Looking Ahead to the Back Half of July in the Tropics

The pregnant pause in the aftermath of Beryl continues this week

It’s only been a week since Hurricane Beryl slammed the Texas coast, knocking out power to over 2 million customers – most notably in the Houston area – for days on end, leaving Texans basting under the stifling summer heat. Although power has returned to about 90% of those affected as of Monday, cleanup and repair efforts continue, leaving many Texans vulnerable during the early stages of the season, with 95% of historical hurricane season activity in front of us.

Mercifully for stormstruck Texas as well as an anxious U.S. coastline, the tropical Atlantic looks largely shut down for the next week or two.

Tropical systems are like bananas – they come in bunches

Tropical storms and hurricanes tend to cluster together in time. They’re a lot like bananas – we get them in bunches.

A big reason why tropical storms and hurricanes cluster in episodic outbreaks is because the overall upper-level wind pattern occasionally favors more storminess over the Atlantic. Scientists first discovered this traveling upper-level wind configuration – known as the Madden-Julian Oscillation or MJO – over 50 years ago but only in recent years have we developed tools to better identify and track the MJO, which circles the globe roughly every month or two.

The MJO comes in two flavors: a rising branch and a sinking branch. The rising branch promotes hurricane activity by encouraging stormier conditions in the tropics and reducing storm-busting wind shear. This explains much of the bunchiness in tropical storm formation, especially on the shoulders of the hurricane season like in June, July, October, and November when the atmosphere is somewhat marginal for development. We’re just coming out of one of those periods during which we saw the first three named storms of the season and a few wannabe tropical depressions.

The counterpart to the rising MJO branch is the sinking MJO branch. The sinking branch comes in behind and cleans up after the basinwide hurricane party, turning up the dim bar lights and telling the tropical systems to go home. This can happen fairly quickly on the heels of an active or rising MJO episode, and the flip of the switch can be a little jarring. Over the years, I’ve found the hurricane whiplash can make some even question if the hurricane season is broken.

The hurricane season isn’t broken. It’s quietly building.

The next few weeks appear to be one of those MJO-induced Atlantic hangovers. The sinking branch should generally suppress development in the tropical Atlantic through the end of July. This of course doesn’t mean we can’t (or won’t) see any tropical development, but any development that happens will be isolated.

For now it’s tropical tumbleweeds in the forecast models. This isn’t unusual and even expected, particularly in July following the passage of an active MJO. Although the tropics may stay silent, the background conditions continue to build toward peak season – waters heat up with the summer sun, wind shear plummets, disturbances rolling off Africa are increasingly vigorous.

So the next time the tropics turn on, the ocean and atmosphere will look a lot different, nearing peak form, and having been conditioned to churn out big, strong hurricanes. The extended range models and climatology suggest we shouldn’t expect the next spate of storms until sometime in August, though by August they can come fast and furious.

In the meantime, we enjoy a much-deserved interlude across the tropical Atlantic.