El Nino is a term used to describe changes in global weather patterns, triggered by a series of oceanic and atmospheric changes in the equatorial Pacific region. El Nino is a natural although irregularly occurring climatic phenomenon, which occurs on average every two to seven years. It can last for a period of up to twelve months, with its peak usually during late fall/early winter and lasting through early spring. Its potentially disruptive effects on weather are felt across various regions of the globe.
Although the exact initial trigger of El Nino is not yet proven, the oceanic and atmospheric circulations are in a constant state of flux and heat transfer, moving heat from one part of the globe to another.
Initial signs of an impending El Nino event can be measured in the first instance by changes in sea surface temperatures across the equatorial Pacific region using a series of buoys and satellite observations. Other measurable changes in that region include sea level changes, a weakening or reversal of direction of the local so-called trade winds (nearly constant strong easterly winds that dominate most of the tropics and equatorial region) and changes in the locations of the world’s largest thunderstorms. Sea surface temperature changes alone do not necessarily indicate an impending El Nino event. However, when coupled with any of the additional changes listed above, an El Nino event is already underway.
Measuring the changes once an El Nino event has begun is relatively easy. Predicting exactly when an El Nino event will occur however is much more difficult. Once an El Nino is underway, the ocean surface along the coast of the eastern Pacific from Peru to Ecuador all the way to the center of the equatorial Pacific Ocean sees a rise in temperature of a few degrees Celsius. A temperature rise of 0.5 degrees Celsius above the average indicates a weak El Nino; +1.0 degrees Celsius a moderate one; and a rise above +1.5 degrees Celsius is characterized as a strong El Nino. Sea temperatures in the monitored region in September 2015 showed an anomaly of +3.0 degrees Celsius, with all indicators pointing towards a very strong El Nino, with the potential to last all the way through the spring of 2016.
So how and why does an El Nino event, originating in the Pacific region, affect weather worldwide? Weather systems are driven across the globe by the very fast currents of air in the upper atmosphere known as jet streams. Any changes to the position or strength of these jet streams affect weather at the surface of the planet. During El Nino, the warmer warm waters in the equatorial Pacific release more moisture into the air, causing an increase in thunderstorms and tropical storms over a much larger area than usual. El Nino effects are strongest during the northern hemisphere winter due to the fact that ocean temperatures worldwide are at their warmest. This increase in convective storms in turn affect the tropical jet streams, since the increased intensity of the convective storms can extend deep in the atmosphere where the jet streams are usually located.
Since these jet streams are responsible for transporting weather systems across the globe, typical storm paths in the mid-latitudes of the U.S. and other areas in both hemispheres of the earth are shifted.
For North America during the winter months, the storm track is shifted so that an increase in precipitation occurs along the southern tier of U.S. states and Mexico, which could provide some relief for drought-stricken California. Warmer-than-normal temperatures invade northern latitudes of Canada, Alaska and the northern tier of U.S. states. The effect of El Niño on the central U.S. varies. El Niño events have provided both extremes of weather and some have had no effect at all. Typically though, El Niño reduces the amount of snowfall in central sections of the states. In the southeast, states such as Florida and Georgia may experience much cooler temperatures than normal with greater amounts of precipitation.
Additionally, there is a noticeable decrease of hurricanes during hurricane season (June to November) in the Gulf of Mexico and Western Atlantic. In 2015 for an example, only one named storm occurred during August and September in the Gulf of Mexico and Western Atlantic. This is as a result of the jet stream aligning in such a way that the vertical wind shear (changes in wind direction and strength with increasing height) is increased over the Caribbean and Atlantic. The increased wind shear helps to prevent tropical disturbances from developing into hurricanes, by blowing apart storms before they can intensify. At the same time there is an increase of tropical storms in the Pacific due to the increased atmospheric moisture. The eastern Pacific has suffered an increase in hurricanes and tropical storms during the 2015 season.
Elsewhere, places such as northern Australia, and southeast Indonesia and Asia are more likely to experience drought conditions because moisture-transporting storms are shifted away from these areas. Conversely Argentina, South China, Brazil and Japan can receive an increase in moisture-bearing storms that cause long periods of heavy rains and flooding.
However, even during an El Nino year, there are other major factors affecting the year’s climate. Last winter, the so-called Arctic Oscillation greatly affected winter temperatures and snowfall in the northern hemisphere, as a result of the influx of Arctic air into the mid-latitudes due to a weakened polar vortex (a fast moving current of air located over the Earth’s poles, which normally traps cold Arctic air within its boundaries).
Across the globe there are numerous natural climate fluctuations, or ‘oscillations’, which naturally oscillate between two main states. El Nino is part of the El Nino Southern Oscillation (ENSO), which also includes its opposite state, known as La Nina.
In a La Nina phase, the sea surface temperatures in the same Pacific region are much lower than usual with different effects on global weather. Therefore the ENSO fluctuates between warm (El Nino) and cold (La Nina) conditions.
Other oscillations include the Madden-Julian Oscillation (affecting tropical convection/rainfall conditions); the North Atlantic Oscillation (affecting the strength of surface westerly winds across the Atlantic); the Interdecadal Pacific Oscillation (affecting atmospheric pressure and sea surface temperatures over the central North Pacific); the Antarctic Oscillation (southern hemisphere) and Arctic Oscillation (northern hemisphere) affecting the wind circulations around both poles as a result of atmospheric pressure differences.
Several factors link all of these oscillations:
- They can have either two or three phases; a positive, negative and a neutral phase
- Sea surface temperatures and atmospheric wind circulations are directly affected
- They have a large effect on daily weather and longer term climate, both locally and on a global scale
- They do not occur to a regular or predictable time period and are therefore extremely difficult to forecast
- Once underway however, satellites and other methods of measuring can monitor their strength and local/global impact
- A given oscillation can impact another, if they occur at the same time
- No two oscillations, even if they have the same strength, are exactly the same
The current El Nino event is predicted to be strong, given the size of the measured temperature anomaly in the Pacific. The strongest ever El Nino recorded to-date occurred in 1997-1998. How this year’s El Nino will affect the coming winter weather is difficult to forecast on a seasonal timescale, particularly with the potential interaction of the Arctic Oscillation, which played a large role in the Northern hemisphere winter of 2014. However there is a strong likelihood of greater precipitation in the southern part of the US, and above average temperatures across much of the west and northern half of the contiguous US.
In a warming world, can we expect the frequency or strength of future El Nino’s to change? It would seem likely that this would be the case, given the increasingly warm sea surface temperatures in that region, but without long- term accurate data records it is difficult to prove.
Measurements of the El Nino phenomenon started in 1950. The six decades worth of data gathered so far is too short a timescale for accurate long-term predictions. However, all measurements so far indicate that this year’s El Nino could prove to have significant consequences.