Weather Impacts of ENSO

The Jetstream

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.

A very strong El Niño 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 (below) 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 Cyclones

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 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).

From Australia Bureau of Meteorology

Region		El Niño Years		Non-El Niño Years
		Number of Storms	Intensity	Number of Storms	Intensity
North Atlantic		Large Decrease	Small Decrease	Small Increase	Small Increase
Eastern North Pacific		Slight Increase	Increase	Slight Decrease	Decrease
Western North Pacific	Eastern half	Increase	No Change	Decrease	No Change
	Western half	Decrease	No Change	Increase	No Change
Indian Ocean (North / South)		No Change	No Change	No Change	No Change
Australian Region	Western	Slight Decrease	No Change	Slight Increase	No Change
	Central and East	Decrease	Slight Decrease	Increase	Slight Increase
South / Central Pacific (>160°E)		Increase	Increase	Decrease	Slight Decrease