The Caribbean Sea serves as a major pathway for global thermohaline circulation (THC), which is a complex and vital component of the Earth’s climate system, influencing global heat distribution and oceanic circulation. Though relatively stratified, it is the boundary layer that distributes mass and temperature between the surface waters and the deep ocean where we observe various multiscale mixing processes from mesoscale to fine-scale. In regions where bathymetry is shallower and mechanical mixing forces, such as winds and tides, are more dominant, diapycnal diffusivity is typically stronger, driving vertical mixing. This type of mixing occurs at small scales, typically as internal waves break within the internal ocean, making it difficult to quantify and observe. Through the combination of seismic images and oceanographic data, known as seismic oceanography, we can qualitatively and quantitatively observe the variability of the ocean’s internal wave field and its diverse components, which include the turbulent and internal wave subranges from vertical displacement spectra. Exploiting these subranges allows us to quantify vertical mixing behaviors across isopycnal layers, effectively representing the cascade of energy for mixing. Quantitatively constraining these energy components is essential to comprehensively understand the total energy budget of the THC.
This research focuses on mapping and quantifying diapycnal diffusivity in the southeastern Caribbean Sea, a region characterized by the convergence of two primary water masses, North Atlantic water (NAW), and South Atlantic water (SAW), as they spill into the Caribbean Sea through the Lesser Antilles passages. This convergence introduces perturbations in temperature, salinity, and nutrients, resulting in the formation of the Caribbean Current. The current’s predominant westward direction, driven by surface winds, is influenced at depth by interactions with deeper water masses and the irregular coastal bathymetry.
We utilize five seismic profiles, totaling approximately 1000 km in length, collected in 2004 in the southeastern Caribbean Sea, in conjunction with oceanographic data and models, to identify fine-scale and mesoscale structures in the thermocline (300-1200m). Additionally, we quantify diffusivity along sections of each seismic profile using displacement slope spectra techniques calculated from seismic reflectors.
Our analysis reveals average diffusivity values of ∼10−5.2 m2/s, which are comparable to the global average of 10−5 m2/s of the open ocean. Notably, we observe an increase in diffusivity to values of >10−4 m2/s in regions characterized by elevated or rugged bathymetry, confirming the hypothesis of the existence of regions of concentrated, high diffusivity (i.e., hot spots) and greater mixing. The existence of these regions has been advocated to explain the discrepancy between the observed average diapycnal diffusivity value of >10−5 m2/s and the theoretical globally averaged values of >10−4 m2/s derived from an advective diffusive balance. Our findings contribute to a better understanding of mixing processes in the SE Caribbean Sea, shedding light on the quantification of diapycnal diffusivity and its spatial variations, ultimately enhancing our grasp of the THC’s energy budget.
Maria Beatrice Magnani
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Renzaglia, Joseph, "Transport and Mixing of Water Masses Across the Southeast Caribbean Ocean Imaged by Seismic Reflection Data" (2023). Earth Sciences Theses and Dissertations. 32.