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Subtropical Indian Ocean Dipole
The Subtropical Indian Ocean Dipole (SIOD) is featured by the oscillation of sea surface temperatures (SST) in which the southwest Indian Ocean i.e. south of Madagascar is warmer and then colder than the eastern part i.e. off Australia.〔Behera SK, Yamagata T. 2001. (A dipole mode in the tropical Indian Ocean ), Geophysical Research Letters 28: 327–330.〕 It was first identified in the studies of the relationship between the SST anomaly and the south-central Africa rainfall anomaly; the existence of such a dipole was identified from both observational studies and model simulations〔Behera SK, P.S. Salvekar, and Yamagata T. 2000. (Simulation of interannual SST variability in the tropical Indian Ocean ), J. Clim, 13, 3487-3499.〕
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==The phenomenon==
Positive phase of Subtropical Indian Ocean Dipole is characterized by warmer-than-normal sea surface temperature in the southwestern part, south of Madagascar, and colder-than-normal sea surface temperature off Australia, causing above-than-normal precipitation in many regions over south and central Africa. Stronger winds prevail along the eastern edge of the subtropical high, which become intensified and shifted slightly to the south during the positive events, leading to the enhanced evaporation in the eastern Indian Ocean, and therefore result in the cooling SST off Australia. On the other hand, reduced evaporation in the southwestern part causes reduced seasonal latent heat loss, and therefore results in increased temperature in the southwestern part, south of Madagascar. The negative phase of the SIOD is featured by the opposite conditions, with warmer SSTs in the eastern part, and cooler SSTs over the southwestern part. The physical condition favoring negative events is also just opposite. Also, Ekman transport accompanied with surface mixing process also plays a role in the formation of the SST dipole.〔
Generally speaking, the Subtropical Indian Ocean Dipole mode develops in December–January, peaks in February, then decays in the following two months, and finally dies down in May–June. The evolution and deformation process of the Subtropical Indian Ocean Dipole event is highly affected by the position of the subtropical high; atmospheric forcing plays a significant role in the evolution process of the Subtropical Indian Ocean Dipole event.〔
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