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Abstract

The Indian Ocean Dipole (IOD) is a major source of seasonal climate variability. This dipole has strong impacts on the Indian Ocean region and through teleconnections can influence the seasonal climate of remote regions. The strong IOD of 2019/2020 influenced the state and predictability of the Northern Hemisphere extratropics. Thus, a good understanding of the mechanism of this teleconnection is desirable. In this contribution we investigate the special teleconnection of the winter 2019/2020. In model experiments with the OpenIFS from ECMWF we show that the NAO in the winter 2019/2020 is influenced by the IOD. We use hindcast ensembles of the DJF season 2019/2020 to analyse this teleconnection. In model experiments, the Sea Surface Temperature (SST) boundary conditions are perturbed. With these experiments we identify the relative importance of individual ocean regions to the state of the NAO in the winter of 2019/2020. We contrast the experiments to the operational ECMWF System5 forecast and ERA5. Experiments with the 2019/2020 SSTs in the Indian Ocean (with other boundary conditions set to climatology) reproduce many of the observed atmospheric 2019/2020 features. In contrast, experiments with SSTs in the Pacific show very different patterns. We identify eddy-mean-flow interactions as a mechanism that connects and transports information from the Indian Ocean to the North Atlantic. With Hoskins E-Vectors we show that anomalous eddy activity during IOD events impacts the position and strength of the Northern Hemisphere extratropical jet. This interaction provides a teleconnection mechanism in addition to the Rossby-wavetrain discussed in other studies.