El Niño and Global Warming

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:

-Delayed Oscillator: Adjustments in the oceans as a whole, such as response to changes in the surface wind stress, happens through wave propagation. Ocean serves as the memory whereby slow oceanic Rossby waves and Kelvin waves propagate through the basin and affect the depth of the oceanic surface layer of warm water. Through waves, wind forcing in one location may have an effect on a remote location. Kelvin waves can travel eastward along the equator and poleward on the coasts along the eastern boundary of the ocean basins, but not in the ocean interior. Rossby waves, however, can travel westward and away from the equator and the coasts. Model results have shown that these waves can account for some of the irregular appearances of ENSO.

-Coupled Mode: The notion involves unstable air-sea coupling, where a local region deep convection enhances the trades to the east and reduces the surface winds to the west. These local wind alterations produce changes in the Ekman pumping.

-Noise forced and volcanoes: The idea is that weather events kick off El Niños due to unstable conditions. The system is ‘charged up’, and El Niño events are then initiated by a ‘kick’ (eg the Madden-Julian Oscillation or westerly wind bursts).

What are the effects of ENSO?

Although ENSO represent ‘modest’ shifts in the weather patterns world-wide, El Niño events may, according to the ‘Report to the Nation’, have dire social and economical consequences. Increased rain in the Andean states due to El Niño can have devastating effects, as can the increase in corresponding increase in drought in Indoneasia – and this can lead to big increases in forest fires and air pollution. There is also a clear statistical relationship between El Niños and hurricanes/typhoons. Whereas the number of hurricanes in the Atlantic basin tend to drop (~50%) during El Niño years, the number of typhoons tend to increase in the Pacific. Drought in the American West is associated with La Niña conditions. The effects can also be felt as far away as Antarctica where a ‘dipole’ of sea level pressure between the Bellinghausen and Weddell Seas is highly correlated to the ENSO phase and can have important effects on sea ice and Antarctic ecology.

When will next El Niño take place?

This is always the exciting question around June. After the notorious spring barrier the predictive skill of ENSO models increase, and our confidence in the ENSO prognoses are higher. The BOM provides a nice summary of a number of different model prognoses. At the moment, there are no indication of an El Nino for the end of this year.

How will the El Niño phenomenon be affected by a global warming?

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