Variational Data Assimilation for the Global Ocean - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 296

Chapter 13
Variational Data Assimilation
for the Global Ocean

James A. Cummings and Ole Martin Smedstad

Abstract A fully three dimensional, multivariate, variational ocean data assimila-
tion 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.

13.1 Introduction

Eddy-resolving global ocean prediction requires high resolution since the charac-
teristic scale of ocean eddies is on the order of a few tens of kilometers. Only
recently have sufficient data and computer power become available to nowcast
and forecast the ocean weather at eddy-resolving scales, including processes that
control the surface mixed layer, the formation of ocean eddies, meandering ocean