Metr 503 Spring 2005

Quiz #1: Snap -- Part II

Execute the script:

sathts

Note the split flow over North America.

The branch of the flow over the southern tier of states in these patterns is often erroneously referred to as "the subtropical jet". It should really be referred to as either "a southern branch of the polar jet" or "a subtropical branch of the polar jet".

Using the script "eta_maps eta_thick -ft=init" and "jetstream -pac -lev=500", "jetstream -pac -lev=400", "jetstream -pac -lev=300", discuss why the jet branch over the southern tier of states is not the subtropical jet (but the polar jet); and (b) that there is a subtropical jet, however, over the Pacific.

The polar jet stream is due to the increasing westerly winds with height (implied by the thermal wind relation) in the zone of deep temperature discontinuity between subtropical and polar air masses. The subtropical jet stream, at first explanation, is due to the development of west wind speeds relative to the earth in the extreme upper troposphere in the air moving out of the Hadley Circulation in the upper troposphere. The polar jet stream will always be evidenced in the thickness field (since without a thickness field there is no thermal wind) as is the case here, in eta_thick. Thus, if there is a deep thickness gradient, the associated jet stream will be the polar jet or a branch of the polar jet. Also, the subtropical jet should not be evident at all in the middle troposphere, as is the case here when looking at the 500 mb and 300 mb jetstream plots.