El Niño and La Niña can impact rainfall across different seasons. These maps show the probabilities of above normal rainfall across the El Niño weather cycle, and for each season.
El Niño and climate forecasting
On this page
- What is El Niño?
- El Niño’s effect on NZ
- 1997/98’s El Niño and NZ
- Climate Forecasting
- Some Useful References
El Niño is a natural feature of the global climate system. Originally it was the name given to the periodic development of unusually warm ocean waters along the tropical South American coast and out along the Equator to the dateline, but now it is more generally used to describe the whole "El Niño - Southern Oscillation (ENSO) phenomenon", the major systematic global climate fluctuation that occurs at the time of the ocean warming event. El Niño and La Niña refer to opposite extremes of the ENSO cycle, when major changes in the Pacific atmospheric and oceanic circulation occur.
When neither El Niño nor La Niña are present, (usually referred to as "neutral" or normal conditions), trade winds blow westward across the Pacific, piling up warm surface water so that Indonesian sea levels are about 50 cm higher than those in Ecuador. Cool, nutrient-rich sea water "wells up" off the South American coast, supporting marine ecosystems and fisheries. Relatively cold sea temperatures also extend along the equator from South America towards the central Pacific. High rainfall occurs in the rising air over the warmest water to the west, whereas the colder east Pacific is relatively dry.
During El Niño, the trade winds weaken, leading to a rise in sea surface temperature in the eastern equatorial Pacific and a reduction of upwelling off South America. Heavy rainfall and flooding occur over Peru, and drought over Indonesia and Australia. The supplies of nutrient rich water off the South American coast are cut off due to the reduced upwelling, adversely affecting fisheries in that region. In the tropical South Pacific the pattern of occurrence of tropical cyclones shifts eastward, so there are more cyclones than normal in areas such as the Cook Islands and French Polynesia.
During La Niña events, the trade winds strengthen, and the pattern is a more intense version of the "normal conditions", with an even colder tongue of sea surface temperatures in the eastern equatorial Pacific.
The Southern Oscillation Index (SOI) is calculated from the pressure difference between Tahiti and Darwin. Anomalously low values of this index correspond to El Niño conditions, while the opposite conditions with an anomalously high SOI value are called La Niña episodes. El Niño events occur about 3 to 7 years apart, typically becoming established around April or May and persisting for about a year thereafter.
Some researchers have suggested the frequency (and perhaps intensity) of El Niños will be affected by climate change resulting from continuing enhanced anthropogenic greenhouse gas emissions. However, there is as yet no scientific consensus on this issue, or on whether the relatively higher frequency of El Niños over the last two decades is related to the slow rise in global temperatures this century. This is discussed further on our page about global climate models.
How does El Niño typically affect New Zealand?
New Zealand is not usually affected as strongly by El Niño conditions as are parts of Australia, but there is nevertheless a significant influence. In El Niño years, New Zealand tends to experience stronger or more frequent winds from the west in summer, leading to drought in east coast areas and more rain in the west. In winter, the winds tend to be more from the south, bringing colder conditions to both the land and the surrounding ocean. In spring and autumn southwesterlies tend to be stronger or more frequent, providing a mix of the summer and winter effects.
La Niña events which occur at the opposite extreme of the Southern Oscillation Index cycle have weaker impacts on New Zealand’s climate. New Zealand tends to experience more northeasterly winds, which bring more moist, rainy conditions to the northeast parts of the North Island, and reduced rainfall to the south and south-west of the South Island. Therefore, some areas, such as central Otago and South Canterbury, can experience drought in both El Niño and La Niña. Warmer than normal temperatures typically occur over much of the country during La Niña, although there are regional and seasonal exceptions. The last La Niña event was in 2007/08.
Although El Niño has an important influence on New Zealand’s climate, it accounts for less than 25% of the year to year variance in seasonal rainfall and temperature at most New Zealand measurement sites. East coast droughts may be common during El Niños, but they can also happen in non El Niño years (for example, the severe 1988-89 drought). Serious east coast droughts do not occur in every El Niño, and the districts where droughts occur can vary from one El Niño to another (although some are more consistently affected than others). However the probabilities of the climate variations discussed above happening in association with El Niño are sufficient to warrant management actions and planning to be taken when an El Niño episode is expected or in progress.
Brett Mullan of NIWA has published a useful summary of the typical effects of El Niño on New Zealand and the South Pacific in the proceedings of a 1996 Royal Society workshop on prospects for climate forecasting. Al McKerchar and Charles Pearson published a paper on relationships between the SOI and summer inflows to South Island hydroelectricity storage lakes in the same proceedings (see references below).
Most of the material in this section is summarised from the report: The 1997/98 El Niño event – Impacts, Responses and Outlook for New Zealand. This was compiled by Dr Reid Basher of NIWA, with assistance from staff of the Ministry of Agriculture and Fisheries and from other CRIs (see references below).
A major El Niño event developed rapidly over the second quarter of 1997, and has been cited as the cause of many climatic disasters around the world. These have included:
- severe drought and food shortages in Papua New Guinea,
- dry conditions favouring large-scale uncontrolled wildfires in Indonesian tropical forests,
- more tropical cyclones than normal in the South Pacific during the October 1997 – April 1998 tropical cyclone season, with a shift in occurrence towards eastern islands such as the Cook Islands and French Polynesia
- drought and fires in Venezuela, French Guyana and northern Brazil
- severe flooding in Ecuador, Peru and central Chile
- widespread bushfires in New South Wales (Australia).
In New Zealand, it was much drier than normal in the east from July 1997 onwards – the figure below shows the extent of drought areas from Canterbury through to the Bay of Plenty for the summer of 1997/98. In late April 1998 the Ministry of Agriculture and Forestry estimated the likely cost of the drought on farm gate returns would be $256 million for the year ending June 30 1998, and $169 million for the following year, giving a total of $425 million. Given the impact on downstream value-added agricultural production, the likely total cost to the country is likely to be in excess of NZ$1 billion.
Not all of the unusual New Zealand conditions can necessarily be blamed on El Niño. For example the 1997/98 summer was very warm, culminating in the exceptionally warm month of February. This is not typical of El Niño summers.
Links to El Niño forecasts from a number of institutions are available on the internet through the El Niño theme page.
In mid 1998 the atmosphere moved back into a La Niña phase, with a high positive value for the Southern Oscillation Index (see the figure earlier on this page showing the SOI time series).
Ordinary weather forecasts of conditions for a particular day cannot be usefully extended beyond about a week, because of the chaotic nature of the atmosphere. However it is now becoming clear that in some regions and under some existing climate conditions (e.g. during an El Niño), useful predictions can be made of the likelihood of particular climatic conditions in a future month or season. (Such predictions might indicate, for example, whether the season is likely to be warmer, colder, wetter or drier than average). The reasons for this are:
- research on past events shows that El Niño events tend to follow similar patterns of development and decay. Thus once they have commenced, their evolution is partly predictable. The same is true for La Niña events.
- computer models have been developed which simulate the coupled ocean-atmosphere dynamics of the central Pacific. Some of these models have been reasonably successful in forecasting the onset, development and breakdown of El Niño events.
- once an El Niño or La Niña event gets underway, its development can be tracked from the Tropical Atmosphere Ocean (TAO) array of buoys in the tropical Pacific, and using sea surface temperature and altitude data sensed remotely by satellites. These measurements assist with the forecasting of such events.
- local climate anomalies can then be predicted based on statistical links exhibited previously with key indicators of ENSO - for example the typical New Zealand seasonal temperature and rainfall anomalies discussed earlier on this page.
Thus at least during La Niña and El Niño conditions, useful seasonal outlooks are possible for some regions of New Zealand. Such outlooks are prepared through a telephone conference convened monthly by NIWA scientists, with input also from the Meteorological Service. After such conferences, NIWA puts out media releases regarding El Niño and seasonal weather outlooks.
NIWA scientists are researching other factors as well as El Niño which influence New Zealand’s climate. For example there are other modes of atmospheric circulation as well as El Niño which may be at least partially predictable, and major volcanic eruptions (such as Mt Pinatubo) often lead to reduced New Zealand temperatures.
Research led by Dr John Kidson and Dr Xiaogu Zheng (see references below) suggest that about half of the variability in New Zealand seasonal temperatures, and up to 30% of the variability in New Zealand seasonal rainfall, might potentially be predictable. The remaining variability (50% for temperature, and 70% for rainfall) is inherently unpredictable since it arises from the random, chaotic nature of the atmosphere. The scientific methods and knowledge available to us today allow us to reach around half of the potential predictability.
Basher, R.E. (1998). The 1997/98 El Niño Event: Impacts, responses and outlook for New Zealand. Report No. 73. Ministry of Research, Science and Technology, Wellington. 28 p. http://www.morst.govt.nz/publications/elnino/index.htm
Bhaskaran, B., and A. B. Mullan, 2003: El Nino-related variations in the southern Pacific atmospheric circulation: model versus observations. Climate Dynamics, 20, 229-239.
Kidson, J.W. (1996). The potential predictability of seasonal temperature and precipitation in New Zealand. In: Braddock, D. (ed.) Prospects and needs for climate forecasting. Miscellaneous Series 34, pp. 45–48. Royal Society of New Zealand, Wellington.
Kidson, J. W., and J. A. Renwick, 2002: Patterns of convection in the tropical Pacific and their influence on New Zealand weather. Int. J. Climatol., 22, 151-174.
Kidson, J. W., and J. A. Renwick, 2002: The Southern Hemisphere evolution of ENSO during 1981-1999. J. Climate, 15, 847-863.
Madden, R. A., and J. W. Kidson, 1997: The potential long range predictability of temperature over New Zealand. Int. J. Climatol., 17, 483-495
McKerchar, A.I., and Pearson, C.P., 1996: The spring Southern Oscillation Index conditions summer lake inflow probabilities, South Island. In: Braddock, D. (ed.) Prospects and needs for climate forecasting. Miscellaneous Series 34, pp. 33–34. Royal Society of New Zealand, Wellington.
Mullan, B., 1996: Effects of ENSO on New Zealand and the South Pacific. In: Braddock, D. (ed.) Prospects and needs for climate forecasting. Miscellaneous Series 34, pp. 23–27. Royal Society of New Zealand, Wellington.
Mullan, A. B., and C. S. Thompson, 2006: Analogue forecasting of New Zealand climate anomalies. Int. J. Climatol., 26, 485-504.
Philander, S.G.H. (1990). El Nino, La Nina and the Southern Oscillation. Academic Press, San Diego, CA. 289 p.
Tait, A. B., J. A. Renwick, and A. H. Stroombergen, 2005: The economic implications of climate-induced variations in milk production. New Zealand Journal of Agricultural Research, 48, 213-225.
University Corporation for Atmospheric Research (1997). Reports to the Nation – El Niño and Climate Prediction. http://www.pmel.noaa.gov/toga-tao/el-nino-report.html
Velde, M. (1998). El Niño – “The Boy Child”. Gamma Series. Royal Society of New Zealand, Wellington. 4 p.
Zheng, X., H. Nakamura, and J. A. Renwick, 2000: Potential predictability of seasonal means based on monthly time series of meteorological variables. J. Climate, 13, 2591-2604.
Zheng, X., and J. A. Renwick, 2001: Estimating interannual variability arising from weather events. Journal of Applied Probability, 38A, 285-299.
Zheng, X., and J. A. Renwick, 2003: A regression-based scheme for seasonal forecasting of New Zealand temperature. J. Climate, 16, 1843-1853.
Zheng, X. G., M. Sugi, and C. S. Frederiksen, 2004: Interannual variability and predictability in an ensemble of climate simulations with the MRI-JMA AGCM. Journal of the Meteorological Society of Japan, 82, 1-18.
Zheng, X., and C. S. Frederiksen, 2006: A study of predictable patterns for seasonal forecasting of New Zealand rainfall. J. Climate, 19, 3320-3333.
Prepared by David Wratt, Reid Basher, Brett Mullan and Jim Renwick