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An eddy-resolving four-dimensional variational (adjoint) data assimilation and state estimate was constructed for the low- to mid-latitude Pacific, Indian Oceans, and South China Sea based on the framework of "Estimating the Circulation and Climate of the Oceans (ECCO)". It is named as the Two Oceans One Sea State Estimate (TOOSSE). It fits a model to a number of modern observations of 2015–2016, including the Argo float temperature and salinity, satellite altimetric sea surface anomalies, by adjusting initial temperature and salinity, sea surface boundary conditions, and background diapycnal diffusivities. In total, ∼50% of the original model-data misfits have been eliminated, and the estimated state agreed well with a variety of independent observations at meso- to large scales, and on the intra-seasonal to interannual timescales. Mesoscale variability is systematically strengthened in TOOSSE and closer to observations than that without data assimilation, which is especially evidenced by the improved simulation of the mesoscale tropical instability waves (TIWs). Adjustments to ocean surface forcing parameters exhibit both large and frontal/mesoscale structures, and the magnitude reach 20%–100% of the first guesses; the adjustments to diapycnal diffusivity exhibit an obvious elevation (decrement) in (below) the thermocline in the equatorial band. The results indicate that TOOSSE represents a dynamically and thermodynamically consistent ocean state estimate of the 2015–2016 Indo-Pacific Ocean, and can be widely utilized for regional process studies. [ABSTRACT FROM AUTHOR]

An ensemble-based method for the observation system simulation experiment (OSSE) is employed to design optimal observation stations and assess the present observation stations in the northeastern South China Sea (SCS). We employed the 20-year (1992–2012) sea surface height (SSH) data to design an array to monitor the intraseasonal to interannual variability. The results show that the most key region was found located at the northwest of Luzon Island (LI) where the energetic Luzon cyclonic gyre (LCG) occurs; other key regions include the edge of the LCG, the northwest of the Luzon Strait (LS), and the southwest of Taiwan, China. By contrast, we found that the present observation stations might oversample at the northwest of the LS and undersample at the northwest of LI. In addition, the optimal stations perform better in a larger area than the present stations. In vertical direction, the key layer is located within the upper 200-m depth, of which the surface and subsurface layers are most valuable to the observing system. [ABSTRACT FROM AUTHOR]