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A Model Analysis of the Behavior of the Mediterranean Water in the North Atlantic

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A Model Analysis of the Behavior of the Mediterranean Water in the North Atlantic 764 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 37 A Model Analysis of the Behavior of the Mediterranean Water in the North Atlantic YANLI JIA,ANDREW C. COWARD,BEVERLY A. DE CUEVAS, AND DAVID J. WEBB National Oceanography Centre, Southampton, Southampton, United Kingdom SYBREN S. DRIJFHOUT Koninklijk Nederlands Meteorologisch Instituut, de Bilt, Netherlands (Manuscript received 22 July 2005, in final form 20 June 2006) ABSTRACT The behavior of the Mediterranean Water in the North Atlantic Ocean sector of a global ocean general circulation model is explored, starting from its entry point at the Strait of Gibraltar. The analysis focuses primarily on one experiment in which explicit watermass exchange between the Mediterranean Sea and the Atlantic at the Strait of Gibraltar is permitted. The model produces an exchange rate of approximately 1 Sv (Sv ϵ 106 m3 sϪ1). This is comparable to estimates derived from field measurements. The density of the Mediterranean outflow, however, is lower than observed, mainly because of its high temperature (more than 2°C higher than in reality). The lower density of the outflow and the model’s inadequate representation of the entrainment mixing in the outflow region cause the Mediterranean Water to settle in a depth range ϳ800–1000 m in the North Atlantic, about 200 m shallower than observed. Here an interesting current system forms in response to the intrusion of the Mediterranean Water, involving three main pathways. In the first, the Mediterranean Water heads roughly westward across the basin and joins the deep western boundary current. In the second, the water travels northward along the eastern boundary reaching as far as Iceland, where it turns westward to participate in the deep circulation of the subpolar gyre. In the third, the water initially moves westward to the central Atlantic just north of 30°N before turning northwestward to reach an upwelling region at the Grand Banks off Newfoundland. At this location, the saline Mediterranean Water is drawn upward to the ocean upper layer and entrained into the North Atlantic Current system flowing to the northeastern basin; part of the current system enters the Nordic seas. 1. Introduction boundary undercurrent from the Gulf of Cadiz to the northern end of the Rockall Trough (near 60°N), where The flux of the warm and saline Mediterranean Wa- it rises and flows over the Wyville–Thomson Ridge into ter (MW) into the North Atlantic Ocean through the the Nordic seas as part of the Atlantic inflow. This Strait of Gibraltar, though small in magnitude [on the pathway of the MW has been termed the “deep source order of 1 Sv (Sv ϵ 106 m3 sϪ1); Bryden et al. 1994; hypothesis” by McCartney and Mauritzen (2001) who Candela 2001], is one of several distinct water sources also put forward an alternative “shallow source hypoth- whose interactions determine the overall watermass esis.” They proposed that the MW contributes to the distribution of the North Atlantic. Moreover, it is be- saline characteristic of the North Atlantic Central Wa- lieved that the MW is also a saline source for the Nordic ter (NACW) by blending with other water masses in seas, which implicates its potential impact on the World the thermocline of the subtropical gyre. Thermocline Ocean circulation. At present, there are two hypotheses waters are drawn upward into the North Atlantic Cur- with regard to the route that the MW takes to arrive at rent (NAC) system, branches of which flow into the the Nordic seas. Reid (1979) conjectured that the MW Nordic seas. is transported northward in a deep (Ͼ1000 m) eastern In the Nordic seas, the saline characteristic of the Atlantic inflow, including contributions from the MW either directly as in the deep source hypothesis or in- Corresponding author address: Dr. Yanli Jia, IPRC/SOEST, University of Hawaii, 1680 East West Road, POST Bldg. 401, directly as in the shallow source hypothesis, combined Honolulu, HI 96822. with intense winter cooling increases the density of the E-mail: [email protected] surface water, which sets the condition for deep con- DOI: 10.1175/JPO3020.1 © 2007 American Meteorological Society JPO3020 MARCH 2007 J I A E T A L . 765 vective mixing and the formation of the North Atlantic and joins the NAC, part of which flows into the Nordic Deep Water (NADW). The NADW flows southward in seas. This MW pathway is consistent with the shallow the deep western boundary current (DWBC) as the source hypothesis. lower branch of the Atlantic overturning circulation. In In what follows, we describe the model configuration this way, the MW (or its high salinity content) is thus and experimental design in section 2. Section 3 focuses recognized as an important ingredient of the Atlantic on the watermass exchange at the Strait of Gibraltar overturning circulation, and ultimately the global ther- and the influences from upstream in the western Medi- mohaline circulation. terranean. The downstream evolution of the MW in the One support for the deep source hypothesis is a study Gulf of Cadiz is examined in section 4, and the path- by Iorga and Lozier (1999b) who estimated the velocity ways of the MW in the North Atlantic basin are pre- fields on two representative isopycnal surfaces of the sented in section 5. The results are summarized in sec- Mediterranean Sea outflow with a diagnostic model tion 6. that uses climatological hydrographic data in the North Atlantic and measurements of the water exchange 2. Model description and experimental design through the Strait of Gibraltar. They showed continu- The model used for this study is a fully global ocean ous poleward flow along the eastern boundary on both model built within the U.K. Ocean Circulation and Cli- surfaces carrying the MW all the way to 60°N where the mate Advanced Modeling (OCCAM) project. The nu- model boundary lies. Beyond 60°N, the likelihood of merics are based on the Bryan–Cox–Semtner (BCS) the MW rising and flowing over the Wyville–Thomson formulation (Bryan 1969; Semtner 1974; Cox 1984), Ridge into the Nordic seas is deduced from the clima- which solves the ocean primitive equations using finite tological pressure fields, which show the northward differences on a horizontal Arakawa-B grid (Arakawa shoaling of the isopycnal surfaces to reach above the sill 1966). The present implementation replaces the rigid depth of the ridge. lid of the BCS scheme with a free surface scheme simi- One support for the shallow source hypothesis is a lar to that described by Killworth et al. (1991), with study by New et al. (2001) who analyzed three eddy- modifications to increase computational efficiency. An permitting ocean circulation models of the North At- improved advection scheme, known as the Split- lantic that differ primarily in the discretization of the Quadratic Upstream Interpolation for Convective Ki- vertical coordinate. They showed that although the nematics (QUICK) (Webb et al. 1998) with fourth- MW may reach as far north as 60°N in a deep eastern order accuracy, is used for both the tracer and velocity boundary undercurrent, none of the models allows the fields and in both horizontal and vertical directions. MW to rise and flow over the Wyville–Thomson Ridge The model is configured to make use of multiprocessor into the Nordic seas. Instead, the saline Atlantic inflow computers for computational efficiency (Webb et al. is derived partly from water masses of western origin 1997). Some details of the model relevant to the present carried by the NAC and partly from the eastern North study are described below. Atlantic water transported northward via a “shelf edge The model attains its global coverage by using two current” flowing around the continental margins in the horizontal grids (Coward et al. 1994): a rotated grid for upper ocean. The study does not discuss whether the the North Atlantic, the Mediterranean, and the Arctic MW contributes to the saline characteristic of the in- and a conventional latitude–longitude grid for the rest flowing water masses in these models. However, Mau- of the world oceans. The two grids match exactly at the ritzen et al. (2001) showed, through the analysis of ob- geographical equator in the Atlantic. In the regions of servational data, laboratory experiments, and inverse interest for this study, the Mediterranean and the North modeling, how the MW may increase the salinity of the Atlantic that are connected at the Strait of Gibraltar NACW in the eastern basin by an upward salinity flux and lie within the rotated grid, the model latitude and in the Gulf of Cadiz. In this way, the MW is embedded longitude lines run approximately from south to north in the inflowing water masses to the Nordic seas. and from east to west. The grid resolution is 0.25° in In this study, we provide a detailed description of the both directions. The model has 36 vertical levels, with pathways of the MW in the North Atlantic starting grid space ranging from 20 m near the ocean surface to from its entry point at the Strait of Gibraltar, as seen in 250 m at depth. The model bathymetry is based on the an ocean general circulation model. Stepping ahead of Digital Bathymetry Data Base 5-minute (DBDB5) by the presentation, we find that the model solutions do the U.S. Naval Oceanographic Office, with modifica- not show evidence for the deep source hypothesis. In- tions to important sills and straits (including the Strait stead, we identify a location in the western North At- of Gibraltar). lantic where the MW upwells to the ocean upper layer The model uses Laplacian diffusion and viscosity to 766 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 37 represent horizontal mixing.
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