SAN FRANCISCO STATE UNIVERSITY Metr 430
DEPARTMENT OF GEOSCIENCES Fall 2010
In Class Exercise No. 3
Instructions: Work with other students if you like. However, please do not ask other faculty members to help you with this. I want to see what you come up with on your own.
Thermal Wind (Use Thermal Wind arguments in answering this question. You’ll also need to think about geography)
1. Explain why
(a) In any given season, the polar jet stream is stronger in the S. Hemisphere;
The 1000-500 mb thermal wind is a vector that when added to the near surface (1000 mb) geostrophic wind gives the 500 mb geostrophic wind. Since the 1000 mb wind is usually small or negligible when compared to the 500 mb geostrophic wind, one can assume that the thermal wind estimates the actual 500 mb wind. Since the polar jet stream in the Southern Hemisphere s stronger than its counterpart in the Northern Hemisphere in comparable seasons, the thermal wind vector must be larger in the Southern Hemisphere in comparable seasons.
The Thermal Wind vector at a given latitude is proportional to the meridional temperature gradient. Hence, the meridional temperature gradient must be stronger in the Southern Hemisphere.
The reason for the stronger equator to pole temperature gradient in the Southern Hemisphere must reside with polar temperatures, since equatorial temperatures are relatively constant, and are the end point for meridional temperature gradient calculations in both hemispheres. Polar temperatures are much colder in the Southern Hemisphere because there is a large continent with elevated ice sheets (~13000 feet) at the South Pole. The North Polar region is characterized, really, by a shallow ice sheet laying on an ocean that, by definition, has a temperature greater than 0C.
The Antarctic ice sheets are 10000 to 13000 deep and are grounded on a continent that can cool radiationally very effectively (where the land surface is exposed) and that have high albedo. Thus,temperatures in the polar regions are much colder in the Southern Hemisphere.
(b) There is less of a variation in the strength of the S. Hemisphere jet from
winter to summer in the strength of the N. Hemisphere jet.
The ice sheets in the Southern Hemisphere are grounded on a continent. The greatest meridional temperature gradient occurs on the equatorward periphery of the ice sheets. Since the ice sheets very minimally retract from winter to summer, both the position and strength of the thermal wind vector stay fixed.