Australia Guide for Oct. 1997

Climate Outlook - Rainfall

AUSTRALIA October 1997 - March 1998

The IRI Climate Forecast Division has prepared this Climate Forecast Outlook for Australia and Indonesia for October 1997 - March 1998. Of special relevance in the preparation of this Outlook has been the exceptionally strong El Niño episode now underway in the tropical Pacific, and the expectation that strong El Niño conditions will persist through much of the forecast period. This present El Niño is comparable, if not stronger than the major event in 1982- 83 (at the time the strongest of the century) which was associated with severe climate anomalies around the globe that caused major socio-economic losses in many regions.

METHODS -

This Outlook was prepared using the following procedures and information:

A) Statistical analyses of the observed historical response of the climate system over Australia and Indonesia during the strongest 12 El Niño events over the period 1890-1989.

B) Coupled ocean-atmosphere model predictions of tropical Pacific sea surface temperatures (SST) covering the forecast period. Particularly heavy weighting has been given to predictions from the coupled model operated by the NOAA National Centers for Environmental Prediction, Climate Modeling Branch. This model suggests moderating, but still relatively strong, El Niño conditions will persist in the tropical Pacific through the forecast period.

C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted SST patterns in the tropical Pacific (especially) and in other oceans as well. These models have been shown to reproduce important aspects of the climatic response to El Niño in simulations covering such significant events as the 1982-83 event mentioned above.

The procedures, models, and data used to derive this Climate Outlook are somewhat different from those used by the national meteorological services in Australia and Indonesia. Thus, this product may differ from the official forecasts issued in those areas. This Outlook has been prepared in the time available, using all information that was reasonably accessible. Inclusion of other climate information and guidance requires further arrangements. The IRI is engaged in establishing such collaborative arrangements with the goal of improving its capability to provide the best and most complete global climate guidance. The Climate Outlook for January-March, especially, are dependent of the quality of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information. However, there is considerable spread in coupled model predictions concerning the rate at which tropical Pacific SSTs will decline in early 1998 (assuming the El Niño peaks near the end of 1997). This spread in predictions is a primary source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SSTs will decline at a modest rate during early 1998. Another source of uncertainty is the spread in prediction of the Indian Ocean SSTs. Warm SST anomalies in central/eastern tropical Pacific, associated with El Niño, are generally accompanied by warm SST anomalies in the central tropical Indian Ocean and cool SST anomalies in the western Pacific / eastern Indian Ocean. Also, a significant positive trend in central tropical Indian Ocean SSTs has been noticed over the last several decades. What is uncertain is the detailed evolution of SST anomalies in the Indian Ocean and their exact spatial pattern, which may influence impacts locally. It is stressed that the current status of seasonal-to-interannual climate forecasting allows prediction of spatial and temporal averages, and does not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal time scales and relatively large areas; local variations should be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.

OUTLOOK -

This Outlook covers two seasons; October-December 1997 and January-March 1998. Maps are given showing expected probabilities that the seasonal rainfall will fall into the driest third of the years, the middle third of the years, or the wettest third of the year (see Map Caption below for further details, cautions and caveats).

October-November-December 1997:

The map is dominated by the following main regions of enhanced probabilities of rainfall anomalies:

A) Slightly enhanced probability for above normal rainfall over the western third of Australia.

B) Enhanced probabilities for below normal rainfall for the eastern part of Australia.

C) Enhanced probabilities for below normal rainfall over most of the maritime sub-continent.

D) Slightly enhanced probability for above normal precipitation over northern portions of Sumatra and Malaysia.

E) Slightly enhanced probability for above normal precipitation over the South Island of New Zealand.

January-February-March 1998:

The pattern of probabilities shows considerable similarity to that for October - December 1997. The main features of this season are as follows:

i) The region of elevated probabilities for normal-to-above normal rainfall over the western part of Australia (see A above) shifts eastward and occupies the central third of the continent. The western coast for this season shows slightly enhanced probabilities for below normal rainfall.

ii) The of enhanced probabilities for below normal rainfall over eastern Australia (see B above) extends farther west into the center of the continent.

iii) Continued enhanced probabilities for below normal precipitation over the maritime sub-continent and Malaysia (see C and D above). The probabilities for below normal rainfall are higher over the northeastern part of the maritime continent.

iv) Continued slightly enhanced probability for above normal precipitation over the South Island of New Zealand, and slightly enhanced probabilities for below normal precipitation over the North Island (see E above).

The probabilities given here are based mainly on the statistical analysis of the historical data. Further confidence is added from the atmospheric GCMs forced with a variety of predicted SST scenarios including persistence, statistical estimates, and dynamical forecasts. These atmospheric GCM climate predictions are in agreement with each other and with the statistical analysis for the regions of enhanced probabilities for below normal precipitation over the Pacific coast of Australia and over Indonesia, particularly for the October-December 1997 season.

Map Caption

For each sub-region, during the 1997-98 season, the likelihood associated with this Outlook is represented by the probabilities of that sub-region receiving below-normal, near-normal or above normal rainfall (see keys on Maps), based on model results and expert interpretation. Over many years, rainfall will fall equally frequently into three categories. For example, between 1900 and 1989 30 years will fall into the below-normal category, 30 years will be normal and 30 will fall into the above-normal category.

If all three outcomes were equally likely this season, the chance for each category would be 33%. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Otherwise, for example, in the case of western Australia in January-March 1998 (Map B), there is a 20% probability that the rainfall will be in the wettest third of the years, a 30% chance it will be in the near-normal third of the years, and a 50% chance that the rainfall will be in the driest third of the years. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

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