Understanding The MJO

November 30, 2021

In the winter the effects of the MJO are more pronounced in the United States. I have not talked about it for a while, so here is a refresher for some and a (relatively) brief explanation for new followers.

I am going to do my best to keep this as easy to understand as possible, and as unboring as possible.

The MJO is short for Madden Julian Oscillation. Yes, it was discovered by a guy named Madden and a guy named Julian in the early 70s. The MJO is a mostly eastward-moving tropical disturbance that circles the planet through the tropical latitudes about every 60 days or so.

Forecasters refer to the location that the tropical disturbance (the MJO) is in as a phase. There are 8 phases of the MJO.  Different phases at different times of the year can contribute to the weather across the planet. In the United States, the biggest effect on the weather occurs in (roughly) October through April. The winter months are impacted the most.

Here is the latest GFS forecast for the MJO.

It sometimes helps when you see this for the first time to consider this the earth. I know it is a square but it helped me to understand it better years ago when I first saw this.

The numbers 1 through 8 refer to the particular phases (locations) of the MJO. The red squiggly lines which, in this case, travel from phase 1 to phase 5 are the past 40 days of where the MJO has been.

The green lines are where the GFS ensemble model is forecasting the MJO to go. The yellow lines are where the individual GFS ensemble family members are forecasting the MJO to go. The green lines are the average of all of the yellow lines. The fat green line is week 1, the skinny green line is week 2.

On the left side of the diagram you see the numbers o through 4 on top, and through -4 on the bottom. These represent the magnitude of the particular phase. The circle in the middle lines up with number 1  (-1). This is considered the “null” phase. This means the effects of the MJO will be minimal.

Once you get out of that circle you approach the higher magnitudes of a particular phase reaching 2 (-2) or beyond.  You can expect some extreme weather events when you reach the higher magnitudes of 3 (-3) and 4 (-4)

As I said different phases have different implications at different times of the year. I don’t have all of the implications of all the phases at all times of year memorized, I have a chart I refer to.  Here is the best thing to remember.  Phase 1 and 2 in the winter months usually indicate below-average temperatures and above-average precipitation for our area. Phases 7 and 8 usually imply slightly below-average temperatures and average precipitation. Phase 6 is warm in early winter for most of the United States.

So what is the forecast showing us? Straight into the heart of phase 6 until December 7th.

Here is what phase 6 looks like.

Here are forecasted temperature anomalies for the next 7 days.


Hopefully, we will make a short trip through phases 6 and 7 and end up in 8, 1, 2 from late December through February!

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