Wednesday, June 9, 2010

Hurricane Watch: "Expert: La Niña might arrive by July"

From Dr. Jeff Masters' Wunder Blog:
El Niño rapidly dissipated in May, and we are now very close to entering into a La Niña event, according to the latest sea surface temperature (SST) data over the tropical Eastern Pacific. The weekly SST readings in the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region", fell to 0.4°C below average on June 7, a full 1°C drop in just one and a half months. This puts us very close to the -0.5°C threshold needed to be considered a La Niña event, according to NOAA's latest El Niño Discussion. The Australian Bureau of Meteorology showed conditions in the Niña 3.4 region were not quite that cool--0.2°C below average for the week ending June 6. Nevertheless, the speed of the collapse of El Niño makes it likely that a La Niña event is on its way this summer, and NOAA's Climate Prediction Center has issued a La Niña watch. Ten of the 23 El Niño models (updated as of May 19) are predicting La Niña conditions for hurricane season. However, as NOAA's Climate Prediction Center commented in their June 3 advisory, a number of the more reliable models are now calling for La Niña to develop this summer. They comment, "there is an increasing confidence in these colder model forecasts, which is supported by recent observations that show cooling trends in the Pacific Ocean and signs of coupling with the atmospheric circulation." Historically, about 35 - 40% of El Niño events are followed by a La Niña within the same year.

Figure 1. Atlantic named storm, hurricane, and intense hurricane activity since the active hurricane period we are in began in 1995. Both La Niña and neutral years have shown similar levels of Atlantic hurricane activity, though the figures are somewhat skewed by the record-setting year of 2005. Background photo: Hurricane Dean, taken from the Space Shuttle.

It is interesting to note that the last time we had a strong El Niño event, in 1998, El Niño collapsed dramatically in May, and a strong La Niña event developed by hurricane season. History appears to be repeating itself, and I predict the emergence of La Niña by July. Since La Niña events tend to bring lower amounts of wind shear to the tropical Atlantic, we can expect a much more active Atlantic hurricane season than usual in 2010. Since 2010 is similar to 1998 in the behavior of the El Niño/La Niña cycle, it is possible that this year's hurricane season could resemble the 1998 Atlantic hurricane season. That year had about 40% above-average activity, with 14 named storms, 10 hurricanes, and 3 intense hurricanes. The season was relatively late-starting, with only one named storm occurring before August 20. Once the season got going, six named storms affected the Gulf of Mexico, including two hurricanes, Earl and Georges, that passed directly over the location of the Deepwater Horizon oil spill.

Figure 2. Tracks of all named storms for the 1998 Atlantic hurricane season.

Figure 3. Typical regional weather anomalies observed during June - August when La Niña conditions are present. The Caribbean tends to be cloudier and wetter than average, but there is typically little change to temperature and precipitations patterns over North America. Image credit: NOAA Climate Prediction Center.
Here's the current Sea Surface Temperature Anomaly Map from Unisys: 

Current Sea Surface Temperature Anomaly Plot

The area's to watch are right at the equator:

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