Observations and EOF Analysis of Low-Frequency Variability in the Western Part of the Gulf Stream Recirculation

Abstract

This study of low-frequency oceanic variability is based on data collected during the Long Term Upper Ocean Study (LOTUS), which was a two year program of (mainly) moored meteorological and oceanographic measurements. The mooring arrays were centered at 34°N, 70°W over the Hatteras Abyssal Plain. With a distance of about 300 km to the mean Gulf Stream axis and the continental slope, LOTUS was the most northern and western long-term mooring site in the Gulf Stream recirculation region to date. The observed low-frequency variability is dominated by zonally elongated motions of the secular time scale (periods <100 days) even great depths (4000 m). In contrast to observations in other parts of the recirculation region, the spectral shapes are strongly depth dependent. The vertical structure of the variability was examined by EOF analysis. Different kinds of EOFs were tested; the best representation of the observed variability was obtained by an EOF representing a undirectional nonrotating (with depth) flow. The first and second EOF together explain 96% of the observed energy. The first EOF (66%) is almost barotropic with a slight increase at the surface and the bottom of the ocean; the second mode closely resembles the first baroclinic dynamical mode. The barotropicity decreases with increasing frequency. Motions with periods less than 30 days are almost surface trapped, and a strong bottom intensification additionally exists for periods between 10 and 30 days. The low-frequency flow at LOTUS is consistent with a stochastic, quasi-geostrophic wave field that may be affected by mean current, but our observations are insufficient to test this hypothesis.