Long-term Modulation And Response To Greenhouse Gases Increasing Of The Tropical Indian Ocean SST Interannual Variability

Based on observational datasets,the intergrations of a coupled ocean-atmosphere general circulation model and various statistical methods,the low-frequency modulation and response to greenhouse gases of the dominating modes of tropical Indian Ocean SST interannual variability——Indian Ocean Basin (IOB) mode and Indian Ocean dipole (IOD) mode are investigated in this paper. It is first time that low-frequency modulation and change under greenhouse gas increasing of the IOD mode are investigated systematically with a pair of multicentury integrations of a coupled ocean-atmosphere general circulation model.Using various observations, it is confirmed the multidecadal change of IOB mode and its "capacitor" effect in the 1970s and revealed its dynamic mechanism. Then multidecadal modulation of IOB mode duo to ENSO's strength changing is confirmed by analyzing the intergration of a coupled model. The change of IOB mode under greenhouse gases increasing is investigated. Some new results are found as follows:1.It is found that IOD mode has remarkably low-frequency natural modulation, which is influenced by the mean thermocline depth in the eastern equatorial Indian Ocean. With a shoaling of the EEIO thermocline, the thermocline feedback strengthens,and this leads to an increase in IOD variance and other characteristics changing.It is suggested that the IOD variance is not only associated with the thermocline feedback but also with the atmospheric feedback. In response to greenhouse gases increasing, the thermocline feedback of IOD is enhanced, the atmospheric feedback is weakened and therefore there are little change of IOD variance.The zonal wind anomalies associated IOD are found to weaken, likely due to increased static stability of the troposphere from global warming.The opposing changes in thermocline and atmospheric feedbacks result in little change in IOD variance.2.It is revealed that the IOB mode is enhanced after the climate regime shift of the 1970s due to the ENSO's intensity increasing.All processes in the formation of IOB mode including oceanic Rossby wave in Southwestern Indian Ocean, the Northern Indian Ocean warming in boreal summer and anti-symmetric wind pattern are different between the pre-regime and post-regime shift epoch. Due to the intensification of IOB mode and its "capacitor" effect after 1970s, the surface anti-cyclone circulation over Northwestern Pacific is enhanced. Furthermore, based on the ocean-atmosphere coupled model it is found that the change of ENSO strength could cause the multidecadal of IOB mode.3.After greenhouse gases doubling, it is found that the IOB mode is enhanced in boreal summer and more persistent, especially for the anti-symmetric wind pattern and North Indian Ocean warming in summer. A hypothesis is raised that it is associated with the stronger interannual variability of Western Pacific SST after greenhouse gases doubling. The stronger IOB mode enhances its "capacitor" effect, including the NW Pacific surface anti-cyclonic circulation.The above study investigates the formation and persistence of dominating modes of the tropical Indian Ocean SST interannual variability systematically and reveals the roles of dynamic processes in the low-frequency modulation and change under greenhouse gases increasing. The results could provide more understanding of ocean-atmospheric interactions over Indo-Pacific oceans and provide theoretical proof for long-term climatic prediction.