Unusual IOD event of 2007

The positive Indian Ocean Dipole (IOD) of 2007 evolved together with a La Nina in the Pacific and consecutive to a positive IOD event of 2006. It was an extremely rare blend of climate anomalies in those two basins. The evolution of a negative IOD, which normally follows a positive IOD, was reversed during boreal summer of 2007. This was associated with a pattern of sea surface temperature (SST) anomalies in which the colder seas of the Maritime Continent were flanked by the warmer central Pacific and Indian Oceans. The associated subsidence over the Maritime Continent caused divergent easterly wind anomalies in the eastern equatorial Indian Ocean to trigger a successive positive IOD event through ocean dynamics. Discrete occurrences of such events are found only twice in historical records; the positive IOD of 1967 evolved concurrently with a La Nina and the positive IODs of 1913 and 1914 evolved consecutively. Those historical events were recognized by a rainfall dipole index in addition to the customary SST dipole index.