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N. Nicholls

Abstract

An examination of data from 1950 to 1975 has suggested that interannual variations in the number of tropical cyclones are related to pressure anomalies at Darwin in the preceding winter. The closest relationship is with the number of early season (October–December) cyclones.

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N. Nicholls

Abstract

The stability of simple linear regression equations for the long-range prediction of Australian spring rainfall was studied. Specifically, the way in which the accuracy of the forecasts depends on the number of years of data used to derive the equations and the length of the period between the end of the data used in forecast equation derivation and the application of the equations in prediction were examined. An optimum period of data of about 15 years was found; the use of more or less data in deriving the forecast equations led to deterioration in the forecasts. The forecasts also deteriorated if the equations were used for more than a few years after the end of the period of data from which they were derived, suggesting a need for routine updating of the forecast equations. The lack of stability in the forecast equations presumably reflects nonstationarity in the data series possibly resulting from changes in the general circulation patterns. If this is so, the results might be applicable to similar statistical long-range forecast methods in other areas.

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N. Nicholls

Abstract

The relationship of the Southern Oscillation and El Niño phenomena to sea surface temperature anomalies in the Indonesian region is investigated. The three are closely related and the relationship has a strong annual cycle. The Indonesian sea surface temperature anomalies show arena persistence approximately from January through October with a tendency to dissipate or change sign around November. Changes of Indonesian sea surface temperature anomalies lead by about a season changes in the Southern Oscillation and east Pacific sea surface temperature.

It is demonstrated that a simple ad hoc model representing a stochastically-forced, seasonally-varying interaction between the atmosphere and the ocean in the Indonesian region can product simulated anomalies at Darwin pressure and Indonesian sea surface temperature that reproduce the observed statistical behavior of them anomalies without the inclusion of the effects of oceanic and atmospheric events external to the Indonesian region. It suggested that the El Niño-Southern Oscillation might be the dynamic response of the Pacific Ocean and overlying atmosphere to anomalies produced by such an interaction in the Indonesian region. A speculation is raised involving the possible physical basis for such an interaction.

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N. Nicholls

Abstract

The relationship between mean rainfall and the relative variability of annual rainfall is shown to be different for stations that have consistent relationships with ENSO compared with stations not affected by ENSO. Variability is typically one-third to one-half higher for ENSO-affected stations. It appears that ENSO causes a snore variable climate in the areas it affects.

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N. Nicholls and K. K. Wong

Abstract

The relationship between the relative variability of annual rainfall, the long-term mean annual rainfall, the latitude, and the correlation between annual rainfall and the Southern Oscillation Index is examined, using data from 974 stations. A nonlinear relationship between these variables accounts for 94% of the variance in annual rainfall variability. Relative variability typically increases as mean annual rainfall decreases, as latitude decreases, and as the effect of the Southern Oscillation increases. There is an interaction between latitude and the Southern Oscillation so that the effect of the Southern Oscillation on variability weakens as latitude increases.

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J. L. McBride and N. Nicholls

Abstract

Correlations between indices of the Southern Oscillation (SO) and areal average rainfall for 107 Australian rainfall districts for the period December 1932 to November 1974 have been calculated. Simultaneous correlations between the SO and rainfall show a clear annual cycle with the best relationship occurring in spring (September-November). The season with the weakest relationship is summer (December-February). In all seasons, seasonal rainfalls in some parts of Australia are significantly correlated with the SO in the preceding season. The strongest lag correlations occur with spring rainfall, which for some areas is also significantly correlated with the SO two seasons (six months) earlier.

Correlations were also calculated with the data divided into two subseries from 1932 to 1953 and from 1954 to 1974. These calculations suggest a westward shift with time of the correlation pattern, associated with substantial changes in the magnitude of the correlations in some areas.

Some speculations on the possible causes of certain aspects of the observed seasonal cycle in the correlations are advanced.

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N. Nicholls, J. L. McBride, and R. J. Ormerod

Abstract

An index of the date of onset of the North Australian wet season is defined based on rainfall received at a single station (Darwin). It is demonstrated that this index can be predicted some months ahead.

The amount of rain received during the wet season is only weakly related to the date of onset, and the amount of rainfall received in the middle and late portion of the season is totally unrelated to either the date of onset or to the amount of rain received in the early part of the season.

Discussion is presented on the relationship between the wet-season onset as here defined and the Australian monsoon onset as defined by Troup (1961). A distinction is made between the monsoon portion of the season and an earlier transition season which also accounts for a large proportion of the total rainfall.

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