Subtropical Indian Ocean Dipole

(a)Regressed SIOD Index on detrended Sea Surface Temperature (SST) (1958-2007). The western and eastern box used to calculate SIOD Index are indicated. Water temperatures in the southwestern Southern Indian Ocean are significantly higher than water temperature in the eastern part of South Indian Ocean, off Australia. In this image blue areas are colder than normal, while red areas are warmer than normal. (b)The normalized SIOD index during 1958-2007. The values are normalized by stardard deviation of 0.65. Blue line indicates the original time series and the red curve are the 10-season running mean.

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.

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 and parts of Australia. 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. The physical condition favoring negative events is also just opposite. Also, Ekman transport accompanied with surface mixing process also play a role in the formation of the SST dipole.

The SIOD and Global Climate

Subtropical Indian Ocean Dipole related anomalies over the Southeastern Indian Ocean is also suggested to impact the position of Madagascar high and thus the Indian summer monsoon. Positive (negative) Subtropical Indian Ocean dipole events during boreal winter are always followed by strong (weak) Indian Summer Monsoons. During positive (negative) SIOD event, the Mascarene High shifting southeastward (northwestward) from austral to boreal summer causes a weakening (strengthening) of the monsoon circulation system by modulating of the local Hadley cell during the Indian Sumer Monsoon event.

Southwest Australia dry(wet) years are corresponding to positive(negative) SIOD events and these appear to in phase with the large-scale winds over the tropical/subtropical Indian Ocean, which modify SST anomalies through anomalous Ekman transport in tropical Indian Ocean and through anomalous air–sea heat fluxes in the subtropics, which also alter the large-scale advection of moisture to the Southwestern Australia coast (England et al. 2005). Positive SIOD events also causes increased summer rains over large parts of southeastern Africa by bringing enhanced convergence of moisture. Higher temperature over the Southwestern Indian Ocean warm pole results in increased evaporation, and this moist air is advected to Mozambique and eastern South Africa, which is strengthened by the low pressure anomaly generated over this warm pole.

The SIOD Index

The Subtropical Indian Ocean Dipole Index is computed from SST anomaly difference between western (55E-65E,37S-27S) and eastern (90E-100E,28S-18S) Indian Ocean.

See also

• Indian Ocean Dipole
• Monsoon of Indian subcontinent

External links

• Subtropical Indian Ocean Dipole
• Subtropical dipole mode events in the Indian Ocean