Bisits, J. I., Zika, J. D., Drake, H. F., Holmes, R. M., Griffies, S. M., Gibson, A. H., & Hogg, A. M. G. (In prep.). A simple formula to infer rates of numerical mixing in finite volume ocean models part two: application.
Zika, J. D., Bisits, J. I., Drake, H. F., Holmes, R. M., Griffies, S. M., Gibson, A. H., & Hogg, A. M. G. (In prep.). A simple formula to infer rates of numerical mixing in finite volume ocean models part one: theory and example.
Bisits, J. I., McDougall, T. J., & Zika, J. D. (2026). Determining how nonlinearities in the equation of state affect diffusive interfaces in the ocean. Journal of Fluid Mechanics, 1040, A33. https://doi.org/10.1017/jfm.2026.11889
In the ocean, favourable vertical gradients and the differing molecular diffusivities of salinity and temperature cause double diffusive instabilities. At high latitudes, double diffusive instabilities are in the ‘diffusive’ convection regime due to the colder, fresher waters that form atop warmer, saltier waters. Cold conditions at high latitudes also enhance effects of the nonlinear equation of state. Modelling studies of diffusive convection typically use a linear equation of state, thereby not including processes such as cabbeling, the gain in density upon mixing, which arises from nonlinearities in the equation of state. Here, we use a fully nonlinear equation of state in direct numerical simulations (DNS) to investigate the impact of cabbeling on diffusive convection and the resulting ‘diffusive’ interfaces. A one-dimensional molecular diffusion model shows that cabbeling affects the diffusive convection instability by forming unequal density anomalies within the layers either side of a diffusive interface. This asymmetry is not present when the equation of state is linear. The primary driver of the asymmetry between the density anomalies in the nonlinear case is the temperature difference across an interface. In a two-layer system, a larger density anomaly forms below the interface, and our DNS experiments show that this drives an upward migration of the interface agreeing with earlier laboratory results. Our results indicate that cabbeling’s impact on diffusive interfaces in the Arctic Ocean is subtle, while in the Southern Ocean, cabbeling can drive sustained upward migration of interfaces that may influence thermohaline staircase formation and maintenance.
Lloyd, S. D., Bisits, J. I., Popovic, G., Osmond, P., Roser, D., & Khan, S. J. (2026). Swimming in urban estuaries: understanding stormwater contamination events and recovery from historical data. Water Research, 125686. https://doi.org/https://doi.org/10.1016/j.watres.2026.125686
Cities globally are responding to public demand for water-based recreation by transforming urban rivers and estuaries into swimming destinations. Historical water quality compliance records may help diagnose contamination and recovery dynamics and serve as a valuable reference point for assessing the suitability of proposed urban swimming sites. This study analysed historical bathing water compliance data and concurrent hydrometeorological records to characterise long and short-term contamination and recovery patterns at current and proposed bathing sites in Sydney’s Parramatta River estuary. Generalised Linear Mixed Modelling (GLMM) was used to estimate site-specific recovery parameters and explore environmental drivers of variability in the faecal indicator bacteria (FIB) enterococci. Recovery times to health-relevant thresholds ranged from 3 to 125 hours, with background levels reached between 10 to 142 hours depending on initial contamination. Marine-dominated bathing sites were temporarily diluted to near-freshwater conditions following rainfall, as indicated by reductions of up to 97% in baseline salinity. A 10 mm rainfall threshold was identified as a practical trigger for event-based sampling. Salinity and solar exposure were the most influential environmental factors affecting recovery rates. This study demonstrates how routine, compliance datasets can be repurposed to derive short-term, site-specific recovery metrics, quantify stormwater intrusion using a practical salinity proxy (RSR%), and establish actionable rainfall triggers for event-based monitoring, providing an evidence-based blueprint to inform risk management for emerging urban bathing sites when high-frequency data are unavailable.
Bisits, J. I., Zika, J. D., & Sohail, T. (2025). Cabbeling as a catalyst and driver of turbulent mixing. Journal of Fluid Mechanics, 1011, A17. https://doi.org/10.1017/jfm.2025.349
At constant pressure, a mixture of water parcels with equal density but differing salinity and temperature will be denser than the parent water parcels. This is known as cabbeling and is a consequence of the nonlinear equation of state for seawater density. With a source of turbulent vertical mixing, cabbeling has the potential to trigger and drive convection in gravitationally stable water columns and there is observational evidence that this process shapes the thermohaline structure of high-latitude oceans. However, the evolution and maintenance of turbulent mixing due to cabbeling has not been fully explored. Here, we use turbulence-resolving direct numerical simulations to investigate cabbeling’s impact on vertical mixing and pathways of energy in closed systems. We find that cabbeling can sustain convection in an initially gravitationally stable two-layer configuration where relatively cold/fresh water sits atop warm/salty water. We show the mixture of the cold/fresh and warm/salty water is constrained by a density maximum and that cabbeling enhances mixing rates by four orders of magnitude. Cabbeling’s effect is amplified as the static stability limit is approached, leading to convection being sustained for longer. We find that available potential energy, which is classically thought to only decrease with mixing, can increase with mixing due to cabbeling’s densification of the mixed water. Our direct numerical simulations support the notion that cabbeling could be a source of enhanced ocean mixing and that conventional definitions of energetic pathways may need to be reconsidered to take into account densification under mixing.
Bisits, J. I., Zika, J. D., & Evans, D. G. (2024). Does Cabbeling Shape the Thermohaline Structure of High-Latitude Oceans? Journal of Physical Oceanography, 54(12), 2419–2430. https://doi.org/10.1175/JPO-D-24-0061.1
Vertical exchange of heat and carbon in the ocean regulates Earth’s climate. Convection, a driver of near-surface exchange, occurs when dense water overlies light water. In a 1957 study, N. Fofonoff pointed out that when lighter overlying cold-fresh water mixes with denser underlying warm-salty water, the mixture can become denser than the underlying water due to a nonlinear process known as cabbeling. He suggested that such profiles, despite being gravitationally stable, could be classed as being unstable to cabbeling. Fofonoff hypothesized that, by mixing away such profiles, cabbeling may be shaping the thermohaline structure of polar oceans. We investigate this hypothesis here. In a one-dimensional model, we find that convective mixing occurs in temperature inverted profiles that are unstable to cabbeling even when they are initially gravitationally stable. In data from an observationally constrained global circulation model, we find profiles with a temperature inversion larger than −0.5°C are unstable to cabbeling less than 0.02% of the time, and in high-quality in situ observations, they are unstable less than 12% of the time. We find that due to cabbeling, larger temperature inversions, which should weaken stratification, make profiles more stable. Our results suggest that cabbeling limits the stability behavior of temperature inverted profiles and influences the thermohaline structure in parts of the ocean where cold-fresh water overlays warm-salty water.
Bisits, J. I., Stanley, G. J., & Zika, J. D. (2023). Can We Accurately Quantify a Lateral Diffusivity from a Single Tracer Release? Journal of Physical Oceanography, 53(2), 647–659. https://doi.org/10.1175/JPO-D-22-0145.1
Mixing along sloping isopycnals plays a key role in the transport and uptake of heat and carbon by the ocean. This mixing is quantified by a lateral diffusivity, which can be measured by tracking the lateral spreading of point release tracer patches. We present a definition for the area of a tracer patch, the time derivative of which provides the lateral diffusivity. To accurately estimate the diffusivity, an ensemble mean concentration field of many tracer release experiments is required. We use numerical experiments to quantify how accurately the “true” lateral diffusivity (obtained from the ensemble mean concentration field) can be estimated from a single tracer release experiment (one ensemble member). To simulate observational campaigns, we also estimate the diffusivity from a single tracer release that is spatially and/or temporally subsampled, quantifying how the error between the estimated diffusivity and the true diffusivity grows as this sampling resolution worsens. We perform these numerical experiments in a two-layer quasigeostrophic model of turbulent flow on a β plane, using an ensemble of 50 passive tracer release experiments, each initialized as a 2D Gaussian but with differing realizations of the turbulent flow. We find that the diffusivity estimates from the single tracer releases have a relative root-mean-square error (RMSE) of 1.43% from the true diffusivity. Subsampling a single tracer release experiment every 956 km increases the relative RMSE from the true diffusivity to 3.1%; also subsampling every 277 days raises this figure to 6.5%.
Selected oral presentations
Bisits, J. I., Zika, J. D., & Sohail, T. (2026, February). Cabbeling as a catalyst and driver of turbulent mixing. Ocean Sciences.
Bisits, J. I. (2025, October). Non-linear controls on ocean circulation and mixing in the high-latitude oceans. Australis Centre for Excellence in Antarctic Science Seminar Series.
Bisits, J. I., Zika, J. D., & Evans, D. G. (2024, June). Does cabbeling shape the thermohaline structure of high-latitude oceans? Gordon Research Seminar on Ocean Mixing.
Bisits, J. I., Zika, J. D., & Evans, D. G. (2024, February). Does cabbeling shape the thermohaline structure of high-latitude oceans? Australian Meteorological and Oceanographic Society Annual Conference.
Bisits, J. I., Stanley, G. J., & Zika, J. D. (2023, July). Can we accurately quantify a lateral diffusivity from a single tracer release? XVIII General Assembly of the International Union of Geodesy and Geophysics (IUGG.
Bisits, J. I., Stanley, G. J., & Zika, J. D. (2022, November). Can we accurately quantify a lateral diffusivity from a single tracer release? Australian Meteorological and Oceanographic Society Annual Conference.
Selected poster presentations
Bisits, J. I., Zika, J. D., & Sohail, T. (2024, November). The effects of cabbeling on mixing and energetics in polar oceans. Australian Antarctic Research Conference.
Bisits, J. I., Zika, J. D., & Sohail. (2024, June). The effect of non-linear processes on mixing. Gordon Research Conference on Ocean Mixing.
Bisits, J. I., Zika, J. D., & Evans, D. G. (2023, July). Does cabbeling shape the thermohaline structure of high-latitude oceans? Physics of the Ocean Summer School.
Bisits, J. I., Stanley, G. J., & Zika, J. D. (2022, November). Can we accurately quantify a lateral diffusivity from a single tracer release? Australian Centre for Excellence in Antarctic Science Workshop .
