During a typical La Niña, the easterly trade winds across the equatorial Pacific become stronger than normal. This, in turn, pushes warm water west with cooler water upwelling off the west coast of South America. The cooler water tends to weaken wind shear across the tropical Atlantic Ocean -- making conditions ideal for hurricanes to develop, and all signs are pointing to a burgeoning La Niña this fall.
However, certain atmospheric conditions have been stifling the formation of tropical systems. And all signs point to an uncommon weather phenomenon known as the Atlantic Niña.
It's the same type of natural pattern as a La Niña, just smaller in scale and concentrated over the equatorial, tropical Atlantic off the African coast. An area of cooler sea surface temperatures, the Atlantic Niña doesn't occur often -- only about half a dozen times since the early 1980s while La Niña occurs roughly every three to five years. And it's rare for both to occur at the same time.
In the map below you can see the cooler waters emerging during June and July, and then slipping a bit in August below, but staying mostly cooler than average during the last month of meteorological summer.
The equatorial Atlantic was running exceptionally warm leading up to the start of hurricane season -- temperatures at one point exceeded 86 degrees and marked the strongest warm-up before a hurricane season since 1982. It was followed by a seemingly record swing back in the other direction during what should have been the warmest time of the year.
An Atlantic Niña will be determined after August numbers are finalized, but the previous confirmed Atlantic Niña was back in 2012. By definition, for a Niña event to be confirmed, there needs to be a period of three months or longer where surface temperatures run at least half of a degree cooler Celsius, or about 1 degree Fahrenheit.
This is where things get interesting. Because Atlantic Niñas occur so infrequently, the jury is still out on what definitive connections the phenomena has to the tropical storm and hurricane formation during the Atlantic season.
But something definitely happened and called for the rapid cooling of equatorial ocean water to be at the very least -- influential. Cooler sea surface temperatures mean that there is simply less moisture available to evaporate in the atmosphere in order build and fuel hurricanes. And this was occurring in portions of the eastern Main Development Region.
With less moisture in the air, more dense, dry air can actually redirect tropical waves coming off the African coast farther north -- which we've seen happen multiple times during July and August. Tropical storms need to have persistently warm water to draw copious amounts of moisture to grow.
With storms trying to originate over these relatively cooler waters, it's like putting a restrictor plate on a race car, reducing available power and speed to the vehicle.
Hurricane season reached its climatological peak on Tuesday, Sept. 10 -- so there is still a long way to go as we're essentially just at half-time (Atlantic hurricane season runs until Nov. 31).
And of the half-dozen or so Atlantic Niña events in the past, all of them rose and fell pretty quickly. This signals that the active part of the season is simply delayed and that the bulk of the storms is yet to come.
As of Wednesday, there were four disturbances in the Atlantic waters, including a tropical depression that has formed in the eastern tropical Atlantic that will likely be named Tropical Storm Gordon in the next day or two.
In addition, there's a 20 percent chance for a system to develop off the southeast coast, but a strong high pressure should do a great job of protecting New England as the system attempts to organize over the next week.
Despite the slow emergence of tropical storms, NOAA's National Hurricane Center is still holding firm to an extremely active season.
The climatological average for the fourth named hurricane is Sept. 16, so with Francine strengthening to hurricane status, the hurricane season is back to above-average pace.