Climate Outlook - Rainfall & Temperature

ASIA January 1998 - June 1998

The IRI Climate Forecast Division has prepared this Climate Forecast Outlook for Asia 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 Asia 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.

The procedures, models, and data used to derive this Climate Outlook are somewhat different from those used by the national meteorological services in Asia. 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 possible source of uncertainties is the spread in prediction of the Indian ocean SSTs, which may affect parts of southern Asia. Warm SST anomalies in central/eastern tropical Pacific, associated with El Niņo, are 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 also possible that tropical and/or mid-latitude Atlantic SSTs may play some role in modulating rainfall changes over central Asia.The impacts of possible changes in Atlantic SSTs during the forecast period have not been considered in this report. It is also 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) Slightly enhanced probability for normal-to-below normal rainfall over Siberia.

P.B) Slight enhancement of probabilities for below normal rainfall extending over much of southern Asia. In particular, eastern China, east of the Tibetan Plateau, shows a significant enhancement of probabilities for below normal rainfall, as does the region covering Borneo and much of the Philippine Islands.

P.C) Enhanced probability for above normal precipitation over parts of central Asia, including Mongolia and western China. Greater probabilities for above normal rainfall exist over western Mongolia and northwestern China. There is also a small region of increased likelihood of above normal rainfall to the west and north of Afghanistan.

P.D) Enhanced probability for above normal precipitation over eastern China, particularly along the subtropical Pacific coast and over Taiwan.

P.E) Slightly enhanced probability for above normal precipitation over the southern-most part of India. However, note that the guidance for this region is based on the statistical analyses; this region experienced below normal rainfall during this season in 1972 and 1982.

TEMPERATURE:

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

T.A) Slightly enhanced probabilities of cooler than normal temperatures over central Asia, with greater likelihood of colder temperatures east of Mongolia. (Note: no forecast over Siberia due to lack of observed data) Be aware that the persistent blocking patterns over the interior of Asia may significantly influence the anomaly patterns suggested here, as was the case in 1982/83.

T.B) Enhanced probabilities of warmer than normal temperatures over south- and southeast- Asia, particularly over India, western China and the maritime continent.

April-May-June 1998:

The pattern of probabilities shows some similarities to that for January-March 1998. The main differences can be described as follows:

PRECIPITATION:

P.i) The region of enhanced probability for below normal rainfall across southern Asia (see P.B above) is more spatially coherent in this season extending from India to south-east Asia, and including Indonesia and the Philippine Islands.

P.ii) The area of slightly enhanced probability for above normal precipitation over central Asia (see P.C above) becomes more zonal, stretching across most of the continent, toward the eastern coast of China, although the coastal area is most likely to experience near-normal rainfall conditions in this season. Higher probabilities for above normal rainfall are evident over far western China, northern Pakistan and northern Afghanistan.

TEMPERATURE:

T.i) Weakened enhancement of probabilities for colder than normal temperatures over central eastern Asia. (see T.A above)

T.ii) Region of enhanced probabilities of above normal temperatures over southern Asia/India has shifted southward (see T.B. above) and extends only as far north as central India.

The probabilities given here are strongly influenced by 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 region of enhanced probabilities for below normal precipitation over the southern part of Asia and for the region of enhanced probabilities of above normal precipitation over the sub-tropical part of eastern Asia. They are also in agreement concerning the enhancement of probabilities for above normal temperatures over south- and southeast- Asia.

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 the northern half of Siberia in January-March 1998 (Map A), there is a 25% 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 45% 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.