Wave processes along 26N

University Of Washington, Seattle WA

Investigators

Abstract

Despite our firm understanding of large-scale and long-period waves in the ocean from theoretical and numerical studies, many paradigms of planetary propagation have not yet been tested in the real ocean for lack of suitable data that resolve the long temporal and large spatial scales involved. The simplifying assumptions necessary for theoretical and numerical treatment (e.g. vertical side-walls, idealized stratification, and constrained temporal responses) all reduce the range of allowable motion and so are incomplete descriptions of the real ocean. However, nearly the entire wave spectrum across a subtropical gyre can now be examined using extensive and prolonged measurements from the Atlantic Meridional Overturning Circulation (AMOC) moored array located at 26°N (the MOCHA/WBTS/RAPID project). Seven years of AMOC array data extend the previous satellite observational basis below the surface and over long periods and allows testing of hypotheses about wave signals and pathways in a subtropical basin. Through the analysis of subsurface observations, this study will investigate wave energies and wave patterns over a broad frequency range. Particular attention will be given to planetary waves (Kelvin and Rossby) and internal waves (tidal and inertial), resolving waves with periods ranging from 2 hours to 2 years using reprocessed high-frequency full-depth measurements of dynamic height and bottom pressure at 8 moorings across the basin and horizontal velocity at the 4 western moorings. With diagnosis of potential energy (dynamic height) and kinetic energy (velocity), the energy equation can be partially diagnosed as a function of frequency at the western boundary. Inferences about energy transfer between frequencies and between moorings can be made by examining spectral slopes in depth- and mode-space. Decomposition of pressure and velocity into mode-space allows the calculation of energy flux and the array's linear geometry enables a 2D estimate of energy flux divergence, lending insight into the propagation and possible dissipation of these waves. A particularly salient aspect of the energy flux decomposition is the ability to examine the low-frequency variability of wind and tidal internal waves, which is not well understood due to a lack of long term subsurface data. Though there have been clear observations of cross-basin Rossby wave propagation in the open ocean, their fate upon reaching the western boundaries remains unclear. These fates might include local dissipation, reflection or even transformation to Kelvin waves that radiate southward along the western boundary. Due to the long time and large spatial scales, previous observational studies have not resolved these processes. Utilizing the temporal and spatial resolution of the array and applying theoretical relations (ratios of kinetic to potential energy, polarization relations and dispersion equations), will help identify the variance contained by these signals, and quantify the energy pathways for these classes of waves at the western boundary. To complement the subsurface sampling, a high frequency satellite altimetry product will be investigated in collaboration to examine horizontal scales and propagation. At any frequency the sources, pathways, and sinks of large-scale waves are largely unknown, and this study is a first step toward quantifying their roles in the western subtropical Atlantic. Intellectual Merit: This project will characterize a broad spectrum of wave motion and elucidate pathways of waves, increasing our understanding of both large-scale, low-frequency planetary waves and tidal and wind-generated internal waves. These analyses provide added benefit to the data already collected by the AMOC array. Broader Impacts: This project supports two junior scientists in establishing their research careers. Scientific results will develop methods for investigating large-scale waves and will greatly extend the observational basis of waves that participate in meridional coherence, ocean predictability, and ocean-atmosphere feedback patterns. Educational components consist of training an undergraduate student and of disseminating results through a web-based outreach program.