East Pacific Oscillation Is The Climate Signal To Watch For Winter Cold Risk

The Last Fortified La Nina Modoki
10/05/2021, 7:40 am EDT
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Traders/Analysts: Why it’s the East Pacific Oscillation and not necessarily the arctic oscillation/North Atlantic oscillation to look for a “polar vortex” pattern.

Fig. 1-3: The 15-day arctic oscillation forecast indicates a moderate-strength negative phase the next several days. AO of -1.0 to -1.5 climatology yields below normal temperature across Canada into the Northeast U.S. However, the 5-day forecast indicates prohibitive warmth for this sector.  

Discussion: Historically, the arctic oscillation (AO) is a good guide to judge whether high-latitude blocking develops and to compensate the polar vortex splits and shifts southward. Traditionally, the -AO pattern is the lead diagnostic to identify a cold pattern in the middle latitudes during wintertime. However, it’s not that simple (anymore). Current AO forecasts are moderate negative phase for the next few days (Fig. 1) which historically corresponds to colder than normal climate across East Canada/Northeast U.S. (Fig. 2). But note the 5-day forecast which is prohibitively warm for this zone (Fig. 3).

Why is the -AO forecast not producing eastern North America chill? The non-tropical North Atlantic SSTA are largely warmer than normal (Fig. 4). The warm SSTA correlates to widespread anomalous warmth in the low-level atmosphere. Additionally, a warm bias to the real-time climate is caused by the constricted polar ice cap (Fig. 5) which leaves comparatively warm ocean waters along and north of all continents. These conditions are forecast to persist through upcoming mid-winter (Fig. 6).

Fig. 4-5: Global SSTA analysis reveals widespread warm SSTA across the northern hemisphere oceans. The polar vortex is constricted well below normal.

Nowadays, the tendency of the polar vortex to form and amplify is over mid-continents away from the moderating influence of the anomalous warm oceans. As an example, the February (2014, 2015 and 2021) polar vortex was centered on Western Ontario (Fig. 7). The arctic oscillation was very negative FEB-21 but neutral to positive in FEB-14/FEB-15. The 2013-14/2014-15 polar vortex winters gave rise to the East Pacific oscillation (EPO). The EPO identifies the strength of the upper air pattern over North-central/Central North America, a perfect index for the new polar vortex patterns – much more so than the arctic oscillation.

Fig. 6: The forecast global SSTA for January 2022 by the IMME model reveals persistent warm SSTA in the middle latitudes of the northern hemisphere.

 Fig. 7: The polar vortex patterns of February 2014, 2015 and 2021 were dominated by -EPO index and not necessarily -AO.