Ocean Initialization for Seasonal Forecasts

The potential for climate predictability at seasonal time scales resides in information provided by the ocean initial conditions, in particular the upper thermal structure. Currently, several operational centres issue routine seasonal forecasts produced with coupled ocean-atmosphere models, requiring real-time knowledge of the state of the global ocean. Seasonal forecasting needs the calibration of the numerical output of the coupled model, which in turn requires an historical ocean reanalysis, as will be discussed in this paper. Assimilation of observations into an ocean model forced by prescribed atmospheric fluxes is the most common practice for initialization of the ocean component of a coupled model. It is shown that the assimilation of ocean data reduces the uncertainty in the ocean estimation arising from the uncertainty in the forcing fluxes. Although data assimilation also improves the skill of seasonal forecasts in many cases, its impact is often overshadowed by errors in the coupled models. This paper offers a review of the existing ocean analysis efforts aiming at the initialization of seasonal forecasts. The current practice, known as "uncoupled" initialization, has often been criticized as having several shortcomings, the initialization shock being one of them. On the other hand, the uncoupled initialization usually benefits from better knowledge of the atmospheric forcing fluxes, an advantage that should not be overlooked. In recent years, the idea of obtaining truly "coupled" initialization, where the different components of the coupled system are well balanced, has stimulated several research activities that will be reviewed in light of their application to seasonal forecasts.