Global observations of nonlinear mesoscale eddies

Climate phenomena and their relevance for future regional climate change

Spatial variations in sea surface temperature (SST) and rainfall changes over the tropics are investigated based on ensemble simulations for the first half of the twenty-first century under the greenhouse gas (GHG) emission scenario A1B with coupled ocean–atmosphere general circulation models of the Geophysical Fluid Dynamics Laboratory (GFDL) and National Center for Atmospheric Research (NCAR). Despite a GHG increase that is nearly uniform in space, pronounced patterns emerge in both SST and precipitation. Regional differences in SST warming can be as large as the tropical-mean warming. Specifically, the tropical Pacific warming features a conspicuous maximum along the equator and a minimum in the southeast subtropics. The former is associated with westerly wind anomalies whereas the latter is linked to intensified southeast trade winds, suggestive of wind–evaporation–SST feedback. There is a tendency for a greater warming in the northern subtropics than in the southern subtropics in accordance ...

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Global warming pattern formation: sea surface temperature and rainfall.

Global Warming Pattern Formation: Sea Surface Temperature and Rainfall

Wind Erosion and Wind-Erosion Research.- Wind-Erosion Climatology.- Atmospheric Boundary Layer and Atmospheric Modelling.- Land-Surface Modelling.- Basic Aspects of Wind Erosion.- The Dynamics and Modelling of Saltation.- Dust Emission.- Dust Transport and Deposition.- Integrated Wind-Erosion Modelling.- Sand Dunes, Dynamics and Modelling.- Techniques for Wind-Erosion Measurements.- Concluding Remarks.

Physics and Modelling of Wind Erosion

A general analysis is developed for turbulent shear flows over two- and three-dimensional hills with low-slopes which allows for a wide range of upwind velocity profiles, such as those caused by wakes of upwind hills, roughness changes, or changes in stratification. In this paper the atmosphere is assumed to be neutrally stable and the half-lengths of the hills, L, are large compared with their heights, H, which are very large compared with the roughness length zo. The general structure of the solution is defined by dividing the flow into two regions, each of which is divided into two sublayers: an inviscid outer region composed of an upper layer in which there is potential flow when the atmosphere is neutrally stable, and a middle layer in which the wind shear dominates; and an inner region of thickness l ≤ L in which the effects of perturbation shear stresses are confined. The latter region is divided into two: a shear stress layer where the shear stresses, although weak, determine that the maximum of the perturbation velocity is located in this layer; and an inner surface layer of thickness ls where the shear stress gradient varies rapidly and the perturbation velocity tends to zero. The details of the middle layer are given here for different kinds of upwind profiles, including logarithmic, ‘power law’ and linear profiles. It is shown that the analysis can be extended to allow for nonlinear inertial effects in the middle layer. Analytical solutions are derived for the inner region as asymptotic expansions in δ = [ln(l/zo)]−1, which is assumed to be small, and this shows that ls ∼ zo(l/zo)1/2.



The results of the analytical model are compared with our own and with previously published numerical computations of the full equations (applying the same assumptions used for calculating the turbulent shear stresses as used in the analytical work), which have largely been validated against full-scale measurements. These results confirm that the relative increase of surface stress is significantly greater than the increase of wind speed near the surface except when there is no upwind shear (as for example in a logarithmic boundary layer when the roughness length tends to zero).



Finally, the paper shows that the outer regions of laminar (or constant eddy viscosity) and of turbulent flows over hills are broadly similar, but that the effects of the flow in the inner region on the outer regions are much smaller in the latter case.

Turbulent shear flows over low hills

Prompted by observational evidence for an enhanced source of surface nutrients within an anticyclonic eddy in the NE Atlantic, we investigate vertical transport processes that may produce such a phenomenon. For the eddy investigated, the dominant mechanism is found to be ageostrophic circulation resulting from a perturbation of the circular flow of the eddy. This can produce upwelling velocities of order 10 m d −1 . Ekman pumping due to wind stress on the eddy also produces upwelling within, but on a smaller scale of ∼0.5 m d −1 . There is no evidence that self-propagation of the eddy leads to an enhanced nutrient flux by displacement upwards of nutrient-laden deeper isopycnals over the core of the eddy. Deeper winter mixing within the eddy relative to the surroundings, however, may contribute to the elevated nutrient levels.

Mechanisms for vertical nutrient transport within a North Atlantic mesoscale eddy

A two-dimensional stability analysis is presented of flow of low Froude number over an erodible bed. Particular regard is given to the modelling of the turbulent flow close to the bed. In contrast to previous theories that use a constant eddy-viscosity approach the present theory predicts the occurrence of two separate modes of instability, with wavelengths related to the roughness of the bed and the depth of the flow. It is postulated that these two modes correspond to the formation of ripples and dunes respectively. The results are strongly dependent on the two parameters z0, the roughness length of the bed, and β, the effect of the local bed slope on the bed-load transport. Using physically plausible estimates for these parameters the results of the analysis are in good agreement with observations for both ripples and dunes.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S002211208000078X

The formation of ripples and dunes on an erodible bed

eddies and meandering frontal jets. Some trajectories of deep floats also indicate the prevalence of zonal currents, but their ensemble fails to resolve individual jets, if any. [4] At present, data with sufficient horizontal and temporal resolution and coverage are only available from remote sensing and are confined to the sea surface. Attempts to detect zonal jets (ZJs) of the nature discussed above are missing in previous studies. This may be partly because the theory of 2D turbulence is more applicable to the deep ocean layers and/or because the wind-driven circulation overwhelms the upper ocean currents. Here, we describe a method to extract the surface signature of ZJs from maps of satellite altimetry. We also use the OFES output to justify the relevance of the extracted signal to deep ZJs. Both datasets are then used to evaluate properties of the jets.

Observational evidence of alternating zonal jets in the world ocean

Wind tunnel measurements have been made of the streamwise mean and turbulent velocities over a rough, bell-shaped, two-dimensional hill, with height h and maximum slope 0.26, placed in a neutrally stable boundary layer of thickness 10 h and roughness length zo = 0.02 h. Close agreement is found between the mean velocity and predictions obtained from Taylor's (1977) computational model and Jackson and Hunt's (1975) analytical linearized model, for locations at or upwind of the hill top but not in the wake. Examination of the models shows that the shear stresses are only important in an inner region close to the hill surface, so that, as suggested by Jackson and Hunt (1975), the perturbation to the mean flow outside this region is essentially inviscid. the theory shows that over very rough surfaces, such as wooded or urban terrain, the height of this inner region can be of the same order as the height of the roughness elements (so that in our experiments no measurements could be made in this region).

In a second experiment flow over a smooth hill on a rough surface was studied. the additional increase of wind speed over the hill top can be estimated by assuming a linear superposition of the velocity changes produced by the changes in elevation and in surface roughness (in this case rough to smooth). In the lee of a hill, however, the change in roughness significantly alters the flow with flow separation being suppressed and here a linear superposition is not appropriate.

Finally we consider why observed changes in turbulence structure close to the surface differ from those well above the surface. Calculations of these changes based on the simple theoretical arguments of equilibrium shear layers and rapidly distorted turbulent flows agree well with turbulence measurements in wind tunnels and in the field.

Kelvin J. Richards

Papers

Turbulent shear flows over low hills

Mechanisms for vertical nutrient transport within a North Atlantic mesoscale eddy

The formation of ripples and dunes on an erodible bed

Observational evidence of alternating zonal jets in the world ocean

Air flow over a two-dimensional hill: studies of velocity speed-up, roughness effects and turbulence