Abstract

In the winter of 1978–79 practically the entire contiguous United States averaged below normal in temperature—a rare event, particularly with respect to the last two decades. The following events appear to have played a role in producing these extremes:

1) A strong mid-tropospheric ridge developed late in fall in the eastern North Pacific along with the expected accompanying strong trough over the western United States.

2) In association with the ridge, the North Pacific circulation appears to have undergone the “negative feedback” effect, wherein cold surface water in summer in the northeastern Pacific usually brings about higher pressures than normal at surface and aloft in the subsequent fall, The new fall pattern remained quasi-stable throughout the winter perhaps in part due to coupled air-sea interaction. Premonitory signs of these events were evident as early as November 1978.

3) Normally, the consequence of the strong eastern Pacific ridge would be a strong Bermuda high and upper level ridge affecting the southeast according to statistically derived teleconnections. However, the eastern ridge did not materialize because of strong and extensive blocking, dominating the area from Scandinavia through northern Canada and Alaska. This immense block demolished the Bermuda high as storm tracks were displaced southward. A satisfactory explanation of the blocking is not at hand.

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