Why the MJO and stratosphere are so critical for wintry weather in February

As we move into February we are likely to see another change in the hemispheric weather pattern. Once again, changes with MJO/tropical forcing and in the stratosphere will play a major role in this shift. In our long-range outlook update on Saturday, we discussed these features and how they may potentially influence the atmospheric pattern. Here we will discuss what influence these climate phenomenon will have on pattern during the February and into first half March.

The MJO will likely influence the atmosphere over next few weeks. Latest model and ensembles forecasts show an MJO wave propagating phases 5/6 in a strong fashion over the next week. This will result in tropical forcing over Indonesia and Western Pacific enhancing a stronger pacific jet, with deeper troughines and more phasing occuring over Western US.  Some cooler weather conditions are likely over the West Coast, with pacific systems putting even more of a dent in the drought in California. Over the Eastern US, more zonal mid level heights or ridging with warmer temperatures will occur. This was lead to storm track over the the Appalachians or into the Great Lakes/Ohio Valley.

As the MJO continues to propagate into phase 7 and 8, the Pacific will undergo a retrogression. A large ridge over the Bering Sea will retrograde further west into Asia — which in turn will support troughiness over the Aleutian Islands. In turn, more ridging will gradually build over the over the West Coast of the USA as we approach the last two weeks of February. 500mb composite map height anomalies in this phase — with neutral ENSO conditions —  show high-latitude blocking with -AO/NAO. We are anticipating the MJO/tropical forcing to enhance wave activity flux and cause another major disruption to the stratospheric polar vortex after February 15.

Latest model and ensemble guidance are starting to pick this up during this time period. The question that we have had several times before is: Despite these changes in the stratosphere, what impacts will we see in the troposphere? Will we see more prolific high latitude blocking with both the AO and NAO in a negative phase? Or will we see more muted or transient blocking as we get closer?

A sudden but relatively minor stratospheric warming event occurred over the past 5 days. The disruption came with enough reversal of zonal winds to lead to a displacement of the stratospheric polar vortex towards Eurasia. The stratospheric polar vortex will gradually recover over the next 5 days.

However, a wave-breaking event with stronger ridging developing over Scandinavia and the Kara Sea will cause another disruption in multiple levels of the atmosphere during the next week. When we see a positive, anomalous 500mb height anomaly in the Kara Sea, we tend to see a splitting polar vortex in multiple levels of the troposphere and stratosphere. Here we see the GFS and ECMWF models showing that again in 7 to 10 days.

There is some concern that the MJO/tropical forcing influence may be modulated somewhat by the strong westerly phase of the QBO (quasi-biennial oscillation). These westerly winds zonal winds at 30mb and 50mb may keep wave activity and EP flux reduced from the Atlantic side. When looking GFS beyond 10 period, zonal winds become westerly again and the polar vortex begins to quickly recover and move back over North Pole.

So we will have to continue monitoring carefully over next week to see if this next wave of tropospheric forcing impacts the polar vortex and the atmospheric pattern overall. While the pattern is changing, there are still significant questions as to the longevity and significance of the pattern change — including the potential for wintry weather events.

Stay tuned!