The Indian Ocean Dipole (often referred to as simply the IOD) is a major feature of the tropical and subtropical flow in the Northern and Southern Hemispheres that often has implications for seasonal weather. The IOD is not widely known to the public or even to some whose activities are directly impacted by this weather phenomenon. And yet, the IOD is an important climate index for long-range forecasters.
What is the Indian Ocean Dipole – Introduction
The Indian Ocean Dipole can be thought of as the Indian Ocean branch of the Walker Cell, which in turn, is associated with the El Niño Southern Oscillation. The IOD is defined by the Dipole Mode Index (DMI) which is a measure of the anomalous sea surface temperature (SST) gradient between the western equatorial Indian Ocean (50E-70E and 10S-10N) and the southeastern equatorial Indian Ocean (90E-110E and 10S-0N). Changes in SSTs on either side of the Indian Ocean (the dipole) drive convection and alter the Walker Cell circulation.
The Indian Ocean Dipole generally moves in sympathy with the phase of the El Niño Southern Oscillation (ENSO) but there are occasions when it doesn’t behave quite as expected, and it can really come into its own during a neutral ENSO, such as the strong positive event in 2019. Another difference is that ENSO normally peaks in the Northern Hemisphere winter, while the IOD can peak at any time of year.
The impacts of the IOD are not only felt in the tropics, but can also influence weather patterns across much of the mid-latitudes.
Indian Ocean Dipole: Negative Phase
The negative phase of the IOD is driven by cooler than normal SSTs off the coast of Africa and warmer than normal SSTs to the west of Indonesia. This configuration of surface sea temperatures gives the normal circulation a boost.
The stronger circulation brings more consistent westerly winds over the ocean, stronger convection over Indonesia, and often drought conditions for the Horn of Africa. This pattern is consistent with La Niña (negative phase of ENSO).
Indian Ocean Dipole: Negative Phase Impacts during US Winter
During the negative phase of the IOD in the wintertime, warmer than normal temperatures are more likely in the lower 48 states, especially over the Midwest (Figure 3). During a strongly negative IOD, the warm anomalies are even more likely and more widespread.
While this pattern is mostly consistent with a negative ENSO (La Niña) event, it is worth noting that during a negative IOD event, much of the Central US sees the opposite anomaly to that normally seen during La Niña.
During a negative IOD in winter, much of the South and West is likely to be wetter than normal, with dry weather more likely for the Midwest (Figure 4).
Indian Ocean Dipole: Negative Phase Impacts during US Summer
During the negative phase of the IOD in summer, there are few significant impacts as indicated by the the maps below showing a mixture of above and below normal probabilities. It is interesting to note that the Mid-Atlantic is usually cooler and wetter than normal, and Texas is normally warmer and drier than normal.
What is the Indian Ocean Dipole – Positive Phase
The positive phase of the IOD is driven by warmer than normal SSTs off the coast of Africa and cooler than normal SSTs to the west of Indonesia. This configuration of surface sea temperatures reverses the normal circulation with easterly winds setting up across the equatorial Pacific.
The reversed circulation suppresses convection over Indonesia, and leads to predominately dry weather and potential for drought. Over the Horn of Africa, convection is enhanced and rainfall is much more abundant than usual. This pattern is consistent with El Niño (positive phase of ENSO).
Indian Ocean Dipole: Positive Phase Impacts during the Australian Winter
During the positive phase of the IOD in the wintertime over Australia, Western Australia is usually warmer than normal while the rest of Australia is cooler than normal (Figure 8).
During a positive IOD in winter, much of Australia is drier than normal and the risk of drought significantly increases (Figure 9).
Indian Ocean Dipole: Positive Phase Impacts during the Australian Summer
In summertime when the IOD is positive, much of Australia is warmer than normal, but the interior of the north and east is usually cooler than normal (Figure 10).
During a positive IOD in summer, Western Australia is usually drier than normal. For the rest of the country above normal rainfall brings a heightened risk of flooding (Figure 11).
The World Climate Service provides a powerful data analysis system that enables users to quickly explore the Indian Ocean Dipole, and many other indices, by phase for each month of the year, showing impacts on all the major weather parameters right around the globe.
The Indian Ocean Dipole and Long-Range Forecasting
The Indian Ocean Dipole typically remains in a given phase from a few weeks to many months. Thus, the IOD is very much a seasonal climate index (as opposed to a more short-lived subseasonal climate index such as the PNA). The IOD is generally well predicted by dynamical modelling and thus we can usually be fairly confident about which phase the IOD will favor in the coming months.
The World Climate Service provides calibrated monthly forecasts of many climate indices, including the IOD. For example, Figure 13 shows a table with the 5 month forecasts of the IOD made in August 2021 by all of the leading global weather models. The suite of dynamical models is unanimous in its assertion that the IOD will be in the negative phase through fall 2021. (The cells with bold borders indicate the model shows statistically significant forecast skill for that particular month and lead time).
The Indian Ocean Dipole – Conclusion
The Indian Ocean Dipole is defined by the sea surface temperatures within the Indian Ocean, and can be thought of as the Indian Ocean branch of the Walker Cell.
Phases of the Indian Ocean Dipole often persist for many months and are generally well predicted by dynamical modelling. The Indian Ocean Dipole is therefore a vitally important consideration in seasonal forecasting over both the tropics, and the mid-latitudes.