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La Nina no more Seven out of eight climate models agree: Weird weather world-wide will abate and normality will resume ... eventually! July 2000 If you live in Australia, expect the big wet to dry out - unless you live in Tasmania or south-eastern Victoria. If you live in Mozambique, expect less wet this coming wet season. Southern Brazil might get some rain, as should the midwestern United States. And the Atlantic hurricane season should ease up on what it has been for the past two years. Whoa! Easy does it But don't expect this change in the weather for a while. While climate models are almost unanimous in their prediction of normal climate conditions through to March of 2001, signs on the ground still point to a La Nina, despite Southern Oscillation Index and ENSO figures showing neutral values since June. Hurricane forecaster Dr Bill Gray has declared the 2000 Atlantic basin hurricane season will be much like last year, because "the existing La Niña has weakened but is holding strongly in the Central Pacific. (and) there has never been a case where the La Niña has remained as cold as at present and an El Niño has then developed before the season's over." (El Nino tends to inhibit hurricane development). While trade winds in the Western Pacific have eased up, a sign of a La Nina phase in decline, other climate indicators aren't so co-operative. Vernon Kousky, of the National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center, has this to say: "La Nina does appear to be weaker this year than it was a year ago. The ENSO or Nino indices are weaker, reflecting smaller negative sea surface temperature anomalies. We also monitor the subsurface ocean temperatures using an array of moored bouys located in the tropical Pacific. Subsurface temperatures have warmed throughout the eastern equatorial Pacific, indicating a return towards normal. In spite of these indications, many atmospheric features continue to reflect La Nina conditions. The monsoons of Southeast Asia/ India and Mexico/ Southwest US are stronger than normal, which is typical of La Nina. "Our research has indicated a general lack of consistent US impacts from either La Nina or El Nino during the summer months. However, on average during a La Nina summer there is enhanced upper-level ridging over the mid-latitudes of the Northern Hemisphere. "The lack of a consistent signal for any specific region of the US is a result of variability in where the enhanced upper-level ridges actually develop. "Last year the ridge developed over the center of the US, resulting in very dry conditions in many sections of the Midwest and even east of there to the Mid-Atlantic states. This year the ridge is farther west, and the eastern half of the nation has experienced frequent cold frontal passages that have served to kick off showers and thundershowers and have brought cooler temperatures to the region. "These year-to-year differences are not related to La Nina, but are what we call natural variability. Because of the existing La Nina we were confident that a strong upper-level ridge would develop somewhere between the West Coast and the Midwest, but knowing exactly where the ridge would develop was not possible in advance." Take heed of models Richard Kleeman, Associate Professor at the Courant Institute for Mathematical Sciences, New York University, adds to Kouskys observations: "My experience with operational forecasting (around 7 years) tells me that the degree of consistency amongst (climate) models should be taken very seriously. The La Nina does seem to be hanging on in the atmosphere however. Another example is Australia where rainfall over the North and east of the country has been well above normal for the past 3 months. A possible reason for this inconsistency lies in decadal (longer term variability) which operates in a somewhat different area of the ocean to El Nino/La Nina (the central rather than eastern Pacific). This has shown signs in recent years of entering a cool phase which would be associated with the atmospheric anomalies mentioned by Vernon and the Australian anomalies mentioned above. " Useful links: Australian Bureau of Meteorology has compiled the eight recognised climate prediction models, with links to each forecast and an explanation of how the models are created. NOAA's Climate Prediction Centre has done the same. The Climate Prediction Centre also issues regular La Nina and El Nino advisory bulletins. This El Nino / Southern Oscillation (ENSO) chart shows changes in Pacific sea surface temperatures at various locations along the equator. Positive values, coloured orange, indicate a warm period - or El Nino, and negative values - coloured blue - indicate a cool phase, or La Nina. NOTE: A La Nina under ENSO is shown as a negative value, while the SOI, or Southern Oscillation Index (based on atmospheric pressure differences rather than sea surface temperatures) shows La Nina as a positive value. Confusing, isn't it? Dr Bill Gray's hurricane prediction page. Gray is a legend when it comes to hurricane forecasting. And to think he does this from a little office at Colorado State University! Intellicast's Dr Dewpoint has an interesting essay on the effects of La Nina on the continental US. © weather-wise.com 2000 | Did you know? The decay of La Nina in the first six months of this year is almost identical to the first six months of 1999. One year ago, easing Pacific trade winds (low level wind anomolies, in science speak) changing sea surface temperatures along the equator and atmospheric pressure differences between Darwin and Tahiti - all tools to help create SOI and ENSO indexes - showed La Nina in decline. This year, however, statistical models are almost unanimous in their prediction of further decay and a return to normal, or neutral conditions in the Pacific. The Climate Prediction Centre has a colourful graph showing the changing wind anomolies and atmosphereic pressure anomolies between July 1997 ands April 2000.
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