Dispersion of wind-induced inertial waves by a barotropic jet

This note attempts to reinterpret previous results on the dispersion of wind-induced inertial waves by a geostrophic barotropic jet in the ocean. The approach is to consider the jet vorticity influence on the different baroclinic modes using a vertical normal mode expansion. Numerical and analytical analysis of the linear equations shows that vorticity effects on a single baroclinic mode strongly depend on the ratio of its Rossby radius and the length scale of the geostrophic vorticity: trapping of the near-inertial energy occurs when this ratio is small. When this ratio is of order one, inertial waves are almost unaffected by the geostrophic vorticity because dispersion efficiently overcomes the jet vorticity effects. A 2-D primitive-equation model is used to examine the scattering of wind-induced inertial waves in realistic situations. Results indicate that contribution of the lowest baroclinic modes, unaffected by the jet vorticity, explain some striking features reported in previous studies as the downward phase propagation of near-inertial waves in the positive vorticity region

Modeling the Nd isotopic composition in the North Atlantic basin using an eddy-permitting model

International audienceBoundary Exchange (BE – exchange of elementsbetween continental margins and the open ocean) hasbeen emphasized as a key process in the oceanic cycle ofneodymium (Nd) (Lacan and Jeandel, 2005a). Here, we usea regional eddy-permitting resolution Ocean General CirculationModel (1/4) of the North Atlantic basin to simulatethe distribution of the Nd isotopic composition, consideringBE as the only source. Results show good agreementwith the data, confirming previous results obtained using thesame parameterization of the source in a coarse resolutionglobal model (Arsouze et al., 2007), and therefore the majorcontrol played by the BE processes in the Nd cycle onthe regional scale. We quantified the exchange rate of theBE, and found that the time needed for the continental marginsto significantly imprint the chemical composition of thesurrounding seawater (further referred as characteristic exchangetime) is of the order of 0.2 years. However, thetimescale of the BE may be subject to large variations as avery short exchange time (a few days) is needed to reproducethe highly negative values of surface waters in the LabradorSea, whereas a longer one (up to 0.5 years) is required tosimulate the radiogenic influence of basaltic margins and distinguishthe negative isotopic signatures of North AtlanticDeep Water from the more radiogenic southern origin watermasses. This likely represents geographical variations inerosion fluxes and the subsequent particle load onto the continentalmargins. Although the parameterization of the BEis the same in both configurations of the model, the characteristicexchange time in the eddy-permitting configuration issignificantly lower than the previous evaluations using a lowresolution configuration (6 months to 10 years), but howeverin agreement with the available seawater Nd isotope data.This results highlights the importance of the model dynamicsin simulating the BE proces

The present and future system for measuring the Atlantic meridional overturning circulation and heat transport

of the global combined atmosphere-ocean heat flux and so is important for the mean climate of the Atlantic sector of the Northern Hemisphere. This meridional heat flux is accomplished by both the Atlantic Meridional Overturning Circulation (AMOC) and by basin-wide horizontal gyre circulations. In the North Atlantic subtropical latitudes the AMOC dominates the meridional heat flux, while in subpolar latitudes and in the subtropical South Atlantic the gyre circulations are also important. Climate models suggest the AMOC will slow over the coming decades as the earth warms, causing widespread cooling in the Northern hemisphere and additional sea-level rise. Monitoring systems for selected components of the AMOC have been in place in some areas for decades, nevertheless the present observational network provides only a partial view of the AMOC, and does not unambiguously resolve the full variability of the circulation. Additional observations, building on existing measurements, are required to more completely quantify the Atlantic meridional heat transport. A basin-wide monitoring array along 26.5°N has been continuously measuring the strength and vertical structure of the AMOC and meridional heat transport since March 31, 2004. The array has demonstrated its ability to observe the AMOC variability at that latitude and also a variety of surprising variability that will require substantially longer time series to understand fully. Here we propose monitoring the Atlantic meridional heat transport throughout the Atlantic at selected critical latitudes that have already been identified as regions of interest for the study of deep water formation and the strength of the subpolar gyre, transport variability of the Deep Western Boundary Current (DWBC) as well as the upper limb of the AMOC, and inter-ocean and intrabasin exchanges with the ultimate goal of determining regional and global controls for the AMOC in the North and South Atlantic Oceans. These new arrays will continuously measure the full depth, basin-wide or choke-point circulation and heat transport at a number of latitudes, to establish the dynamics and variability at each latitude and then their meridional connectivity. Modeling studies indicate that adaptations of the 26.5°N type of array may provide successful AMOC monitoring at other latitudes. However, further analysis and the development of new technologies will be needed to optimize cost effective systems for providing long term monitoring and data recovery at climate time scales. These arrays will provide benchmark observations of the AMOC that are fundamental for assimilation, initialization, and the verification of coupled hindcast/forecast climate models

The North Atlantic subpolar gyre in four high resolution models

The authors present the first quantitative comparison between new velocity datasets and high-resolution models in the North Atlantic subpolar gyre [1/10° Parallel Ocean Program model (POPNA10), Miami Isopycnic Coordinate Ocean Model (MICOM), ° Atlantic model (ATL6), and Family of Linked Atlantic Ocean Model Experiments (FLAME)]. At the surface, the model velocities agree generally well with World Ocean Circulation Experiment (WOCE) drifter data. Two noticeable exceptions are the weakness of the East Greenland coastal current in models and the presence in the surface layers of a strong southwestward East Reykjanes Ridge Current. At depths, the most prominent feature of the circulation is the boundary current following the continental slope. In this narrow flow, it is found that gridded float datasets cannot be used for a quantitative comparison with models. The models have very different patterns of deep convection, and it is suggested that this could be related to the differences in their barotropic transport at Cape Farewell. Models show a large drift in watermass properties with a salinization of the Labrador Sea Water. The authors believe that the main cause is related to horizontal transports of salt because models with different forcing and vertical mixing share the same salinization problem. A remarkable feature of the model solutions is the large westward transport over Reykjanes Ridge [10 Sv (Sv ≡ 106 m3 s−1) or more

Eddy-Permitting Ocean Circulation Hindcasts of Past Decades

International audienc

The mechanism of force transmission at bacterial focal adhesion complexes

Various rod-shaped bacteria mysteriously glide on surfaces in the absence of appendages such as flagella or pili. In the deltaproteobacterium Myxococcus xanthus, a putative gliding motility machinery (the Agl–Glt complex) localizes to so-called focal adhesion sites (FASs) that form stationary contact points with the underlying surface. Here we show that the Agl–Glt machinery contains an inner-membrane motor complex that moves intracellularly along a right-handed helical path; when the machinery becomes stationary at FASs, the motor complex powers a left-handed rotation of the cell around its long axis. At FASs, force transmission requires cyclic interactions between the molecular motor and the adhesion proteins of the outer membrane via a periplasmic interaction platform, which presumably involves contractile activity of motor components and possible interactions with peptidoglycan. Our results provide a molecular model of bacterial gliding motility