Ventilation is an important process within the global ocean, where waters sink to deeper layers, are transported by deep currents, and eventually get upwelled back to the surface. This process affects the distribution of oxygen and carbon in the global ocean by transporting these elements from the surface to deeper regions of the ocean.

Scientists are still trying to understand the exact dynamics and circulation pathways that determine how dense, cold water from deep in the Gulf of Mexico gets circulated within the Gulf. Current theories suggest that deep water is transformed into intermediate water by small-scale mixing which drives upwelling before flowing out of the Gulf through the Yucatán Channel.

Research used underwater gliders to observe and measure small scale mixing in the Gulf of Mexico to try to verify these theories. The results were published in Ocean-Land-Atmosphere Research on 16 October.

“Deep ventilation of the Gulf of Mexico is not yet well understood. Recent studies by Ochoa et al. in 2021 and Amon et al. in 2023 proposed a conceptual model for deep Gulf of Mexico ventilation. In this paper, we tried to complement their conceptual model by investigating the role mixing along steep bathymetry plays in the ventilation of the deep Gulf of Mexico,” said Sergey Molodtsov, who was a researcher at Texas A&M University in Galveston, Texas, while working on this study (now at Los Alamos National Laboratory).

In 2017, the researchers completed microstructure observations of the continental slope of the western Gulf of Mexico. Microstructure observations are measurements of very small variations in temperature, salinity, and small-scale velocity shear. These microstructure measurements assist oceanographers in estimating small scale mixing rates within a body of water, which is crucial for understanding ocean processes.

The microstructure measurements were taken with underwater gliders, which are small submarine-shaped autonomous underwater vehicles. This was the first time that an underwater glider was used to conduct microstructure measurements on the continental slope of the western Gulf of Mexico.

Researchers were specifically looking for microstructure measurements that could show evidence of elevated diapycnal mixing in this part of the Gulf of Mexico. Diapycnal mixing happens vertically across the water layers of different densities and can result in upwelling of waters.

Researchers suspected that there would be a higher level of mixing in areas where the continental slope is steeper. This area is described as having “steep bathymetry.” Bathymetry is the topography of the ocean bottom. Steep bathymetry, such as in the western Gulf of Mexico, may lead to the result of enhanced internal breaking of internal waves (similar to breaking of surface waves) and thus elevated diapycnal mixing levels.

“Using various observational data, we confirmed the existence of elevated mixing levels along areas with steep bathymetry in the western Gulf of Mexico, which, in turn, may lead to upwelling-favorable conditions. We hypothesize that these regions could serve as pathways for North Atlantic Deep Water entering through the Yucatán Channel, filling the deep Gulf, upwelling, and transforming into an intermediate water layer that forms the outflow from the deep Gulf of Mexico,” said Molodstov.

In addition to the steep slope of continental slope, there is a cyclonic boundary current that runs along the perimeter of the Gulf of Mexico that could be another factor contributing to the elevated mixing rates.

Looking ahead, researchers are hoping to conduct additional observational missions. “Additional research could include microstructure turbulence measurements, targeting other areas in the deep Gulf of Mexico. These studies could further support the proposed hypothesis of ventilation in the deep Gulf of Mexico” said Molodstov.

Other contributors include Ayal Anis and Rainer M. W. Amon, at Texas A&M University; and Paula Perez-Brunius, Julio Sheinbaum, and Julio Candela at the Center for Scientific Research and Higher Education at Ensenada (CICESE).