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The feasibility of global ocean weather prediction was just emerging as GODAE began in 1997. Ocean weather includes phenomena such as meandering currents and fronts, eddies, the surface mixed layer and sea surface temperature (SST), equatorial and coastally trapped waves, upwelling of cold water, and Rossby waves, all influencing ocean variables such as temperature (T), salinity (S), currents, and sea surface height (SSH). Adequate real-time data input, computing power, numerical ocean models, data assimilation capabilities, atmospheric forcing, and bathymetric/boundary constraints are essential to make such prediction possible. The key observing systems and real-time data inputs are SSH from satellite altimetry, satellite and in situ SST, T or T&S profiles (eg Argo, TAO, PIRATA, BTs), and atmospheric forcing. The ocean models dynamically interpolate the data in conjunction with the data assimilation, convert atmospheric forcing into oceanic responses, and forecast the ocean weather, applying the bathymetric/boundary constraints in the process. The results are substantially influenced by ocean model simulation skill and it is advantageous to use an ocean model that is eddy-resolving (typically< 8 km grid increments), not just eddy-permitting. Since the most abundant ocean observations are satellite surface data and subsurface data are very sparse in relation to the space scales of the mesoscale ocean features that dominate the ocean interior, downward projection of surface data is a key challenge in ocean data assimilation. The need for accurate prediction of ocean features that are inadequately observed, such as the mixed …