El Niño Southern Oscillation (ENSO) is a sloshing of the Pacific Ocean in response to a sloshing in the atmosphere, exacerbated at times by the rhythm of some forcing mechanism.
The eastward phase of the slosh brings warm water to the central and eastern Pacific, and (if the water gets warm enough) is called El Niño.
The westward phase brings upwelling and therefore colder water to the central and eastern Pacific and (if the water gets cold enough) is called La Niña.
El Niño and La Niña conditions affect weather worldwide.
Waves In The Atmosphere And Ocean
There are many types of waves in the real world, but the gravity wave is the one we are most familiar with. Ocean surface waves are a common example. A gravity wave forms when there is a departure from average on a boundary. Gravitational forces then try to restore equilibrium, and the departure oscillates between above and below average. If two children hold a rope and one of them shakes her end, the ripple that flows down the rope is a good approximation of a gravity wave.
There are sloshes of many sizes in the atmosphere and ocean, from tiny whirlwinds to hurricanes and from ripples to tides. When a slosh is in synch with something that is pushing or pulling, it is analogous to a child on a swing being pushed by a babysitter. The babysitter need only give the child a light nudge at the right time in the arc of the swing to push it higher. A nudge at the wrong time dampens the motion.
In the ocean, the best example of an amplified slosh is the enormous tide in the Bay of Fundy. The Atlantic Basin is just about the right size to receive a nudge by the daily gravitational pull of the moon which is in synch with a natural period of sloshing. The result is a tidal range of up to 50 feet.
In the Gulf of Mexico, where the forcing is out of synch with natural harmonics, the tides are very small — often under a foot.
The Role Of Kelvin Waves In El Niño
In the ocean, a type of wave known as a Kelvin wave can form along a boundary, such as the equator. Fluids flow counter-clockwise around low pressure in the northern hemisphere and clockwise around low pressure in the southern hemisphere. B. Wang of the University of Hawaii put it succinctly: “The Kelvin wave is a special type of gravity wave that is affected by the Earth’s rotation and trapped at the Equator ….” A Kelvin wave can be initiated by the wind field at the ocean’s surface.
As a Kelvin wave propagates eastward in the Pacific, alternating warm and cold phases, the result of up- and down-welling, heat and cool the ocean’s surface. Early in 2014, a strong Kelvin wave appeared and was the primary reason for the El Niño forecasts. The warm phase of this Kelvin wave has now passed, and that is the reason for reduced probability of an El Niño.
Nevertheless, even with the approach of the cold phase of the Kelvin wave, warm surface water persists across the equatorial Pacific.
The Current Situation Has Confounded Forecasters For Many Months
Since last spring, NOAA has been issuing El Niño Watches. First, the El Niño was supposed to start in the summer, then the fall, and now late winter — but the conditions in the Pacific Ocean don’t quite measure up to an El Niño by NOAA’s definition.
Decoded Science has a different view: If it looks like an El Niño, and has at least some of the effects of an El Niño, then it’s an El Niño, whether or not it satisfies some arbitrary criteria.
Differences Of Opinion Are Not Confined To Decoded Science And NOAA
Meteorological agencies in other countries frequently affected by El Niño have their own metrics for declaring El Niños. In the current case, Australia has issued an El Niño alert, while Japan has declared a full-fledged El Niño. So this will be El Niño Eggplant in the English-speaking world, and Nasu, the Japanese word for eggplant, in Japan.
The Observations That Led Decoded Science To Declare An El Niño
The sea surface temperatures (SST) across the Pacific have been above normal for some time. Until recently, the jet stream flow has not responded. However, the recent rains in California were a result of a change in the jet stream pattern consistent with El Niño. In addition, the recent warmth in Australia is consistent with El Niño.
What Can We Learn From Previous El Niños?
The current period of ENSO-neutral conditions (as defined by NOAA) is the third longest since record-keeping began in 1950. The only longer ones followed El Niños; this period of calm follows a La Niña. This is the longest ENSO-neutral period following a La Niña on record. So we are, so to speak, in uncharted waters.
The only two longer neutral periods — both following El Niños — were both followed by another El Niño.
The only comparable period of neutral conditions following a La Niña came in 1989 to 1991. This was followed by a period in which the Pacific slowly turned from cool to warm, and finally reached an El Niño in the spring of 1991. That El Niño was weak through the summer of 1991, but ramped up to a strong El Niño by late fall and continued through winter of 1992.
This suggests that the real El Niño may be yet to come and that it may occur not this winter and spring, but next fall and winter.
What Are The Major Ramifications Of El Niño?
The El Niño phase of the Southern Oscillation is correlated with the following:
- Warmer than normal in northwest Canada and Alaska,
- Warmer than normal in India,
- Warmer than normal in western and northern South America,
- Colder than normal over the southern US,
- Colder than normal in China, and
- Warmer than normal in Australia.
- Wetter than normal in the southwest US,
- Drier than normal in Australia and southeast Asia,
- Drier than normal in India,
- Drier than normal in northern South America, and
- Wetter than normal in most of Europe.
- Above normal activity in the central and western Pacific Ocean, and
- Below normal activity in the Gulf of Mexico and Caribbean Sea.
The Future Of Eggplant
As we pointed out, there is precedent for this phase of ENSO to blossom into a strong El Niño next fall and winter.
All of the statistical models predict that El Niño conditions, though perhaps not sufficient to meet the rigid NOAA definition, will persist at least through the summer.
There is insufficient data on which to base more than an educated guess about Eggplant. Our educated guess is that it will continue to be a minor El Niño, perhaps never reaching the NOAA threshold.
However, significant rain has already fallen in California, and Australia is suffering through a drought. Since these conditions are consistent with El Niño, anyone in an area normally affected by this phase of the ENSO cycle should be alert for possible extreme weather.