El Niño and Global Warming

It is important not to confuse the Walker Circulation with the Hadley Circulation (also known as the ‘Hadley Cell’), which also involves deep convection in the tropics. Whereas the Walker Circulation (or ‘Walker Cell’) refers to an air flow parallel with the equator – all in the tropics – the Hadley Cell involves air rising in the tropics (follows the solar equator and gives rise to the Inter-Tropical Convergence Zone (ITCZ) which then flows polewards before sinking in the subtropics. The Walker Circulation involves an east-west asymmetry, whereas the Hadley Cell in principle does not.

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?

Page 2 of 4 | Previous page | Next page