Outline Further Reading: Chapter 07 of the text book - Geostrophic Winds - Cyclones and Anti-Cyclones - Jet Streams Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (1 of 9) - Upper Atmospheric Circulation

Geostrophic Winds-1 Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (2 of 9) Previously, we talked about pressure gradient forces and the Coriolis Force We want to continue to look at how these forces affect winds Before we looked at how the Coriolis force affects winds at the surface. What happens to winds aloft where there is no frictional force? Produces Geostrophic winds L H PGF CF 1016mb 1004mb 1008mb 1012mb

Geostrophic Winds-2 Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (3 of 9) –These winds occur when the only important forces are the Pressure gradient force and the coriolis force –Originally the winds blow down the PGF –As they do so, they now have a velocity so they are affected by the Coriolis force –This makes the winds veer to the right (in the northern hemisphere) –The winds continue to veer to the right until the coriolis force is in the opposite direction as the pressure gradient force –Now there are no net forces, i.e. the winds continue blowing in the direction they’re going –The net effect is that for geostrophic winds, they blow parallel to isobars, not perpendicular to them –The other important result is that for geostrophic winds, the Coriolis force is always equal to and in the opposite direction from the pressure gradient force

Cyclones-1 Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (4 of 9) Almost all winds except for those at the surface are geostrophic Knowing this, how can we determine the direction the winds are blowing just by looking at a map of isobars? First, we when we look at maps of isobars, we typically see “cyclones” and “anti- cyclones” L PGFCF

Cyclones-2 Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (5 of 9) –For a cyclone, we know the pressure gradient points into the center perpendicular to the isobars –Because the winds are geostrophic, we know by definition that the coriolis force has to be equal and in the opposite direction of the PGF –If we are in the NH, we know the CF is to the right of the winds, i.e. the winds are to the left of the CF, i.e. the winds blow to the north –On the other side of the cyclone we can do the same thing and we find the winds blow to the south –In general, we find that in the northern hemisphere winds blow counterclockwise around cyclones

Anti-Cyclones Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (6 of 9) H PGFCF For an anti-cyclone, we know the pressure gradient points out of the center perpendicular to the isobars Because the winds are geostrophic, we know by definition that the coriolis force has to be equal and in the opposite direction of the PGF If we are in the NH, we know the CF is to the right of the winds, i.e. the winds are to the left of the CF, i.e. the winds blow to the south On the other side of the cyclone we can do the same thing and we find the winds blow to the north In general, we find that in the northern hemisphere winds blow clockwise around cyclones

Upper Atmospheric Circulation Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (7 of 9) - Pressure decreases much less rapidly in a warm atmosphere relative to a cold atmosphere. - Therefore, at a fixed height, there is higher pressure in the tropics compared to the poles - Thus, a pressure gradient exists in the upper atmosphere from the tropics to the poles Upper Air Westerlies: Blow in a complete circuit around the Earth, from 25N to the poles. At high latitudes, the westerlies form a circum-polar spiral circling the polar low. Tropical High Pressure Belt: High altitude part of the surface sub-tropical high belt, slightly shifted to the equator. Equatorial Easterlies: A zone of low pressure, with light easterly winds called equatorial easterlies.

Rossby Waves Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (8 of 9) The smooth westward flow of the upper air westerlies can form undulations (disturbances), forming Rossby waves, especially when the cold polar air comes into contact with warm tropical air. In these undulations, warm air pushes poleward and tongues of cold air can be brought equatorward. Eventually, the tongue of cold air can be pinched off, leaving it a far southerly location. This Rossby wave circulation is an important mechanism by which heat is transported to the polar regions! This is also the reason why the weather at mid-latitudes is so variable, as pools of warm moist tropical air clash with cold and dry polar air masses.

Jet Streams Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 18: Winds Aloft Mar (9 of 9) Narrow belt of high velocity winds (~ mph) Located at high altitudes. Polar Front Jet Stream: The polar jet is found in both hemispheres, somewhere between 30-60N, with intense westerly winds. Subtropical Jet Stream: Located just above the subtropical high pressure cells, in both hemispheres, this westerly jet is a result of conservation of angular momentum (the picture). Tropical Easterly Jet Stream: It is a summer-time jet, seen in Northern hemisphere only, over southeast Asia, India and Africa.