| Summary: | Surface electromagnetic propagation over the ocean is highly sensitive to near-surface atmospheric variability, particularly the height of the evaporation duct. Seasonal variation in near-surface meterological factors and sea surface temperatures impact the evaporation duct height (EDH). Present U.S. Navy EDH climatology is based on sparse ship observations over a relatively short time period and an outdated evaporation duct (ED) model. This EDH climatology does not utilize smart, or modern, climatology datasets or methods and provides only long term mean (LTM) values of EDH. We have used existing, civilian, dynamically balanced reanalysis data, for 1970 to 2006, and a state-of-the-art ED model, to produce a spatially and temporally refined EDH climatology for the Indian Ocean (IO) and nearby seas. Comparisons of the present U.S. Navy EDH climatology with our climatology show a number of differences. These differences, and the differences in the methods used to generate the two climatologies, indicate that the EDH climatology we have generated provides a more accurate depiction of EDH. The EDH climatology we have produced provides LTM EDH values. But the data and methods we used to create this climatology also allowed us to examine the impacts of climate variations on EDH. Climate variations can have major impacts on the upper ocean and overlying lower troposphere. These impacts can lead to major fluctuations in the factors that determine EDH, and can thereby alter the propagation of EM signals through the atmosphere. The IO and nearby seas are strongly affected by a number of climate variations (e.g., El NinÌ o-La NinÌ a (ENLN), Indian Ocean Zonal Mode (IOZM)). These climate variations are known to lead to large anomalies in sea surface temperature, air temperature, winds, humidity, and other variables in the IO; however, the associated impacts on EDH and EM propagation have not been identified. To assess these impacts, reanalysis data composited by season and climate variation were processed using: (1) the NPS ...
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