Indian Ocean Dipole: The Key Culprit Behind the Recent Intensive Floods in Africa

In past blogs we have talked about how does climate change affect Africa’s countries via extreme events at different locations. So, today’s focus will be the driver of these events, which is a climate phenomenon that is similar to El Niño called “Indian Ocean Dipole”. It has been declared that there has been a negative Indian Ocean Dipole in July of 2021 -- the first negative event in five years. In this blog, I will explain what is this and how does this affect Africa’s water system.

 

The frequent flooding and landslides in East Africa each year have caused catastrophic damages to the region. This has been linked to the Indian Ocean Dipole(IOD) - the difference in sea-surface temperatures in opposite parts of the Indian Ocean (the western Indian Ocean and the eastern Indian Ocean south of Indonesia), which results in changes in the air moisture and contributes to the rainfall variability in all countries around the Indian Ocean Basin. It can be considered as “the Indian Ocean cousin of the famous El Niño and La Niña events” in the Pacific, as they bring dry weather and wet weather via positive and negative phases, respectively. 

 

What are El Niño and La Niña events and how does IOD link with ENSO?

 

El Niño and La Niña are the climate pattern that can affect weather worldwide. Normally, there is an exchange of warm and cold water in the ocean when the trade winds blow along the equator, and from west towards the east to bring warm water. When this normal condition of the Pacific Ocean and atmosphere above it is changed, there is a cycle of warming and cooling events for every two to seven years intervals called the El Niño-Southern Oscillation (ENSO) cycle is achieved, which then have global impacts on the weather: on seasonal precipitation and temperature patterns. A warming of the central and eastern tropical Pacific has been associated with El Niño events, while cooling of the same areas is brought by La Niña events. This ocean oscillation between warm and cold create three different phases called “positive”, “neutral” and “negative”, which creates different sea temperatures at different parts of the ocean that are reflected in ocean temperature gradients.

 

The IOD is very similar to ENSO, which is also a coupled ocean-atmosphere phenomenon that involves interaction between these two but occurs in the equatorial Indian Ocean. It is linked to ENSO as the extension of the Walker Circulation to the west, which is the warming of air and water by the easterly trade wind, and the flow of warm tropical ocean water from the Pacific into the Indian Ocean.

 

 

How does this affect Africa’s weather?

 

IOD events have affected Africa mainly via the positive Indian Ocean Dipole (pIODs), which has brought heavy downpours and deadly floods to the eastern region. Normally, IOD events start in the late autumn or winter and last until the end of next spring, and the pIODs peaked in mid-October when the temperature of the water around East Africa was about 2 degrees Celsius higher than that near Australia. This thus favoured heavy downpours in countries of East Africa over the last two months, such as Mozambique, with the rainfall being up to 300 per cent above average from October to mid-November. 

 

Illustrated in figure 1, a positive dipole phase is associated with La Niña, which is an irregular oscillation of sea surface temperature that brings a wetter, warmer condition in the eastern Indian Ocean, and cooler, drier conditions in the west. The warmer sea surface temperature increases the change of rain due to more convection at the lower troposphere over East Africa. As a result, east Africa would suffer from unusual and deadly flooding, whereas the other side, western Australia, will suffer from droughts and bushfires.

Figure 1 Diagram shows the positive phase of the Indian Ocean Dipole 

 

 

The IOD thus became a key culprit behind Africa's recent natural disasters and brought more future uncertainty to the continent with its instability.