Meteorological services rebutts report by Botswana society
Correspondent | Monday March 3, 2014 15:02
The Department would like to clarify a few issues in relation to the above article by the Botswana Society regarding drought and rainfall projections. It is true that Botswana is facing acute water shortages particularly in the south-eastern parts of the country and that conservation measures have to be undertaken at all times.
This, being due to the fact that the country is by nature a semi-arid area, and rainfall amounts are low and unreliable. This is sometimes compounded by the presence of droughts from time to time.
However, it would be misleading to predict a drought for next year at this stage as well as classifying the current season (2013/14) as another drought year. Thus far, most of the country has been receiving average to above average rains, with the west having received above average rains. The only exception is the south-eastern parts of the country where rains have been below average. Average rains range from 300mm in the extreme southwest to 600mm in Chobe.
The rain season ends in March and more rain is still possible. As such classifying the whole country as having a drought this year is inaccurate. The drought assessment for this year will be conducted in April 2014 where a report will be produced on the impact of meteorological parameters on livestock, crop production, vegetation, wild animals, dam levels and surface water. For example, in 2011/12 was declared, partial drought, whilst 2012/13 season was declared a drought.
Predicting a drought based only on the assumption that a weak El-Nino is forming in the Pacific Ocean would not be accurate. Nearly all model forecasts indicate the persistence of ENSO-neutral (Niño-3.4 index between -0.5°C and 0.5°C) through the Southern Hemisphere autumn 2014, but afterwards, an increasing number of models predict either ENSO-neutral or El Niño (greater or equal to +0.5ºC) during the Southern Hemisphere winter 2014.
Whichever the case, the El Niño is expected to be weak and although the southern part of Africa generally receives below-normal rainfall during El Nino years and La Nina usually brings normal or above-normal rainfall, it cannot be accepted as a rule.
Botswana can be divided into numerous rainfall regions, each region having a different correlation with ENSO. Also, ENSO explains only approximately 30% of the rainfall variability, which means that other factors should also be taken into account when predicting seasonal rainfall.
Other factors affecting rainfall in our region include the Indian Ocean Dipole (IOD), Southern Annual Mode (SAM), the positioning of the mid-latitude Jet Stream and presence of Tropical Cyclones in the Southwest Indian Ocean, amongst others.
High temperatures do contribute to higher evaporation rates, however they are not correlated to lower rainfall as suggested in the article. In fact, record temperatures do not even correlate to El Niño years. Elevated Sea Surface Temperatures (SST’s) correlate to El Niño events, however continental temperatures are much more affected by other additional factors including wind direction and speed, prevailing air masses etc, which can have a huge impact on temperature and evaporation rates. In conclusion therefore, the prediction for drought over the next 12 months cannot be concluded from the likelihood of a weak El Niño alone, as other factors have to be taken into consideration.