A historic Atlantic hurricane season?Aug 11, 2015
In the last newsletter, I wrote about the upcoming hurricane season and the potential effects of a developing El Nino pattern in the Pacific. Now we are further along in the hurricane season here in the first half of August, and strengthening El Nino conditions are present across most of the equatorial Pacific. The most significant differences between average sea surface temperatures and current conditions are occurring in the eastern Pacific. Figure 1 shows the departure from average sea surface temperatures across the world’s oceans, and the orange and red colors along the equatorial portions of the central and eastern Pacific clearly show sea surface temperatures running 3.5 to 5 degrees centigrade above normal values for the season.
As noted in the last newsletter, typically the presence of an El Nino pattern leads to less tropical cyclone activity in the Atlantic, but greater activity in the Pacific. So far this has been borne out with only three systems to date in the Atlantic, but 10 in the eastern Pacific and five in the central Pacific (two of which moved into that region from the eastern Pacific). In the western Pacific, 17 tropical cyclones have occurred since the start of 2015.
The Pacific numbers are all above normal for this point in the season. In fact, in the central Pacific there have already been as many systems as there were during the entire 2014 season, and only one less than the 2013 season. The most active season during the past 10 years was 2009 when seven systems were observed — and not coincidentally. This was the last time a strong El Nino was present.
In the Atlantic, the three storms that have formed so far have all been short-lived tropical storms, and none have developed in the areas that typically lead to strong hurricanes. Two (Ana and Bill) developed fairly close to the U.S. coast and made landfall fairly quickly, then weakened; the third (Claudette) developed near a frontal zone in the western Atlantic and was absorbed into the frontal zone within a couple of days, leading to its loss of tropical characteristics. In addition to the presence of El Nino, there are other factors that point toward a quieter than normal Atlantic season, one of which is that the sea surface temperatures are running below seasonal norms. This can be seen in Figure 1 by noting the patches of blue around the Caribbean and to its east over the tropical Atlantic.
Meteorologists specializing in climate prediction are forecasting that the El Nino pattern will persist through the rest of 2015 and well into 2016, and will continue to strengthen, perhaps becoming one of the stronger El Ninos in recent memory. This is likely to continue the trends seen so far in the tropical cyclone seasons of both the Atlantic and the Pacific, and this has led to a recent update in the seasonal hurricane forecast for the Atlantic. The seasonal forecast for the Pacific has not been updated, and the previous outlooks for above-normal seasons in the central and eastern Pacific remain in effect.
For the Atlantic, forecasters now indicate a 90 percent chance of below-normal activity this season, compared with a 70 percent chance of that outcome at the start of the season. This is the highest confidence level ever provided for a seasonal hurricane forecast. As part of the forecast, the outlook calls for three to seven named storms for the remainder of the season, of which one to four are forecast to become hurricanes. Should only one system reach hurricane strength during the 2015 season (there have been none so far), this would eclipse the record low of two hurricanes, which occurred in 1982 and 2013. Thus the 2015 hurricane season has the potential to be historic on the low end. The details of the updated seasonal forecast for the Atlantic can be found at the following link: http://www.cpc.ncep.noaa.gov/products/outlooks/hurricane.shtml
It is always important to note that even in a season of below-normal activity, significant impacts can still occur in some spots, and the seasonal forecasts do not have the ability to provide any guidance as to whether or not tropical storms or hurricanes that form will make landfall, or where they might do so. Also, the hurricane season has not yet reached its halfway point with the peak of the season generally occurring in the first half of September, so there is still a fair amount of time remaining. Therefore, continued monitoring of tropical activity remains important for all those in areas of potential impact.