The seasonal teleconnections of the Indian Ocean Dipole to the North Atlantic region

Abstract

The Indian Ocean Dipole (IOD) is a major source of seasonal climate variability in theIndian Ocean. This dipole has strong impacts on the Indian Ocean region and throughteleconnections can influence the seasonal climate of remote regions like the North Pacificand North Atlantic. A prominent example of this teleconnection from the IOD occurredin the winter 2019/2020, where the IOD was in a positive state. This influenced the stateand predictability of the Northern Hemisphere extratropics. Thus, a good understand-ing of the mechanism that transports information from the Indian Ocean to the NorthAtlantic is desirable. In this contribution we investigate the special teleconnection of thewinter 2019/2020 and analyse the transport mechanism.In model experiments with the OpenIFS from ECMWF we show that the NAO in thewinter 2019/2020 is influenced by the IOD and analyse the teleconnection mechanisms.We use hindcast ensemble model experiments of the DJF season 2019/2020 to analysethe behaviour of the IOD and its impact on the NAO. In the uncoupled OpenIFS the SeaSurface Temperature (SST) boundary conditions are perturbed in regions of importanceto the NAO (like the ENSO region and the Indian Ocean). With these perturbations weidentify the relative importance of individual ocean regions to the state of the NAO inthe winter of 2019/2020.We contrast the experiments with the perturbed SST conditions to the operational ECMWFSystem5 forecast and ERA5. Experiments with the 2019/2020 SST’s in the In-dian Ocean (with other boundary conditions set to climatology) reproduce many of theobserved atmospheric 2019/2020 features. In contrast, experiments with SST’s in thePacific show very different patterns to the observed 2019/2020 ones.We identify eddy-mean-flow interactions as a mechanism that connects and transportsinformation from the Indian Ocean to the North Atlantic. With Hoskins E-Vectors weshow that anomalous eddy activity during IOD events impacts the position and strengthof the Northern Hemisphere extratropical jet. This interaction provides a teleconnectionmechanism in addition to the Rossby-wavetrain discussed in other studies.