Generating ocean initial condition for coupled forecasts through nudged NEMO experiments

Accurate ocean initial conditions are beneficial to coupled ocean - atmosphere forecasts in several ways depending on context. On short time scale (days), some extreme, societally high-impacting meteorological events such as tropical cyclones are associated with exceptionally intense air-sea exchanges, thus requiring good knowledge of the initial state of the upper ocean layers. As the forecast evaluation time scale gets longer (seasonal, decadal), the information contained within the atmospheric initial conditions becomes virtually ineffective to the advantage of that contained within the ocean’s.

In phase one of the Destination Earth (DestinE) initiative of the European Union, ECMWF is responsible for delivering the first two digital twins, on weather extremes and climate change adaptation. These will rely on the fusion of observations and cutting-edge, high-resolution versions of Earth system models. Developing a cheap, affordable method for generating realistic high-resolution ocean initial conditions is particularly critical to both these forthcoming digital twins developed in DestinE. In this presentation, we thus introduce a new ocean initial condition generation method built to meet these specific needs.

This method consists in running preliminary nudged ocean-standalone experiments, using the NEMO ocean model, which is part of the ECMWF’s Integrated Forecasting System. Its main technical novelty is the adaptation of a former sea-ice nudging scheme to NEMO’s multicategory SI3 sea-ice model, and it can be significantly cheaper than relying on ocean data assimilation, particularly at high resolutions. However, this method touches on several matters related to the ability of ocean models to be diverted from their natural equilibrium by being constrained towards increasingly realistic states. We investigate the criteria for obtaining realistic ocean initial condition using distinct combinations of model and nudging dataset resolutions, and evaluate the resulting skill of coupled ocean - atmosphere forecasts.