On the air last night, I mentioned a Kelvin Wave.
These things are pretty cool. And pretty important when it comes to forecasting during hurricane season. The MJO and Kelvin waves can help dictate how – generally speaking – likely tropical development may be on a non-specific basis. However, I don’t usually dive into these things since…
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.
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.
The video spells it out pretty well. But, for you bookworms, here is a textbook definition 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 a seasonal forecaster. In this case, 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.
And in other areas, at the same time it can suppress the development of thunderstorms. Here is another video that does a good job to explain it a bit more thoroughly.
The MJO marches all the way around the world doing this, too. Increasing thunderstorm activity in some places, while suppressing it in others.
The above image may be hard to see, So here 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.
And what is full of storms and spins horizontally in the atmosphere?
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.”
