The Representation of the Circumglobal Teleconnection in the ECMWF Seasonal Forecast Model

Recent research has led to improvements in European winter seasonal forecasts, however there has been less of a focus on the summer season and so summer forecast skill remains relatively low. The European climate is affected by a large range of influences, which include influences from tropical regions, and better understanding of the mechanisms behind these tropical-extratropical teleconnections can inform our evaluation of seasonal forecast systems and priorities for model development.

One such potential source of predictability for Europe is the Indian summer monsoon (ISM), which can affect European weather via a global wave train known as the “Circumglobal Teleconnection” (CGT, Ding and Wang 2005). Here we assess the ability of the ECMWF coupled seasonal forecast model to represent this teleconnection mechanism. We use seasonal hindcasts for JJA which are initialised on the 1^st May, with 25 ensemble members, for the period 1981-2014. We show that the model has errors in forecasting geopotential height and jet stream location in several key regions (“centres of action”) for the teleconnection mechanism, and that the representation of the CGT wave pattern in the model is much weaker than observed. We will present results from several relaxation experiments, including relaxing regions over west-central Asia and northwest Europe, to further understand the cause of these errors and to determine how they relate to errors in the CGT pattern and poor forecast skill over Europe.