Decadal Variability And Dynamics Of The IOD-ENSO Precursive Teleconnection

In this study, the observational datasets, CCSM4(Community Climate System Model version 4) simulations and CESM1(Community Earth System Model version 1) numerical experiments, are used in the lag correlation analyses between the oceanic anomalies in the southeastern tropical Indian Ocean(STIO) in fall and the Indo-Pacific oceanic anomalies in the following four seasons, to investigate the decadal variability of 1-year time lag correlation between Indian Ocean Dipole(IOD) and El Ni?o-Southern Oscillation(ENSO) and the role of the oceanic channel dynamics in the relation(the IOD-ENSO precursive teleconnection), and to examine the effect of the SST(sea surface temperature) variabilities in the Indo-Pacific Warm Pool(IPWP) on the IOD-ENSO precursive teleconnection by modifying the Walker circulation.The lag correlations between the observed SST anomalies(SSTA) in STIO in fall and those in the Pacific cold tongue at the one-year time lag are shown to have decadal variability after filtering with running windows of 7-11 years. Similar decadal variability has also been identified in the historical simulations of CCSM4 that participates in the Coupled Model Intercomparison Project phase-5(CMIP5). The dynamics of the inter-basin teleconnection during the positive phases of the decadal variability are diagnosed in the simulations using composite lag correlations, which suggest that the interannual variations of the Indonesian Throughflow(ITF) associated with IOD produces subsurface temperature anomalies in the western equatorial Pacific Ocean to propagate to the east. This connection is called the ―oceanic channel‖ dynamics of the IOD-ENSO precursory relation and is shown to be consistent with the observational analyses. In comparison, the subsurface lag correlations in the western Pacific Ocean during the negative phases of the decadal variability are still dominated by the oceanic channel dynamics, but do not correlate well with the SSTA in the cold tongue due to a deeper thermocline in the eastern equatorial Pacific. The CCSM4 model is found to underestimate the interannual variability of the ITF transport but overestimate the westerly wind anomalies in the western-central equatorial Pacific, which force unrealistic anomalies in the equatorial Pacific Ocean associated with the IOD, indicating model deficiency in simulating the ―atmospheric bridge‖.Lag correlation analyses are compared between the CESM1 control experiment, in which the atmosphere and the ocean over the Indo-Pacific Oceans are fully coupled, and the sensitivity experiment, in which the time-varying SST over the IPWP are replaced with annual mean climatological SST when forcing the atmosphere. The comparison of the control and the sensitivity experiments suggests that the removal of IPWP SST variability has little effects on the dominance of the oceanic channel dynamics during the positive phases of the decadal variability of the IOD-ENSO precursive relation. However, the seasonal variations of the tropical Indo-Pacific atmospheric bridge are greatly altered by the modification of the IPWP SST variability. In the sensitivity experiment, the Walker circulation is greatly enhanced in summer and fall so that a shallower thermocline in the central and eastern equatorial Pacific Ocean is established, which strengthens the positive phases of the decadal variability and weakens the negative.