The Climate Prediction Center weekly ENSO update came out today and it has some interesting information. El Niño weakened earlier in 2016 with equatorial Pacific waters cooling. Over the last 6 weeks, the equatorial waters are cool enough for ENSO neutral conditions. Neutral means that monthly water temperatures are within a half of a degree of normal. So, the very warm waters of El Niño in 2015 and early 2016, over the tropical Pacific, have cooled to neutral and are now entering cooler than average temperatures in the very same region.
What is so interesting about this morning’s report? This is the first week that ENSO water temperatures in Nino Region 3.4 have reached the threshold of La Nina.
La Nina registers, when the surface water temperature anomalies in Nino 3.4 drop below -.5 degrees Celsius. Today’s update shows that region with an anomaly of -.6 degrees Celsius. To officially register as La Nina, the surface water temperature anomaly, in this region, must stay below -.5 degrees Celsius for three consecutive months. If we are entering a La Nina, we are still months from it being official. However, the significance of entering La Nina during the peak months of the Atlantic Hurricane season is real.
The July ENSO forecast shows a slight drop in the confidence for a developing La Niña for late summer through the winter. However, it is still the most likely phase. Climate records show a La Niña follows strong El Ninos. The 2015/2016 El Niño was one of the strongest. Here is the July 2016 CPC forecast:
The June forecast probabilities were 70% confidence for late summer through winter. Those probabilities have dropped in the 55 to 60% range in the July forecast. In regards to the Atlantic Hurricane Season, August, September and October (ASO on the chart) is the most important time frame. Those three months account for about 90% of all historical tropical systems and major hurricanes in the tropical Atlantic.
If you look at Accumulated Cyclonic Energy (ACE), a measure of wind energy to classify between low, average, active seasons, you will see that La Nina experiences higher ACE values versus all other phases.
So, not only is more tropical wind energy measured in La Niña years but, not surprisingly, they are more active.
There is a lot of information in those last two graphics. Lets look at ACE first. Since 1950, ACE across all years averages 103. In a La Niña, it spikes to 135, a 31% increase over all years, and much higher than neutral and El Nino phases. Now lets shift from ACE and focus just on the number of storms and most importantly, the number of tropical storms and hurricanes than make landfall in the continental United States. Looking at historical records of all four phases, La Nina ranks highest in the average number of storms, average landfalling storms and more importantly the average landfalling hurricanes and major hurricanes. La Nina average 3.25 landfalls of tropical systems in the United States. La Niña averages 1.75 landfalls of a hurricane and .75 landfall of a major hurricane. All of those numbers are for the United States and are higher than El Niño, Neutral and All Years.
Here is one last chart to look at. This chart shows United States landfalling hurricanes and major hurricanes per season by phase.
Since 1950, hurricanes have made landfall in 13 of the 16 years with La Nina. There have, also, been 8 years with a landfalling major hurricane. Now, that doesn’t mean that a landfalling hurricane, or major hurricane, will hit the United States this year, but it does mean probabilities are higher.
So, in summary, La Niña during ASO sees higher numbers of ACE, tropical storms, hurricanes, major hurricanes, landfalling tropical storms, landfalling hurricanes and landfalling major hurricanes. Sounds foreboding right? Well, there are three occasions when no hurricanes hit the United States during an ASO La Niña. However, there is enough data to show why tracking La Nina weekly is important over the next three to four months. Also, fluctuations in Pacific weekly temperature anomalies are very possible. We could have a few weeks oscillating back and forth the -.5 degree Celsius threshold.
Other factors for tropical development in the Atlantic look favorable moving closer to August and the peak of hurricane season – above average surface water temperatures in the western Atlantic Ocean, above average rainfall in western Africa and less dry Saharan air. There many moving parts but add them up and you see why the NOAA 2016 Hurricane Forecast favors an average to above average season.