Abstract
Ocean submesoscale dynamics are thought to play a key role in both the climate system and ocean productivity, however, subsurface observations at these scales remain rare. Seismic oceanography, an established acoustic imaging method, provides a unique tool for capturing oceanic structure throughout the water column with spatial resolutions of tens of meters. A drawback to the seismic method is that temperature and salinity are not measured directly, limiting the quantitative interpretation of imaged features. The Markov Chain Monte Carlo (MCMC) inversion approach has been used to invert for temperature and salinity from seismic data, with spatially quantified uncertainties. However, the requisite prior model used in previous studies relied upon highly continuous acoustic reflection horizons rarely present in real oceanic environments due to instabilities and turbulence. Here we adapt the MCMC inversion approach with an iteratively updated prior model based on hydrographic data, sidestepping the necessity of continuous reflection horizons. Furthermore, uncertainties introduced by the starting model thermohaline fields as well as those from the MCMC inversion itself are accounted for. The impact on uncertainties of varying the resolution of hydrographic data used to produce the inversion starting model is also investigated. The inversion is applied to a mid-depth Mediterranean water eddy (or meddy) captured with seismic imaging in the Gulf of Cadiz in 2007. The meddy boundary exhibits regions of disrupted seismic reflectivity and rapid horizontal changes of temperature and salinity. Inverted temperature and salinity values typically have uncertainties of 0.16°C and 0.055 psu, respectively, and agree well with direct measurements. Uncertainties of inverted results are found to be highly dependent on the resolution of the hydrographic data used to produce the prior model, particularly in regions where background temperature and salinity vary rapidly, such as at the edge of the meddy. This further advancement of inversion techniques to extract temperature and salinity from seismic data will help expand the use of ocean acoustics for understanding the mesoscale to finescale structure of the interior ocean.
Highlights
Mediterranean water eddies, or “meddies,” are anti-cyclonically rotating, sub-surface lenses of warm, salty water formed where the Mediterranean Sea outflows into the Atlantic Ocean (e.g., Richardson et al, 2000)
On the northeast of the section, along the upper edge of the meddy, there is one reflection horizon with anomalously low temperatures, high salinities and unstable density: this region should be interpreted with caution due to the high posterior Markov Chain Monte Carlo (MCMC) inversion uncertainties here
The Bayesian MCMC approach has been applied to a seismic oceanographic dataset to recover the temperature and salinity of a meddy, with lateral and vertical resolutions of O(10 m)
Summary
Mediterranean water eddies, or “meddies,” are anti-cyclonically rotating, sub-surface lenses of warm, salty water formed where the Mediterranean Sea outflows into the Atlantic Ocean (e.g., Richardson et al, 2000). While the cores of meddies are largely homogeneous, high gradients of temperature and salinity, with interleaving, thermohaline intrusions and “layering” are commonly found at the meddy periphery (Armi and Zenk, 1984; Ruddick, 1992; Ménesguen et al, 2009; Pinheiro et al, 2010; Biescas et al, 2014) These layering structures typically have vertical scales of 20–75 m and are thought to be generated by both stirring and double diffusive processes (Ruddick and Hebert, 1988; Pinheiro et al, 2010; Song et al, 2011; Meunier et al, 2015). With translation speeds of a few cm/s, typically south-westward, they can transport Mediterranean water more than a thousand kilometers from its source (Richardson et al, 2000)
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