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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

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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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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