REPOST: La Nina and the 2007 Atlantic Hurricane Season

The official hurricane forecast from NOAA is for 13-17 named storms including 7-10 hurricanes. These numbers are very similar to what would be expected from the statistical average of a La Nina summer. Over three months ago I posted here that a simple statistical analysis suggests that the 2007 Atlantic hurrican season should be above average based on the idea that a weak La Nina was expected to form during the summer. The most recent ENSO diagnostic discussion continues to suggest a weak La Nina is expected. Here is that original post.

Original Post: February 22, 2007
While it is still early to project what type of hurricane season the Atlantic basin will have in 2007, it is worth nothing that computer models anticipate that La Nina conditions are expected to rapidly develop over the next few months. What does this mean for the 2007 hurricane season?

The latest ENSO diagnostic report (a weekly “state of the climate report” that is a must read for anyone who considers themselves a synoptic climatologist) shows that the ensembles predict the rapid development of a moderate La Nina by May, which suggests that the 2007 hurricane season should be much more active than 2006 and should feature greater than normal hurricane activity.

Looking at hurricane activity from 1995-2006 (which coincides with the recent upturn in hurricane activity in the Atlantic basin as well as the warm phase of the AMO, which along with global warming, is likely the reason for the upturn), shows the following statistics.

Average (total storms, tropical storms, hurricanes)
14.3; 6.1; 8.3

El Nino (total storms, tropical storms, hurricanes)
10.5; 5.5; 5.0

La Nina (total storms, tropical storms, hurricanes)
17.2; 6.8; 10.4

Neutral (total storms, tropical storms, hurricanes)
14.7; 6.4; 8.3

It should be noted that the main difference between El Nino and La Nina is not the number of weak, tropical storms, but rather the number of hurricanes that form. And while this does represent a small n size (12 years), the differences between El Nino and La Nina for hurricanes (and thus, total storms), is highly statistically significant (p < 0.01). However, it is important to note that nearly all of the variability between El Nino and La Nina can be explained by only four of the twelve years.

Eight of the years were nearly identical in their number of total storms (1996, 1998-2004) and were an even mix of 3 La Nina, 2 El Nino and 3 Neutral years. The average of this 8-year period was 13.8 and the standard deviation was a paltry 1.4 storms. The range during these eight years was between 12-16 total storms, and the only impact of El Nino and La Nina was that during La Nina years, those 12-16 storms were slightly more likely to be hurricanes than tropical storms.

The other four years (1995; 1997; 2005-06) showed the following numbers…

La Nina (total storms, tropical storms, hurricanes)
1995 – 19, 8, 11
2005 – 27, 12, 15

El Nino (total storms, tropical storms, hurricanes)
1997 – 7, 3, 4
2006 – 9, 4, 5

Obviously something else in the atmosphere/ocean was contributing to the impressive increase or decrease with the respective La Nina or El Nino event during those four years. Some researchers have suggested it could be the Quasi-Biennial Oscillation (QBO) which affects stratospheric winds near the equator; Saharan dust storms, or some other yet unknown perturbation. The point is, just because a La Nina is expected for this summer, a repeat of 2005 is not necessarily in store. However, recent history does show that the 2007 hurricane season in the Atlantic Basin should be at least above the average of the past twelve years. How far above the average is unknown.

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