The 2010 hurricane season is underway, and with all eyes on the spill in the Gulf there are strong signals from nature that this year will be much different from 2009.
Last year, there were only three hurricanes. The long-term average is 10 tropical storms, six that develop into hurricanes. The season runs from the first of June to the end of November — hurricanes only form over water that is 80 degrees or warmer, as the Atlantic Ocean is usually not warm enough to support hurricanes in June and July. But this year, we’ve already got Alex, so we are off and running. Generally the warmer the water, the greater the chance there will be a stormy season. But there are other important factors.
As a reference for how active the Atlantic can get, we need only to go back to the amazing season of 2005. That year the very warm water temperatures generated 27 named storms and 15 hurricanes — both of these figures are records. In fact, the National Hurricane Center ran out of names! After Hurricane Wilma, the Greek alphabet was used to name the storms. This was the first time this had ever happened since the naming of tropical storms and hurricanes started in 1953.
So what made 2005 such a crazy and deadly season? There are several factors involved, but one of the most important is not even directly connected to the Atlantic Ocean. There is a warming and cooling of the Pacific Ocean known as El Nino-Southern Oscillation, or ENSO. Straddling the equator, the water from the coast of South America out to the International Date Line will warm significantly for up to a year and a half — this is the well known El Nino. Sometimes the water will cool significantly for a similar period, which is the less known but equally significant La Nina.
Hurricanes are strongly affected by strong winds. You might say, wait a minute: hurricanes are storms with very strong winds — how can they be affected by other strong winds? It starts with what El Nino does to the atmosphere. El Nino’s warm water heats up the air above it. This warming causes rising motion in the air as the atmosphere becomes more buoyant. This warm rising air collects more moisture, and makes clouds and storms. The way the atmosphere compensates for this unusual storminess is to create strong winds high above the ocean. In a way, it’s similar to what we do when we become overheated — we turn on a fan to carry the heat away from out bodies.
The winds blow from the west to the east up at thirty to sixty thousand feet. As these winds blow out over the Atlantic Ocean during the hurricane season, they can literally decapitate developing hurricanes. As a fledgling tropical storm tries to stick its head higher and higher into the sky, the El Nino winds lop it off, effectively killing the storm. As a result, we usually see fewer and weaker hurricanes during El Nino hurricane seasons.
La Nina conditions have the opposite effect.