In 1995, the World Meteorological Organization (WMO) Second International Symposium on Assimilation of Observation in Meteorology and Oceanography was held in Tokyo, which initiated the Japanese Summer School on Data Assimilation. Over the past 20 years, the ocean data assimilation research community in Japan has accomplished remarkable achievements, including the successful prediction of the Kuroshio large meander and the development of a coupled atmosphere-ocean four-dimensional variational data assimilation system, which is the only one in the world. Ocean data assimilation systems are now operationally used to monitor and forecast the ocean state, and their products, or ocean reanalysis datasets, are broadly provided for oceanographic and climate studies, fisheries management, the prevention of oceanic disasters, and the maintenance of coastal areas. This paper provides a summary of the achievements of the research community, the current status of ocean data assimilation products, and ongoing research subjects in the community. Additionally, the requirements for successful progress in this research areas are discussed from a future perspective.

Large-scale circulations such as the Kuroshio flows and water masses characterized by specific temperatures and salinities, exist in the surface ocean. The author focuses his study on surface ocean variability in the western part of the North Pacific subtropics and its role in climate, using observational datasets, model outputs, and simple experiments. It revealed (1) the spatial structure of the reemergence mechanism of sea surface temperature anomalies; (2) the influence of the oceanic Rossby waves formed as a result of the decadal-scale meridional movement of the Aleutian Low in the central North Pacific on the formation of the North Pacific Subtropical Mode Water, Kuroshio transport south of Japan, and North Pacific Intermediate Water distribution south of Japan; (3) the temporal variability in the path state of the Kuroshio Extension; and (4) the roles of the surface ocean in the heat exchange between the ocean and atmosphere in the Kuroshio-Oyashio confluence region. In this article, the author reviews and discusses these results.