Climate Outlook - Rainfall & Temperature

AFRICA January - June 1998

The IRI Climate Forecast Division has prepared this Climate Forecast Outlook for Africa 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 at least for the first half of the forecast period. This present El Niņo 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 Africa 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 moderate, but still relatively strong, El Niņo 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 Niņo in simulations covering such significant events as the 1982-83 event described above.

D) A consensus guidance for January-March 1998 for southern Africa, produced at the Southern Africa Climate Outlook Forum held in Windhoek, Namibia, on 18-19 December 1997, forms an important input to Map A. Contributors to the consensus guidance included the Meteorological Services from eleven SADC countries (Botswana, Lesotho, Malawi, Mauritius, Mozambique, Namibia, South Africa, Swaziland, Tanzania, Zambia and Zimbabwe) and national, regional and international institutes (Drought Monitoring Centers - Harare and Nairobi; University of Zululand; Clark University; SADC Food Security Technical and Administrative Unit; Southern Africa Transport and Communications Commission; World Meteorological Organization; Food and Agriculture Organization; International Research Institute for climate prediction; US National Oceanic and Atmospheric Administration Office of Global Programs and Climate Prediction Center; US Agency for International Development; UK Meteorological Office; IMGA/CNR-Bologna). The guidance was drawn up by assessing output from coupled ocean-atmosphere models and physically-based statistical models. Additional information available from these sources was used in the April-June Outlook.

The procedures, models, and data used to derive this Climate Outlook are somewhat different from those used by the national meteorological services in Africa. 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 early 1998. Another possible source of uncertainties 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. In addition, 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. Also, it has been suggested that tropical and/or mid-latitude Atlantic SSTs may play some role in modulating rainfall changes over Africa. 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/temperature will fall into the wettest/warmest third of the years, the middle third of the years, or the driest/coolest third of the year (see Map Caption below for further details, cautions and caveats).

January-February-March 1998:

PRECIPITATION

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

P.A) Enhanced probability for above normal rainfall over tropical east Africa with the greatest probabilities for the above normal precipitation over coastal areas between about 0 and 10N.

P.B) Slight enhancement of probabilities for below normal rainfall for the northern part of the tropical belt extending from the western coast of Africa into the center of the continent.

P.C) Enhancement of probabilities for below normal rainfall over much of southern Africa south of about 15S. Over south-eastern parts of southern Africa, the probability of below normal rainfall is less severe and the highest probability is for near-normal conditions.

P.D) Slightly enhanced probability for below normal precipitation over the region located to the north-east of the Red Sea.

TEMPERATURE:

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

T.A) Enhanced probability for above normal temperature south of the equator.

T.B) Enhanced probability for above normal temperature along the south and west coasts of West Africa and along the Mediterranean coast.

T.C) Enhanced probability of below normal temperature along the coast of East Africa.

T.D) Enhanced probability of near-normal temperature over tropical northern Africa.

April-May-June 1998:

PRECIPITATION

The pattern of probabilities shows a number of changes from that for January-March 1998. The main features of this season are as follows:

P.i) The probabilities of above normal rainfall over the eastern part of Africa (see P.A above) weaken as the Long-Rains begin.

P.ii) There is an area of increased likelihood for above normal rainfall along the south coast of West Africa.

P.iii) The region of elevated probabilities for below normal rainfall over southern Africa (see P.C above) moves northward. Near-normal rainfall is expected in the far south and there is a possibility of above normal rainfall along the west coast.

P.iv) A slightly enhancement of the probability for below normal precipitation over the region located to the north-east of the Red Sea (see P.D above).

TEMPERATURE:

The pattern of probabilities remains largely unchanged. The main differences are:

T.i) The probabilities of below normal temperature over the eastern part of Africa (see T.C above) weaken as the Long-Rains begin.

T.ii) There is an area of increased likelihood for below normal temperature over Egypt.

The probabilities given here are based partly 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.

Map Caption

The numbers for each region indicate the probabilities of rainfall/temperature in each of the three categories, below-, near- and above-normal. Above-normal rainfall/temperature is defined as within the wettest/warmest third of recorded precipitation totals/temperature in each region; below-normal rainfall/temperature is defined as within the driest/coolest third of rainfall totals; near-normal is the third centered around the climatological median. The top number indicates the probability of rainfall/temperature occurring in the above-normal category, the middle number is for near-normal and the bottom for below-normal. In the case of Madagascar in January-March 1998 Map A), there is a 40% probability that the rainfall occurring in the above-normal category (i.e. within the range of the wettest third of recorded precipitation); a 35% chance in the near-normal category; and a 25% chance in the below-normal category. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct. A "C" stands for "climatology", which indicates that there is no clear scientific basis for favoring a prediction of below-normal, normal, or above-normal rainfall/temperature.

Cutoffs and Max/Min extremes for JFM and AMJ.