Double duct propagation in the Arctic

Abstract

Recent experiments in the Beaufort Sea encountered the “Beaufort Lens,” a hydrographic feature named by Russian acousticians, where warmer Pacific water enters the Bering Straits and rests above Atlantic water. This creates a “double duct” subsurface duct with this “lens” being near ubiquitous in space and time throughout the Beaufort. As part of the ONR Task Force Ocean program we found subsurface ducts in the Irminger Sea over the Reykjanes Ridge not reliably predicted by HYCOM. Propagation in these “double ducts” has some remarkable features and is well understood using normal modes and analogies to potential wells in quantum mechanics. These include the (i) filling of individual wells preferentially according to the modal phase speeds; (ii) tunneling phenomena when the phase speeds approaches the height of the potential barrier or maximum speed separating the ducts; (iii) issues of degeneracy when the modal phase speeds appear to cross; (iv) appearance of “mini convergence zones” for the transmission losses in each duct; (v) differences in modal group speeds with very small differences in phase speeds; and (vi) very low transmission loss when both source and receiver are in the subsurface duct separated from the water and/or ice surfaces.