Indian Ocean Dipole Variations During the Last Millennium in PMIP3 Simulations

Earlier proxy-observational studies, and a sole modeling study, suggest that the Indian Ocean Dipole (IOD), an important global climate driver, exhibited multi-scale temporal variability during the Last Millennium (LM; CE 0851-1849, with relatively high number of strong positive IOD events during the Little Ice Age (LIA; CE 1550-1749), and strong negative IOD events during the Medieval Warm Period (MWP; CE 1000-1199). Using nine model simulations from the PMIP3, we study the IOD variability during the LM after due validation of the simulated current day (CE 1850-2005) IOD variability. Majority of the models simulate relatively higher number of positive IOD events during the MWP, and negative IOD events in the LIA, commensurate with simulated background conditions. However, higher number of strong positive IOD events are simulated relative to the negative IODs during the LIA, in agreement with proxy-observations, apparently owing to increased coupled feedback during positive IODs. The Indian Ocean Dipole (IOD) is a natural climate phenomenon in the tropical Indian Ocean with significant global impacts. Positive IOD (pIOD) events are apparently occurring more frequently in recent decades, which may also be due to under-sampling associated with limited observations span. Analyzing outputs for last millennium (CE 850-1850) from climate models, validated for historical period, helps in generating relatively longer-period the paleo-IOD records. Our analysis of simulations of the last thousand years from multiple models indicates relatively more positive (negative) IOD events in medieval warm period-CE 1000-1200 (Little Ice Age-CE 1550-1749). While during the ICA, background conditions similar to a negative IOD were simulated, models also simulate an increase in relatively-stronger positive IOD events in its latter part, in agreement with a proxy-climate record. The simulated centennial changes in positive and negative IOD frequencies are associated with changes in coupled ocean-atmospheric feedback mechanisms. Change in the Indian Ocean mean state from the medieval warm period (MWP) to the Little Ice Age (LIA) Despite negative IOD-like background conditions in the LIA, models and paleo-data show more stronger positive IODs then There are significant changes in feedback mechanisms of IODs from the MWP to LIA