A 4D-Var Analysis System for the California Current: A Prototype for an Operational Regional Ocean Data Assimilation System - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 338

Global ocean data assimilation efforts have been propelled in part by the
coordinated efforts of the Global Ocean Data Assimilation Experiment (GODAE),
the growing need for seasonal forecasts, as well as growing concerns over climate
change. An increasing number of groups produce global ocean analyses of the
circulation of the past (see for example http://www.godae.org/Ocean-products.
html ) . In addition, some operational centers routinely generate analyses for present
conditions. Global ocean data assimilation, however, presents a considerable chal-
lenge because of the size of the inverse problem involved, so the resolution
(horizontal and vertical) of global assimilation products is often limited. The highest
resolution products currently available are performed on grids with horizontal grid-
spacing typically 1=4 degree, so the effective resolutions are probably 2-3

times lower than this considering that anything less than 3 or 4 grid lengths is
poorly resolved. This is marginal for resolving much of the important mesoscale
variability in the open ocean, and certainly inadequate for capturing important
circulation features in coastal regions. For this reason, there has also been a push
to develop regional ocean data assimilation systems which utilize higher resolution
grids. Two approaches to regional ocean data assimilation are typically used: either
the regional model is nested within a global data assimilating model, or the regional
model is run stand-alone and boundary condition information is provided by a global
assimilating model. While there are clear advantages and disadvantages to both
approaches, the stand-alone approach offers greater flexibility since the analyses can
be produced using a variety of global circulation estimates as boundary conditions,
thus providing a range of uncertainty estimates. Some current regional ocean data
assimilation efforts can also be found at http://www.godae.org/Ocean-products.
html .
In this chapter we will review a state-of-the-art regional ocean data assimilation
system that is run routinely for the U.S. west coast to provide both near-real
time and historical analyses for the California Current System (CCS). This is one
of several such systems currently in operation in support of the U.S. Integrated
Ocean Observing System (IOOS) which comprises seven regional centers, three
of which are focused on different parts of the CCS. The CCS is one of 65 Large
Marine Ecosystems (LMEs) that have been identified by NOAA and the United
Nations Environment Program (UNEP) which collectively account for 95

- 1=6

%of
global fisheries biomass (see http://www.lme.noaa.gov ) . The CCS is particularly
noteworthy because it is one of five LMEs that are subject to seasonal variations
in coastal upwelling in which cold, nutrient rich water is brought to the surface,
creating conditions that are favorable for high levels of primary productivity.
The CCS is therefore a region of considerable environmental and socio-economic
importance.
In Sect. 14.2 we describe the Regional Ocean Modeling System (ROMS) and a
detailed summary of the important features of the ROMS 4-dimensional variational
(4D-Var) data assimilation algorithms. The specific configuration of ROMS and
4D-Var for the CCS is introduced in Sect. 14.3 , while in Sects. 14.4 and 14.5 we
describe two ongoing applications: an historical analysis of the CCS circulation,
and a near real-time analysis system. We end with a summary in Sect. 14.6 .