The relative contribution of initial condition versus atmospheric forcing in 10-day forecasts of the Mediterranean Sea

Using a suite of initial-condition (IC) ensemble forecasting experiments for the Mediterranean Sea, we assess the relative contribution of initial conditions versus atmospheric forcing in 10-day forecasts. Each ensemble member is forced at the surface by ECMWF fields and forecasts the same 10-day period starting from a different initial condition, which is taken from an ocean analysis estimate of the preceding 10-day period. This IC-time-shifted ensemble scheme allows us to explore the forecast dependency from both the IC and the atmospheric forcing (i.e., ECMWF).  Generally, the surface forcing dominates the forecast trajectory at the surface, leading to an inter-member spread that decreases with time. There are also cases in which the initial spread at the surface increases with time, indicating the dominant role of the uncertainty in the IC. While much less common, there are specific times and regions in which the initial condition determines the forecast trajectory during multiple days, indicating that certain oceanic structures are intrinsically more predictable. The identification of these highly-predictable oceanic states might prove extremely valuable in predicting the uncertainty in the forecast.