The role of the MJO and stratosphere in the long range pattern

The pattern is changing — again. We discussed in an article on Monday how the Pacific Ocean will play a large role in the long-range pattern going forward. Currently, a jet extension in the Pacific Ocean is facilitating a progressive, mild pattern across the United States with storms tracking over the Central part of the country towards the Great Lakes. But tropical forcing and an MJO pulse will cause a deepening trough over Eastern Asia, with a developing standing wave (ridge) over the Eastern Pacific Ocean.

A huge change from the current state of the pattern, these developing waves in the Pacific Ocean are being caused by the MJO and tropical forcing developments, with additional impacts from the stratosphere. Lets dive in to the details of what is really causing the changing pattern — and why it’s so important.

The Madden Julian Oscillation

The MJO is monitored as “major fluctuations in tropical weather on weekly to monthly timescales” by NOAA. We monitor convection and rainfall in the tropical pacific including its intensity and location. Westerly winds near the dateline support strengthening forcing in the tropics and a pulse in the MJO. The location of the forcing supports a trough developing near the Aleutian Islands, with a ridge over the Northeast Pacific Ocean and the Western USA.

Most models and ensembles have trouble observing and forecasting the MJO. But the ECMWF and its ensembles tends do better than the rest. At this time, the ECMWF and its ensemble members suggest  the MJO will propagate strongly into phase 2 before weakening significantly . During phase 2, a ridge over the Western US typically forms with a trough developing in the Central and Eastern parts of the country.  The latest ECMWF weeklies actually suggest the MJO will reemerge and propagate into phases 7/8 later in February. While the GFS and GEFS have different ideas, with a more -PNA pattern with strong negative height anomaly.

slack-imgs.com

GFS showing 850mb westerly zonal wind anomalies near dateline next week. This supports more MJO/tropical forcing near dateline, enhancing troughiness (standing wave) near Alleutians and ridge over West Coast.

Currently, cool sea-surface temperature anomalies continue to diminish with a very weak La Nina this winter over the Pacific. The AAM (Atmospheric Angular Momentum) forecasts and 30-day SOI data support the more neutral ENSO conditions in the Pacific Ocean going into February. Based on the 500mb mean height anomaly composites, during a neutral ENSO, Phase 8 would support more high-latitude blocking and a trough in the Eastern US with below normal temperatures later in February. We will have to continue to monitor forecasts to see how the MJO phases are handled moving into the next few weeks.

Overall, support for a ridge in the Western USA is growing in the end of January towards the beginning of February — while its longevity remains in question.

The Stratosphere

A number of wave-breaking events induced by tropical forcing and MJO progression are likely to cause  significant disruptions to the stratospheric polar vortex late this month and into February. These disruptions will likely result in significant displacement of the polar vortex off the north pole, at multiple levels of the atmosphere. The first of these disruptions occurs over the next few days with tropical forcing over the Atlantic. Another wave-breaking event from the MJO in 7 to 10 days will result in a large stratospheric high over Alaska/Western Canada.

The ECMWF and GFS shows zonal winds decreasing in the longer range — but not quite reversing into a westerly direction yet. If you’re wondering, a westerly reversal in stratospheric winds is relatively rare, and constitutes what is called a “Sudden stratospheric warming event”. This is not modeled to occur. Instead, the stratospheric vortex looks likely to be simply disrupted. What does this mean for us?

Here’s how it works: In order for the stratosphere to help enhance blocking in the high latitudes (which helps bring colder air down towards our area), warmer air needs to downwell from the stratosphere to the troposphere (the troposphere is where most of our weather happens). In order for this to happen, EP vectors need to be pointed poleward during warming in the stratosphere, to help move that warmer than normal air towards those regions. All of these things work together to help downwell warmer than normal air and create an environment more favorable for high latitude blocking.

In this case, the EP vectors are still pointed mostly equatorward. This will help keep the stratospheric vortex intact, instead of facilitating a true split. It will also mean that warmer air will have trouble downwelling towards the troposphere. So although the stratospheric vortex is disturbed, it may not facilitate a pattern that is colder or snowier than normal for the Northeast US just yet.

ECMWF showing zonal winds decreasing. But EPV still remain mostly equatorward

ECMWF model showing zonal winds decreasing. But EPV still remain mostly equatorward, supporting no complete breakdown of polar vortex

Forecast model guidance has been suggesting the development of high latitude blocking (known as a -NAO) for almost two years now. But each time, the blocking trends more impressive as we approach it. With the stratospheric vortex still relatively profound and the QBO in a strong westerly phase, we are skeptical of any massive or classic blocking over Greenland/Davis Straits.

Still, with the changing Pacific pattern, it appears likely that colder air will be drawn southward from Canada. While not a classic cold and snowy pattern by any means, support will exist — thanks to tropical forcing and the Western USA ridge — for cold air and opportunities for wintry weather in our area from late January through the first week or two of February.

We updated the long-range outlook last week, with an extended overview and highlight of most of February — mostly for the reasons discussed above. A new long range update with a February outlook with more confidence will be out next week.