Figure 2
Source: www.geog.ucsb.edu/~joel/g110_w05/lecture_notes/general_circulation/agburt08_04a.jpg
Surface ocean currents, those above the thermocline, are generally wind driven and the Walker circulation has a number of important oceanic impacts. The easterly trade winds of the Walker circulation cause a movement of surface water westerly along the Pacific. This leads to a build of warm water at the western end of the equatorial Pacific around Indonesia. Barley and Chorley (1998) identified this pool of warm water as the world’s warmest at 28°C due to the intense insolation of the area and retention of latent heat of evaporation in this region due to light winds.
These easterly trade winds result in a 60cm (Wikipedia, 2005) increase in water height in the western Pacific and there is an even more marked increase in warm below the surface where the thermocline can reach depths of inexcess of 200m (Barley and Chorley, 1998). While water height in the eastern Pacific does not decline by an equal proportion the depth of the thermocline is significant reduced to around 40m.
Figure 3
Source: Ahrens, 1999
This reduction of water and decline of depth of the thermocline off the South American coast allows for upwelling of the Humboldt (also known as the Peru) current. Upwelling is the process of cold water moving from below the thermocline mixing with the surface water.
The Humboldt current is a cold nutrient rich ocean current that flows northward off the west coast of South America and it becomes a major part of the westward South Equatorial Current. The Humboldt current originates in the Southern ocean near the Antarctic, and is thus about 7-8 °C cooler than the ocean at similar latitudes. Figure 3 shows both the Humboldt current (17) and the south equatorial current (9) and its associated counter currents (8&10).
During a major El Niño event many of the predominant major atmospheric and oceanic processes of the Pacific described above significantly transform. The following diagrams and summaries below provide an overview of these Pacific systems that occur during non-El Niño, El Niño and El Nina years.
Figure 4
Source: http://www.pmel.noaa.gov/tao/proj_over/diagrams/index.html
The Walker circulation is the main factor in maintaining non El Niño conditions and when this circulation weakens a number of different effects are found. If the pressure gradient of the Pacific ocean weakens or reverses then the easterly trade winds also weaken or transpose into westerly winds. This allows the warm water which has built up and been maintained in the western Pacific to equalise across the ocean.
If the easterly trade winds weaken sufficiently or reversed then a surge known as a Kelvin wave (Ahrens, 1999) can form. This wave which can stretch for hundreds of kilometres north and south of the equator can return vast quantities of warm water to the Peruvian coast. If the prevailing trade winds weaken for long enough then the pool of warm water normally found around Indonesia will collect along the equator off the Peruvian coast as seen in the classic sea surface temperature chart in figure 1.
This movement of warm has a number of significant impacts. Firstly the location of rising air moves from the western Pacific towards the central Pacific and this has huge impact on global weather systems. The thermocline depth equalised across the Pacific which reduce the amount of upwelling of cold nutrient rich water from the Humboldt current which has a number of ecological effects.
Figure 5
Source: NOAA, 2005
One of the major impacts of El Niño events is the change in global weather systems. Figure 5 shows the changes in climate around the world. During a strong El Niño event may trigger a response in nearly of the indicated areas while a weak event will only effect some areas. In brief during the early part of an El Niño event as the warm pool of water moves eastwards from it normal position it leaves the western Pacific warmer and dryer. Increased rainfall occurs across the central and eastern Pacific ocean.
Other areas receiving driers weather are northern South America, Madagascar and SE Africa and northern India. Areas experiencing above average temperatures are Japan, Alaska and NW Canada and USA and the eastern seaboard of Canada. Finally those areas along with the central pacific which are wetter include Peruvian and Argentinean coast, SE USA, Rocky mountains, East Africa and Southern India and Sri Lanka (Ahrens, 1999).
The above highlights the wide ranging climate effects of El Niño but it should be noted that the impacts are highly seasonally. For example the Peruvian coast is wetter during November to March while Southern India and Sri Lanka received additional precipitation between October and December.
Caviedes (2001) noted a number of impacts of these changes in weather which can be extreme resulting in major flooding in Peru and Ecuador. The drier conditions of Indonesia, Philippines and northern Australia can lead to major forest fires. While there can be a greater occurrence of ice around Antarctica.
The major non-climate effect cited is to the changes of fish populations off the South American coast (D’Aleo, 2002). As the upwelling of the Humboldt current is suppressed, which normally provide nutrient rich water that can sustain large fish populations, fish stocks dwindle which has huge economic impacts the communities of the Peruvian and Ecuadorian coasts. Also noted is the decline of sea bird due to the reduce fish stocks and this in turn leads to a reduction in guano, bird dropping which are used as a fertiliser.
The Southern Oscillation Index has been developed for monitoring and as an attempt to help predict the occurrences of El Niño events. The index is multivariable and includes six different data sets including air temperature, surface water temperature, sea level pressure, cloudiness and wind speed and direction. Figure 6 shows the data collected between 1876 and 1998 a period o over 120 years it clearly illustrates the frequency of El Niño and La Nina events but also shows the intensity and duration of each event.
Figure 6
Source: NOAA, 2005
The mechanisms which might cause an El Niño event are still being researched and it is difficult to find patterns which may show causes or allow forecasts. Global warming and increasing carbon dioxide levels in particular are creating additional difficulties in determining the causes of El Niño. NOAA (2005) has identified global warming, though there is no clear consensus, as contributing to increases in the frequency of El Niño events. Wang (2001) while reviewing the triggers to El Niño events identified the following major theories:
- Delayed Oscillator – westerly winds force downwelling on the equator and upwelling to the north and south which creates Kelvin and Rossby wave – the theory looks at the reflections of these waves as delayed triggers to El Niño.
- Recharge/Discharge Theory – this theory is based on the warm pool of the western pacific and proposes that where heat energy builds up then has to be dispersed to higher latitudes by an El Niño event. After the release of energy the area than has to recharge before another event can take place.
- Western Pacific Oscillator -. emphasises the effect winds outside of the equatorial zone have on the Walker circulation and how small anomalies could trigger an El Niño event.
- Advective-Reflective Oscillator - emphasises the importance of regional advections associated with wave reflection at both the western and eastern boundaries
- Unified Oscillator – current theory for the triggers of El Niño events that incorporates the four theories above and is based upon chaos theory.
El Niño is a highly complex and multi-factorial climate event of the equatorial Pacific ocean that has far reaching global impacts particularly in terms of disruption to normal climate patterns. These disruption to weather can be hugely damaging in terms of human life and economic cost. While the underlying principles and processes of an El Niño event are now well understood in terms of the changes in atmospheric and oceanic systems the mechanism which triggers an El Niño event are still not fully understood nor are the impacts of global warming.
Bibliography:
Ahrens, C.D. (1999) Meteorology Today: An Introduction to Weather, Climate, and the Environment. 6th edn. California: Brooks/Cole
Barry, G.B. and Chorley, R.J. (1998) Atmosphere, Weather and Climate. 7th edn. London: Routledge
Caviedes, C. (2001) El Niño in history: storming through the ages. Florida: University Press of Florida
D’Aleo, J.S. (2002) The Oryx resource guide to El Niño and La Nina. Connecticut: Oryx Press
NOAA (2005) Available at: http://www.elnino.noaa.gov/. (Accessed: November 2005)
Philander, S.G. (1990) El Niño, La Nina, and the southern oscillation. San Diego: Academic Press
Wikipedia (2005) Available at: http://en.wikipedia.org/wiki/El_nino/. (Accessed: November 2005)
Wang, B (2001) ‘Transition From a Cold to a Warm State of the El Niño-Southern Oscillation Cycle’. Metrological Atmospheric Physics, 56 pp17-32
