As MJO passes, Atlantic basin could become more favorable for development in July

Author: || Published: 6/28/2019 at 2:07pm

I don’t often get onto sub-seasonal oscillations on here for a handful of reasons…

1. I don’t know as much as I wished I did about them
2. In general, it doesn’t apply to everyday life for people
3. It can be really complicated

But today, I’m going to set all of that aside. Because I know some smart people, I think you’ll care, and I’m going to do my best to break it down

First, though, we have to lay a foundation: What the heck is an MJO?!

The Madden Julian Oscillation

The Madden-Julian Oscillation, or the MJO, is a wave in the atmosphere that propagates eastward around the globe. It is in the tropics. And it has a 30 to 60 day cycle.

From NOAA:

The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics. There is evidence that the MJO influences the ENSO cycle. It does not cause El Niño or La Niña, but can contribute to the speed of development and intensity of El Niño and La Niña episodes.

The MJO is characterized by eastward propagation of regions of enhanced and suppressed tropical rainfall, primarily over the Indian and Pacific Oceans. The anomalous rainfall is often first evident over the Indian Ocean, and remains apparent as it propagates eastward over the very warm waters of the western and central tropical Pacific. Over the cooler ocean waters of the eastern Pacific, the pattern of tropical rainfall generally becomes nondescript, but often re-appears over the tropical Atlantic and Africa.

In English? For that we turn to Matt Lanza.

“The MJO has a lot of value for long range forecasting,” Matt Lanza, meteorologist at Space City Weather, said. “Particularly when it’s fairly active. There are often a lot of similar patterns that recur when an MJO wave is in a particular position of the globe. So it can enhance predictability at timescales you wouldn’t always trust. But the issue with using it is that it’s not always going to be active and it can be fickle.”

Speaking of “active” phases. This wave can promote the development of thunderstorms and clusters of thunderstorms in the tropics as it passes.

As the wave moves, it promotes thunderstorm development in some areas and suppresses it in others // Courtesy: NOAA.gov

And in other areas, at the same time it can suppress the development of thunderstorms.

And it marches all the way around the world doing this.

MJO propagation around the world // Courtesy: NOAA.gov

The above image may be hard to see, Sohere is a link to a much large image. If you look closely it shows that the maps on the bottom of each image stack, you can follow the pattern of lower surface pressure, higher rainfall amounts and higher vorticity in the 850mb heights (fancy weather term for lower in the atmosphere, around 3,000 to 5,000 feet) across the board during the bottom three images of each stack.

That is when the MJO makes it “most favorable” to see development in the Atlantic basin. And sub-seasonal forecasters call that time the Week 1-3 time frame.

But sometimes, it doesn’t propagate like it should.

“If it’s a little faster or slower, that can shift your forecast timing around,” Lanza said. “It can also stall out in place or just fall apart. That can ruin whatever predictability you planned on. Even more frustrating can be when you have multiple areas of convection around the equator at the same time. Then it becomes, which one is your signal? That kinda deal.”

How does the MJO affect tropical development?

Since the MJO promotes the development of thunderstorms and, as shown in the image stack above, it also increases 850mb (lower level) vorticity. And recall from my Tropical Update videos I post every weekday that vorticity is a fancy weather word for horizontal spin. So, the MJO increases the potential for storms and increase the potential for horizontal spinning in the atmosphere.

During the Tropical Update I use the 700mb moisture / 850mb vorticity map to highlight areas of interest

And what is full of storms and spins horizontally in the atmosphere?

Hurricane Irma // Courtesy: College of DuPage Meteorology

Bingo! As Ross Whitley points out, it can really increase the chances of tropical development:

Increase doesn’t mean guarantee

An increased potential doesn’t promise anything. It is just an indicator to meteorologists to be on the lookout with a bit more diligence. Because this thing cruises by every month or two, every summer, since forever. And there have been hurricanes when it wasn’t nearby. And it has been dead quiet while it passed, too.

In fact, Hurricane Irma, pictured above, formed and moved through during a non-peak MJO time.

And that is normal, too.

“While it has a ton of value sometimes, it can lead you down a path that you may not like at other times,” Lanza said. “That’s why I don’t think you can utilize it exclusively for forecasting and have to factor in the bigger picture. And no two MJO events are always perfectly alike. So, as with any forecasting principle, you just have to be careful with how you utilize it.

So what does it mean this time?

This time is no different than any other time with the MJO, really. But, now that you know what to pay attention to, it can give a bit of an advanced warning on where to watch.

MJO forecast from 6/28/19 from the GEFS // Courtesy: NOAA.gov

The forecast is for the MJO to peak in the Caribbean and Atlantic in about two weeks. Guidance today shows that it will peak for the Caribbean a bit more than the open Atlantic, so tropical waves will need extra attention in the Caribbean and the Gulf during mid-July. That also coincides with a time when the Saharan Dust will subside.

So there are two factors (MJO and lack of dust) leading toward an increased likelihood that tropical waves will have an easier time getting organized. Again, no guarantee. Just something worth watching given that currently there are factors working against development. And that is why it has been so quiet.

Then as the MJO continues to push east, away from the Atlantic, during August. But, as Matt Lanza pointed out, sometimes things change and the MJO doesn’t act as it is forecast.

If something forms, where will it go? Is it going to hit where you live?

No one knows that. No one can know that. But it is a good reminder to check that Hurricane Preparedness Kit. And to have a plan.


Author of the article:


Nick Lilja

Nick is former television meteorologist with stints in Amarillo and Hattiesburg. During his time in Hattiesburg, he was also an adjunct professor at the University of Southern Mississippi. He is a graduate of both Oregon State and Syracuse University that now calls Houston home. Now that he is retired from TV, he maintains this blog in his spare time.