arXiv subjects

Callum J. Shakespeare

Publications and source records attributed to Callum J. Shakespeare.

4 recordsLinked to original sources

arXiv2025

Exciting Standing Rossby Waves in the Large Rotating Annulus

Laboratory experiments with rotating tanks remain the premier physical analogue for atmospheric dynamics. Often, the equipment involved is engineered to be sufficiently versatile and modular so as to be able to accommodate experiments that explore a wide range of atmospheric processes. The exercise of initially configuring the apparatus then involves running experiments that sweep through parameter space to identify the specific dynamical regimes of interest. This process is typically considered part of the development and testing phase of a given project, and these initial experiments are generally left unreported; while many of these test experiments may not be directly relevant to the project at hand, they may be useful for other applications or scientific communities. Here we report on a series of 93 different laboratory experiments run with the intention of identifying suitable experimental configurations that excite and sustain standing Rossby waves.

physics.flu-dyn↗

arXiv2025

Dynamical Consequences of Polar Amplification on Standing Rossby Waves: a Laboratory Perspective

Polar amplification describes the predicted reduction in the latitudinal surface temperature gradient, which will have physical implications for mid-latitude dynamics. The precise nature of these dynamical consequences remains unclear. Here we explore aspects of polar amplification by way of 24 distinct idealised laboratory experiments. The apparatus employed can independently prescribe laboratory analogues for the latitudinal temperature gradient ($Δ{T}$, which controls the stratification $N$), background zonal flow speed ($U_{b}$), and strength of the background gradient in potential vorticity ($β$). The ability to control these processes individually is beneficial as decoupling them from one another enables their influences can be examined separately. Reducing the sidewall temperature difference substantially reduces small-scale and high frequency dynamics, but does not affect the large scale features of the flow, including the north-south amplitude of standing meanders. Reducing the zonal flow speed does reduces the length-scales and amplitudes of the standing Rossby waves, while reducing the potential vorticity gradient has the opposite effect; these responses are well described by the canonical expression relating the standing Rossby wavelength to $\sqrt{U_{b}/β}$. Variability is partitioned into components that are standing and transient; the response of this variability partitioning depends on all 3 experimental parameters, and a non-dimensional term is developed ($U_{b}β/N^{2}$) which captures the behaviour of the variability. These findings suggest that the dynamical consequences of polar amplification is a tendency for mid-latitude weather to shift away from transient storms towards more persistent events, however the zonal wavelength and north-south extent of these persistent events will tend to decrease.

physics.flu-dyn↗

arXiv2025

Evaluating and improving wave and non-wave stress parametrisations for oceanic flows

Whenever oceanic currents flow over rough topography, there is an associated stress that acts to modify the flow. In the deep ocean, this stress is predominantly a form drag due to pressure differentials across topography, caused by the formation of internal waves and other baroclinic motions: processes that act on such small scales most global ocean models cannot resolve. Despite the need to incorporate this stress into ocean models, existing parametrisations are limited in their applicability. For instance, most parametrisations are only suitable for small-scale topography and are either for periodic or steady flows, but rarely a combination thereof. Here we summarise some of the most widely used parametrisations and evaluate the accuracy of a carefully selected subset using hundreds of idealised two-dimensional and three-dimensional simulations spanning a wide parameter space. We focus on the case of an isolated Gaussian hill as an idealised representation of a seamount. In cases where the parametrisations prove to be inaccurate, we use our data to suggest improved formulations. Our results thus provide a starting point for a comprehensive parameterisation of topographic stresses in ocean models where fine scale topography is unresolved.

physics.ao-ph↗

arXiv2025

Rapid topographic scatter of near-inertial waves generated by storms

Internal waves propagate on the ocean's stratification, carrying energy and redistributing momentum through the ocean. When internal waves break, they contribute to diapycnal mixing in the ocean interior, but this breaking behaviour depends upon the scale of the waves. Low-mode internal waves have larger horizontal and vertical scales, and thus break less readily than higher-mode waves. The scattering of internal waves by topography is an important mechanism in transferring internal wave energy to smaller scales that are more conducive to wave breaking and mixing processes. In this study, we propose and investigate a mechanism in which storm-generated low-mode internal waves scatter at topography. We hypothesise that horizontally propagating internal wave modes generated by strong winds (i.e., due to a storm) can rapidly dephase; these dephased waves can then be scattered from topography, resulting in higher-mode upward-propagating waves within hours of the passage of a storm. We investigate this phenomenon in an idealised numerical model of a storm passing over a prominent ridge. Bottom-scattered near-inertial internal waves propagate away from the ridge rapidly in the wake of the storm. We perform several perturbation experiments varying the properties of the ocean, the winds and the topography. The bottom-scattered waves exhibit spatial downscaling, and have an energy flux equivalent to 10% the magnitude of the energy flux from surface-generated near-inertial waves in our domain. Although small in a globally averaged sense, we argue that the topographic scatter of storm-generated near-inertial waves could account for the unexplained near-inertial wave signals found in ocean observations and numerical studies.

physics.ao-ph↗