Kuroshio ("black current" in japanese) and its extension is one of the major currents in the Earth's oceans. Its path can vacillate between a dynamically stable and unstable state on the decadal timescales. As indicated in Jan. 2005 (see Image of the Month "Current heat"), the place and path of the Kuroshio (as well as the Gulf Stream) has an impact on climate, by modifying the heat content of the upper layers of the ocean, and the ocean-atmosphere exchanges. Thus understanding, monitoring (through indicators, in particular) and forecasting this is important for e.g. seasonal predictions.

With 20 years of continuous data, altimetry enable to catch year-to-year variation in the path of a current, and to better understand its mechanisms. The length of the path, its variability give an indication of the situation on a given year, which can be translated in figures, i.e. "indicators" or "indexes".