Linia Mashawi Gopo and Shingirai Nangombe
El Nino is a climate cycle in the Pacific Ocean with a global impact on weather patterns. It is a naturally occurring phenomenon in the equatorial region which causes temporary changes in the world climate. It occurs when sea surface temperatures in the central and eastern tropical Pacific Ocean become substantially warmer than average.
During the El Nino phase the relationship between winds and ocean currents in the Pacific Ocean changes with impact on weather conditions around the world especially on snow, rainfall and temperature.
The 2015-16 El Nino phenomenon is poised to be the strongest ever on record thereby surpassing the 1997-98 El Nino which was recorded as the strongest till now.
Research is showing that climate change is causing more frequent El Nino occurrences; however, it is important to note that El Nino itself is not climate change. El Nino events are a prominent feature of climate variability with global climatic impacts.
The season 1997-98 El Nino episode has been referred to as the climate event of the 20th century because of the enormous and severe effect it had on weather globally. Extreme El Nino episodes disrupt global weather patterns affecting the amount of rainfall in that particular season, tropical cyclone activity, and other extreme weather events the world over.
The relationship between El Nino and rainfall over Zimbabwe is misunderstood by many where most people are of the view that El Nino implies drought; but is this correct?
Normally El Nino is associated with reduced rainfall over mostly the southern parts of Southern Africa including Zimbabwe. However, a closer analysis of the past 21 El Nino years in relation to Zimbabwe rainfall was undertaken; it is quite interesting to note that out of the 21 El Nino years which occurred in the past 60 years, only eight (38 percent) had above normal rainfall while 13 (62 percent) had below normal rainfall.
These statistics clearly show that the greater number of El Nino episodes led to below normal rainfall, however, the eight episodes where above normal rainfall was received during an El Nino episode explain that El Nino episode and below normal rainfall is not a one-one relationship.
An El Nino episode does not translate into below normal rainfall (meteorological drought). However, the fact that 62 percent of the El Nino years had below normal rainfall shows that most of the El Nino years may result in a meteorological drought (below normal rainfall).
Seasonal climate forecasting is not only guided by El Nino, it is just but one of the many indicators that are critically assessed and monitored when coming up with a seasonal climate forecast hence El Nino should not be looked at in isolation. This explains the reason why there are El Nino years which had above normal rainfall.
El Nino is NOT the only seasonal climate forecast indicator. The other seasonal climate forecast indicators include but are not limited to sea surface temperatures (SSTs) globally, prior heating before the start of the rainfall season, behaviour of the winds during the winter season, persistent westerly cloud bands at the start of the season.
A close assessment of these indicators: prior heating – a very hot September and October is a sign of a good summer rainfall season for Zimbabwe, while persistent cool south-easterlies that extend into September and October are a sign for a poor rainfall season.
On the other hand, persistent westerly cloud bands in October and November are a sign of good rains over Zimbabwe. Thus, it is of paramount importance to understand that El Nino is just but one of the seasonal climate forecast indicators which should not be looked at in isolation.
Summer rainfall season outlook for 2015-16 was generally going for below normal rainfall for the whole country for October-November-December and January-February-March. El Nino, one of the seasonal climate forecast indicators, was forecast to be at its peak throughout the summer rainfall season (above 95 percent).
On the other hand, cool south-easterlies prevailed throughout the winter period extending to September and part of October. Temperatures only started to increase quite significantly to break all-time records in Zimbabwe in November 2015 onwards. The westerly cloud bands were not as active neither were they persistent at the beginning of the season.
Looking at a few of the indicators besides the El Nino they were showing a tendency towards a poor rainfall season over Zimbabwe; which is what the country is experiencing and below normal rainfall is highly likely to continue up to the end of the summer rainfall season.
Lastly, it is important to note that the worst meteorological drought was experienced during the 1991-92 summer rainfall season which was not even an El Nino episode.
Inasmuch as the 1997-98 El Nino was strong the rainfall performance over Zimbabwe was not bad. This serves to strengthen the argument that El Nino and meteorological drought (below normal rainfall) is not a one-one relationship: El Nino does not imply drought!
- The writers are principal meteorologists at the Met Department.