El Niño effects on water availability

So now I hope we all have a better understanding of how El Niño Southern Oscillation can affect weather globally, and the two extremes that are El Niño and La Niña. To tie this in with the theme of water and development in Africa, the next two blogs will be exploring some ways ENSO can affect water resources in Africa.

Water resources can be impacted by ENSO through many different factors (see the table below for a quick summary). There are aspects (conditions) that ENSO affects like sea temperature, droughts, rainfall variability and storms. These natural changes (ENSO is a natural phenomenon after all, but arguably likely to be intensified due to climate change) can exacerbate man-made aspects of water resources such as flooding in poorly drained cities, contamination of water sources (due to flooding or drought) and more. This blog post will focus on water availability, or how ENSO can affect natural variability in water (such as rainfall), and the next blog post will focus on water access, or how ENSO can affect the ability of people to access water resources for use (such as contamination).

Condition: Effect: warmer sea temperatures plankton and fish kills in coastal waters lower sea levels exposure of underwater, fragile coral reefs higher sea levels salt water intrusion into water supplies coastal erosion and damage to shoreline and property flooding/increased rainfall contamination of drinking water systems flooding of wastewater systems contamination of recreational sites and estuaries waterborne illness droughts crop failure increase in disease due to lack of water for sanitation and hygiene blowing dust pollution of viable water sources decrease in near-shore coastal water quality warmer, wetter, more humid weather boom in mosquito population and subsequent increase in malaria and dengue fever boom in termite population resulting in damage to buildings and homes

Source: Water Quality and Health Council

In Africa, east and southeast Africa are the main areas affected by ENSO. During an El Niño winter, southeast Africa tends to be drier and warmer, and east Africa tends to be wetter. During a La Niña winter, the opposite occurs, with southeast Africa wetter than normal and east Africa drier than normal. These generic changes are by no means easily predicted and vary hugely within El Niño and La Niña years (and one must also remember that El Niño and La Niña years are not easily predicted either, nor does a La Niña year necessarily follow an El Niño year or vice versa). But this general pattern that occurs with ENSO dictates changes in weather that affect water resources.

For Africa, the biggest effects I would say ENSO has on water availability are affecting rainfall and influencing droughts. However, as I mentioned, this is highly variable even within El Niño years. For example, the 1997-1998 El Niño that is argued to be the last "truly massive" El Niño we've seen globally, had less of a disastrous effect on southern Africa than expected. The 1997-1998 El Niño may have cost an estimated $35 billion in destruction and 23,000 deaths around the world, but areas like southern Africa were surprised when the predicted catastrophic drought failed to materialise and seasonal rainfall was largely near or even above average. This is argued to have been because of a warm sea surface temperature anomaly in the South Atlantic that created upper easterly winds to counteract the ENSO-induced upper westerly winds over the Atlantic, and shielded southeastern Africa from ENSO (Jury, 1998).

In general though, ENSO tends to lead to a decrease in rainfall in southeastern Africa and drought periods. Rouault and Richard (2003) used the Standardised Precipitation Index to quantify intensity and spatial extension of droughts that have occurred in the 93 rainfall "districts" of the South Africa Weather Service. They found that there have been 10 dry years with an average of 44 dry districts since 1962, seven of which were El Niño years (1966, 1970, 1973, 1979, 1983, 1987, 1992 and 1995). This evidently shows that El Niño can play a major role in whether or not there are droughts in southeastern Africa. Available water resources during droughts are diminished as the water table drops, and effective management is crucial. In southeastern Africa, droughts can put pressure on important water sources like rivers, aquifers, dams and boreholes. This region of Africa is also historically dependent on rainfall for agriculture, and more intense and more frequent drought periods have large impacts on agriculture.

Cartoon depicting the unpredictability of El Niño

On the other end of the spectrum, ENSO can also bring bouts of increased rainfall and flooding to Africa. In the news recently, the 2015 El Niño has brought "the worst drought in 30 years" to Ethiopia, at the same time as flooding and landslides to its neighbour Somalia. This variability within the area of eastern Africa shows how harsh and uneven ENSO can be. Eastern Africa in particular is vulnerable to huge variability of rainfall during El Niño years, with coastal countries like Somalia and Kenya facing increased rainfall and areas more inland like Ethiopia and South Sudan facing drought. Have a look at this great snapshot of predicted El Niño impacts on eastern Africa.

Therefore, water availability can be increased during El Niño (and decreased during La Niña) in eastern Africa due to rainfall. This can be highly variable though as more rain does not always mean more water resources that are available for use. For example, flash floods and corresponding mudslides can occur in eastern Africa during the rainy season created by El Niño, thus damaging crops and livestock. Eltahir (1996) studied the natural variability of the flow of the Nile River and suggests that 25% of natural variability is associated with ENSO. This was due to a positive correlation of sea level pressure anomalies in the Ethiopian Plateau (the source of the Nile) and at Darwin, Australia (where surface pressure can be an indicator of the onset of ENSO). A positive anomaly in annual sea level pressure over the Ethiopian Plateau is associated with decreased rainfall and hence a negative anomaly of the annual flow of the Blue Nile. Since the Blue Nile originates in Ethiopia (and contributes most of the inter-annual variability of the Nile), the negative relationship between ENSO and the flood of the Nile can have a strong forcing on water availability in Ethiopia.

This look into southern Africa and eastern Africa and how they respond to ENSO events gives an indication to how something as simple as changing rainfall patterns due to ENSO can have a major effect on water availability. In the future, I think it's vital that countries have plans for potential El Niño and La Niña years. Governments, farmers, development agencies and other interested parties desperately need to have prediction systems, mitigation strategies and continuing research to understand ENSO and to ensure water availability for all. The scary thing is that El Niño and La Niña events are occurring more frequently, so much so that even if one extreme follows the other (such as increased rainfall following drought due to La Niña following El Niño), they cannot compensate for each other. Jennifer Olson sums it up: "Those kinds of shocks to the system, whether it's drought or flooding, are usually just enough that people can’t recover anymore.”