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Neville Nicholls

A number of studies in meteorological journals have documented some of the constraints to the effective use of climate forecasts. One major constraint, the considerable difficulty people have in estimating and dealing with probabilities, risk, and uncertainty, has received relatively little attention in the climate field. Some of this difficulty arises from problems known as cognitive illusions or biases. These illusions, and ways to avoid them impacting on decision making, have been studied in the fields of law, medicine, and business. The relevance of some of these illusions to climate prediction is discussed here. The optimal use of climate predictions requires providers of forecasts to understand these difficulties and to make adjustments for them in the way forecasts are prepared and disseminated.

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Neville Nicholls

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Neville Nicholls

Abstract

The absence of an upward trend in normalized building damage in Australian bushfires may reflect reduced vulnerability (due to improved weather forecasts and other factors) offsetting increases in the frequency or intensity of bushfires.

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Neville Nicholls

Abstract

A rotated principal component analysis of Australian winter (June–August) rainfall revealed two large-scale patterns of variation which together accounted for more than half of the total rainfall variance. The first pattern was a broadband stretching from the northwest to the southeast corners of the country. The second was centered in the eastern third of the continent. The two patterns were correlated to sea surface temperatures in the Indian and Pacific oceans. The first rainfall pattern was best related to the difference in sea temperatures between the Indonesian region and the central Indian Ocean. The second rainfall pattern was related to equatorial Pacific sea surface temperatures. This relationship reflects the influence of the Southern Oscillation on both sea surface temperatures and Australian rainfall but the relationship between the first rainfall pattern and the difference between Indonesian and central Indian Ocean sea surface temperatures is largely independent of the Southern Oscillation. This sea surface temperature difference may be another factor influencing Australian rainfall, some-what separate from the well-known effect of the Southern Oscillation.

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Neville Nicholls

The El Nino-Southern Oscillation (ENSO) phenomenon affects the atmosphere and ocean over much of the globe. The resultant atmospheric and oceanic anomalies can produce a variety of biological and societal impacts. Three examples of impacts that may be predictable by monitoring simple indices of ENSO are discussed. The advantages and disadvantages of such “direct” prediction of impacts are considered.

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Neville Nicholls

Abstract

Evidence is presented supporting the hypothesis (first expressed over 60 years ago) that interannual fluctuations of early wet season rainfall in the Indonesian Archipelago can be successfully predicted from prior observations of atmospheric pressure anomalies. It is shown that this predictability is related to sea surface temperature anomalies. The postulated mechanism for this predictability is interaction of the atmosphere and ocean leading to a tendency for anomalies in the two media to persist. Experiments to testthis postulate are suggested.

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Neville Nicholls

Abstract

Solutions to a set of differential equations representing a system of postulated interactions between the ocean and the atmosphere can reproduce certain hitherto unexplained aspects of atmospheric and oceanic behavior in the Indonesia-North Australia region. These solutions also represent a biennial oscillation and it is concluded that the postulated air-sea interaction could be the source of the tropospheric quasi-biennial oscillation.

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Neville Nicholls

Abstract

The number of tropical cyclones observed in the Australian region during a single cyclone season has ranged from one to nineteen since 1909. Previous studies, using limited data sets, have suggested that interannual variations in the number of cyclones are related to the Southern Oscillation and that an index of the Southern Oscillation (e.g., Darwin pressure) can be used to predict the number of cyclones expected in the coming season. This study uses a 74 year time series of tropical cyclone numbers, from the 1909/10 season to the 1982/83 season to confirm this. Strong and stable correlations are found between cyclone numbers and Darwin pressures before and during the cyclone season. Even stronger relationships are found between Darwin pressure and the number of cyclone days in a cyclone season. The correlations are strong and stable enough to allow prediction of seasonal cyclone activity from several months prior to the start of the tropical cyclone season. A simple equation for predicting seasonal cyclone activity is derived.

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Neville Nicholls

Abstract

The relationship between Indian summer (June–September) monsoon rainfall and sea surface temperatures around northern Australia–Indonesia has been explored using data from 1949 to 1991. Warm sea surface temperatures are generally associated with a good monsoon; a poor monsoon is usually accompanied and preceded by low sea surface temperatures. This finding confirms, on independent data, a suggestion made a decade ago. This study also confirms a relationship between changes in Darwin pressure and Indian monsoon rainfall. Thew two relationships appear to provide a method for predicting Indian summer monsoon rainfall a month or two before the onset of the monsoon season. Two predictors (April sea surface temperatures and the change in Darwin pressure from January to April) together account for about 50% of the variance in Indian monsoon rainfall if the data are adjusted to remove possible artificial trends in the ocean temperatures. The northern Australia–Indonesia region is clearly an important component in the large-scale interaction between the Indian monsoon and the El Niño/Southern Oscillation.

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Neville Nicholls

Abstract

The mouths of October 1971 and 1972 were, respectively, months of heavy and light rainfall over much of Indonesia and New Guinea. The two months also showed considerable differences in the atmospheric circulation in the higher latitudes of the Australian region, with the wetter month (1971) exhibiting stronger westerlies between 30° and 45°S and weaker tropospheric westerlies poleward of 45°S. These changes are similar to those observed in the eastern Pacific during periods of heavy or light equatorial rainfall. Examination of 14 years of data in the Australian region suggests the presence of 1) significant coherence between tropical rainfall and the strength of the subtropical westerlies for periodicities greater than six months with no apparent lag, and 2) a significant out-of-phase relationship between tropical rainfall and high-latitude westerlies at periods greater than 24 months, the latter lagging the former by several months. The relevance of these results to tropical-extratropical interactions and numerical modeling is discussed.

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