Another Negative Atmospheric Angular Momentum Episode Ahead Causing Weather Extremes

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Another Intense –AAM Sequence Ahead

Discussion: Forecast models are in agreement that another northern hemisphere upper level wind slowdown is ahead. A vigorous negative phase of the atmospheric angular momentum (AAM) index is predicted for the last half of June. Implied is increasing amplitude of long wave trough (and ridge) patterns leading to increased risk of weather extremes. The last –AAM sequence occurred May 7-21 and featured strong upper troughs over northeast Canada, the Aleutian Islands and Central Russia (Fig. 1). The GFS ENS northern hemisphere 15-day 500 MB anomaly forecast indicates these same features will return (Fig. 2).

Fig. 1: The last –AAM sequence occurred May 7-21 and produced 3 deep long wave trough patterns (with equally strong ridge in-between).

Fig. 2: The next 2 weeks 500 MB anomaly forecast by the GFS ENS indicates another –AAM pattern with the same long wave troughs as in May returning.

Implications: High impact weather events will occur due to the –AAM evolution. Some of the high impact events expected for all sectors of the northern hemisphere for next week are listed.

  • Hot and dry as western Canada drought intensifies and fire risk increases dramatically.
  • Lurking back door cold fronts into the Northeast U.S. moving south from a cold source region over still snow covered northeast Canada. South of these fronts extreme heat can occur in the U.S.
  • A wet monsoon into the southern Continental Divide.
  • Hot and dry across Southern Europe while Northern Europe to Western Russia are just the opposite, showery and much cooler.
  • Dry northwest flow continuing to dry-out northern China.

Summary: Yet another mode of climate variability causing weather extremes and in an active phase this northern hemisphere spring season is the global atmospheric angular momentum regime. A strong negative phase is developing and forecast to peak in intensity next week. Dramatic weather changes will affect North America, Europe/Western Russia and Eastern Asia. The negative phase of AAM occurs when the mean westerly flow in the middle latitudes slows down. There are many contributions by the ocean and atmosphere causing these episodes although not well understood. A likely contribution to the –AAM pattern ahead is heavy convection across Mexico/Southern U.S. ejecting heat pole ward in contrast with an unusually cold atmosphere over expansive snow cover in northeast Canada and vast cooling of the North Atlantic to the south of Greenland.