Lots of talk this year about the strong El Nino event that we are/going to experience. Some of you guys already understand the ramifications of this, but a lot may not. Thought I would put a little something together to help understand what El Nino is, and what it means for us.
Background
El Niño is part of the larger cyclical climate phenomenon called the “El Niño-Southern Oscillation”, which is characterized by warmer-than-average sea surface temperatures in the equatorial Pacific Ocean. Inversely, La Niña is when the water is cooler than normal.
The interaction of the ocean and atmosphere means that these changes in sea surface temperatures are translated into changes in wind direction and strength. During El Niño, the prevailing easterlies over the tropical Pacific weaken and can even reverse. The disruption to the wind pattern can reverberate through the atmosphere, impacting the flow of air from the equator to the poles which in turn can affect the jet stream and weather around the globe.
The whole process of the warming and cooling of the water is called El Niño Southern Oscillation, or ENSO. It’s a fairly new phenomenon, only being studied within the last 30 years or so
Strong La Niñas have been associated with more volatile weather patterns across North America, including tornado outbreaks. El Niños are most noticeable during the winter months. The latest outlook shows an almost certain El Niño for the winter of 2015-2016. Some predict it could even be a record-breaking El Niño stronger than what we had in 1997-1998.
Regional Impacts
Looking at past strong El Niño conditions, below are the expectations for temperatures and precipitation from late fall through winter in the U.S.:
- Wetter: Southern U.S. from California to the Carolinas then up parts of the East Coast
- Drier: Parts of the Ohio Valley, Great Lakes, Northwest and Northern Rockies
- Cooler: Desert Southwest, Southern Plains, northern Gulf Coast
- Warmer: Northern tier of states from the Pacific Northwest to the Northern Plains, Great Lakes, and Northeast
Southwest
The winter of 1997-98 is remembered for the catastrophic amounts of water dumped on California, leading to flooding and mudslides.
Even if El Niño could bring enormous amounts of rain to California, it will almost certainly not wipe out the state’s four years of drought. Central and Northern California supply much of the state’s water but do not typically receive as much precipitation from an El Niño as Southern California. Meeting the current water deficit for the state would require more than twice the average amount of precipitation for a year — something in excess of the wettest year on record.
While a great deal of rain could recharge many of the state’s reservoirs, much of the state’s water supply depends on the amount of snow on the Sierras. El Niño does have the potential to bring a lot of water to California but it doesn’t necessarily bring a lot of snow.
Northeast
In the Northeast snowy conditions are expected but it is worth mentioning that the impact of El Niño is just one piece of the puzzle in predicting weather in regions that are a long distance away from where El Niño is measured, in the equatorial eastern and central Pacific Ocean.
One or two big storms can define a season in the Northeast, especially in places like New York, Boston or Hartford where blockbuster snow isn’t typical. On average, New England has seen an above average amount of precipitation during an El Niño winter. But it is worth noting that El Niño winters with little snow get dwarfed by the blockbuster El Niño snowfall seasons, and the “above-average” snowfall signal is muddled or perhaps even misleading.
Midwest
In the Midwestern United States, El Niño almost always means less snow. However, there are a few El Niño winters that buck the trend. Quite simply, if we have a few huge winter storms in a season, it’s possible to have an above-average snow with less frequent snow systems, but overwhelming years have seen a lot less snow and warmer temperatures.
Southeast
One of the major effects of El Niño in the Southeast is a less active hurricane season. The El Niño events create high wind shear conditions over the regions of the Caribbean Sea, Gulf of Mexico, and Atlantic Ocean where hurricane formation takes place. These regions of high wind shear can prevent hurricane development and weaken storms that do manage to form.
Fall weather tends to be drier than usual during an El Niño event. This results from a quieter hurricane season, since most of the Southeast's fall precipitation comes from tropical systems. Average temperatures are not influenced by El Niño during fall months.
Wetter, cooler winter and spring weather is another major effect. The higher-than-usual amounts of rain and cloud cover resulting from El Niño are believed to be the cause of the lower-than-usual average temperatures during the winter months.
Despite lowering average temperatures, El Niño conditions decrease the risk of freezes in the Southeast. The event causes a strong subtropical jet stream that keeps cold arctic air masses from moving into the Southeast U.S., making severe cold outbreaks less likely during an El Niño event
Hope that helps you make sense of some of the terms getting tossed about in the long term weather forecasts for this season. Feel free to ask questions.
