The state of the Indian Ocean dipole representing the SST anomaly difference between the western and southeastern regions of the ocean is investigated using historical SST reconstructions from 1880 to 2004. First, the western and eastern poles of the SST-based dipole mode index are analyzed separately. Both the western and eastern poles display warming trends over this period, particularly after the 1950s. The western pole tends to be anomalously colder than the eastern pole from 1880 to 1919, whereas in the interval 1950-2004 the SST anomalies over the western pole are comparable to those over the eastern pole though there are occasional outliers where the eastern pole is anomalously colder than the western pole.The tendencies of the occurrences of positive and negative dipole events in September-November show three distinct regimes during the period analyzed. In 1880-1919, negative dipole events associated with La Nina events occur more frequently than positive events. In 1920-49, some weak positive events occur relatively independently of El Nino events over the Pacific. The period of 1960-2004 is characterized by strong and frequent occurrences of positive events associated with El Nino events.
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