Talks

Baroclinic instability is a phenomenon in rotating stratified systems, and is known to play an important role in the Earth's atmosphere and ocean. The Eady problem is a text book example of baroclinic instability widely invoked for various applications (along with the Charney and Phillips problem). In the first part we recap some of the features of the standard geostrophic Eady problem (e.g., as described in most standard GFD textbooks), such as the linear instability characteristics, method of solution by analytical means, instability mechanism in terms of Counter-propagating Rossby Waves (CRWs), relations to baroclinic lifecycles, and its parameterisation in atmospheric and/or oceanic systems (e.g., works of Green, Gent-McWilliams and/or Greatbach-Lamb schemes). In the second part, we proceed to highlight and/or clarify some features of a modified Eady linear instability problem in the presence of a slope, namely (i) links of the Eady problem with quantum mechanics under the umbrella...

Submesoscale density fronts (1s-10s km wide) are a common feature of the ocean’s surface layer and are thought to be important for mediating buoyancy, heat, and energy exchanges which in turn have a substantial impact on the biogeochemistry. These fronts are challenging to observe due to their fast time and short length scales, so to date observations on regional and global scales have been limited. To fill this gap, this study uses a global database of along-track salinity and temperature data in combination with satellite data to identify 250,000 submesoscale density fronts across the globe.  On average, frontal buoyancy gradients scale with the frontal width, which is consistent with the expected dynamics. The submesoscale frontal gradients also exhibit global geographic variability that is correlated with the large scale density gradient, and inversely correlated with the large scale horizontal Turner angle and the mixed layer depth. Potential future research trajectories that coul...

Comprehending how submesoscale dynamics and their potential interplay with tides affect climate models is challenging due to their small scales and high computational demands. To address this challenge, our approach integrates modelling and observational methods. In this study, we investigate the impact of internal tides, eddies and submesoscale currents on the frequency energy spectrum of the ocean. To this end, we apply a novel simulation with telescopic grid refinement to achieve a horizontal resolution finer than 600 m over large regions of the South Atlantic. This refined resolution allows us to accurately capture submesoscale turbulence and a relatively large part of the internal wave spectrum under realistic atmospheric conditions. By comparing simulations with and without tides, we find that without tidal forcing there is significantly less energy at the high frequency end of the spectrum. Validation with mooring and Pressure Inverted Echo Sounder data sets deployed over a two ...

 Batchelor's self-similarity and Kraichnan's inertial range work on two-dimensional turbulence are unsuccessful due to the formation of coherent vortices which generate spatial hierarchical structures with time. In particular, the vortices create spatial intermittency and non-Gaussianity, and mechanisms for inverse energy cascade and direct enstrophy transfer are still open to probe. Via numerical simulations and self-similar vortex theory, we quantified the vortex populations and found the energy spectrum at high wave numbers follows a steeper slope than that predicted by the Batchelor and Kraichnan theories. Also, we discuss the reasons for the decay of enstrophy, which is due to the debris that is created by the vortex collisions.

With the aim of assessing internal wave-driven mixing in the ocean, we develop a new technique for numerical simulations of stratified turbulence. Since the spatial scales of energetic ocean currents and internal gravity waves are typically much larger than that of turbulence, fully incorporating both in a model would require a high computational cost and is therefore out of our scope. Alternatively, we cut out a small domain periodically distorted by an unresolved large-scale flow field and locally simulate the energy cascade to the smallest scales. This technique enables us to concentrate the computational resources on resolving the breaking process of small-scale waves and the resulting turbulence while properly incorporating energy supply from large-scale flow components. This time, we demonstrate the results of two typical problems: the parametric subharmonic instability (PSI) of a plane internal gravity wave and the ageostrophic anticyclonic instability (AAI) of an elliptic vorte...

In turbulent convection, a complex nonlinear problem, quantification of heat transport remains a challenge. Two competing theories for extreme turbulence or large Raleigh number (Ra) are (a) classical 1/3 scaling where the heat transport is proportional to Ra^{1/3}, and (b) ultimate ½ scaling where heat transport is proportional to Ra^{1/2}. We simulate compressible turbulent convection up to Ra = 10^{18}, which is the highest Ra achieved so far, and show agreement with the classical 1/3 scaling. We also show that the Reynolds number scales as Ra^{1/2}. Our work is a major step towards the resolution of heat transport in turbulent convection.