Layman’s Guide To El Nino
By Paul Homewood
Given that El Nino is making big news at the moment, I thought it would be useful to publish this short resume from NWS on ENSO processes:
Normally, sea surface temperature is about 14°F (8°C) higher in the Western Pacific than the waters off South America.
This is due to the trade winds blowing from east to west along the equator allowing the upwelling of cold, nutrient rich water from deeper levels off the northwest coast of South America.
Also, these same trade winds push water west which piles higher in the Western Pacific. The average sea-level height is about 1½ feet (46 cm) higher at Indonesia than at Peru.
The trade winds, in piling up water in the Western Pacific, make a deep 450 feet (150 meter) warm layer in the west that pushes the thermocline down, while it rises in the east.
The shallow 90 feet (30 meter) eastern thermocline allows the winds to pull up water from below, water that is generally much richer in nutrients than the surface layer.
El Niño Conditions
However, when the air pressure patterns in the South Pacific reverse direction (the air pressure at Darwin, Australia is higher than at Tahiti), the trade winds decrease in strength (and can reverse direction).
The result is the normal flow of water away from South America decreases and ocean water piles up off South America. This pushes the thermocline deeper and a decrease in the upwelling.
With a deeper thermocline and decreased westward transport of water, the sea surface temperature increases to greater than normal in the Eastern Pacific. This is the warm phase of ENSO, called El Niño.
The net result is a shift of the prevailing rain pattern from the normal Western Pacific to the Central Pacific. The effect is the rainfall is more common in the Central Pacific while the Western Pacific becomes relatively dry.
La Niña Conditions
There are occasions when the trade winds that blow west across the tropical Pacific are stronger than normal leading to increased upwelling off South America and hence the lower than normal sea surface temperatures.
The prevailing rain pattern also shifts farther west than normal. These winds pile up warm surface water in the West Pacific. This is the cool phase of ENSO called La Niña.
What is surprising is these changes in sea surface temperatures are not large, plus or minus 6°F (3°C) and generally much less.
El Niño effect during December through February
El Niño effect during June through August
La Niña effect during December through February
La Niña effect during June through August
As the position of the warm water along the equator shifts back and forth across the Pacific Ocean, the position where the greatest evaporation of water into the atmosphere also shifts with it. This has a profound effect on the average position of the jet stream which, in turn, effects the storm track.
During El Niño (warm phase of ENSO), the jet stream’s position shows a dip in the Eastern Pacific. The stronger the El Niño, the farther east in the Eastern Pacific the dip in the jet stream occurs. Conversely, during La Niña’s, this dip in the jet stream shifts west of its normal position toward the Central Pacific.
The position of this dip in the jet stream, called a trough, can have a huge effect on the type of weather experienced in North America.
During the warm episode of ENSO (El Niño) the eastern shift in the trough typically sends the storm track, with huge amounts of tropical moisture, into California, south of its normal position of the Pacific Northwest.
Very strong El Niños will cause the trough to shift further south with the average storm track position moving into Southern California.
During these times, rainfall in California can be significantly above normal, leading to numerous occurrences of flash flood and debris flows. With the storm track shifted south, the Pacific Northwest becomes drier and drier as the tropical moisture is shunted south of the region.
The maps (right) show the regions where the greatest impacts due to the shift in the jet stream as a result of ENSO. The highlighted areas indicate significant changes from normal weather occur. The the magnitude of the change from normal is dependent upon the strength of the El Niño or La Niña.
Tropical cyclone activity in the North Atlantic is more sensitive to El Niño influences than in any other ocean basin. In years with moderate to strong El Niño, the North Atlantic basin experiences:
- A substantial reduction in cyclone numbers,
- A 60% reduction in numbers of hurricane days, and
- An overall reduction in system intensity.
This significant change is believed to be due to stronger than normal westerly winds that develop in the western North Atlantic and Caribbean region during El Niño years. Other regions around the world show no affect or are only slightly affected.
The table below gives the trend in number and intensity of cyclones around the world due to the effects of El Niño. (However, as with most meteorological phenomena, there are always exceptions to these trends).