The Seasonal Teleconnections of the Indian Ocean Dipole to the North Atlantic region

The Indian Ocean Dipole (IOD) is a major source of seasonal climate variability in theIndian Ocean. This dipole has major impacts on the Indian Ocean region and throughteleconnections can influence the seasonal climate of remote regions as well. In late 2019 amajor IOD event contributed to the strong positive North Atlantic Oscillation (NAO) of thatwinter. Thus, a good understanding of the mechanism that transports information fromthe Indian Ocean to the North Atlantic is desirable. In this contribution we investigatethe special teleconnection of the winter of 2019 and analyse the transport mechanism.In model experiments with the OpenIFS from ECMWF we show that the NAO in thewinter 2019 is influenced by the Indian Ocean Dipole. We use hindcast ensemble modelexperiments to analyse the behaviour of the IOD and its impact on the NAO. Theseseasonal hindcast experiments are started from the 01. November 2019 and run for theDJF season 2019/2020. Since the OpenIFS is uncoupled we change the Sea SurfaceTemperature (SST) boundary conditions in regions of importance to the NAO (like theENSO region, the North Atlantic, and also the Indian Ocean). With these perturbationswe identify the relative importance of individual ocean regions to the state of the NAOin the winter of 2019.We contrast the experiments with the perturbed SST conditions to a control forecast andERA5 reanalysis. We find that removing the IOD has a significant impact on the NAO ofthe 2019/2020 DJF season, pushing the NAO to a more negative state. Additionally, thecontrast between control forecast and model experiments shows Rossby Waves emanatingfrom the Indian Ocean over the North Pacific and the Arabian Peninsular.Experiments with perturbations in other ocean regions show that some signals, like ENSO,can suppress the impact of the IOD on the NAO, but in their absence the positive IODevent of 2019 did likely contribute to the strong positive NAO of 2019/2020.