Multi-model ensemble predicts positive rainfall conditions for Namibia
There is a 50% probability that most of Namibia will receive normal to substantially above-normal rainfal for the three-month period, March, April and May this year, according to the latest precipitation forecast from the International Research Institute for Climate and Society (IRI) at Columbia University’s Climate School in the United States.
In a weather assessment for Africa, released earlier in February, IRI noted that sea surface temperatures (SSTs) in the equatorial Pacific were still cooler than normal during January this year. This shows a continuation, but also gradual weakening, of La Nina conditions that have been in place for almost two years. For instance, the Nino3.4 index value in January 2023 was -0.75°C and -0.5°C during the first half of February.
Nino3.4 is a very large area in the equatorial Pacific ocean covering most of Nino 3 and Nino 4. For the purposes of climate research, the Pacific Ocean between the South American coastline and Indonesia, about 12 kilometres, is devided into four zones that are monitored individually and collectively. Nino 3.4 is an overlapping area between Nino3 and Nino4, west of the dateline, covering a large part of the Eastern Pacific. The Nino3.4 index is generally regarded as a representative benchmark value for oceanic conditions in the rest of the Pacific.
Based on the February El Nino Southern Oscillation (ENSO) forecast of the Climate Prediction Centre in the National Oceanic and Atmospheric Administration of the US Government, the current La Nina is expected to transition to ENSO neutral during the next three months and remain neutral during the summer of the northern hemisphere (June, July, August).
“No strong SST anomalies are present in the tropical Indian Ocean, while the Atlantic Ocean shows weak warm anomalies in both observations and the model forecasts,” stated the IRI.
“Enhanced probabilities of above-normal precipitation are forecast over much of northeastern South America, parts of southern Africa, Southeast Asia, and especially the Philippines during Mar-May and Apr-Jun.”
The IRI probabilistic seasonal climate forecast is based on a re-calibration of model output from the U.S. National Oceanographic and Atmospheric Administration (NOAA)’s North American Multi-Model Ensemble Project (NMME) and on models from NASA, NCAR and COLA at the University of Miami. The output from each NMME model is re-calibrated prior to multi-model ensembling to form reliable probability forecasts.
A probability forecast does not predict rainfall conditions per se, but indicate the chances for departures from the standard (expected) conditions. Rainfall in Namibia, because of its arid climate and huge variability in actual precipitation outcomes, is notoriously difficult to predict accurately.