It’s only late March (at the time of writing this). Hurricane season doesn’t start for another 2 months. So what made me randomly want to write about the upcoming hurricane season? Well, I’ve had people ask me if hurricanes will form earlier than normal because of the above normal temperatures across 2/3rds of the U.S. The short answer would be….. most likely not. There have been studies that show that there is no correlation between above average land temperatures and development of tropical cyclones. There are many, many reasons why. I will go over a couple of them, briefly.
Please note that I will NOT be discussing “global warming” by any means. This is a completely different topic, and more of a theory. I will go over facts about tropical cyclone development.
It’s important to understand the effects that climate has on the development of tropical cyclones. What I mean by climate is: El Niño, La Niña, or Neutral environment. For a description of what El Niño and La Niña are, please click here.
During a La Niña period in the northern hemisphere, tropical systems tend to be more active in the Atlantic Basin. La Niña produces easterly wind departures at upper levels of the atmosphere and westerly departures in the lower levels, across the eastern tropical Pacific Ocean. These wind patterns are in phase in the eastern Pacific, therefore is stronger vertical wind shear. In the Atlantic Basin, La Niña produces winds opposite normally observed, which means less vertical wind shear across the Atlantic. Because tropical cyclones can’t develop well in high vertical wind shear, La Niña typically means a less active tropical season over the eastern Pacific, and a more active Atlantic Basin.
El Niño is the opposite of La Niña. It produces winds opposite of what is normally observed along the eastern Pacific and in sync with winds across the Atlantic Basin. This means, during an El Niño period, tropical systems tend to form more along the eastern Pacific than the Atlantic.
So why does this matter? What does this have to do with an early start to hurricane season?
Well, it takes a lot more than above average temperatures during winter and spring to generate an early start to hurricane season. One must know the ingredients of how tropical systems develop before coming to a conclusion that it will or won’t start early. There are six accepted conditions for hurricane development:
1.) Ocean waters must be above 26 degrees Celsius (79 degrees F).
2.) Must be more than 5 degrees north of the Equator.
3.) There must be rotation and saturated lapse rates near the rotation.
4.) Low vertical wind shear.
5.) High relative humidity values from the surface to the mid-levels.
6.) Must be thunderstorm complex to form a tropical wave.
Notice there is no mention of “above normal air temperatures”, or anything to do with continental air temperatures. Now, getting back to El Niño and La Niña. It’s important to understand where we are right now. Climate Prediction Center wrote a great discussion about where we stand right now, and can be read here. Basically, we have been in La Niña conditions since the second half of 2010. Below is a graph showing model predictions of ENSO.
Climatologists believe we will be entering ENSO-neutral conditions by the end of April. This means La Niña is weakening.
The real question isn’t about land temperatures, it is about water temperatures.
As you can see, there are not many areas above 26 degrees C, right now. Now how does this compare to “average” temperatures around this time of the year?
There are a few areas in the Gulf of Mexico and off the coast of the Northeast that above average, but for the most part, water temperatures seem to be around average for this time of the year.
Because of near normal sea surface temperatures and a weakening La Niña, hurricane season will most likely not come early this year. However, that is NOT to say that a hurricane CAN’T form early. Conditions (low wind shear, no fronts, low level circulation, etc.) and water temperatures have to be just right in order for a tropical cyclone to develop. This can happen any time of the year, during a La Niña or El Niño period. But in the case of above normal temperature across the U.S., there is no correlation between air temperature and hurricane development.
In conclusion, with the weakening of La Niña, this SHOULD be a less active hurricane season for the Atlantic Basin.