Note: I originally wrote this article for the General Aviation Council of Hawaii Spring 2014 newsletter. While this seasonal outlook was from last spring, the typical impacts during El Niño remain consistent. Hopefully you will still find it interesting and useful. –JB
The spring 2014 forecast [view the latest forecast] from the National Weather Service Climate Prediction Center indicates that there is a greater than 50 percent chance that El Niño conditions will develop during the summer of 2014, and continue into the winter of 2014-2015. What is El Niño and what does it mean for weather in Hawaii?
What is El Niño?
El Niño is the warm phase of the El Niño/Southern Oscillation (ENSO) cycle. It is identified by warmer than normal water along the equatorial Pacific. It is the opposite of La Niña, which is identified by colder than normal water along the equatorial Pacific. ENSO episodes typically last 9-12 months, and reach their peak strength during late winter (from December to April). The episodes typically occur every 2-7 years.
The phenomenon was first noticed by Peruvian fishermen. Normally water along the surface of the ocean moves westward from South America, and deeper water rises to the surface along the coast. This upwelling brings colder and more nutrient-rich water to the surface. The fishermen noticed that, every few years, fish were much harder to find. During these periods the water along the coast was much warmer than normal. This was usually most noticeable during December and January, near the Christmas season, and they gave it the name El Niño (The Boy).
Weather Impacts to Hawaii
One of the main impacts that we see during the summer in El Niño years is a greater than normal number of tropical cyclones in the Pacific. In a normal year there are four to five tropical cyclones in the Central Pacific, which includes tropical depressions, tropical storms, and hurricanes. Tropical cyclones get their energy from warm ocean water, and during El Niño years the water in the central and eastern Pacific is warmer than normal. Also, hurricane season tends to last longer, as the warmer water allows cyclones to form later in the year.
On a side note, while the Pacific hurricane season is usually more active during El Niño years, the Atlantic hurricane season is usually less active. In addition to warm water, tropical cyclones need an environment with low wind shear (little change in wind with height) in order to develop and strengthen. During El Niño years, there is greater wind shear in the Atlantic basin, which leads to a less active season for them.
During the winter of El Niño years, the large scale flow across the North Pacific tends to be straight from west to east, or what is referred to as zonal. There are frequent storm systems, but because of the zonal pattern, they tend to stay far north of the state. Locally, these systems shift the subtropical ridge southward over the islands, leading to periods of light winds with stable conditions and little rainfall. Therefore El Niño years tend to be drier than normal across the state. Because of the active weather systems across the north Pacific, we also tend to see higher than normal surf during the winter season.
The most recent El Niño event occurred during 2009-2010, and it was considered a moderate event. The 2009 hurricane season in the Central Pacific was more active than normal, with seven tropical cyclones (three hurricanes, three tropical storms, and one tropical depression). This included Hurricane Neki late in October, which struck the Northwest Hawaiian Islands. Conditions during the winter were also drier than normal, with limited rainfall exacerbating drought conditions in many areas. The surf season was also much more active than normal.
The ENSO Forecast
There is higher uncertainty in early season forecasts, due to what is known as the spring predictability barrier. Conditions associated with the changing season are difficult to account for in computer models, and can mask the longer-term climate signals. ENSO forecasts tend to improve by July, but it is important for long-term planning to have a reliable forecast as early as possible. As of the latest forecast from the NWS Climate Prediction Center, issued on April 10, 2014, the chances for El Niño conditions increase through the year. There is a 50 percent chance that we will see El Niño conditions by this summer, and a 65 percent chance that we will see them by this winter.
Stay up to date with the latest warnings, advisories, and forecasts from your National Weather Service. If you have questions about any NWS forecast products, you can call the office at 808-973-5286. (Just remember that while we can answer your questions, we can’t provide flight briefings.)
John Bravender
Aviation Program Manager
National Weather Service Honolulu