118 research outputs found
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 â 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 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 m3âsâ1) or more
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
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On the response of the Antarctic Circumpolar Current transport to climate change in coupled climate models
The CMIP3 (IPCC AR4) models show a consistent intensification and poleward shift of the westerly winds over the Southern Ocean during the 21st century. However, the responses of the Antarctic Circumpolar Currents (ACC) show great diversity in these models, with many even
showing reductions in transport. To obtain some understanding of diverse responses in the ACC transport, we investigate both external atmospheric and internal oceanic processes that control the ACC transport responses in
these models. While the strengthened westerlies act to increase the tilt of isopycnal surfaces and hence the ACC transport through Ekman pumping effects, the associated changes in buoyancy forcing generally tend to reduce the surface
meridional density gradient. The steepening of isopycnal surfaces induced
by increased wind forcing leads to enhanced (parameterized) eddy-induced
transports that act to reduce the isopycnal slopes. There is also considerable narrowing of the ACC that tends to reduce the ACC transport, caused mainly by the poleward shifts of the subtropical gyres and to a lesser extent by the equatorward expansions of the subpolar gyres in some models. If the combined effect of these retarding processes is larger than that of enhanced Ekman pumping, the ACC transport will be reduced. In addition, the effect of Ekman pumping on the ACC is reduced in weakly stratified models. These findings give insight into the reliability of IPCC-class model predictions of the Southern Ocean circulation, and into the observed decadal-scale steady ACC transport
The Atlantic meridional overturning circulation in high resolution models
The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N â a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by highâresolution models. Furthermore, we will describe how highâresolution model simulations can help resolve outstanding challenges in our understanding of the AMOC
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