La Nina event strongest since 1974 Wet seasons inundate Mozambique and Australia Rain, rain. Will it ever go away? For the last two years it seems our planet has been inundated with one record flood event followed by another. In the news of late Mozambique and central Australia have borne the brunt of super-strong wet seasons, with disastrous results. What the heck is going on? Of course, the big news events are only a part of this weather story, because La Nina effects our climate in many more subtle ways. For the last two cruising seasons in Queensland, Australia, yachts sailing south after a winter in the tropics were hampered by headwinds when they expected a downhill slide. Those making winter crossings from the east coast to the Pacific islands were dogged by storms in the Coral Sea and sailors in the last two Sydney to Hobart races were clobbered by the best that Bass Strait could dish out. During the Austral summer, 1999, rain soaked depressions and cyclones in Queensland caused extensive flooding along the Capricorn Coast while intense low pressure cells off the NSW coast created beach erosion not seen for 20 years. Of course, Australia fared fine when compared to the likes of Central America and Asia where constant drenching and poor development control created natural disasters of epic proportions. What's going on? For starters, 1998 was the warmest year on a global average since records began last century. According to Australian Weather Bureau meteorologists Scott Power and Jeff Callaghan. It was also Australia's warmest year on record. And, they say, British researchers found recent decades to be the warmest of the last millennium based on data gleaned for coral cores. Blame La Nina Certainly global warming is happening, but did it create the difficult sailing conditions, the steady beach erosion and the terrible overseas disasters of last year? According to Dr Scott Power, meteorologist with the Bureau of Meteorology's National Climate Centre in Melbourne, global warming is only a background factor in the equation. What is happening up front involves fluctuating Pacific Ocean temperatures and a weather event called La Nina. Spanish for 'Girl Child', La Nina is the opposite of El Nino and its multi-month cycle brings wet and cool weather to Eastern Australia.
Long-term factor Complementing both La Nina and its opposite El Nino is another, even longer-term factor in the current weather equation. Mr Power says there is evidence emerging that there may be 'something' in the background that stretches over decades rather than years and that these much slower changes may disrupt both La Nina weather patterns and their opposite, El Nino. These long-term, background events are being heralded as newly discovered seasons on a grander time scale. The meteorological professionals prefer the title: Interdecadal Pacific Oscillation. If you look below at the jagged graph representing Multivariate ENSO (El Nino Southern Oscilation) figures since 1950 you will notice it tends to dip into negative figures (indicating a La Nina, or wet period) between 1950 and mid 1970 and rises into positive figures (indicating an El Nino, or dry period) from the mid 1970s through the early 1990s. One wetter-than-normal period was followed by a drier and warmer-than-normal period. (Click on the image for a better view.)The latest La Nina appears to have reached maturity and could even be turning around - which means that all this rain and associated storm activity may have peaked. However, Jeff Callaghan, meteorologist with the Bureau's severe storm centre in Brisbane, reckons that in the medium to long term, we may be in for more La Nina episodes than we had over the last 20 years. US hurricane expert, Bill Gray of Colorado University, agrees. Having said that, British researchers argue exactly the opposite. "This is why the science is still in the research phase," notes Scott Power. What can we do? For people who play outdoors, all this means keeping an eye to weather. Scientists from New Zealand's National Institute of Water and Atmospheric Research (NIWA), say waters around New Caledonia, the Solomon Islands and Vanuatu are more likely to experience cyclones during La Nina years. Fiji, Tonga, Cook Islands and Tahiti, however, should have fewer than average
tropical cyclones. Jeff Callaghan elaborates by pointing out that stronger than normal easterlies along the equator during La Nina and a weaker than usual westerly jet stream over tropical Queensland means middle-level winds are able to help steer cyclones toward our coast. A typical situation is this: An upper trough lies stationary over central Australia in a blocking pattern, while a high sits in the Tasman. To the north of the high a cyclone develops and the middle level winds in the counter-clockwise spinning high then tend to steer that cyclone toward our shores. East Coast lows, meanwhile, tend to form within the cloud bands associated with stalled troughs and migrate to the south east. During warm and dry El Nino years, the threat moves offshore. Troughs and their cloud bands tend to stall well east of Australia, dumping rain over central Polynesia but preventing cyclones and low pressure cells from coming towards Australia. Help yourself Look at Australian cloud satellite images - taken every six hours - and compare them with the weather map for the same period. If you're keen, insert sequential cloud satellite images into a GIF animation program (available as shareware on the Internet - try searching for GIF animator) and see for yourself how clouds move and develop over Australia. © weather-wise.com 2000 |