CoAuthors

Norihisa Usui (Meteorological Research Institute, Japan Meteorological Agency, Japan); Yosuke Fujii (Meteorological Research Institute, Japan Meteorological Agency, Japan)

A new estimation method for the mean sea surface dynamic height (MSDH) is applied for a variational ocean data assimilation system developed in the Meteorological Research Institute, Japan Meteorological Agency (MOVE system). MOVE system constrains temperature and salinity as state variables (Fujii and Kamachi, 2003) with assimilating in-situ profiles, sea surface temperature, and sea level anomalies (SLAs). For the calculation of the cost function associated with the SLAs, a MSDH is necessary to refer to the model sea surface dynamic height (SDH). A conventional MSDH in the MOVE system is produced by assimilating temperature and salinity from the in-situ profiles during 1993-2012 on the climatology of World Ocean Atlas without numerical simulations. However, it significantly depends on the density of the observations. For instance, it is difficult to conduct in-situ observations in winter in the Sea of Okhotsk, where sea ice covers sea surface. In addition, the number of the observation in the 1990s, before the deployment of the Argo floats, was sparse compared to the 2000s, after the deployment of the Argo floats. A new MSDH proposed here is estimated by subtracting the observed SDH, which is calculated by using in-situ profiles, from the SLAs. It can reduce seasonal dependency of the in-situ observations and extent the period of SLA data from 1993 to the latest. The MSDH used in the MOVE system consist of a steric component while the observed SLAs includes both steric and non-steric components. The non-steric variations derived from the barotropic response to atmospheric forcing and the global ocean mass change must be corrected, and the correction was conducted based on the method in Hirose et al. (2019). The steric MSDH values on each in-situ profile referenced 2000 m depth are gridded on the eddy-resolving model for the North Pacific Ocean. The new MSDH shows a similar spatial pattern against the conventional MSDH and has a bias of 0.5 cm averaged over the whole domain, indicating that the new approach basically worked well. In the south of Japan, however, the significant differences between the new and conventional MSDHs, which are larger than 10 cm, were obtained corresponding to the shift of the mean Kuroshio axis. We will present an impact of the new MSDH on the MOVE system in the presentation.