Last week, NOAA announced the issuance of an El Niño watch as they predict about a 50/50 chance of the development of an El Niño later this summer or fall. Increasing sea surface temperatures in the western Pacific Ocean and above-average water temperatures below the surface of the central Pacific are the reasons why experts think that an El Niño could develop within the next six months. Depending on the strength of the possible El Niño, it could have a very real impact on the upcoming Atlantic hurricane season.
What is an El Niño?
An El Niño occurs when warm waters off in the western Pacific move eastward and come to rest along the west coast of South America. The technical definition of El Niño requires that water temperatures in the eastern Pacific rise at least a half a degree Celsius above normal for three consecutive months. That rise in temperatures doesn't sound like much, but it can have far-reaching consequences around the world.
How does an El Niño occur?
Under normal conditions, an atmospheric circulation called the Walker Circulation (figure a in the diagram to the left) is present across the Pacific Ocean. Easterly winds blow across the Pacific from South America towards the west, pushing warm surface water up against Australia and the islands of Oceania. This air rises in thunderstorm activity and heads back towards the east in the upper atmosphere. Once this air reaches South America again, it sinks and creates high pressure off the coast of Peru and Ecuador.
In the Pacific Ocean itself, cold deep-sea water rises along the west coast of South America in a process called upwelling. This cold water (along with high pressure caused by the Walker Circulation) further cools and stabilizes the air, preventing any sort of rain or thunderstorm activity from developing. In addition to giving the region a thriving fishing industry, it keeps the west coast of South America so dry that NASA uses the Atacama Desert as a Mars simulator.
The most widely accepted theory as to why El Niño occurs lies in the breakdown of the Walker Circulation. The easterly winds that flow across the Pacific either stop blowing altogether or reverse, allowing the warm water in the western pacific to flow eastward (figure b in the picture to the left).
Consequences for the Atlantic hurricane season
As an El Niño builds and waters in the eastern Pacific begin to warm up, it allows the air to become warm and unstable, leading to the development of thunderstorms. Thunderstorms ingest this unstable air and vent it out into the upper atmosphere. This upper-level outflow travels eastward into the Caribbean and western Atlantic Ocean and shreds apart the tops of thunderstorm activity, disrupting the storms' updrafts and preventing them from organizing into tropical cyclone activity. Wind shear also proves lethal to existing tropical cyclone activity, as seen by the above picture documenting the demise of the eastern Pacific's Hurricane Sergio in 2006.
Through three degrees of separation, El Niño years tend to dampen hurricane activity in the Atlantic basin. The strongest El Niño ever recorded occurred in 1997 and deep-sixed that year's hurricane season. This isn't an absolute certainty, though. Depending on when or even if El Niño conditions form this summer and fall, it could form too late (or too weak) to have a dampening effect on this year's tropical cyclone activity. This was the case a decade ago. Despite a strengthening El Niño during the summer of 2004, the season saw 4 devastating hurricanes in Charley, Frances, Ivan, and Jeanne.
While the upcoming Atlantic hurricane season could be blunted by a potential El Niño, the prudent coastal resident will heed the paraphrased advice of the great James Spann. When asked online if he thinks the upcoming tornado season will be a bad one, he usually responds by saying that "it's a bad tornado season if one hits your house." In this case, and as Florida learned in 1992, it only takes one storm making landfall to make it a bad season.