Using Acoustic Travel Time to Determine Dynamic Height Variations in the North Atlantic Ocean

Abstract

There is often an approximately linear relationship between various water-column integrals, in particular between surface dynamic height anomaly ?D and acoustic round-trip travel time τ. Consequently, the record from an inverted echo sounder, which measures τ, can be interpreted in terms of ?D. Nevertheless, the slope m of this linear relation is not everywhere well defined, and varies from place to place. This study seeks to establish where, in the extratropical North Atlantic, one can reasonably assume a linear relation between ?D and τ, and for these regions compute m. Using climatological atlas data and historic hydrographic data, it is shown that a well-defined, linear relation exists between ?D and τ in a region centered on the Gulf Stream and extending from the northern Sargasso Sea almost to Ireland. Where m is well defined, it is negative, and its value is usually similar to that associated with first-baroclinic-mode excitation. Its magnitude generally decreases with increasing latitude. The value of m typically ranges from ?40 dyn m s?1 in the northern Sargasso Sea to ?20 dyn m s?1 in the North Atlantic Current. In the Gulf Stream it is typically between ?30 and ?35 dyn m s?1.