Ensemble optimal interpolation schemes for assimilating Argo profiles into a hybrid coordinate ocean model

Abstract

Abstract To develop a capable ensemble-based scheme for operationally assimilating Argo profile observations into a hybrid coordinate ocean model (HYCOM), we compared some different ensemble optimal interpolation (EnOI) schemes, which can be divided into two different kinds. The first kind is straightforward, i.e., updating the model variables (i.e., layer thickness, layer velocity and layer temperature/salinity) at the same time from the temperature and salinity profiles observed from Argo floats. In the second kind of schemes (will be referred as the modified schemes), which are based on Thacker and Esenkov (2002) , the Argo profiles are first converted to the “observations” of layer thicknesses and they are assimilated to adjust the model layer thickness and model velocity fields. Then the T (or S ) profiles are assimilated to adjust the model layer temperatures (or salinities), followed by deriving the model layer salinity (or temperature) from the equation of seawater state. In this study we showed that the two kinds of EnOI schemes can be implemented in various setups. Firstly for the straightforward schemes an analysis can be done using the thinned (to model layers) observations of Argo profiles or using the full vertical information provided by the Argo profiles. Secondly one can make an analysis applying or not applying localization vertically. Thirdly for the modified schemes either the temperature or the salinity field can be diagnosed. Then we designed six assimilation experiments, each is attached to a different setup in order to test and compare their performances. In all experiments, Argo profiles were assimilated into HYCOM in the Pacific for a four-year period (January, 2004–December, 2007). A large amount of Argo profiles were withheld to validate the assimilation results. The results show the significant improvement by the modified schemes over the straightforward schemes. The best setup of the modified scheme is to diagnose the temperature and to apply the vertical localization at the step of assimilating the layer temperatures.