Early winter underestimated Indian Ocean teleconnections to the Northern extra-tropics in seasonal forecast models

There has been increased attention to the Indian Ocean as a source of predictability for the early winter climate over the North Atlantic European (NAE) region in the form of a North Atlantic Oscillation (NAO)-like pattern. Particularly, the October Indian Ocean Dipole (IOD) triggers a delayed precipitation response over the Indian Ocean in early winter, which mediates the Rossby wave that propagates to the NAE sector. Seasonal forecast models have been found to be severely underestimating this teleconnection, and finding the underlying causes is crucial to the improvement of the models’ performance. We demonstrate that models lack an adequate December Indian Ocean convective response, both in terms of magnitude and spatial pattern. This misrepresentation of the heating influences the generation of Rossby Wave Source (RWS) along the subtropical jet. Employing a dry linear baroclinic model, we also find that the December upper-tropospheric mean state does not support accurate downstream wave propagation in the forecast models. Particularly, a combination of a stronger subtropical jet and erroneous polar waveguide zonally traps the wave train in the models. Furthermore, our linear model simulations also suggest that this inaccurate downstream wave propagation potentially affects the strengthening of the wave over the North Pacific.