to be done in this area. Face recognition is a problem that scarcely clamoured for attention before the computer age but, having surfaced, has involved a wide range of techniques and has attracted the attention of some fine minds (David Mumford was a Fields Medallist in 1974). This singular application of mathematical modelling to a messy applied problem of obvious utility and importance but with no unique solution is a pretty one to share with students: perhaps, returning to the source of our opening quotation, we may invert Duncan's earlier observation, 'There is an art to find the mind's construction in the face!'.

Linear and nonlinear waves, by G. B. Whitham. Pp.636. £50. 1999. ISBN 0 471 35942 4 (Wiley).

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.

Salt marshes protect coastlines against waves. Wave flume experiments show that marsh vegetation causes substantial wave dissipation and prevents erosion of the underlying surface, even during extreme storm surge conditions.

/pdf/wave-attenuation-over-coastal-salt-marshes-under-storm-surge-cenihcxco5.pdf

Wave attenuation over coastal salt marshes under storm surge conditions

Because of its relevance for the global climate the Atlantic meridional overturning circulation (AMOC) has been a major research focus for many years. Yet the question of which physical mechanisms ultimately drive the AMOC, in the sense of providing its energy supply, remains a matter of controversy. Here we review both observational data and model results concerning the two main candidates: vertical mixing processes in the ocean's interior and wind-induced Ekman upwelling in the Southern Ocean. In distinction to the energy source we also discuss the role of surface heat and freshwater fluxes, which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state. We conclude that both wind-driven upwelling and vertical mixing are likely contributing to driving the observed circulation. To quantify their respective contributions, future research needs to address some open questions, which we outline.

/pdf/on-the-driving-processes-of-the-atlantic-meridional-4ze95ojzss.pdf

On the driving processes of the Atlantic meridional overturning circulation

Natural ventilation of buildings is the flow generated by temperature differences and by the wind. The governing feature of this flow is the exchange between an interior space and the external ambient. Although the wind may often appear to be the dominant driving mechanism, in many circumstances temperature variations play a controlling feature on the ventilation since the directional buoyancy force has a large influence on the flow patterns within the space and on the nature of the exchange with the outside. Two forms of ventilation are discussed: mixing ventilation, in which the interior is at an approximately uniform temperature, and displacement ventilation, where there is strong internal stratification. The dynamics of these buoyancy-driven flows are considered, and the effects of wind on them are examined. The aim behind this work is to give designers rules and intuition on how air moves within a building; the research reveals a fascinating branch of fluid mechanics.

The fluid mechanics of natural ventilation

We report laboratory and numerical experiments with the convective circulation that develops in a long channel driven by heating and cooling through opposite halves of the horizontal base. The problem is similar to that posed by Stommel (Proc. Natl Acad. Sci. vol. 48, 1962, p. 766) and Rossby (Deep-Sea Res. vol. 12, 1965, p. 9; Tellus vol. 50, 1998, p. 242), where flow forced by a linear temperature variation along the ocean surface or the base of a tank presented a demonstration of the smallness of sinking regions in the meridional overturning circulation of the oceans. In contrast to the previous experiments, we use small aspect ratio, larger Rayleigh numbers, piecewise uniform boundary conditions and an imposed input heat flux. The flow is characterized by a vigorous overturning circulation cell filling the box length and depth. A stable thermocline forms above the cooled base and is advected over the heated part of the base, where it is eroded from below by small-scale three-dimensional convection, forming a ‘convective mixed layer’. At the endwall, the convective mixing is overshadowed by a narrow but turbulent plume rising through the full depth of the box. The return flow along the top of the box is turbulent with large slowly migrating eddies, and occupies approximately a third of the total depth. Theoretical scaling laws give temperature differences, thermocline thickness and velocities that are in good agreement with the experimental data and two-dimensional numerical solutions. The measured and computed density structure is largely similar to the thermocline and abyssal stratification in the oceans.

Convection driven by differential heating at a horizontal boundary

of water and of the sedge Schoenoplectus americanus observed (using synchronized current meters and video) in a shallow salt marsh (depth < 1 m). Consistent with theory, sedge motion led water motion, with the phase decreasing (from 90 to 0 degrees) with increasing wave frequency. After tuning of a single free parameter (Young’s modulus), the theory successfully predicted the transfer function between measured water and stem motion. Formulae predicting frequency‐dependent wave dissipation by flexible vegetation are derived. For the moderately flexible stems observed, the model predicted total dissipation was about 30% of the dissipation for equivalent rigid stems.

Wave-forced motion of submerged single stem vegetation

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JC006448

Wave‐forced motion of submerged single‐stem vegetation

This paper examines the role of mangrove pneumatophore roots as a spatial control over the turbulent kinetic energy (TKE) dissipation rate within a natural mangrove forest. Measurements of turbulence at millimeter scales were compared with vegetation geometries reconstructed using a novel photogrammetric technique. These small-scale relationships were then averaged to show larger-scale patterns in turbulence across the mudflat and mangrove fringe-forest transition. Although turbulence estimates varied with across-shore position, TKE dissipation was always elevated in the fringe relative to mudflat and forest interior sample sites. The largest dissipation rates (4.5 × 10 −3  W kg −1 ) were measured as breaking waves propagated over canopies in very shallow water. Dissipation was reduced, but often remained intense (10 −5 –10 −4  W kg −1 ) under non-breaking waves at the fringe, likely indicating turbulent generation in pneumatophore wakes. Pneumatophore density was positively correlated with the spatial distribution of TKE dissipation. Turbulence was also correlated positively with wave height and negatively with water depth. Fringe sediments were more sandy and less muddy than sediments onshore and offshore, suggesting that the intense turbulence may lead to winnowing of fine-grained sediments at the fringe.

Julia C. Mullarney

Papers

Convection driven by differential heating at a horizontal boundary

Wave-forced motion of submerged single stem vegetation

Wave‐forced motion of submerged single‐stem vegetation

The Effect of Pneumatophore Density on Turbulence: A Field Study in a Sonneratia-dominated Mangrove Forest, Vietnam

The influence of wind and waves on the existence of stable intertidal morphology in meso-tidal estuaries