Atmospheric and Oceanic Fluid Dynamics

The potential for sea ice-albedo feedback to give rise to nonlinear climate change in the Arctic Ocean – defined as a nonlinear relationship between polar and global temperature change or, equivalently, a time-varying polar amplification – is explored in IPCC AR4 climate models. Five models supplying SRES A1B ensembles for the 21 st century are examined and very linear relationships are found between polar and global temperatures (indicating linear Arctic Ocean climate change), and between polar temperature and albedo (the potential source of nonlinearity). Two of the climate models have Arctic Ocean simulations that become annually sea ice-free under the stronger CO 2 increase to quadrupling forcing. Both of these runs show increases in polar amplification at polar temperatures above-5 o C and one exhibits heat budget changes that are consistent with the small ice cap instability of simple energy balance models. Both models show linear warming up to a polar temperature of-5 o C, well above the disappearance of their September ice covers at about-9 o C. Below-5 o C, surface albedo decreases smoothly as reductions move, progressively, to earlier parts of the sunlit period. Atmospheric heat transport exerts a strong cooling effect during the transition to annually ice-free conditions. Specialized experiments with atmosphere and coupled models show that the main damping mechanism for sea ice region surface temperature is reduced upward heat flux through the adjacent ice-free oceans resulting in reduced atmospheric heat transport into the region.

Geophysical fluid dynamics.

We review what is known about the convective process in the open ocean, in which the properties of large volumes of water are changed by intermittent, deep-reaching convection, triggered by winter storms. Observational, laboratory, and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales, the large-scale circulation of the ocean, and the prevailing meteorology. Two aspects make ocean convection interesting from a theoretical point of view. First, the timescales of the convective process in the ocean are sufficiently long that it may be modified by the Earth's rotation; second, the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability. Moreover, the convective and geostrophic scales are not very disparate from one another. Detailed observations of the process in the Labrador, Greenland, and Mediterranean Seas are described, which were made possible by new observing technology. When interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection, great progress in our description and understanding of the processes at work is being made.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98RG02739

Open-ocean convection: Observations, theory, and models

Fluid dynamics is fundamental to our understanding of the atmosphere and oceans. Although many of the same principles of fluid dynamics apply to both the atmosphere and oceans, textbooks tend to concentrate on the atmosphere, the ocean, or the theory of geophysical fluid dynamics (GFD). This textbook provides a comprehensive unified treatment of atmospheric and oceanic fluid dynamics. The book introduces the fundamentals of geophysical fluid dynamics, including rotation and stratification, vorticity and potential vorticity, and scaling and approximations. It discusses baroclinic and barotropic instabilities, wave-mean flow interactions and turbulence, and the general circulation of the atmosphere and ocean. Student problems and exercises are included at the end of each chapter. Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation will be an invaluable graduate textbook on advanced courses in GFD, meteorology, atmospheric science and oceanography, and an excellent review volume for researchers. Additional resources are available at www.cambridge.org/9780521849692.

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Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation

The meridional overturning circulation of the ocean plays a central role in the climate and its variability. This Review of recent studies emphasizes the importance of wind-driven upwelling in the Southern Ocean for global ocean circulation.

http://oceans.mit.edu/JohnMarshall/wp-content/uploads/2013/08/Closure-of-the-meridional-overturning_134.pdf

Closure of the meridional overturning circulation through Southern Ocean upwelling

Ventilation of the upper ocean plays an important role in climate variability on interannual to decadal timescales by influencing the exchange of heat and carbon dioxide between the atmosphere and ocean. The turbulent nature of ocean circulation, manifest in a vigorous mesoscale eddy field, means that pathways of ventilation, once thought to be quasi-laminar, are in fact highly chaotic. We characterise the chaotic nature of ventilation pathways according to a nondimensional ‘filamentation number', which estimates the reduction in filament width of a ventilated fluid parcel due to mesoscale strain. In the subtropical North Atlantic of an eddy-permitting ocean model, the filamentation number is large everywhere across three upper ocean density surfaces — implying highly chaotic ventilation pathways — and increases with depth. By mapping surface ocean properties onto these density surfaces, we directly resolve the highly filamented structure and confirm that the filamentation number captures its spatial variability. These results have implications for the spreading of atmospherically-derived tracers into the ocean interior.

Characterizing the chaotic nature of ocean ventilation

Closing a gyre with a western boundary current imposes a constraint on its vertical structure that requires there to be no net vertical flux of potential vorticity through any closed pressure contour in steady state. This constraint resembles the traditional similarity balance for the internal thermocline, with additional terms representing friction in the western boundary current and convection in the mixed layer. The terms in the integral constraint are diagnosed in a planetary geostrophic ocean model and are used to understand the coexistence of ventilated and internal thermoclines within the subtropical gyre.

/pdf/understanding-the-structure-of-the-subtropical-thermocline-3sb2uvq110.pdf

Understanding the Structure of the Subtropical Thermocline

Recent studies show that the western boundary acts as a ‘graveyard’ for westward-propagating ocean eddies However, how the eddy energy incident on the western boundary is dissipated remains unclear Here we investigate the energetics of eddy-western boundary interaction using an idealised MIT ocean circulation model with a spatially variable grid resolution Four types of model experiments are conducted: (1) single eddy cases, (2) a sea of random eddies, (3) with a smooth topography and (4) with a rough topography We find significant dissipation of incident eddy energy at the western boundary, regardless of whether the model topography at the western boundary is smooth or rough However, in the presence of rough topography, not only the eddy energy dissipation rate is enhanced, but more importantly, the leading process for removing eddy energy in the model switches from bottom frictional drag as in the case of smooth topography to viscous dissipation in the ocean interior above the rough topography Further analysis shows that the enhanced eddy energy dissipation in the experiment with rough topography is associated with greater anticyclonic-ageostrophic instability (AAI), possibly as a result of lee wave generation and non-propagating form drag effect

/pdf/an-idealised-model-study-of-eddy-energetics-in-the-western-34a0v597sh.pdf

An Idealised Model Study of Eddy Energetics in the Western Boundary 'Graveyard'

Great progress has been made in understanding the role of small- and mesoscale processes on the large-scale structure and circulation of the oceans. However, many questions remain regarding the sensitivity of the large-scale ocean circulation and hence the earth's climate, to rates of stirring, mixing, and dissipation supported by ocean eddies, internal waves, and boundary layers. Although such relatively small-scale oceanic phenomena are ubiquitous, we lack detailed knowledge about their interaction with the large-scale motions, as well as their interactions amongst themselves. Contributions are presented from observational, theoretical, and modeling studies that focus on small- and mesoscale ocean processes, and some parameterizations of these processes for use in studies of the large-scale circulation. The range of topics is relatively broad, with most focus on internal wave generation, flow–topography interaction, and small- and mesoscale processes involved in ocean mixing. General conclusions emphasize the notion of localized mixing and the importance of interaction between waves, eddies, and topography, and the importance of interactions between oceanic motions and the background density stratification.

Small and mesoscale processes and their impact on the large scale:an introduction

Atmospheric reanalyses are commonly used to force numerical ocean models, but despite large discrepancies reported between different products, the impact of reanalysis uncertainty on the si...

/pdf/impacts-of-atmospheric-reanalysis-uncertainty-on-atlantic-16fcd0jux2.pdf

David P. Marshall

Papers

Characterizing the chaotic nature of ocean ventilation

Understanding the Structure of the Subtropical Thermocline

An Idealised Model Study of Eddy Energetics in the Western Boundary 'Graveyard'

Small and mesoscale processes and their impact on the large scale:an introduction

Impacts of Atmospheric Reanalysis Uncertainty on Atlantic Overturning Estimates at 25°N