Transport of salt and freshwater in the Atlantic Subpolar Gyre (original) (raw)
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Coupling of eastern and western subpolar North Atlantic: salt transport in the Irminger Current
Ocean Science Discussions, 2013
Salt transport in the Irminger Current and thus the coupling between eastern and western subpolar North Atlantic plays an important role for climate variability across a wide range of time scales. High-resolution ocean modeling and observations indicate that salinities in the eastern subpolar North Atlantic decrease with enhanced circulation of the North Atlantic subpolar gyre (SPG). This has led to the perception that a stronger SPG also transports less salt westward. In this study, we analyze a regional ocean model and a comprehensive global coupled climate model, and show that a stronger SPG transports more salt in the Irminger Current irrespective of lower salinities in its source region. The additional salt converges in the Labrador Sea and the Irminger Basin by eddy transports, increases surface salinity in the western SPG, and favors more intense deep convection. This is part of a positive feedback mechanism with potentially large implications for climate variability and predictability.
The North Atlantic Subpolar Gyre in Four High-Resolution Models
Journal of Physical Oceanography, 2005
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), 1 ⁄6°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 ϵ 10 6 m 3 s Ϫ1 ) or more].
Deep Sea Research Part I: Oceanographic Research Papers, 2011
The role of Mediterranean Overflow Water (MOW) in creating subsurface salinity anomalies within the Rockall Trough, a gateway to high latitude areas of deep convection, has been examined closely in recent years. Eulerian investigations of high latitude property fields have suggested that these subsurface anomalies are likely the result of variability in the zonal extent of the eastern limb of the subpolar gyre: when expanded into the eastern North Atlantic, the gyre is presumed to limit the extent to which MOW is able to penetrate northward to subpolar latitudes. However, though the depth of the subsurface salinity anomalies in the Rockall Trough supports the hypothesis that the intermittent presence of MOW is involved in creating the anomalies, MOW pathways to the Rockall Trough have not yet been established. Here, Lagrangian trajectories from floats released in the eastern North Atlantic between 1996 and 1997 and synthetic trajectories launched within an eddy-resolving ocean general circulation model are used to demonstrate that two main density neutral transport pathways lead to the Rockall Trough. One pathway involves the transport of relatively fresh waters as part of the North Atlantic Current and the other involves the transport of relatively salty waters from the eastern reaches of the subtropical North Atlantic. The results from this study indicate that changes in these pathways over time can explain the subsurface salinity variability in the Rockall Trough.
Analysis of an 80-Year Integration of a 1/3-Degree Ocean Model of the Subpolar North Atlantic
Journal of Oceanography, 2005
Previous work had examined an ocean model of the subpolar gyre of the North Atlantic Ocean that used the Gent and McWilliams parameterization with a variable eddytransfer coefficient, and showed significant improvements to the model's circulation and hydrography. This note examines an extended (80-year-long) integration of the same model and focuses on the adjustment of the intermediate and deep waters as well as on model stability. It is shown that the model is able to retain a good representation of the water masses, especially in the Labrador Sea, through the full integration. Labrador Sea Water dispersal is well simulated by the model in the western basin, with a good correspondence between the model and observational salinities on the σ σ σ σ σ 2 = 36.95 isopycnal surface. Labrador Sea Water dispersal to the eastern basin is not nearly as well represented, as this water mass has trouble passing over the Mid-Atlantic Ridge in the model. The variable eddy-transfer coefficient significantly improves the model representation of the Cold Intermediate Layer on the Labrador shelf by reducing spurious diapycnal mixing. Finally, the evidence in this note suggests that open boundary conditions do not generate significant model drift, even for integrations approaching a century in length. (10 7 cm 2 s-1) was used in a model of the North Atlantic (England and Holloway, 1998). Since the intensity of the eddy-induced tracer transport is variable in the ocean, there have been attempts to take it into account. The variable eddy-transfer coefficient proposed by Visbeck et al. (1997) (henceforth referred to as VMHS) for the GM scheme outperformed a constant coefficient in simulations with a 1.25°-resolution model of Wright (1997), and a discussion of its effect in an eddy-permitting global ocean model was presented by Gent et al. (2002). Myers and Deacu (2004) modelled the subpolar North Atlantic with an eddy-permitting model (1/3°-resolution) and found an unrealistic drift of the model Labrador Sea salinity. They suggested that enhanced baroclinic eddy activity along the Labrador slope strengthened the countercurrent adjacent to the Labrador Current, which entrained too much high-salinity water of North Atlantic Current origin into the Labrador Sea. Deacu and Myers (2005) considered the inclusion of a VMHS eddy-transfer coefficient and found it had a positive effect on the model results. Improvements to the simulated velocity field included a better representation of the Labrador
2011
On interdecadal timescales, the Atlantic meridional overturning circulation (AMOC) is thought to be in phase with the North Atlantic Sea Surface Temperatures (as measured by the Atlantic Multidecadal Oscillation-AMO-index). However, it appears that we have entered a positive phase of the AMO since 1995-2000 although we fear the Atlantic meridional overturning may be on a declining trend, as suggested by several observational and modelling studies. Here we constrain ocean models with temperature and salinity fields built on observations, and compare the results with various simple methods (namely diagnostic, robust diagnostic and prognostic), models (North Atlantic and global configurations at various resolutions), and forcings. Mean transports of heat and mass are sensitive to the method and model configuration, but their decadal variability is much more coherent and does not depend explicitly on the variations of the surface forcing, its influence being imprinted in the thermohaline structure. Multidecadal variations are of the order of 20% (0.15 PW in heat transport and 4 Sv in overturning), with large transports in the subpolar gyre in the early 1960's and mid 1990's, and low values in the mid 1970's. Declining transports of heat and mass are coherent in several models and methods since 1995, especially in the subpolar gyre, and opposite to the long term tendency from 1958 to 2008.
The impact of salinity perturbations on the future uptake of heat by the Atlantic Ocean
Geophysical Research Letters, 2014
Anthropogenic ocean heat uptake is a key factor in determining climate change and sea-level rise. There is considerable uncertainty in projections of freshwater forcing of the ocean, with the potential to influence ocean heat uptake. We investigate this by adding either-0.1 Sv or +0.1 Sv freshwater to the Atlantic in global climate model simulations, simultaneously imposing an atmospheric CO 2 increase. The resulting changes in the Atlantic meridional overturning circulation are roughly equal and opposite (±2Sv). The impact of the perturbation on ocean heat content is more complex, although it is relatively small (∼5%) compared to the total anthropogenic heat uptake. Several competing processes either accelerate or retard warming at different depths. Whilst positive freshwater perturbations cause an overall heating of the Atlantic, negative perturbations produce insignificant net changes in heat content. The processes active in our model appear robust, although their net result is likely model-and experiment-dependent.
Climate Dynamics, 2006
On the time scale of a century, the Atlantic thermohaline circulation (THC) is sensitive to the global surface salinity distribution. The advection of salinity toward the deep convection sites of the North Atlantic is one of the driving mechanisms for the THC. There is both a northward and a southward contributions. The northward salinity advection (Nsa) is related to the evaporation in the subtropics, and contributes to increased salinity in the convection sites. The southward salinity advection (Ssa) is related to the Arctic freshwater forcing and tends on the contrary to diminish salinity in the convection sites. The THC changes results from a delicate balance between these opposing mechanisms. In this study we evaluate these two effects using the IPSL-CM4 ocean-atmospheresea-ice coupled model (used for IPCC AR4). Perturbation experiments have been integrated for 100 years under modern insolation and trace gases. River runoff and evaporation minus precipitation are successively set to zero for the ocean during the coupling procedure. This allows the effect of processes Nsa and Ssa to be estimated with their specific time scales. It is shown that the convection sites in the North Atlantic exhibit various sensitivities to these processes. The Labrador Sea exhibits a dominant sensitivity to local forcing and Ssa with a typical time scale of 10 years, whereas the Irminger Sea is mostly sensitive to Nsa with a 15 year time scale. The GIN Seas respond to both effects with a time scale of 10 years for Ssa and 20 years for Nsa. It is concluded that, in the IPSL-CM4, the global freshwater forcing damps the THC on centennial time scales.
Meridional transport of salt in the global ocean from an eddy-resolving model
Ocean Science, 2014
The meridional transport of salt is computed in a global eddy-resolving numerical model (1/12 • resolution) in order to improve our understanding of the ocean salinity budget. A methodology is proposed that allows a global analysis of the salinity balance in relation to surface water fluxes, without defining a "freshwater anomaly" based on an arbitrary reference salinity. The method consists of a decomposition of the meridional transport into (i) the transport by the time-longitude-depth mean velocity, (ii) time-mean velocity recirculations and (iii) transient eddy perturbations. Water is added (rainfall and rivers) or removed (evaporation) at the ocean surface at different latitudes, which creates convergences and divergences of mass transport with maximum and minimum values close to ±1 Sv. The resulting meridional velocity effects a net transport of salt at each latitude (±30 Sv PSU), which is balanced by the time-mean recirculations and by the net effect of eddy salinity-velocity correlations. This balance ensures that the total meridional transport of salt is close to zero, a necessary condition for maintaining a quasi-stationary salinity distribution. Our model confirms that the eddy salt transport cannot be neglected: it is comparable to the transport by the time-mean recirculation (up to 15 Sv PSU) at the poleward and equatorial boundaries of the subtropical gyres. Two different mechanisms are found: eddy contributions are localized in intense currents such as the Kuroshio at the poleward boundary of the subtropical gyres, while they are distributed across the basins at the equatorward boundaries. Closer to the Equator, salinityvelocity correlations are mainly due to the seasonal cycle and large-scale perturbations such as tropical instability waves. This is the concept of "using the ocean as a rain gauge for the Published by Copernicus Publications on behalf of the European Geosciences Union. 244 A. M. Treguier et al.: Meridional transport of salt www.ocean-sci.net/10/243/2014/ Ocean Sci., 10, 243-255, 2014