The relationship of low-frequency deep variability near the HEBBLE site to Gulf Stream fluctuations

Abstract

Current meter measurements from two recent moored array experiments on the Continental Rise and in the Gulf Stream south of Nova Scotia are examined to determine horizontal and vertical structures of energetic time variable motions; the intent being to put the so-called “abyssal storms” observed at the HEBBLE site into the context of the larger-scale energetics of the region. Empirical orthogonal functions are used to examine characteristics of the motions as a function of frequency. One array spanned the Continental Rise and the Stream. In many low-frequency bands, motions exhibit properties of nearly barotropic topographic Rossby waves and are coherent over the array. Phase propagation of the waves is to the southwest, indicating onshore energy propagation. Data from a second current meter array within the Stream are examined to determine source region structure and relationship to Rise variability. In most frequency bands, Stream motions are too complex to be clearly separable into empirical modes, but a significant mode does exist in the band centered at 30 days. Westward propagation is observed in the meridional component of the velocity at 4000 m depth and the associated zonal wavenumber matches that observed on the Rise. The hypothesis that westward propagating meanders can directly excite far field motions is explored and found to be consistent with observations in the 30-day band.