Optimal adjustment of the atmospheric forcing parameters of ocean models using sea surface temperature data assimilation

[摘要] In ocean general circulation models, near-surface atmospheric variables usedto specify the atmospheric boundary condition remain one of the main sourcesof error. The objective of this research is to constrain the surface forcingfunction of an ocean model by sea surface temperature (SST) dataassimilation. 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 toevaluate the impact of uncertain atmospheric forcing on the ocean state. Thecontrol vector of the assimilation method is extended to atmosphericvariables to obtain monthly mean parameter corrections by assimilatingmonthly SST and sea surface salinity (SSS) climatological data in a lowresolution global configuration of the NEMO model. In this context, thecareful determination of the prior probability distribution of the parametersis an important matter. This paper demonstrates the importance of isolatingthe impact of forcing errors in the model to perform relevant ensembleexperiments.

The results obtained for every month of the period between 1989 and 2007 showthat the estimated parameters produce the same kind of impact on the SST asthe analysis itself. The objective is then to evaluate the long-termtime series of the forcing parameters focusing on trends and mean errorcorrections of air–sea fluxes. Our corrections tend to equilibrate the netheat-flux balance at the global scale (highly positive in ERAi database), andto remove the potentially unrealistic negative trend (leading to oceancooling) in the ERAi net heat flux over the whole time period. Morespecifically in the intertropical band, we reduce the warm bias of ERAi databy mostly modifying the latent heat flux by wind speed intensification.Consistently, when used to force the model, the corrected parameters lead toa better agreement between the mean SST produced by the model and mean SSTobservations over the period of 1989–2007 in the intertropical band.

[学科分类] 海洋学与技术