Tropical Atlantic rainfall drives bias in extratropicalseasonal forecasts

We investigate the impact of Tropical Atlantic seasonal forecast biases on the North Atlantic. The analysis focuses on the hindcasts of the Met Office Global Seasonal forecast system (GloSea).  A tripolar bias occurs in Tropical Atlantic rainfall, where the seasonal forecast system is too dry on the equator and too wet to the north and south of the equator, showing a “double ITCZ” pattern. Our analysis uses a novel ensemble-based method to estimate the impact of this tropical rainfall bias on forecasts of the Extratropical North Atlantic. Prior to completing the analysis, ENSO is regressed out of the fields to ensure this common teleconnection is not the source of this bias. The inter-ensemble spread of the forecast model is used to estimate the impact of the bias in Tropical Atlantic rainfall on the North Atlantic by selecting model members that happen to produce forecast anomalies that most closely resemble the tropical rainfall bias and using these as a proxy for the model error. The Tropical Atlantic rainfall bias impacts Rossby wave sources over the Subtropical Atlantic and there is a clear Rossby wave pattern originating from this area which is comparable to the mean bias in hindcasts. The bias impacts the PMSL over the North Atlantic and projects onto the NAO pattern. We show that the bias is not limited to the GloSea System and is also seen in other seasonal forecast systems. Our results suggest that Tropical Atlantic rainfall errors explain a significant amount of the mean bias in seasonal forecasts over the Extratropical North Atlantic.