Climate Outlook - Rainfall & Temperature

AUSTRALIA and INDONESIA January 1998 - June 1998

The IRI Climate Forecast Division has prepared this Climate Forecast Outlook for Australia and Indonesia for January - June 1998. Of special relevance in the preparation of this Outlook has been the exceptionally strong El Niņo episode now underway in the tropical Pacific, and the expectation that strong El Niņo conditions will persist through at least the first half of the forecast period. This present El Niņo appears to have peaked in December 1997 with SST anomalies stronger than the major event in 1982-83 (at that 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 Niņo events over the period 1890-1989, and also the strongest 8 El Niņo events since 1950.

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 the slow decline of relatively strong El Niņo conditions, indicating that warm anomalies 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 Niņo in simulations covering such significant events as the 1982-83 event mentioned above.

D) Statistical model predictions of the upcoming rainfall probabilities based on the phase of the SOI (Southern Oscillation Index), as defined by Roger Stone at Queensland Department of Primary Industries.

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 April-June, especially, are dependent on 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 1998. 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 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 Niņo, 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; January-March 1998 and April-June 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). Similar maps are given for seasonal temperature.

January-February-March 1998:

PRECIPITATION:

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

P.A) Enhanced probabilities for below normal rainfall over most of the maritime sub-continent, with greater likelihood of below normal rainfall in the east (eastern Borneo, western New Guinea) than in the west (Sumatra). Note that farther to the east, near the Solomon Islands, the rainfall probabilities favor above normal rainfall.

P.B) Slightly enhanced probability for above normal rainfall over central Australia, extending down through South Australia and including Tasmania.

P.C1) Enhanced probabilities for below normal rainfall for most eastern parts of Australia and over the far western coast of Western Australia.

P.C2) Note that over northern Queensland enhanced probabilities of near-normal are indicated. Although the historical record shows that Queensland almost invariably receives below normal rainfall in October-March during El Niņo events, the past three months have not followed this very typical behavior. Northern Queensland has been repeatedly hit by tropical cyclones, which the global climate models are unable to simulate. Given the very warm SSTs off Australia's east coast and the fact that the previous season has disobeyed the statistics, it is likely that this atypical behavior will continue through the January-March season.

P.D) Slightly enhanced probability for below normal precipitation over the South Island of New Zealand.

TEMPERATURE:

T.A) Enhanced probabilities of above normal temperatures for eastern Australia extending upward over the maritime continent.

T.B) Very slight enhancement of cooler than normal temperatures over the western coast of Western Australia.

April-May-June 1998:

PRECIPITATION:

The pattern of probabilities shows considerable similarity to that for April - June 1998. The main features of this season are as follows:

P.i) Continued enhanced probabilities for below normal precipitation over the maritime sub-continent.

P.ii) The area with enhanced probabilities for above normal rainfall near the Solomon Islands (see P.A above) expands westward, indicating a slight enhancement of probabilities for normal-to-above normal rainfall over central-eastern Queensland. Enhanced probabilities for below normal rainfall remain over western and southern areas of Queensland.

P.iii) The enhanced probabilities for below normal rainfall over eastern and western Australia (see P.C above) are less biased in favor of below normal rainfall and are contained in the southern-most part of the continent, over south-west Western Australia, and over Victoria and Tasmania.

P.iv) Slightly enhanced probability for above normal precipitation over South Island, New Zealand, and slightly enhanced probabilities for below normal precipitation over North Island (see P.D above).

TEMPERATURE:

T.i) Area with enhanced probabilities of above normal temperature over eastern Australia (see T.A above) spreads farther inland covering the eastern 2/3 of the continent.

T.ii) Region over western Australia now suggests slight enhancement of probabilities for near-normal to above-normal temperatures.

T.iii) A slight enhancement of probabilities of below normal temperatures is suggested for Tasmania.

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 January-March 1998 season. They are also in good agreement concerning the enhanced probabilities of above normal temperatures, particularly over eastern Australia.

Map Caption

The numbers for each region indicate the probabilities of rainfall in each of the three catagories, below-, near-, and above-normal. Above-normal rainfall is defined as within the wettest third of recorded precipitation totals in each region for the given season; below-normal rainfall is defined as within the driest third of rainfall totals; near-normal is the third centered around the climatological median. The top number indicates the probability of rainfall occuring in the above-normal category, the middle numberer is for near-normal, and the bottom for below-normal. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. In the case of western Australia in January-March 1998 (Map A), there is a 20% probability that the rainfall will be in above-normal category (i.e. within the range of the wettest third of the years), a 30% chance it will be in the near-normal category, and a 50% chance in the below normal category. Similarly for temperature, the numbers over each region 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.

Cutoffs and Max/Min extremes for JFM and AMJ.