Some Background Regarding An El Nino

Currently the weather is being strongly affected by an El Nino. El Nino is but one part of a weather/climate system known as the El Nino Southern Oscillation (ENSO). There are three phases of ENSO — El Nino, La Nina and neutral.   ENSO is important because of its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe. The global atmospheric circulation is called the Walker Cycle and we will look  at that in the next posting.

Many of you already are fully informed about the ENSO but my guess is that some of you are not. I thought it might be helpful to provide some background information. (1)

First of all, we are talking about the Pacific Ocean. The Pacific Ocean from South America to the Maritime Continent(2), a distance of about 10k miles along the equator. Usually the trade winds blow along the equator toward the west. This moves the hot surface water to the Western Pacific. The sea surface is about ½ meter higher in Indonesian than it is in Ecuador. 
Usually, sea-surface temperatures off South America’s west coast range from the 60°s to 70°s F, while they exceed 80°F in the “warm pool.” This description is essentially that of the neutral phase.

On occasions the easterly winds weaken and the hot water begins to flow eastward toward South America. This is the beginning of an El Nino. It typically starts in the May-June timeframe as the water flows eastward. It reaches it peak strength about December. January through March/April typically are the months that the El Nino begins to lose it strength. Some El Ninos maintain strength longer such as the 1998/1999 El Nino, which is considered one of the strongest ever.

The El Nino is usually followed by a La Nina when the trade winds begin to blow strongly to the west and the warm water reverses and goes west. Upwelling of cold water replaces the departing warm water and the effect on the atmospheric circulation is reversed as well.

Below are of the depictions of the 1988 La Nina during the month of December and the 1997 El Nino during December.   The stretch of cool Pacific Ocean water along the equator happened during the 1988 La Nina. The powerful 1997 El Nino shows a stretch of hot Pacific Ocean water from about the International Date Line all the way to the coast of South America.   When ENSO enters the neutral phase, the warm water begins to collect at the Maritime Continent.

The ENSO cycle is not regular.   It ranges from 2 to 8 years.

What causes the ENSO is not known.   NOAA says this:

“The jury is still out on this. Are we likely to see more El Nino’s because of global warming? Will they be more intense? These are questions facing the science community today. Research will help us separate the natural climate variability from any trends due to man’s activities. If we cannot sort out what the natural variability does, then we cannot identify the “fingerprint” of global warming. We also need to look at the link between decadal changes in natural variability and global warming. At this time we cannot preclude the possibility of links but it is too early to say there is a definite link.”

The skeptics don’t know either but think it is primarily natural climate variability.

Because a picture is worth a thousand words, please look at the following video:

NOAA won’t call an El Nino unless it changes global atmospheric circulation. The Walker cycle will be discussed in the next posting.

cbdakota

(1)If you are going to debate someone about ENSO, better go to some expert like Bob Tisdale. He has written extensively about this topic. His book can be accessed by clicking here.

(2) From Wiki: Maritime Continent is the name given primarily by meteorologists and oceanographers to the region of Southeast Asia which comprises, amongst other countries, Indonesia, Philippines and Papua New Guinea. Located between the Indian and Pacific Oceans, it is situated within a warm ocean region known as the Tropical Warm Pool.