The Atlantic hurricane season is already underway (started June 1st), and so far there has been one named storm, Subtropical Storm Alberto. Alberto, reached a maximum intensity of 65 mph winds and made landfall on the western Florida Panhandle on Memorial Day before hurricane season officially began.
There are a multitude of elements that can have an effect on a given hurricane season. We have already discussed how the cooler sea surface temperatures are present in the Main Development Region as well the dry air that may hinder the development of tropical cyclones. Now there is another significant feature worth honing in on: ENSO, or the El Niño Southern Oscillation.
Earlier this year, NOAA (National Oceanic and Atmospheric Administration) declared the La Niña was over, which began in 2017 and ended during the start of 2018. Now officially as of today there is an ‘El Niño Watch’ in effect. This means that El Niño conditions are possible in the long-range time frame: “ENSO-neutral is favored through Northern Hemisphere summer 2018, with the chance for El Niño increasing to 50% during fall, and ~65% during winter 2018-19.”
Let’s first refresh ourselves on what an El Niño is. During an El Niño phase, ocean waters near the equatorial Pacific Ocean are above average and have an effect on where the thunderstorms over this region set up and the direction of the surface winds. The exact opposite occurs during a La Niña, which is when the equatorial Pacific waters are cooler than normal.
Based on the latest data from satellite imagery, the below loop shows the change in sea surface temperature anomalies in the equatorial Pacific Ocean over the past twelve weeks. Notice how the blues shades become more difficult to find while the orange shades become more expansive. This signifies the warming of the ocean temperatures relative to normal, and indicates a potential developing El Niño.
There are other ways to observe how the ocean is behaving as well, such as the OSTM/Jason-2 satellite. According to NASA, this satellite ” is using radar altimetry to collect sea surface height data of all the world’s oceans. These images are processed to highlight the interannual signal of sea surface height. The mean signal, seasonal signal, and the trend have been removed.” One component of the data this satellite collects is the sea surface height anomalies in millimeters. Below we compare the sea surface height anomalies between April 24th and May 24th. These two images illustrate how the temperature profile of the Pacific Ocean has already changed drastically in just one month. One of the key differences is the increase in warming along the Equator, in particular the eastern half. This is another indicator of a looming El Niño.
So how does El Niño effect the Atlantic hurricane season? Surprisingly, the sea surface temperatures in the Pacific can have a significant impact on how hurricane season behaves. Tropical cyclones hate wind shear because it prevents their thunderstorms from organizing and rising up. Oftentimes with an El Niño, wind shear is above normal over the Atlantic basin as opposed to below-normal in a La Niña phase.
Based on the current outlook, an El Niño may develop during the second half of hurricane season, which would likely further hinder any tropical development. Therefore, a near-normal to perhaps below average Atlantic hurricane season can be expected this year.