El Niño and Global Warming

Professor Jacob BjerknesIt was not until 1969 that Jacob Bjerknes (photo to the left) proposed that there was a physical connection between the oceanographic and atmospheric variations on the year-to-year (inter-annual) time scales, and now the oceanic and atmospheric aspects are combined in the term ‘El Niño Southern Oscillation’ (ENSO) that encompasses both the ocean and the atmosphere (also see the IRI link, the Australian Bureau of Meteorology, National Geographic, and a discussion on Wikipedia). El Niño events tend to recur every 3-8 years. The last El Niño as of today was in 1997-98, and was the strongest or second strongest (after 1982-83, depending on what you look at) event observed in modern times. The Bureau of Meteorology (BOM) in Australia provides an Internet page on ENSO with a nice ENSO wrap-up for up-dated information. Another resource for keeping up-to-date with ENSO is the TAO-array. The seasonal migration of both the ITCZ and the South Pacific Convergence Zone (SPCZ) are affected by the presence of El Niño.

What is La Niña?

La Niña usually refers to the opposite state to an El Niño: low sea surface temperatures in the eastern part of the eastern tropical Pacific. Intense trade winds and strong uppwelling along a region near the equator, known as the cold tongue and caused by Ekman pumping, bringing up cold and nutrient water from the deep sea. Note that Ekman pumping does not penetrate deep into the oceanic interior, but since the trades advect the surface waters westward, the upper layer of warm sea water is deeper in the west than in the east. Underneath this layer lies cold ocean water, and the Ekman pumping reaches sufficients depths in the east to bring some of this up to the surface.

As an aside, it’s amusing to note that in some early papers, the opposite of El Niño was described as the ‘anti-El Niño’ but given the religious connotations described above, this usage did not get a lot of support…

Why does ENSO arise?

The background conditions are essential for the existence of ENSO. The prevailing surface winds over the tropical Pacific blow from east-to-west (easterlies), and tend drive a surface current, pushing (advecting) the warm surface water westward. The winds are known as ‘trade winds’ and have played an important role in the world history in terms of the ship routes for sailing vessels. The western tropical Pacific is known as the ‘warm pool’ with the highest sea surface temperature (SST) in the world (on average). The trade winds ‘pile’ up water masses in the west, resulting in a slightly higher sea level in the west. The higher sea level near the western ocean boundary creates a west-east pressure difference in the ocean, that results in the equatorial undercurrent flowing from west to east below the surface.

Mechanisms:

The usual pattern is that an El Niño event gradually builds up between June and December, peaking around Christmas time (the reason for this seasonality is not yet fully understood). There are several explanations as to why there are fluctuations in the Walker circulations/trade winds, and the ocean currents. The different explanations do not exclude the others, and it is possible that more than one of these may take place. Here is a simple desciption of the main mechanisms:

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