Coupled Ocean–Atmosphere Assimilation for NWP at ECMWF

As of June 2018, ECMWF has made seamless its forecasting systems, with all forecasts including a three-dimensional ocean and sea-ice model. This complements the existing atmosphere-land coupling present in the forecasts. With such an Earth-system approach being employed, initial conditions for all components need to be supplied. In this talk I will describe the current and future strategies for ocean-atmosphere coupled data assimilation at ECMWF.

As a first step towards having a consistent analysis across the atmosphere and the ocean components, weakly coupled data assimilation (WCDA) has been implemented. In this talk I will describe the impact of WCDA through the transfer of sea-ice concentration and sea surface temperature from the ocean analysis to the atmosphere. Further I will discuss why we choose to limit sea surface temperature to the tropics and not the extra-tropics.

In parallel a stronger form of coupled analysis has been developed - quasi-strongly coupled assimilation (or outer loop coupling). This type of coupling was pioneered at ECMWF for the CERA-20C and CERA-SAT reanalyses. I will show results that suggest improvements from this stronger form of coupling extend further into the atmosphere than by using WCDA.

There are a number of challenges that need to be overcome to make quasi-strongly coupled assimilation an operationally viable method. These include differing timescales in the 2 components, different timescales in the latency of observations, and biases in the ocean component of the coupled model. I will present a method that combines weakly and quasi-strongly coupled assimilation which has the potential to address all these issues.