Climate of the Indian Ocean and the Indian Ocean Dipole
The Indian Ocean (IO) is the third largest water body in the world extending from the coast of East Africa eastward to the Maritime Continent and Australia. Observations have shown that on average, the IO is relatively warmer in the east and central sector than in the west, creating a temperature difference between the east and west. The IO features a unique climate variability associated with changes in sea surface temperature and winds across the basin.
The most dominant variation is a warming or cooling of the entire IO basin referred to as Indian Ocean basin mode (IOBM). The warming of the IO basin corresponds to a positive phase of the IOBM, while a negative phase of the IOBM refers to the cooling of the IO basin. The IOBM is inherently associated with the El Niňo Southern Oscillation (ENSO) such that El Niňo induces warming across the IO basin with a lag of a few months. During El Niňo, low-level easterly winds weaken across the tropical Pacific Ocean and strengthen across the tropical IO. These changes in wind patterns reverse in the upper atmosphere. Consistent with the impact of El Niňo, a positive IOBM is associated with rainfall surpluses in the western IO and equatorial East Africa, and rainfall suppression in the eastern IO and the Maritime Continent. These conditions reverse during La Niňa and the negative phase of the IOBM as the western IO and equatorial East Africa receive less rainfall, while the eastern IO and the Maritime Continent register above average rainfall. Additional resources about the IOBM are available here.
The second most dominant variation of the IO is in the east – west direction straddling the equator referred to as the Indian Ocean Dipole (IOD). The IOD is a naturally occurring climate phenomenon that appears more frequently and is more intense during the northern hemisphere summer to winter, usually starting around June and peaking between August and October. Studies have shown that the IOD is an intrinsic mode of the IO. However, sometimes ENSO can trigger and modulate the IOD. Meteorologists commonly measure the IOD by calculating an index referred to as the Dipole Mode Index (DMI). The DMI is the difference between SST departures from average in the western equatorial IO and the eastern equatorial IO. The IOD has three phases: positive, negative, and neutral. The positive phase of the IOD is associated with above average SST in the western IO and below average SST in the eastern IO. The SST patterns reverse during the negative phase of the IOD when the surface of the ocean becomes warmer in the eastern IO and cooler in the western IO. Neutral IOD occurs when the difference in the SST changes in the western and eastern IO are negligible.
The IOD is associated with variations in low and upper level winds and affect rainfall patterns around the IO basin. The positive phase of the IOD tends to increase rainfall in the western IO and equatorial East Africa, and decrease rainfall in the eastern IO and the Maritime Continent. The negative phase of the IOD suppresses rainfall in the western IO and equatorial East Africa, and triggers above average rainfall in the eastern IO and the Maritime Continent. Historical IOD events are available here. Additional resources about the IOD is available here.