PER DAY. HIGHER JET STREAM SPEEDS IN THE WINTER. JETS SHIFTS SOUTH IN THE WINTER, NORTH IN THE SUMMER.
WESTERLIES AND THE HEAT BUDGET 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.
Fig. 7-19, p. 202 Poleward transport of heat by the oceans and atmosphere
Poleward transport of Energy We noted before that there is an inbalance between the energy from the sun received by the tropics and that received at the Poles. There must be a net movement of energy from the equator to the Poles. This transfer of energy is achieved by the atmosphere and the oceans. The atmosphere moves energy at the mid- latitudes, while the oceans move energy predominately in the tropics.
Fig. 7-20, p. 203 Approximate position of the ITCZ in January and July
Monsoon A monsoon is a weather feature driven by the change in position of the ITCZ In the winter the ITCZ is South of the Equator and the dominant feature over the Himalayas is a High pressure system. This brings winds from the North across India – cool dry air. But in the summer that ITCZ is now North of India, and the dominant weather feature is a Low pressure system. This brings large amounts of rainfall.
Box 7-2, p. 204 Precipitation patterns and topography
Precipitation patterns and Topography At the beginning when the air is lifted up the mountain the air cools at the dry adiabatic lapse rate. The dew point temperature also falls. At I km altitude the dew point temperature equals the air temperature – saturation. As the air goes further up the mountain it now cools at the wet adiabatic lapse, and the dew point temperature must equal the air temperature Why? Because the air is saturated. At the top of the mountain both the air and dew point temperature are at -2 C. The absolute water vapor pressure of the air is set by the dew point temperature at the top of the mountain
Precipitation patterns and Topography As the air descends its temperature will increase (adiabatic compression) and automatically the air is no longer saturated. Hence the temperature of the air will increase at the dry adiabatic lapse rate. At the same time the dew point temperature will increase at about 2 degrees C per kilometer. The net effect is to increase the temperature of the air and decrease the dew point temperature from one side of the mountain to the other: Temperature 20 C to 28 C Dew-point temperature 12 to 4 C. Example is the island of Hawaii.