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

<p><span dir="ltr" role="presentation">The Indian Ocean Dipole (IOD) is a major source of seasonal climate variability in the</span><br role="presentation" /><span dir="ltr" role="presentation">Indian Ocean. This dipole has strong impacts on the Indian Ocean region and through</span><br role="presentation" /><span dir="ltr" role="presentation">teleconnections can influence the seasonal climate of remote regions like the North Pacific</span><br role="presentation" /><span dir="ltr" role="presentation">and North Atlantic. A prominent example of this teleconnection from the IOD occurred</span><br role="presentation" /><span dir="ltr" role="presentation">in the winter 2019/2020, where the IOD was in a positive state. This influenced the state</span><br role="presentation" /><span dir="ltr" role="presentation">and predictability of the Northern Hemisphere extratropics.</span> <span dir="ltr" role="presentation">Thus, a good understand-</span><br role="presentation" /><span dir="ltr" role="presentation">ing of the mechanism that transports information from the Indian Ocean to the North</span><br role="presentation" /><span dir="ltr" role="presentation">Atlantic is desirable. In this contribution we investigate the special teleconnection of the</span><br role="presentation" /><span dir="ltr" role="presentation">winter 2019/2020 and analyse the transport mechanism.</span><br role="presentation" /><span dir="ltr" role="presentation">In model experiments with the OpenIFS from ECMWF we show that the NAO in the</span><br role="presentation" /><span dir="ltr" role="presentation">winter 2019/2020 is influenced by the IOD and analyse the teleconnection mechanisms.</span><br role="presentation" /><span dir="ltr" role="presentation">We use hindcast ensemble model experiments of the DJF season 2019/2020 to analyse</span><br role="presentation" /><span dir="ltr" role="presentation">the behaviour of the IOD and its impact on the NAO. In the uncoupled OpenIFS the Sea</span><br role="presentation" /><span dir="ltr" role="presentation">Surface Temperature (SST) boundary conditions are perturbed in regions of importance</span><br role="presentation" /><span dir="ltr" role="presentation">to the NAO (like the ENSO region and the Indian Ocean). With these perturbations we</span><br role="presentation" /><span dir="ltr" role="presentation">identify the relative importance of individual ocean regions to the state of the NAO in</span><br role="presentation" /><span dir="ltr" role="presentation">the winter of 2019/2020.</span><br role="presentation" /><span dir="ltr" role="presentation">We contrast the experiments with the perturbed SST conditions to the operational ECMWF</span><br role="presentation" /><span dir="ltr" role="presentation">System5 forecast and ERA5. Experiments with the 2019/2020 SST&#8217;s in the In-</span><br role="presentation" /><span dir="ltr" role="presentation">dian Ocean (with other boundary conditions set to climatology) reproduce many of the</span><br role="presentation" /><span dir="ltr" role="presentation">observed atmospheric 2019/2020 features.</span> <span dir="ltr" role="presentation">In contrast, experiments with SST&#8217;s in the</span><br role="presentation" /><span dir="ltr" role="presentation">Pacific show very different patterns to the observed 2019/2020 ones.</span><br role="presentation" /><span dir="ltr" role="presentation">We identify eddy-mean-flow interactions as a mechanism that connects and transports</span><br role="presentation" /><span dir="ltr" role="presentation">information from the Indian Ocean to the North Atlantic.</span> <span dir="ltr" role="presentation">With Hoskins E-Vectors we</span><br role="presentation" /><span dir="ltr" role="presentation">show that anomalous eddy activity during IOD events impacts the position and strength</span><br role="presentation" /><span dir="ltr" role="presentation">of the Northern Hemisphere extratropical jet. This interaction provides a teleconnection</span><br role="presentation" /><span dir="ltr" role="presentation">mechanism in addition to the Rossby-wavetrain discussed in other studies.</span></p>