Strong negative arctic oscillation ahead for late December
Normally, a cold weather producer.
But warm oceans and upper trough positions doubt the cold.
Fig. 1: Today’s 14-day arctic oscillation forecast trends stronger negative!
Discussion: A developing strong negative arctic oscillation during the onset of calendar winter (Fig. 1) usually creates expectations of harsh cold in the middle latitudes of the northern hemisphere. However, due to the constricted polar ice cap and vast areas of warmer than normal ocean surface in the northern hemisphere (Fig. 2) and projected –AO induced trough patterns occurring over the warmer oceans (Fig. 3-4) the outlooks are not as frightfully cold as they would be 20-30 years ago (or more). The –AO is now forecast to reach an index of -3 which is definitely intense. Let’s take a look at strong –AO regimes in December from 20-30 years ago and compare. Strong –AO in December occurred in 1995 and 1976 and each analog produced harsh cold in the East U.S. (Fig. 5-6).
Fig. 2: Warmer than normal northern oceans contribute to the ineffectiveness of –AO to produce widespread continental cold air masses.
Fig. 3-4: The negative arctic oscillation causes blocking high pressure in the polar region forcing the polar vortex to split into long wave troughs. The GFS ENS day 6-10/day 11-15 upper air forecasts indicate the upper troughs are across the North Pacific and North Atlantic causing wind flows across warmer than normal oceans.
Fig. 5-6: Previous months of December from more than 20 years ago when strong negative arctic oscillation was present. Both 1976 and 1995 were very cold across the northeast sector of the U.S. The middle latitude oceans were much cooler.