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E. ADDITIONAL CONSIDERATIONS 



1. GENERALIZED DIGITAL ENVIRONMENTAL 

 MODEL (GDEM) 



The GDEM is one of the Navy' s standard climatology databases 

 providing information on the temperature and salinity of the 

 ocean. It is a derived database in that it does not contain actual 

 oceanographic observations, but instead consists of statistical 

 representations of the temperature and salinity fields drawn 

 from MOODS. MOODS contains about five million 

 observations worldwide. GDEM was constructed by fitting 

 polynomial curves (with the exception of the surface layer, 0- 

 400 meters) to each depth profile from MOODS and averaging 

 all the equivalent coefficients within each 30 arc minute spatial 

 cell. The surface layer coefficients are those that give the 

 appropriate amplitude response to a filter function. The global 

 results were then gridded with interpolated values fiUing in any 

 voids. Using these coefficients this model can reconstruct a 

 representative climatological vertical profile of ocean 

 temperature and salinity at any location in the Northern 

 Hemisphere for any of the four seasons. GDEM profiles are 

 available for ocean areas where the water depths exceed 100 

 meters, except for the Mediterranean, Red Sea, and Sea of Japan 

 where the cutoff depth is 50 meters, and the Persian Gulf and 

 Yellow Sea where it is meters. An update of the public domain 

 GDEM in progress will result in some areas having 10 arc 

 minute resolution. Figure 26 shows where GDEM ocean 

 models exist and some of their important characteristics. 



A recent comparison between GDEM and the Levitus 

 Climatology suggests that GDEM provides results very similar 

 to the Levitus Climatology, which is extensively used by 

 oceanographers to initiahze numerical global and regional 

 circulation models. These models are providing significant 

 insight intoocean circulation and the role of the oceans in global 

 climate. For areas of strong ocean currents, such as the Gulf 

 Stream and Kuroshio, an unsmoothed version of the GDEM 

 results is available. This unsmoothed version has been reported 

 to provide a better representation of the density strucmre than 

 does the Levitus Climatology. 



V-GDEM stands for Variability for the Generalized Digital 

 Environmental Model, and provides an envelope of plus or 



minus two standard deviations around the GDEM profile for 

 each 30 arc minute grid cell. While GDEM can be obtained for 

 various parts of the world oceans via the Internet with anonymous 

 File Transfer Protocol (FTP), V-GDEM and the high-resolution 

 GDEM are not available via the Internet, and the Arctic GDEM, 

 while not classified, is restricted to Navy-approved users. 



GDEM and V-GDEM could be very useful to a number of 

 oceanographers working on circulation, air-sea exchange, and 

 cUmate issues. We suspect that many oceanographers may not 

 be aware of these models, their structure, and how they might 

 be obtained. It would seem desirable to make the oceanographic 

 community as a whole more aware of these databases. We are 

 certain that the oceanographic community would like to have 

 the Arctic GDEM available for use as well as the higher 

 resolution shallow seas GDEM. As a broader segment of the 

 oceanographic community uses these products, we expect 

 these models to become better known and used, and that 

 recommendations for enhancements will be forthcoming. 



This figure shows the ocean areas where GDEM ocean profile data 

 models of various resolutions exist. 



Table 7 summarizes the most important characteristics of the models. 

 While the lowest resolution GDEM ocean data models have been 

 developed for nearly the entire globe, the higher resolution models exist 

 only for limited regional ocean areas. 



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