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The Modular Ocean Data Assimilation System (MODAS)*
ABSTRACT
The Modular Ocean Data Assimilation System (MODAS) is used by the U.S. Navy for depiction of three-- dimensional fields of temperature and salinity over the global ocean. MODAS includes both a static climatology and a dynamic climatology. While the static climatology represents the historical averages, the dynamic climatology assimilates near-real-time observations of sea surface height and sea surface temperature and provides improved temperature and salinity fields. The methodology for the construction of the MODAS climatology is described here. MODAS is compared with Levitus and Generalized Digital Environmental Model climatologies and with temperature and salinity profiles measured by SeaSoar in the Japan/East Sea to illustrate MODAS capabilities. MODAS with assimilated remotely sensed data is able to portray time-varying dynamical features that cannot be represented by static climatologies.
1. Introduction
Climatologies consist of data averaged over well-defined spatial grids and over time periods such as months, seasons, or years. A broad distribution of the data in time and in space is best for the formulation of representative vertical profiles in the construction of climatologies. Data-sparse situations require special averaging and interpolation techniques. Climatologies have many applications, including climate studies, quality control of new data, and design of experiments. In addition, accurate climatologies are particularly important for numerical model development and evaluation. Climatologies provide boundary conditions and first-guess fields for models.
Levitus (1982) published the first worldwide ocean climatology and has made updates in Levitus and Boyer (1994), Levitus et al. (1994), Antonov et al. (1998a,b,c), Boyer et al. (1998a,b,c), and in Conkright et al. (1999). His climatology (hereafter referred to as the Levitus Climatology, or simply Levitus) is based on objectively analyzed, gridded sets of temperature and salinity fields obtained from all data available through 1998 from the National Oceanographic Data Center (NODC), Washington, D.C. The data were analyzed on annual, seasonal, and monthly timescales and were gridded in 1deg latitude-- longitude cells at standard oceanographic levels between the surface and bottom (maximum depth 5500 m).
Another climatology being considered here, the Generalized Digital Environmental Model (GDEM, current version 2.5; Teague et al. 1990), was developed at the Naval Oceanographic Office (NAVOCEANO). GDEM provides global coverage of temperature and salinity on grids ranging from 1/2deg in the...