NOAA’s monthly El Niño update indicates that water temperatures in the equatorial Pacific have risen in the past month.
NOAA estimates the likelihood that the El Niño, named Eggplant by Decoded Science, has a 90% chance of lasting through the summer, and an 80% chance of continuing until the end of the year.
NOAA has increased its category designation of this El Niño to weak-moderate from weak. Decoded Science feels that a further upgrade is very likely.
The Instigator: A Kelvin Wave
Kelvin waves are odd beasts , created by the spin of the earth and a boundary, often a coastline. Kelvin waves propagate around the ocean basins, with the shoreline on the right in the northern hemisphere (on the left in the southern hemisphere).
A special kind of Kelvin wave can also propagate along the equator, which acts like a coastal boundary because the Coriolis force, created by the rotation of the earth, changes sign from the northern to the southern hemisphere. The wave is essentially trapped at the equator, and Kelvin waves often travel from west to east across the Pacific Ocean.
Normally the wind blows from east to west in the tropics, piling water up near New Guinea in the Pacific. If the easterly winds relax, the piled up water will slosh to the east, inducing a Kelvin wave.
Kelvin waves, like all waves, have crests and valleys. These are associated with a downwelling and an upwelling phase of the Kelvin wave. The downwelling phase depresses the thermocline, the subsurface boundary between warmer water near the surface and colder water below. Thus colder water is less likely to mix to the surface, and surface waters tend to warm during the downwelling phase of a Kelvin wave.
A Kelvin wave with a powerful downwelling phase crossed the Pacific from February to May of 2014, and this set off the current El Niño.
By fall, Decoded Science recognized the El Niño with the name Eggplant, and by mid-winter NOAA grudgingly admitted an El Niño was under way.
The downwelling phase of a Kelvin wave is followed by an upwelling phase which cools the ocean surface, but the two phases do not necessarily have the same amplitudes.
The upwelling phase of last summer’s Kelvin wave had little effect, and the surface waters were left warmer than normal. Since last fall, Kelvin waves have been weak and the warm water has persisted.
The Effects Of El Niño
Peruvian anchovy fishermen were probably the first to notice El Niño when their catches drastically decreased. Normally the easterly winds blowing off the South American continent create upwelling of cold water along the coast which mixes nutrients and enhances the biomass — meaning more fish.
An El Niño is accompanied by a weakening of the east winds, a warming of the water, and a consequent reduction in the biomass — fewer fish.
It is now known that the effects of El Niño are felt well beyond the Peruvian anchovy fishery. In fact, El Niño has been correlated with altered temperature and rainfall patterns worldwide.
The correlations with El Niño vary with the season. In summer, El Niño tends to be accompanied by the following:
- There is enhanced rainfall in the western United States.
- There is enhanced rainfall in the central equatorial Pacific.
- There are drier than normal conditions over India due to a weaker than normal monsoon.
- There are dry conditions in Australia.
- There are warm temperatures over much of South and Central America.
In the winter, El Niño is correlated with the following:
- There are warmer than normal temperatures in a wide swath from southern Alaska through much of Canada, and including the northern tier of the United States.
- There are warm temperatures from India through southeast Asia.
- There are cooler than normal in the southeastern US.
- There is enhanced rainfall in the southwestern US.
- It is dry over the northern half of Australia.
- It is warm and dry in southern Africa.
The extent of El Niño’s alteration of weather trends is far-reaching and affects both temperature and precipitation. Not bad for a little sloshing in the equatorial Pacific.
El Niño And Atlantic Hurricanes
The immediate result of an increase in Pacific Ocean water temperatures is enhanced evaporation and hence rainfall, in the local region. But this sets in motion a series of atmospheric changes that has far-off consequences.
The enhanced convection at the equator begins a process that includes a low-latitude jet stream in the eastern Pacific which extends across Central America into the Caribbean, Gulf of Mexico, and tropical and sub-tropical Atlantic Ocean.
This jet stream creates vertical wind shear which is anathema to tropical storms. As a result, El Niño years generally see below normal hurricane activity in the Atlantic basin. In 1997, a year with a powerful El Niño, tropical activity in the Atlantic basin was about half of normal.. In general, El Niño years are characterized by about 2/3 the average number of hurricanes and tropical storms in the Atlantic basin.
El Niño And Pacific Ocean Tropical Cyclones
El Niño has little effect on the number of hurricanes that form in the eastern Pacific, but more of them tend to strike Mexico.
In the western Pacific, El Niño is correlated with a substantial increase in numbers of typhoons, but since the storms originate farther east, there is no increase in the number that strike Asian countries.
Forecasting El Niño Eggplant
The evolution of an El Niño is notoriously hard to predict. El Niño Eggplant has consistently been stronger than NOAA’s forecasts. NOAA has finally caught up to the Decoded Science predictions for the strength and longevity of Eggplant. If a strong El Niño materializes, it has significant consequences for the weather in much of the world.
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