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El Nino: Will it return in 2002? Five out of 12 models say yes,
meanwhile a
pulse of warm water is travelling towards the eastern Pacific. February, 2002 After the El Nino of 1997 everyone, it seems, learnt the meaning of this Pacific climate phenomenon. Not that EL Nino is unusual. Heck, the years 1990 through mid 1995 were weak El Nino years, but 1997 - like its bigger brother in 1982-83 - was a beauty. In fact, 1982-82 and 1997-98 were the biggest El Nino events since 1950. Every surfer in California and Hawaii got the message first hand as one mega wave event after another rolled out of the North Pacific. Sailors in the last Whitbread had to cope with high pressure balloons in unusual places and a remarkably calm Southern Ocean. Farmers in Australia's central west suffered drought - particularly in 1982-83 and less-so in 1997. And of course, coastal fishermen along the coast of Peru and Chile saw their livelihood evaporate for a time - as it has done over and over again since time immemorial. As cold, nutrient-rich deep-ocean was replaced by warm surface, and subsurface water from the western Pacific, the food which their fish preyed upon disappeared. This cold-water/warm-water relationship is crucial to the El Nino phenomenon. Many models used to predict this regular climate shift combine sea temperature data with atmospheric information to figure out what's going on. Enter the Kelvin Wave What triggers an El Nino? Like everything in weather, nothing is cut and dried. But one event which is known to swing the Pacific's climate balance towards a 'warm phase' is a burst of westerly winds along the equator in the Western Pacific. These strong and continuous bursts of wind, pushing against the 'normal' east-west equatorial current, creates a wave of warmer surface water which can travel all the way across the ocean, affecting temperatures well below the surface. Oceanographers call this a 'Kelvin Wave' and when all that warm water arrives in a corner of the Pacific typically dominated by cold deep-ocean water, shit happens! The 1997-98 El Nino was certainly triggered by a Kelvin Wave, which had its roots in December 1996 when Typhoon Fern drove westerly winds along the equator. This wave was deeply enhanced during March 1997, when Cyclone Justin camped in the Coral Sea for three weeks driving 30 knots and more of westerlies along its northern boundary. Justin was a big system that just wouldn't go away, and the warm water it pushed to the east helped to create the second largest El Nino since 1950. In December 2001, a broad trough created two tropical lows on either side of the equator, driving westerly winds from Papua New Guinea eastwards. While the low south of the equator never really fired up, Typhoon Podul did, contributing to the creating of the Kelvin wave mentioned above. Another typhoon, Waka, did the same along the equator near Tonga in mid-December and now, in late January, meteorologists are seeing sea surface and sub-surface temperatures in the mid-Pacific rise abnormally by more than one degree Celsius. Will Australia's cyclone season produce another westerly wind burst to reinforce this Kelvin Wave? Will the existing wave trigger a weak El Nino? Key indicators The key indicator for an El Nino, or its opposite La Nina, is the Southern Oscillation Index (SOI) which measures differences in barometric pressure between Darwin and Tahiti. For the last three months this index has swing wildly from 11 points on the positive scale - indicating La Nina - to nine points in the negative - indicating El Nino. A more refined tool is the multivariate ENSO index is created by including SOI information while at the same time noting the differences in sea surface temperatures across the Pacific. What results is a jagged graph showing wetter and drier than usual seasons with more accuracy than by using SOI information alone. However, this key is somewhat confusing because wet years, or La Nina, are recorded as negative values and El Nino as positive. This is exactly opposite to the scale used to measure SOI. What will the future hold? What does it mean? Too early to tell, but one thing is for certain - this current climatic 'season' is one of the longest periods of neutral conditions since 1950. And during this time we have seen plenty of unusual weather: Record heat and record wind gusts in parts of Australia, the worst North Atlantic storms in 34 years, unusual anomolies in synoptic patterns which would have been attributed to either El Nino or La Nina had they been present. According to the Climate Prediction Centre, five out of 12 climate models are indicating an El Nino during 2002. However, the Australian Bureau of Meteorology is non-committal, saying the evidence is too thin to make a forecast. El Nino's typically evolve from April to November, so keep an eye on the weather and wait. © Jamie Brown/weather-wise.com 2000-02 More great reading El Nino forecast forum with the Climate Prediction Centre, the National Centres for Environmental Prediction and the National Oceanic and Atmospheric Agency. An alternative El Nino discussion with the Australian Bureau of Meteorology El Nino discussion with NOAA-CIRES Climate Diagnostics Centre |