The mark of a busy hurricane season usually has one element missing from it: El Nino. That is to say, El Nino conditions in the Tropical Pacific tend to suppress Atlantic hurricane activity. This is due mainly to strong upper level winds that cut across the breeding grounds for hurricanes, thus limiting their numbers and intensities. However, one must remember infamous exceptions to this rule such as Andrew in 1992, an El Nino year. There are others as well which remind us of the adage “it only takes one”.
What about neutral years? What defines a neutral year anyway? Basically, when we see the sea surface temperature (SST) anomaly in the Tropical Pacific between .50 Celsius above or below normal, it is a neutral year (ENSO neutral). The scale is not tipped in one direction or another. Easy enough, right?
This is important because we are leaving a La Nina pattern that saw SST anomalies dip well below the .50 C threshold for the last several months across a vast region of the Tropical Pacific. This was due in part to abnormally strong trade winds blowing across the Pacific from east to west. This acts to cool the water by stirring it up, mixing it and upwelling more cold water from the depths off the west coast of South America. Why this happens is where the more mysterious aspect of the El Nino/La Nina phenomenon comes in to play. We’ll leave the mystery solving for another day. Let’s instead look at what may be happening and what it could mean for the upcoming Atlantic hurricane season.
First, let’s take a look at the current SST anomalies for the entire globe. In this graphic, we see the area outlined in pink as being mostly blue to fairly deep blue in some locations. This is the current La Nina or cold episode of the ENSO state (ENSO means El Nino Southern Oscillation).
The next graphic shows the global SST anomalies one month ago. Notice the plume of significantly warmer than normal SSTs coming off of South America and extending westward in to the Pacific. This had a lot of people talking on message boards and in blogs about the distinct possibility of an El Nino setting in and this was the first real sign of its arrival. Not so fast… Look at the top graphic again, it is from March 19, one month after the warm plume developed. Where is that warm plume now? It all but vanished? Why? Probably an increase in the trade winds once again, thus stirring up the waters, cooling them off.
We can measure and usually predict when these warming and cooling phases will take place by tracking the SOI or Southern Oscillation Index. It is the pressure difference between Tahiti and Darwin (to put it in very simple terms). The easiest way to understand it: when the SOI is positive and strongly so, La Nina conditions and abnormally strong easterly trade winds are in place. Conversely, when the SOI is negative and strongly so, trade winds typically weaken or reverse and a warming trend usually follows in the Tropical Pacific. This is a broad brush way to understand the SOI and how it relates to ENSO, there are obviously more complicated factors at work.
This leads me to the next graphic. It shows the SOI from January of 2010 until present day. Notice the distinct drop (A) early this year. This lead to a decrease in the trade winds across a portion of the Tropical Pacific and likely helped to increase the SST anomalies off the coast of South America. The timing is excellent. Now notice the abrupt rise (B) to the SOI and consider the consequent cooling off of those same waters off of South America. This is too coincidental for it to not be related. The warming trend was, at least temporarily, halted and reversed.
We now know the current state of the ENSO and we see that the La Nina has clearly peaked and begun to weaken. This does not necessarily mean that an El Nino is inevitable. In fact, looking at the climate models, the majority of them support ENSO neutral conditions in place during the upcoming Atlantic hurricane season. What implications could this have on the season ahead? To help answer that question, let’s look at all of the ENSO neutral years since 1950, according to the Climate Prediction Center’s site which lists all of the warm and cold periods from 1950 to present.
Neutral years: 1952, 1959, 1960, 1961, 1962, 1966, 1967, 1978, 1979, 1980, 1981, 1984, 1990, 1996, 2001, 2003, 2005, 2008* (was a toss up with first half of year cold, second half neutral).
Do any of those years stand out? Some of them have some big landfalls associated with them, others do not. If we look at just the time period of post-1995, then we see 1996, 2001, 2003, 2005 and 2008 as standing out. All but 2001 had significant U.S. landfalls. All were ENSO neutral years, extending in to the Atlantic hurricane season. While it is not wise to draw any conclusions on this data alone, it does perhaps lend some credibility to the notion that ENSO neutral years tend to have more U.S. landfalls. Obviously, many more years of data is needed before we can say with any real certainty that there is a direct correlation.
We are almost through March and the major climate models are no longer indicating a significant chance of a hurricane-killing El Nino to be present. This could have major ramifications on the Atlantic hurricane season, we’ll have to wait and see. There are so many factors in play, most of which simply cannot be modeled this far out in time. So use this info as perhaps a guide while you think about your hurricane planning and preparedness. While it only takes one in your area to make it a bad year, the data is intriguing enough to suggest that if we do in fact observe ENSO neutral conditions this season, that it just might mean an increased risk of landfall. Problem is, no data can tell us where, not this many months out. I guess we’ll just have to be ready no matter what….
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