On the underestimated Indian Ocean teleconnections to the northern extra-tropics in seasonal forecast models

Recent studies show that the October Indian Ocean Dipole (IOD), rather than El Niño Southern Oscillation (ENSO), leads to strong Indian Ocean precipitation anomalies in early Winter, which results in a Rossby wave response that propagates through the extratropical Northern Hemisphere, eventually impacting the North Atlantic Oscillation (NAO). Although seasonal prediction systems reproduce the teleconnection patterns reasonably well, they underestimate the amplitude of the signals. Our study investigates the potential sources of these reduced teleconnection signals to northern extra-tropics, particularly to NAO. These weak signals could be arising from a multitude of factors, including biases in the forcing, basic state, incorrect eddy-mean flow interaction, etc. Using a simple Linear Baroclinic Model (LBM), we analyze whether these discrepancies, at least partially, could arise from the basic state or the forcing over the Indian Ocean. Running the LBM with basic state and forcing from the ECMWF seasonal hindcast dataset SEAS5 shows a more zonally trapped Rossby wave response, while running it with ERA5 basic state and forcing shows a more meridional propagation of the Rossby wave. This could be due to the stronger South Asian Jet (SAJET) in SEAS5 than ERA5. The impact of the differences in forcing is less clear. Using an intermediate complexity atmospheric general circulation model (AGCM), we further investigate the sensitivity to forcing and jet in modulating the extratropical circulation response to Indian Ocean heating. Despite the possibility of errors arising from multiple factors, understanding whether forcing in the source region and/or model climatology plays a role in producing such reduced signals could pave the way towards improving the seasonal forecast for early boreal winter.