Abstract
Abstract. Hydrographic data from full-depth moorings maintained by the Rapid/\\-MOCHA project and spanning the Atlantic at 26° N are decomposed into vertical modes in order to give a dynamical framework for interpreting the observed fluctuations. Vertical modes at each mooring are fit to pressure perturbations using a Gauss-Markov inversion. Away from boundaries, the vertical structure is almost entirely described by the first baroclinic mode, as confirmed by high correlation between the original signal and reconstructions using only the first baroclinic mode. These first baroclinic motions are also highly coherent with altimetric sea surface height (SSH). Within a Rossby radius (45 km) of the western and eastern boundaries, however, the decomposition contains significant variance at higher modes, and there is a corresponding decrease in the agreement between SSH and either the original signal or the first baroclinic mode reconstruction. Compared to the full transport signal, transport fluctuations described by the first baroclinic mode represent <25 km of the variance within 10 km of the western boundary, in contrast to 60 km at other locations. This decrease occurs within a Rossby radius of the western boundary. At the eastern boundary, a linear combination of many baroclinic modes is required to explain the observed vertical density profile of the seasonal cycle, a result that is consistent with an oceanic response to wind-forcing being trapped to the eastern boundary.
Highlights
With increased sampling of the oceans over the past decades, the importance and ubiquitousness of low frequency and large-scale waves has become increasingly clear and central to the understanding of ocean dynamics
Having described the vertical mode decomposition, we interpret how accurate it is and how well it can recover original measurements made by the moorings and independent measurements made by satellite altimetry
Fitting vertical modes is a well known procedure, its application to moored hydrographic data is not straightforward and we present a technique to do so
Summary
With increased sampling of the oceans over the past decades, the importance and ubiquitousness of low frequency and large-scale waves has become increasingly clear and central to the understanding of ocean dynamics. Large-scale waves transmit information and energy through the ocean in response to changing forcing or to instability. Theories of the setup and response of ocean gyres to wind-stress or buoyancy input rely on energy being transmitted through the ocean by planetary Rossby waves, or along the ocean margin by boundary waves (Johnson and Marshall, 2002). For the waves to permanently adjust the ocean, their energy has to be converted out of the wave motion and into steady motion or altered stratification
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