Abstract
Abstract. In ocean general circulation models, near-surface atmospheric variables used to specify the atmospheric boundary condition remain one of the main sources of error. The objective of this research is to constrain the surface forcing function of an ocean model by sea surface temperature (SST) data assimilation. For that purpose, a set of corrections for ERAinterim (hereafter ERAi) reanalysis data is estimated for the period of 1989–2007, using a sequential assimilation method, with ensemble experiments to evaluate the impact of uncertain atmospheric forcing on the ocean state. The control vector of the assimilation method is extended to atmospheric variables to obtain monthly mean parameter corrections by assimilating monthly SST and sea surface salinity (SSS) climatological data in a low resolution global configuration of the NEMO model. In this context, the careful determination of the prior probability distribution of the parameters is an important matter. This paper demonstrates the importance of isolating the impact of forcing errors in the model to perform relevant ensemble experiments. The results obtained for every month of the period between 1989 and 2007 show that the estimated parameters produce the same kind of impact on the SST as the analysis itself. The objective is then to evaluate the long-term time series of the forcing parameters focusing on trends and mean error corrections of air–sea fluxes. Our corrections tend to equilibrate the net heat-flux balance at the global scale (highly positive in ERAi database), and to remove the potentially unrealistic negative trend (leading to ocean cooling) in the ERAi net heat flux over the whole time period. More specifically in the intertropical band, we reduce the warm bias of ERAi data by mostly modifying the latent heat flux by wind speed intensification. Consistently, when used to force the model, the corrected parameters lead to a better agreement between the mean SST produced by the model and mean SST observations over the period of 1989–2007 in the intertropical band.
Highlights
Near-surface variables from atmospheric reanalyses are commonly used to specify surface boundary condition in ocean general circulation models required for operational forecasts, ocean reanalyses, or hindcast simulations of the recent ocean variability
We will look at the results in terms of global scale heat budget resulting from our corrections
The first important step of the method validation is to look at the results over a specific month (January 2004) which corresponds to one assimilation window
Summary
Near-surface variables from atmospheric reanalyses (air temperature and humidity, wind speed, downward radiation and precipitation) are commonly used to specify surface boundary condition in ocean general circulation models (hereafter OGCMs) required for operational forecasts, ocean reanalyses, or hindcast simulations of the recent ocean variability (the last 50 yr). These atmospheric variables are characterized by significant uncertainties at global scale as shown in different studies, such as Milliff et al (1999), Wang and McPhaden (2001), Smith et al (2001) or Sun et al (2003). Models do not benefit, in their forcing, from one of the best observed ocean surface variables
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