publications

2025

1. Observations of Subduction, Downward Heat Flux and Dense Filament Collapse in the Northern Gulf of Mexico

   James P. Hilditch, Kerstin Bergentz, Devon Northcott, and 10 more authors

   Journal of Geophysical Research: Oceans, 2025

   Abs DOI

   Submesoscale processes are important contributors to the global heat budget and generally support upward heat transport through restratification. However, in salinity-stratified regions, such as the northern Gulf of Mexico with its influx of freshwater from the Mississippi-Atchafalaya river system, temperature can act like a passive tracer and submesoscale processes can contribute to downward heat transport. Oceanic heat content is a factor in many environmental risks the region faces, for example, hurricane intensification, and marine heatwaves. During the 2022 field campaign of the Submesoscales Under Near-Resonant Inertial Shear Experiment, a sampling plan was developed to study such submesoscale processes in high resolution. Over 31 hr, four assets (two research ships and two remotely controlled boats) drove in parallel across a dense filament, capturing its evolution in time and space. The observations show that surface waters, warmed by daytime solar radiation, were subducted and that the associated overturning circulation transported heat below the surface layer where it was later irreversibly mixed away. The estimated downward heat flux was as strong as the concurrent net air-sea heat flux into the ocean. The filament was then observed to rapidly collapse which we attribute to boundary layer turbulence and the breakdown of geostrophic balance. The collapsing fronts display behaviors indicative of gravity currents. These observations highlight how in salinity-stratified regions, frontal dynamics can be associated with downward heat flux and how the submesoscale can play an important role in the oceanic heat budget.

2024

1. Characterization of Mixing at the Edge of a Kuroshio Intrusion into the South China Sea: Analysis of Thermal Variance Diffusivity Measurements

   Alejandra Sanchez-Rios, R. Kipp Shearman, Craig M. Lee, and 5 more authors

   May 2024

   Abs DOI

   The Kuroshio occasionally carries warm and salty North Pacific Water into fresher waters of the South China Sea, forming a front with a complex temperature–salinity (T–S) structure to the west of the Luzon Strait. In this study, we examine the T–S interleavings formed by alternating layers of North Pacific Water with South China Sea Water in a front formed during the winter monsoon season of 2014. Using observations from a glider array following a free-floating wave-powered vertical profiling float to calculate the fine-scale parameters Turner angle, Tu, and Richardson number, Ri, we identified areas favorable to double-diffusion convection and shear instability observed in a T–S interleaving. We evaluated the contribution of double-diffusion convection and shear instabilities to the thermal variance diffusivity, χ, using microstructure data and compared it with previous parameterization schemes based on fine-scale properties. We discover that turbulent mixing is not accurately parameterized when both Tu and Ri are within critical ranges (Tu > 60; Ri < \frac14). In particular, χ associated with salt finger processes was an order of magnitude higher (6.7 \texttimes 10-7 K2 s-1) than in regions where only velocity shear was likely to drive mixing (8.7 \texttimes 10-8 K2 s-1).

2023

1. Out-of-Plane Arrivals Recorded by Drifting Hydrophones during the Northern Ocean Rapid Surface Evolution Experiment

   Megan S. Ballard, Jason D. Sagers, Pierre-Marie Poulain, and 3 more authors

   The Journal of the Acoustical Society of America, Nov 2023

   Abs DOI

   This paper reports on an observation of three-dimensional (3D) arrivals for which the change in the direction of horizontally refracted sound is nearly 180^∘. The experimental site is Jan Mayen Channel (JMCh), which connects the Greenland and Norwegian Seas. During the experiment, signals from a moored source transmitting a 500–1500 Hz sweep every 4 h were recorded by three surface drifters equipped with hydrophone arrays. Over a 3-day period, the drifters moved north across JMCh toward the moored source. In each recording, an in-plane arrival is identified. In a subset of these recordings, a second arrival is observed, having travel time consistent with propagation from the moored source, turning at the ridge on the south side of the channel, and arriving at the drifters. In a smaller subset of recordings, a third arrival is also observed having travel time consistent with a turning point on the face of the bathymetric rise on the west end of the channel that forms the Jan Mayen volcano. A 3D ray trace is employed to show the change in direction results from repeated reflections from the seafloor such that it is classified as horizontal refraction and not a single-bounce reflection.

2. Unpacking Diversity: A Grassroots Initiative and Its Institutional Evolution

   Alejandra Sanchez-Rios, Thi B. Truong, Jennifer A.T.K. Wong-Ala, and 2 more authors

   Oceanography, 2023

   Abs

   Here we describe the evolution of a grassroots, graduate student-led initiative, Unpacking Diversity, designed to address the experiences of marginalized BIPOC (Black, Indigenous, and People of Color) scholars in an atmospheric, Earth, and ocean sciences college. Our main activity involved an annual seminar series delivered in partnership with social scientists and humanities scholars to meet the cultural needs of our college. Through this initiative, we held conversations regarding social justice in academia and offered resources to address these issues within the college. Despite high attendance and community-driven efforts to foster inclusivity, a lack of institutional support weakened Unpacking Diversity?s longevity, and organizing efforts became unsustainable for graduate students, primarily women of color. After four years, we decided to retire the initiative. We discuss the factors that contributed to the initiative?s success, the obstacles that hindered substantial institutional change, and how this experience helped us create a resilient network of peers.

2020

1. Observations of Cross-Frontal Exchange Associated with Submesoscale Features along the North Wall of the Gulf Stream

   Alejandra Sanchez-Rios, R. Kipp Shearman, Jody Klymak, and 2 more authors

   Deep Sea Research Part I: Oceanographic Research Papers, Sep 2020

   Abs DOI

   Using high-resolution measurements of the Gulf Stream North Wall (GSNW), we investigated whether detachments from the warm current at the edge of the front, known as streamers, affect the overall heat and salt content of the region. Temperature, salinity, and velocity data were collected across the front from towed CTDs, shipboard ADCPs, and gliders following a Lagrangian drifter that was deployed at the GSNW during winter 2012. Four streamers were identified, all of which expanded laterally 10–15 km, with vertical salinity interleaving down to a 200 m depth. We observed that temperature and salinity (T/S) increased along the trajectory of the Lagrangian float. These trends were density compensated and ranged from the surface down to a 200 m depth and across a 5 km band. The heat and salt budget analysis showed that surface fluxes, advection due to large-scale circulation, and diapycnal mixing could not explain the observed increase in T/S in the mixed layer and in the subsurface area. The only possible source that could explained the increase in T/S was along-isopycnal mixing. Estimates of the Reynolds transport supported this conclusion, although the low number of realizations meant these estimates were not statistically significant. From the heat and salt budgets, we observed that an along-isopycnal diffusivity, κi, of 110\textpm 30m2s-1 accounted for the estimated residual and matched the Reynolds transport estimates. This value of κi is consistent with other studies that assert that lateral mixing is required for the production of Eighteen Degree Water (EDW) subtropical mode water.