Mid-depth Lagrangian pathways in the North Atlantic and their impact on the salinity of the eastern subpolar gyre (original) (raw)
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Transport of salt and freshwater in the Atlantic Subpolar Gyre
Ocean Dynamics, 2016
Transport of salt in the Irminger Current, the northern branch of the Atlantic Subpolar Gyre coupling the 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.
Journal of Geophysical Research, 1999
Part 1 of this study is a descriptive analysis of the spreading of Mediterranean Water based on high-resolution maps of salinity and density in the eastern basin. In this second part of our study, velocity fields for two representative isopycnal surfaces of the Mediterranean outflow (o-0. s = 29.70 and o-0. s = 29.90) are estimated from a diagnostic model that combines climatological hydrographic data from the National Oceanic Data Center with long-term direct measurements of water exchange through the Strait of Gibraltar. The model is constrained by geostrophic dynamics, conservation of mass, noflux conditions at the continental shelf, and specified flow through the Strait of Gibraltar. Our principal data source is a recently assembled database of the North Atlantic that consists of climatological mean property fields averaged on isopycnal surfaces. The mean fields are based on more than 80 years (1909-1990) of data and have a nominal horizontal resolution of 0.5 ø . To provide boundary conditions at the Strait of Gibraltar, we use the results of a model developed from data collected during the Gibraltar Experiment in 1985. The estimated velocity fields show Mediterranean Water exiting the Strait of Gibraltar, following the southern Iberian coast, and then entering the Tagus Basin, where it turns anticyclonically to create a reservoir of this water mass. The flow continues northward along the eastern boundary, penetrating into the Rockall Channel. Finally, the model flow fields do not show a significant westward advection of Mediterranean waters into the subtropical gyre. Mediterranean Water are whether or not there is a westward advection of this water into the subtropical Atlantic and whether or not this water mass is advected northward to latitudes near Rockall Channel [Iorga and Lozier, this issue; Reid, 1994]. The interest in these pathways is particularly focused on the degree to which Mediterranean Water influences the source waters at the deep water formation sites in the Norwegian-Greenland Seas. To achieve an understanding of this influence, an assessment of the climatological pathways in the eastern North Atlantic is necessary. As a first step toward this Paper number 1999JC900204. 0148-0227/99/1999JC900204509.00 goal, Iorga and Lozier [this issue] analyzed the salinity and density fields in the eastern North Atlantic using a recent, high-resolution climatological database of the North Atlantic [Lozier et al., 1995]. They provided a descriptive analysis of the Mediterranean Water pathway derived from its salinity signature. The reader is referred to Iorga and Lozier [this issue] for background material on past synoptic studies of the Mediterranean outflow waters. In this paper a quantitative analysis of the climatological Mediterranean pathway is presented. Specifically, stream function and velocity fields have been computed in the eastern North Atlantic from a diagnostic model that is based on geostrophic dynamics and the conservation of mass and constrained by no-flux boundary conditions and a flow specification at the Strait of Gibraltar. There have been several past efforts at the estimation of the long-term flow field in the eastern North Atlantic with diagnostic models. A decade ago, Hogg [1987] used an inverse model in conjunction with Levitus climatological data to estimate the Montgomery stream function on two isopycnal surfaces (0-1 = 31.8 and 0-1 = 32.3) in the eastern North Atlantic. He found that at the depth of the Mediterranean outflow the flow is down the temperature tongue (westward) with the advective thermal flux balanced by a lateral diffusive thermal flux. However, the eastern extent of Hogg's model domain was at 23ø30'W; thus his model domain did not encompass the source region for the Mediterranean waters. Additionally, the two isopycnal surfaces that Hogg used for his computation are 26,011 26,012 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--DIAGNOSTIC VELOCITY FIELDS centered at the upper and lower edge of the climatological salinity tongue, rather than at the core, which is located between cr I = 32.00 and cr I = 32.20. Using a simple numerical model based on the salinity conservation equation and a given circulation scheme, Richardson and Mooney [1975] showed that for Peclet numbers corresponding to the North Atlantic (3-30) the tongue-like penetration of salty Mediterranean Water into the subtropical gyre could be explained by diffusive processes. They found that the shape of the tongue (its shift to the south and west and the fact that it becomes narrower away from the Strait of Gibraltar), as well as its westward extent, can be attributed to advective processes. Other studies have modeled the flow field in the North Atlantic but without a particular focus on the Mediterranean outflow and its fate as it flows from the Strait of Gibraltar. For example, Maz• et al. [1997] determined the vertical distribution of zonal transports near the eastern boundary of the North Atlantic. Even though their focus was not on the Mediterranean Water, Maz• et al. [1997] argue that a direct entry of these waters into the ocean interior can only occur at Iberian latitudes through westward Meddy propagation. This conclusion is in agreement with inverse model output from Paillet and Mercier [1996], which shows Mediterranean Water (at 1000 m) turning northward out of the Gulf of Cadiz with no evidence of a direct westward advection of Mediterranean Water across the North Atlantic basin. Paillet and Mercier's model also gives a southern branch of Mediterranean Water along the African coast, which they argtie is not realistic. Supporting Hogg's finding of a westward flow are Bogden et al.'s [1993] model results, where the timeaveraged geostrophic velocity field in the North Atlantic was estimated from observations of density, wind stress, and bottom topography, with a prescribed Ekman pumping at the surface and no normal flow condition at the bottom. Bogden et 2.2. Strait of Gibraltar Flow Specification Along the Strait of Gibraltar there are two prominent sills: the Camarinal Sill at --•5ø45'W, with a depth of 286 m, and the Spartel Sill, at --•6øW, slightly deeper at 316 m. The model used for this study is constrained west of Spartel Sill, at 6ø15'W, between 35ø45 ' and 36ø15'N, by a specified flow field that simulates the two-layer exchange through the Strait of Gibraltar. The flow is defined so that it conserves an outflow transport of 0.72 Sv and an inflow transport of 0.68 Sv, in accordance with the results of the model developed by Bryden et al. [1994], which is based on the data collected during the Gibraltar Experiment in 1985. The vertical velocity distribution of the flow at this site (Plate 1) is derived from the velocity profile described by Johnson et al. [1994] in such a way that it distributes the total transport uniformly over the width of the IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--DIAGNOSTIC VELOCITY FIELDS 26,013 Strait of Gibraltar, which for our model is 0.5 ø of latitude. On IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--DIAGNOSTIC VELOCITY FIELDS 26 023
Structure, transports and transformations of the water masses in the Atlantic Subpolar Gyre
Progress in Oceanography, 2015
We discuss the distributions and transports of the main water masses in the North Atlantic 21 Subpolar Gyre (NASPG) for the mean of the period 2002-2010 (OVIDE sections 2002-2010 every 22 Atlantic Basin to the Irminger Basin (9.4 ± 4.7 Sv) into the contributions of the Central Waters (2.1 35 ± 1.8 Sv), Labrador Sea Water (LSW, 2.4 ± 2.0 Sv), Subarctic Intermediate Water (SAIW, 4.0 ± 0.5 36 Sv) and Iceland-Scotland Overflow Water (ISOW, 0.9 ± 0.9 Sv). Once LSW and ISOW cross over 37 the Reykjanes Ridge, favoured by the strong mixing around it, they leave the Irminger Basin 38 through the deep-to-bottom levels. The results also give insights into the water mass 39 transformations within the NASPG, such as the contribution of the Central Waters and SAIW to the 40 formation of the different varieties of SPMW due to air-sea interaction. 41
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].
The influence of intermediate waters on the stability of the eastern North Atlantic
Changes in the quantities and proportions of various intermediate water masses in the eastern North Atlantic have important consequences for the climate of the region. Subarctic Intermediate Water (SAIW) is mostly found within the subpolar gyre and west of 2O " W. Small quantities of this water mass, however, are found on the 2o " W meridian at the southern end of the Rockall Channel and are observed to influence the vertical structure of the water column. Typically the comparatively fresh SAIW is a highly stratified water mass, and its subduction into the eastern North Atlantic over more saline Mediterranean waters results in a stable layer at intermediate depths that restricts the maximum depth of winter mixing by up to 150 m compared to stations northwest of the Rockall-Hatton Bank. Analysis of Ocean Weather Ship (OWS) data from this region shows temporal variability of the vertical structure during the 1960s and 1970s. During the 1970s the " Great Salinity Anomaly " (Dickson et al., 1988) resulted in increasing quantities of comparatively fresh SAIW being present. This in turn resulted in a weakening of the stable layer instead of the expected strengthening. Changes in the stratification of the SAIW source are implied. By influencing the depth of maximum winter mixing, changes in the properties of the intermediate water masses of the eastern North Atlantic have more immediate consequences for ocean-atmosphere heat exchanges west of Europe than do longer term changes to the source waters of Labrador Sea Water (LSW) or Norwegian Sea Deep Water (NSDW). Such variations in the intermediate water column structure should therefore be taken into account in ocean climate models. 0 1997 Elsevier Science Ltd
Climate Dynamics, 2009
Seawater property changes in the North Atlantic Ocean affect the Atlantic meridional overturning circulation (AMOC), which transports warm water northward from the upper ocean and contributes to the temperate climate of Europe, as well as influences climate globally. Previous observational studies have focused on salinity and freshwater variability in the sinking region of the North Atlantic, since it is believed that a freshening North Atlantic basin can slow down or halt the flow of the AMOC. Here we use available data to show the importance of how density patterns over the upper ocean of the North Atlantic affect the strength of the AMOC. For the longterm trend, the upper ocean of the subpolar North Atlantic is becoming cooler and fresher, whereas the subtropical North Atlantic is becoming warmer and saltier. On a multidecadal timescale, the upper ocean of the North Atlantic has generally been warmer and saltier since 1995. The heat and salt content in the subpolar North Atlantic lags that in the subtropical North Atlantic by about 8-9 years, suggesting a lower latitude origin for the temperature and salinity anomalies. Because of the opposite effects of temperature and salinity on density for both longterm trend and multidecadal timescales, these variations do not result in a density reduction in the subpolar North Atlantic for slowing down the AMOC. Indeed, the variations in the meridional density gradient between the subpolar and subtropical North Atlantic Ocean suggest that the AMOC has become stronger over the past five decades. These observed results are supported by and consistent with some oceanic reanalysis products.
Ocean Modelling, 2009
We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland-Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland-Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing.
Climate Dynamics, 2006
We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic.