Fig. 1-2: The mega-cluster ensemble identifies evolution of a Greenland high-pressure “block” (negative North Atlantic oscillation) during the medium range.
Discussion: Evolution of a titanic negative North Atlantic oscillation (-NAO) pattern is indicated during the medium-range. The effect of -NAO is to produce anomalous upper-level high-pressure blocking over Greenland. The high-pressure block is certainly evident in the “most likely” upper air forecast for the northern hemisphere by the mega-cluster ensemble in the 6-10-day forecast (Fig. 1). In the 11-15-day forecast, the blocking high pressure is arguably record-strength according to the model and forecast with a robust 68% confidence which is exceptional beyond 10 days (Fig. 2).
In the old days an emerging -NAO of this amplitude would guarantee East U.S. cold and snow during wintertime. However, in the modern-day climate the evolution of that East Coast threat takes time.
Why? The prevailing climate is tied into the semi-permanent upper-level trough pattern located over the North Atlantic warm hole (NAWH) south of Greenland and the upper ridge over the northeast quadrant of the Pacific Ocean related to the ongoing marine heat wave in that region (Fig. 3-5). These features resist the full effect of an evolving -NAO pattern, at least initially.
Fig. 3-5: In the modern-day climate, a massive area of persistent anomalous warm water in the Northeast Pacific and redeveloping cool pool south of Greenland are well-correlated to a stubborn upper air pattern that can alter or delay effects from leading modes of climate variability such as -NAO patterns.
Note that the feared upper trough developing south of the Greenland ridge is offshore in the 11-15-day period rather than on the East Coast as a new NAWH trough tries to form. The Greenland Block begins to take the feared East Coast trough effect in 15 days (Fig. 6). Note that forecast confidence is 35% therefore open to potential change.
Is the Madden Julian oscillation (MJO) involved with NAO pattern? The short-term forecast indicates the core of the MJO pattern is shifting from phase_8 (West Pacific) now to phase_1 (North Atlantic) in a few days and then weakening while continuing to shift east back to the Indian Ocean and Maritime Continent (phase_3/phase_4) during the medium range (Fig. 7). The MJO contributes to the onset of the -NAO pattern via latent heat release poleward while shifting into the tropical North Atlantic the next 3-5 days. However, the MJO does not take a storm inducing role during the peak -NAO regime after 10 days.
Fig. 6-7: The mega-cluster ensemble “most likely” upper air forecast for the northern hemisphere in 15 days and the 14-day Madden Julian oscillation forecast from NOAA/CPC.