MJO, SAL: Components of Quiet Tropics


Article written by intern Brylee Faith Brown and Chief Meteorologist Jason Simpson.

Tropical Activity at a halt? What are the Contributing Factors?

Hurricane Elsa formed July 1, 2021; it is the last-named storm we’ve seen this summer. The National Hurricane Center is watching an area of interest near the East Coast. Other than that, the tropics have remained quiet. Why is that?  

Let’s look at a few contributing factors and determine when more action may spark up for the tropics! 

The Madden-Julian Oscillation (MJO) can be a huge driver of tropical activity.  

But what is it?  

NOAA defines the MJO like this:   

“an eastward moving disturbance of clouds, rainfall, winds, and pressure that traverses the planet in the tropics and returns to its initial starting point in 30 to 60 days, on average. “  

The MJO affects the large-scale weather two ways.  

The convective phase contributes to enhanced rainfall amounts in the tropics – resulting in greater tropical development. The suppressed rainfall phase is weaker. Therefore, this phase of MJO has less of an impact on tropical weather.  

Currently, the MJO for the past three weeks has favored a quiet Atlantic. This is the suppressive (phases 3-6) phase . There are signs that point to a more active phase for the Atlantic in early August.  

But other things contribute to Atlantic activity. 

One of those is the Saharan Air Layer (SAL.) 

The SAL is a dry, dusty layer of air that moves westward over the Atlantic from the Saharan desert. The dust present in the atmosphere prevents cloud development making it difficult for storms to form. No storms? No hurricanes.  

The dust acts in the same way as the smoke we are seeing from the active wildfires in the West. The dust and smoke suppress daily convection (thunderstorms) – just like the pattern of storms that developed Thursday in North Alabama and Southern Tennessee.

The Saharan dust combined with a weak, suppressive MJO lower the storm development odds in the tropics for now, but the peak of hurricane season lies ahead!

These limiting factors won’t last forever, and a quick ramp-up to more tropical activity is likely.

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