10 October 200 Lec14_Geostrophy_Appendix 1 1MAR 110 LECTURE #14 Geostrophy Appendix Figure 14.A1 The Polar Front The boundary between the subtropical and arctic climatic zones is called the POLAR FRONT. Since the air density depends very strongly on its temperature, the colder air column to the north of the polar front (left) is more dense than a warmer air column to the right and south of the polar front. This density difference across the front produces pressure differences that support a “jet stream” that flows into the page at altitude along the front.10 October 200 Lec14_Geostrophy_Appendix 2 2 LowLowPressurePressureHighHighPressurePressurePressure Pressure --> PGF > PGF Gradient Gradient ForceForceCF <CF <--CoriolisCoriolisForceForcePGF = CFPGF = CFGeostrophic WindGeostrophic Wind Figure 14.A3 Geostrophic Wind Force With a pressure gradient (blue arrow) pushing air from a high pressure zone to a low pressure zone, the Coriolis force (green arrow) causes the wind to deflect to the right until it is perpendicular to the pressure gradient at which point the Coriolis forces and the pressure gradient balance, forming what is called geostrophic wind (red arrow). Figure 14.A2 The Pressure Gradient Force across the Polar Front The actual density depends very strongly on its temperature; with warmer air being less dense. (above)The warmer air to the south (or right) of the “polar front” is less dense than the colder air to the north (or left). Thus AT THE SAME ALTITUDE, the air density and thus air pressure is less to the south than to the north of the polar front. (below) For example, the altitude of a 700mb pressure in warmer air is higher than the 700mb pressure in the colder air. Note that the 700mb isobar is tilted – indicating a horizontal pressure gradient force (PGF) that is tending to force the air northward. But the air never gets there because of Earth rotation. (??)10 October 200 Lec14_Geostrophy_Appendix 3 3 GeostrophicGeostrophicWinds Winds PGFPGFCFCFXXxxXXlookinglookingnorthwardnorthwardPolar FrontalPolar FrontalWind ShearWind ShearProfileProfileinto the screeninto the screenlookinglookingeastwardeastwardFigure 14.A5 Geostrophic Winds The polar front-induced pressure gradients force (PGF) tends to move air northward. But moving air is deflected (eastward in this case) by the Coriolis Force (CF) until PGF = CF where the air flows along the polar front as the geostrophic wind. (??) Figure 14.A4 Geostrophic Winds: Stronger PGF –> Stronger Winds The stronger the pressure gradient, the stronger the geostrophic winds. (NH)10 October 200 Lec14_Geostrophy_Appendix 4 4 Figure 14.A6 The Global Jet Stream The jet stream is a narrow band of strong winds that snakes its way around the globe along the polar front between the polar and subtropical zones. The location of the jet stream loops strongly influence the tracks of high and low pressure storm systems at lower levels. (UWaC) Figure 14.A7 The Jet Stream (top) The jet stream is a narrow band of strong winds that snakes its way around the globe along the polar front between the polar and subtropical zones. (bottom) The location of the jet stream strongly influences the tracks of high and low pressure storm systems (see above) that are confined to the tropospheric layer below it. (UWaC)10 October 200 Lec14_Geostrophy_Appendix 5 5 Figure 14.A8 Geostrophic Winds