Variational data assimilation for the global ocean

Abstract

A fully three dimensional, multivariate, variational ocean data assimilation system has been developed that produces simultaneous analyses of temperature, salinity, geopotential and vector velocity. The analysis is run in real-time and is being evaluated as the data assimilation component of the Hybrid Coordinate Ocean Model (HYCOM) forecast system at the U.S. Naval Oceanographic Office. Global prediction of the ocean weather requires that the ocean model is run at very high resolution. Currently, global HYCOM is executed at 1/12 degree resolution (∼ 7 km mid-latitude grid mesh), with plans to move to a 1/25 degree resolution grid in the near future (∼ 3 km mid-latitude grid mesh). These high resolution global grids present challenges for the analysis given the huge model state vector and the ever increasing number of satellite and in situ ocean observations available for the assimilation. In this paper the development and evaluation of the new oceanographic three-dimensional variational (3DVAR) data assimilation is described. Special emphasis is placed on documenting the capabilities built into the 3DVAR to make the system efficient for use in global HYCOM.