Source Regions of Infragravity Waves Recorded at the Bottom of the Equatorial Atlantic Ocean, Using OBS of the PI‐LAB Experiment

P Bogiatzis,M Srokosz,A Karamitrou,C A Rychert,N Harmon,J Ward Neale

doi:10.1029/2019jc015430

Abstract

AbstractInfragravity waves are generated along coasts, and some small fraction of their energy escapes to the open oceans and propagates with little attenuation. Due to the scarcity of deep‐ocean observations of these waves, the mechanism and the extent of the infragravity waves energy leakage from the coasts remains poorly understood. Understanding the generation and pathways of infragravity wave energy is important among others for understanding the breakup of ice‐shelves and the contamination of high‐resolution satellite radar altimetry measurements of sea level. We examine data from 37 differential pressure gauges of Ocean Bottom Seismometers (OBS) near the equatorial mid‐Atlantic ridge, deployed during the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We use the beamforming technique to investigate the incoming directions of infragravity waves. Next, we develop a graph‐theory‐based global back‐projection method of noise cross‐correlation function envelopes, which minimizes the effects of array geometry using an adaptive weighting scheme. This approach allows us to locate the sources of the infragravity energy. We assess our observations by comparing to a global model of infragravity wave heights. Our results reveal strong coherent energy from sources and/or reflected phases at the west coast of Africa and some sources from South America. These energy sources are in good agreement with the global infragravity wave model. In addition, we also observe infragravity waves arriving from North America during specific events that mostly occur during October–February 2016. Finally, we find indications of waves that propagate with little attenuation, long distances through sea ice, reflecting off Antarctica.

Highlights

Infragravity waves are oceanic surface gravity waves with typical frequencies between 0.003 and 0.03 Hz, and wavelengths of the order of several kilometers
We present the two array techniques that were used in this work to detect the direction of the transient energy that propagates across the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) array and the potential sources of this energy, namely, incoherent beamforming and back‐projection
We introduce a two‐stage technique that uses an initial estimate of the weights and further optimizes them using the Trust Region Reflective (TRR) Algorithm (Coleman & Li, 1994, 1996) which solves the problem by means of quadratic programming, as it is implemented in MATLAB 9.2 [https://uk.mathworks.com/help/optim/ug/least-squares-model-fitting-algorithms.html#broz0i4, last accessed on January 2019]

Summary

Introduction

Infragravity waves are oceanic surface gravity waves with typical frequencies between 0.003 and 0.03 Hz, and wavelengths of the order of several kilometers They are generated from nonlinear difference interactions of the higher frequency wind waves and swells and are more energetic near coastlines (Ardhuin et al, 2014; Biesel, 1952; Herbers et al, 1994; Herbers, Elgar, & Guza, 1995; Smit et al, 2018; Webb et al, 1991). This is more evident in high‐energy coasts with narrow continental shelves (e.g. Smit et al, 2018)

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