The IRI Experimental Forecast Division has prepared this Climate Outlook for Australia and Indonesia for October 1998 - March 1999. Of relevance in the preparation of this Outlook have been the continued evolution of cooler than average conditions in the eastern and central equatorial Pacific Ocean (La Niņa), and the persistence of warmer than average conditions in the western equatorial Pacific. The sea-surface temperatures of the central and western tropical Indian Ocean have been cooling from record high temperatures, and this trend is expected to continue. It has been assumed that the northern and tropical Atlantic Ocean will remain warmer than normal, and that sea-surface temperatures in the South Atlantic will increase during the forecast period.

METHODS -

This Outlook was prepared using the following procedures and information:

A) Coupled ocean-atmosphere model predictions of tropical Pacific 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 the continued development of moderate La Niņa conditions, with the persistence of positive sea-surface temperature anomalies in the region of Indonesia. This sea-surface temperature forecast is consistent with most other numerical and statistical forecasts.

B) Forecasts of the tropical Indian and Atlantic oceans using statistical models developed by the IRI and CPTEC respectively.

C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted sea-surface temperature patterns.

D) Statistical analyses of the observed climate over Australia and Indonesia.

E) Other sources of information include: CPC, the Queensland Department of Natural Resources, and ECMWF.

The procedures, models, and data used to derive this Climate Outlook may be 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 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 October 1998 - March 1999 is dependent on the quality of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some spread in coupled model predictions concerning the evolution of SSTs. This spread in predictions is a source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SSTs will continue to decline at a modest rate, resulting in moderate La Niņa conditions by the end of the year. Also, it is known that Indian Ocean SSTs may play some role in modulating precipitation changes over parts of Australia and Indonesia. Thus, the uncertainties in Indian Ocean SST values during the forecast period may lead to additional uncertainty over some parts of the region.

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 1998 and January - March 1999. Maps are given showing expected tercile probabilities of precipitation. The maps indicate the probabilities that the seasonal precipitation will fall into the wettest third of the years (top number), the middle third of the years (middle number), or the driest third of the years (bottom number). An additional map is provided for the first season indicating probabilities for extreme precipitation anomalies. Extremes are defined as anomalies that fall within the top and bottom 15th percentile of the observed records. Maps of temperature show expected probabilities that the seasonal temperatures will fall into the warmest third of the years, the middle third of the years, or the coldest third of the years. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas marked "D" experience their dry season during the forecast period, typically receiving less than 15% of their annual precipitation total during the three months. Otherwise, for example, in the case of northern New Zealand in October - December 1998 (Map A), there is a 45% probability that the precipitation 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 25% chance that the precipitation will be in the driest third of the years. The numbers for each region on the temperature maps indicate the probabilities of temperatures to fall in each of the three categories, above-, near-, and below-normal. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

October - December 1998:

PRECIPITATION

The following regions of enhanced probabilities of precipitation anomalies are indicated:

P.A) greatly enhanced probabilities of above-normal precipitation in the Philippines, Indonesia excluding Sumatra, eastern Malaysia, and the far northern part of northern Australia (a greatly enhanced risk of extremely wet conditions is indicated over the Philippines and central Indonesia, and an enhanced risk of extremely wet conditions is indicated over southern Indonesia);

P.B) enhanced probabilities of near-normal precipitation over mainland south-east Asia;

P.C) slightly enhanced probabilities of near-normal precipitation over Sumatra;

P.D) enhanced probabilities of near-normal precipitation over Papua New Guinea;

P.E) enhanced probabilities of near-normal precipitation in the vicinity of the Solomon Islands;

P.F) enhanced probabilities of above-normal precipitation over the northern part of New Zealand (a slightly enhanced risk of extremely wet conditions is indicated);

P.G) enhanced probabilities of above-normal precipitation over southern and eastern Australia.

TEMPERATURE

The following regions of enhanced probabilities of temperature anomalies are indicated:

T.A) enhanced probabilities of above-normal temperatures over Indonesia, the Philippines, Malaysia, New Guinea, northern Australia, and the southern and south-eastern parts of south-east Asia (the probabilities are greatest over Indonesia, the Philippines, Malaysia, and New Guinea);

T.B) enhanced probabilities of near-normal temperatures over most of Australia south of about 20S, excluding the east coast;

T.C) slightly enhanced probabilities of above-normal temperatures over northern New Zealand;

T.D) enhanced probabilities of near-normal temperatures over northern Thailand, Laos, northern Vietnam, and southern China;

January - March 1999:

PRECIPITATION

The following regions of enhanced probabilities of precipitation anomalies are indicated:

P.i) enhanced probabilities of above-normal precipitation over the Philippines and central Indonesia;

P.ii) slightly enhanced probabilities of above-normal precipitation over Malaysia and western Indonesia;

P.iii) enhanced probabilities of above-normal precipitation over eastern Australia;

P.iv) slightly enhanced probabilities of near-normal rainfall over southern Indonesia and north-western Australia;

P.v) slightly enhanced probabilities of below-normal rainfall over New Zealand.

TEMPERATURE

The following regions of enhanced probabilities of temperature anomalies are indicated:

T.i) enhanced probabilities of above-normal temperatures over south-east Asia, Indonesia, the Philippines, Malaysia, and New Guinea (the probabilities are greatest over eastern Indonesia and New Guinea);

T.ii) slightly enhanced probabilities of near-normal temperatures over eastern Australia;

Cutoffs and Max/Min extremes for OND and JAS.

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