El Niño Update And Outlook For Temperature And Precipitation During Winter 2015-16 In US

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As of September 10, sea surface temperatures were as much as four degrees Celsius (7.6 degrees Fahrenheit) above normal in the central and eastern Pacific Ocean. Graphic courtesy of NOAA.

As of September 10, sea surface temperatures were as much as four degrees Celsius (7.6 degrees Fahrenheit) above normal in the central and eastern Pacific Ocean. Graphic courtesy of NOAA.

On September 10, NOAA’s Climate Prediction Center (CPC) updated its forecasts for the current El Niño and the coming winter.

CPC now estimates that El Niño conditions have a 95% chance of continuing through the Northern Hemisphere winter of 2015-16, “gradually weakening through spring 2016.”

Named El Niño Eggplant by Decoded Science, this promises to be the most significant El Niño since at least 1998.

The implications for weather across the US and farther are immense.

What Causes El Niño?

The oceans and atmosphere are subject to major forcing cycles that correspond to the revolution of the earth around the sun, the rotation of the earth on its axis, and the revolution of the moon around the earth. These cycles are approximately 365 days, one day, and 29 days, respectively.

Combinations of these cycles with meteorological forces can lead to a significant warming of the central and eastern Pacific waters — El Niño. The period of these warmings ranges from two to seven years, averaging about four years.

Under normal circumstances, easterly winds over the tropical Pacific cause water to pile up near Indonesia. On the other side of the ocean, surface water is pushed away from the coast of South America; colder water from below upwells to take its place. The initial result is a thriving Peruvian anchovy fishery, as the upwelled water is nutrient-rich.

Occasionally, the winds across the Pacific Ocean slack off and the water piled up in the west end begins to slosh back towards the east. A strange phenomenon known as a Kelvin wave can propagate across the ocean, trapped at the equator by the Coriolis force, which is a result of the spinning of the earth.

All moving air or water parcels are pushed to the right in the northern hemisphere and to the left in the southern hemisphere. Any attempt by the Kelvin wave to escape is met by the powerful Coriolis mechanism and the wave is confined to within five degrees of the equator.

The Kelvin wave has a downwelling phase that is correlated with warm surface water. A strong Kelvin wave can have a positive feedback loop in which enhanced precipitation over the tropical Pacific leads to another Kelvin wave.

The current El Niño Eggplant has been enhanced by a series of Kelvin waves.

 How Will El Niño Affect The Weather In The continental US?

The CPC forecast for November through January shows a typical El Nino winter: Temperatures above normal across the north and below normal across the south. Graphic courtesy of NOAA.

The CPC forecast for November through January shows a typical El Niño winter: Temperatures above normal across the north and below normal across the south. Graphic courtesy of NOAA.

The updated CPC forecast shows a typical El Niño effect over the continental United States: Warmer than normal across the north; Cooler than normal across the south.

Precipitation is expected to be enhanced over the south, including parched California. According to the National Climatic Data Center, during the El Nino of 1997-98, California had its fifth wettest January-February in the 103 year record up to that time.

A more immediate impact of this El Niño is its effect on the Atlantic and Pacific hurricane seasons. Typically El Niño enhances tropical cyclone activity in the Pacific and stifles it in the Atlantic. This summer has seen an unusual number of tropical cyclones in the eastern Pacific that headed in the general direction of Hawaii.

In the Atlantic, with the peak of the season now passed, there have been no hurricane or tropical storm landfalls in the US. Hostile mid-level winds over much of the Atlantic, a hallmark of El Niño, have kept the storms that have formed from becoming dangerous hurricanes.

Nevertheless, major hurricanes have struck the United States in El Niño years, During the years 1900 to 1983, there were 54 landfalling major hurricanes in the US. Four of those occurred in the sixteen El Niño years. According to these data, the chance of a landfalling major hurricane hitting the US is about 0.74 per year, while the chance in an El Niño year is just 0.25, about one-third the normal occurrence.

Effects Of El Niño On Australia

Historically, El Nino is highly correlated with sparse spring rain in Australia. Graphic courtesy of BOM.

Historically, El Niño is highly correlated with sparse spring rain in Australia. Graphic courtesy of BOM.

The interaction between El Niño and weather in far-flung parts of the globe is complicated. But statistics tell us that there is a high correlation of El Niño with drought in Australia.

Eastern Australia typically gets the worst of it. So far,the effects of the current El Niño on Australian precipitation have been minimal.

Is El Niño And Its Effects A Done Deal?

El Niño Eggplant is with us and getting stronger. All forecasts call for it to peak in mid-winter.

However, powerful countervailing forces are at work that could alter the precipitation and temperature patterns that normally accompany El Niño.

In particular, the pool of much-warmer-than-normal water over the northwest Pacific Ocean (affectionately known by meteorologists as ‘the ‘blob’) tends to create warm and dry conditions over the west coast and is thought to be a major contributor to the California drought. The weather this summer seems to be a see-saw between the two distinct patterns induced by El Niño and the blob.

Forecasters are fairly certain, and residents of the southwestern US are hopeful, that the El Niño correlation with precipitation across the southern US will hold this winter. Californians have their fingers crossed.

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