During the past 30 days four strong middle latitude trough(s) in the northern hemisphere were evident: The Great Lakes region, northwest Europe, northeast Asia and northeast of Hawaii. In the polar region a blocking high pressure system was present. In the southern hemisphere, despite summertime there was unusually strong upper trough(s) in the middle latitudes.
The pattern described is characteristic of strong negative phase atmospheric angular momentum (-AAM) occurring when the mean westerly flow in the middle latitudes is below normal strength and susceptibility to storm-generating trough(s) increases which also cause thermal extremes. The strong –AAM pattern is less intense after middle January but likely to continue in the negative phase into February and possibly March. –AAM is typical of a La Nina climate and likely to continue until La Nina weakens.
The Madden Julian oscillation is a key driver of the AAM regime. The MJO is moving east across the eastern Indian Ocean and into the far West Pacific the next 2 weeks supporting increased mild influence on the U.S. pattern although mid-January snow cover will resist some of that warming. The MJO is near stationary through late January likely shifting across the Pacific but weakening into early February. The MJO eastern shift weakens the tendency for strong middle latitude trough(s).
The weaker MJO and less intense –AAM regime ahead favors a more typical La Nina climate for late winter to early spring. The current La Nina pattern is in the mature stage and should weaken notably by March and April.
Another contributor to the U.S. climate pattern is the tendency for upper ridging over the North Atlantic due to the very warm phase of both the Atlantic multi-decadal oscillation AND tropical North Atlantic index. The ridge position over the North Atlantic has helped force the upstream trough in North America and is likely to continue into early spring. Therefore a colder revision is indicated in the outlook for February and March.
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