Indian Ocean Dipole
The Indian Ocean Dipole (IOD) quantifies the difference in sea surface temperature (SST) between the western and eastern areas of the Indian Ocean (Figure 1).
Fig. 1. Indian Ocean Dipole regions.
The IOD has positive, negative, and neutral phases. During the positive phase, the western Indian Ocean is warmer than the eastern Indian Ocean, whereas the reverse is true during the negative phase of the IOD (Figure 2). The IOD varies irregularly in time but is usually strongest during August–November.
Fig. 2. SST Patterns of positive and negative IOD.
While the IOD is not always strong during ENSO events, it is unlikely for the IOD to be negative during El Niño and positive during La Niña. A positive IOD event in combination with an El Niño event enhances the El Niño impacts. Likewise, a negative IOD event in combination with a La Niña event enhances the impacts of the latter.
The IOD strongly influences precipitation over East Africa during the October–December season: a positive IOD results in enhanced precipitation over East Africa while a negative IOD results in reduced precipitation, as illustrated by the December precipitation anomalies in Figure 3.
Fig. 3. Precipitation impacts of the IOD in December.
The impacts of a positive or negative IOD on FEWS NET regions are shown in the table below.
|Region||Season||Positive IOD||Negative IOD|
|Southern Africa||Nov-Mar||Minimal impact||Minimal impact|
|West Africa||Apr-Jul||Minimal impact||Minimal impact|
and the Caribbean
|Jun-Aug||Minimal impact||Minimal impact|
|Central Asia||Jul-Aug||Minimal impact||Minimal impact|
|Note: The East Africa March–May season is not included in this table, as a negative or positive IOD is a very unlikely occurrence in this region at that time.|
About FEWS NET
The Famine Early Warning Systems Network is a leading provider of early warning and analysis on food insecurity. Created by USAID in 1985 to help decision-makers plan for humanitarian crises, FEWS NET provides evidence-based analysis on approximately 30 countries. Implementing team members include NASA, NOAA, USDA, USGS, and CHC-UCSB, along with Chemonics International Inc. and Kimetrica.
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