The IRI Experimental Forecast Division has prepared this Climate Outlook for Australia and Indonesia for July - December 1999. Of relevance in the preparation of this Outlook have been the weakening of La Niņa conditions (lower than average sea-surface temperatures (SSTs) in the central equatorial Pacific Ocean), but La Niņa is expected to reamplify toward the end of the year. The persistence of slightly warmer than average conditions in the western Pacific have been assumed also. The SSTs of the southern tropical Indian Ocean were assumed to remain above average. The forecast of tropical South Atlantic SSTs suggests a strong warming trend through December 1999.

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 resurgence of weak to moderate La Niņa conditions, with the persistence of slightly positive sea-surface temperature anomalies in the western Pacific. The return of colder that average conditions in the central and eastern equatorial Pacific 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) Other sources of information include ( ECMWF , Queensland Department of Natural Resources , UKMO ).

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. The Climate Outlook for July - December 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 uncertainty concerning the detailed evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook provided here.

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: July - September 1999 and October - December 1999. Maps are given showing tercile probabilities of precipitation and temperature. The maps for precipitation 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). A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas that are marked by "D" represent regions experiencing their dry season (less than 15% of annual precipitation) and for which less than 5cm of precipitation typically occurs over the season. Otherwise, for example, in the case of Indonesia in July - September 1999 (Map B), there is a 50% 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 20% chance that the precipitation will be in the driest third of the years.

An additional precipitation 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. A priori, there is a 15% probability of being within the extremely wet category, and a 15% probability of being within the extremely dry category, leaving a 70% probability that the precipitation will not be extreme. The maps indicate areas of increased risk of extreme precipitation totals. Three levels of increased risk are defined: slightly enhanced risk, enhanced risk, and greatly enhanced risk. For slightly enhanced risk, there is a 25-40% probability that precipitation will be within the indicated extreme, i.e. wet or dry. This represents an approximate doubling of the climatological risk. For enhanced risk, there is a 40-50% probability that precipitation will be within the indicated extreme. This represents an approximate tripling of the climatological risk. For greatly enhanced risk, the probability that precipitation will be within the indicated extreme exceeds 50%, i.e. the indicated extreme is the most likely outcome.

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(Map A). 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.

July - September 1999:

PRECIPITATION

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

P.A) enhanced probabilities of below-normal precipitation over Thailand, southern Burma, Cambodia and Laos;

P.B) greatly enhanced probabilities of above-normal precipitation over most of Indonesia (there is a slightly enhanced risk of extremely wet conditions over most of Indonesia, and an enhanced risk over Sulawesi and Moluccas);

P.C) greatly enhanced probabilities of below-normal precipitation over the Marshall Islands;

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

P.E) slightly enhanced probabilities of below-normal precipitation over New South Wales.

TEMPERATURE

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

T.A) greatly enhanced probabilities of above-normal temperatures over south-eastern Asia, Malaysia, Indonesia, Philippines, Papua New Guinea, Solomon Islands, Vanuatu, New Caledonia, and northern Australia;

T.B) enhanced probabilities of above-normal temperatures over the east, west and south coasts of Australia

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

October - December 1999:

PRECIPITATION

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

P.i) slightly enhanced probabilities of below-normal precipitation over Java and southern Sumatra;

P.ii) enhanced probabilities of above-normal precipitation over central Indonesia;

P.ii) slightly enhanced probabilities of above-normal precipitation over part of coastal Queensland.

TEMPERATURE

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

T.i) enhanced probabilities of above-normal temperatures over the Philippines and New Guinea;

T.ii) slightly enhanced probabilities of above-normal precipitation over south-western Australia;

T.iii) slightly enhanced probabilities of above-normal precipitation over New Zealand.

Cutoffs and Max/Min extremes for JAS and OND.

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