Tuesday, January 3, 2012

December Circulation Anomalies

Painful as it may be, we take a look back today at the large-scale circulation anomalies that influenced our weather (or lack thereof) last month.

December was marked, not surprisingly by anomalously high 500-mb heights (synonymous with high pressure aloft) over the northern Rockies and northeastern Pacific Ocean.  Conversely, anomalously low 500-mb heights were observed over the southwest United States and northwest Mexico.

Further to the east, a pronounced trough-ridge dipole anomaly persisted over the North Atlantic.  This dipole, with anomalously low 500-mb heights over the high latitudes and high 500-mb heights over the mid latitudes, reflects a strong positive phase of the North Atlantic Oscillation (i.e., NAO), one of the primary large-scale modes of atmospheric variability in the Northern Hemisphere.

These circulation anomalies wreaked havoc on the Northern Hemisphere in December, resulting in the split flow that plagued us and an anomalously strong North Atlantic jet.  Both can be seen in the 250-mb mean and anomalous wind analysis below.

The strong split flow led to a persistent dry period over Utah and much of the adjoining western United States.  Salt Lake City International Airport observed only 0.03" of precipitation, making it the driest December on record.

Why the strong split flow and drought?  That is the $64,000 question, and a good answer requires more than just "La Nina," as we noted over the summer.

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