The El Nino Southern Oscillation

The El Nino Southern Oscillation is probably the most important ocean-atmosphere interaction affecting weather on a near-planetary scale. There are two parts to the cycle which focus on surface temperatures of the Pacific, from far west near New Zealand to the South American Coast in the East, cutting a wide swath along the equator. Temperatures are relatively warm across the Pacific for El Nino whereas they are much colder (lower by 3-5 degrees Centigrade) for La Nina. The latter typically lasts for about five months, but sometimes more. El Nino may last longer.

In El Nino the seas warm and, as they do, the trade winds are diminished. This is the sign of the coming El Nino. A pool of warm water is created near South America. Rains typically fall in the Peruvian desert while the western Pacific typically tends toward drought conditions. Circulation increases and cyclones form both north and south of the equator and the normal upwelling of nutrient rich deep, cold water at the South American end is blocked. Thus, in El Nino the water circulation from east to west is slowed and there is far less food to eat near the surface as El Nino runs its course.  Rains in the Pacific and winds generally prevail from west to east. Snows are reduced and warmer weather is enjoyed in the Northern United States. It is also wetter and warmer in Mexico and the South West United States. El Nino also suppresses landfall of Atlantic hurricanes. Canada may also be warmer and drier due to the prevailing westerly weather which tends to keep jet streams to the north. Interestingly, Europe does not seem at all affected by El Nino.

Over the last 25 years there has been a shift in the point of origination of El Nino. They start now more toward the central Pacific rather than toward the western Pacific and may in some cases have increased in strength or in duration in comparison to La Nina. It is not clear if this effect is due to global warming or even if at this stage an association could be proven. An alternative model suggesting sustained La Nina conditions due to global warming has also been predicted.

Increased die off of the Amazon rainforest has also been predicted with increased El Nino activity. Both elevated temperatures 3-4 degrees centigrade and more persistent El Nino activity would create reduced precipitation and a longer dry season in the Amazon. This is a process that can also be amplified by the cutting down parts of the rainforest by humans. Thus land-use and reduced regional precipitation due to global warming directly and to an altered El Nino Southern Oscillation should together decide the fate of the Amazon.

In La Nina (2010-2011) temperatures increase further to the west in the North East Indian Ocean. Significant floods also occurred in Australia (Queensland). Heavy snows occurred in the Northeast United States and tornadoes in the Midwest and South in the spring of 2011.


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