Tuesday, February 14, 2012

The Jet Stream

Pull up an elementary textbook description of the jet stream and you are bound to see something like the image below, which includes a northern "polar jet stream" and a southern "subtropical jet stream."

Source: NC State, NASA
Such images give the false impression that the jet stream is a narrow, coherent corridor of flowing air that completely encircles the pole and that two jet streams are always present.  In reality, the upper-level flow is more complicated.

Let's have a look at the north Pacific basin today.  On the image below, I've included contours of 300-mb wind speed at 10 m/s intervals, beginning at 30 m/s (warmer colors indicate larger wind speeds) and have annotated an analysis of the jet streams in blue.

Note how there are two regions of strong winds over China.  Most meteorologists would call the southern most wind maximum the subtropical jet, and the northern most the polar jet.  These two jets do not remain distinct, however.  Instead, they extend eastward to the dateline where they merge into a single, unified jet stream between the Aleutian and Hawaiian Islands.

Further to the east, we see additional complexities in what meteorologists call the jet exit region.  Here, as has been the case for much of the winter, the jet stream weakens and the flow is strongly diffluent, meaning it fans out.  I've shown the jet stream turning sharply southward, but one can also find a wind speed maximum extending up into the northern Gulf of Alaska.  If you like, call this flow splitting.  

The branch that turns southward weakens considerably off the coast of California where it encounters yet another jet stream that originates west of Baja and extends northeastward over northern Mexico and the southwest United States.  This is another subtropical jet stream.

I think it is time to abandon simplified depictions of jet stream ribbons encircling the globe.  As nearly all meteorologists are aware, even the climatology of the upper-level (250 mb) flow does not suggest a continuous jet stream encircling the pole.  Instead, there are strong jets over the Pacific and Atlantic that tilt poleward toward the east and are not structurally continuous.
Yes indeed, all generalizations are wrong.


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