Low-frequency variability of currents in the deepwater eastern Gulf of Mexico

摘要

Vertical structure of the low frequency horizontal currents at the northern edge ofthe Loop Current during eddy shedding events is observed using concurrenthydrographic, moored, and satellite altimetry data from 2005. Dynamic modes arecalculated at three deep (~3000 m), full water-column moorings in the eastern Gulf ofMexico. Time-series of the barotropic and first two baroclinic modes are found using aleast squares minimization that fits theoretically derived modes to observed mooredvelocity data.EOF analyses show that the majority of observed variance is explained by asurface-trapped mode that is highly coherent with the temporal amplitudes of the firstbaroclinic mode, and a lower, but significant percentage of variance is captured inbottom-intensified modes. Amplitudes of the second empirical mode indicate thatcurrents are more coherent in the ocean interior approaching the Loop Current, as morevariance is explained by this mode at the southernmost mooring near the Loop Current.A dynamic mode decomposition of the horizontal currents reveals that thebarotropic and first baroclinic modes exhibit low frequency variability and eddy time scales of 10 ? 40 days. Second baroclinic mode amplitudes show higher frequencyvariability and shorter time scales. A model utility test for the least squares fit ofmodeled to observed velocity shows that the second baroclinic mode is useful to thestatistical model during 50 ? 85 % of the mooring deployment, and is particularlynecessary to the model when cyclonic features are present in the study area. Theimportance of the second baroclinic mode to the model increases significantly closer tothe Loop Current.High-speed currents associated with the Loop Current and anticyclones stimulatea strong first baroclinic response, but the second baroclinic mode amplitudes are foundto be similar in magnitude to the first baroclinic mode amplitudes at times. This happensepisodically and could be an indication of higher order dynamics related to frontal eddiesor Loop Current eddy shedding.