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Formation of an Azores Current Due to Mediterranean Overflow in A 2342 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 30 Formation of an Azores Current Due to Mediterranean Over¯ow in a Modeling Study of the North Atlantic YANLI JIA Southampton Oceanography Centre, Southampton, United Kingdom (Manuscript received 24 August 1998, in ®nal form 5 November 1999) ABSTRACT A mechanism for the formation of the Azores Current is proposed. On the basis of observations and model results, it is argued that the primary cause of the Azores Current is the water mass transformation associated with the Mediterranean over¯ow in the Gulf of Cadiz. Observations show that the transport of the Mediterranean out¯ow water through the Strait of Gibraltar increases signi®cantly as it descends the continental slope by entraining the overlying North Atlantic Central Water. This entrainment process introduces a sink at the eastern boundary to the ocean upper layer in addition to the in¯ow into the Mediterranean. Such a sink is capable of inducing strong zonal ¯ows such as the Azores Current. This mechanism is con®rmed by numerical experiments with and without the representation of the Mediterranean over¯ow process. The numerical model is based on the Miami Isopycnic Coordinate Ocean Model. The model does not include the Mediterranean over¯ow explicitly, but restores the model density ®elds in the Gulf of Cadiz toward the observations. This restoring condition produces a reasonable representation of the water mass transformation deduced from observations. The formation of the Azores Current in response to the water mass transformation in the Gulf of Cadiz suggests that the Mediterranean over¯ow is not only a source of warm and saline water at depth, but also has a strong dynamic impact on the ocean upper layer. This study emphasizes the need to improve the representation of the Medi- terranean over¯ow process in general circulation models in order to capture the correct characteristics of the ¯ow ®elds and water masses in the subtropical eastern North Atlantic. 1. Introduction the African coast with southward branches in the Canary Basin as part of the subtropical gyre recirculation Southeast of the Grand Banks of Newfoundland the (Stramma 1984; Olbers et al. 1985; Klein and Siedler Gulf Stream (GS) separates into two branches. The 1989). The hydrographic database of Lozier et al. (1995) northern branch turns northeastward and becomes the reveals a coherent AC that stretches across the eastern North Atlantic Current (NAC). The southern branch, which becomes the Azores Current (AC), heads south- half of the basin, with divergences to the south and eastward across the Mid-Atlantic Ridge to the south of convergences from the north such that the downstream the Azores, then ¯ows mainly eastward at a latitude of transport does not change much. Recent hydrographic about 358N to the Gulf of Cadiz (GoC). surveys also indicate the eastward extension of the AC Associated with the AC is a front with signi®cant to the Moroccan continental slopes (FernaÂndez and Pin- temperature and salinity contrasts. There have been a gree 1996; Pingree 1997). Buoys deployed in the AC number of detailed hydrographic surveys of the front at are found to travel eastward and reach the western side various locations, for example, to the southeast of the of the GoC, and then move northward or southward Grand Banks (Mann 1967, 1972; Clarke et al. 1980), along the continental slopes. in the region of the Mid-Atlantic Ridge (Gould 1985; Based on the above surveys, the AC is observed to Sy 1988; Stramma and Muller 1989), and southeast of be a meandering jet 60±100 km wide with an eastward the Azores (KaÈse and Siedler 1982; KaÈse et al. 1985; velocity of 25±50 cm s21. The eastward ¯ow is mostly Siedler et al. 1985; Rios et al. 1992). Geostrophic trans- in the upper few hundred meters but can reach as deep port ®elds obtained from historical hydrographic data as 2000 m. The current carries a large fraction of the indicate that the eastward ¯ow extends all the way to water entering the eastern recirculation region of the Canary Basin. The estimates of the AC transport are in the range of 10±15 Sv (Sv [ 106 m3 s21). The surface temperature and salinity changes across the front can Corresponding author address: Dr. Yanli Jia, Southampton Ocean- be as large as 2 C and 0.3 psu. The front marks the ography Centre, Empress Dock, Southampton SO14 3ZH, United 8 Kingdom. northern boundary of the 188C Sargasso Sea water in E-mail: [email protected] the central North Atlantic. q 2000 American Meteorological Society Unauthenticated | Downloaded 09/24/21 02:10 PM UTC SEPTEMBER 2000 JIA 2343 Both drifter data (Richardson 1983; Krauss and KaÈse argued that under such a variable condition, nonlinear 1994; BruÈgge 1995) and satellite altimetry (e.g., Le inertial effects may become the dominant dynamic con- Traon et al. 1990; Wunsch and Stammer 1995; Stammer trol in the GS extension region, thus the separation of 1997) show a band of high eddy kinetic energy (EKE) the GS into the NAC and the AC. They also suggested associated with the AC. KaÈse and Siedler (1982) ob- that a local relative minimum in the magnitude of the served considerable meandering of the front southeast wind stress curl exists in the inner subtropical region, of the Azores with mesoscale eddies on both sides of which permits a quasi-zonal ¯ow to continue to the the front. Baroclinic instability has been identi®ed as eastern basin, hence the eastward extension of the AC. one of the mechanisms for the high energy level (Beck- In this study, a complementary mechanism for the mann et al. 1994a). The isopycnic potential vorticity formation and maintenance of the AC is proposed. It is analysis of climatological hydrographic data (Stammer suggested that the water mass transformation associated and Woods 1987) indicates that a necessary condition with the Mediterranean over¯ow in the GoC may induce for baroclinic instability, the reversal at depth of the the AC. The streamfunction ®eld for potential density meridional gradient of potential vorticity, is present at surface s 0 5 27.00 in Lozier et al. (1995) clearly shows the AC. the convergence of the streamlines associated with the The primary mechanism for the formation and main- AC in the GoC, which suggests a possible connection tenance of the Azores Front (AF) is unknown. For this between the two. The Mediterranean over¯ow is often very reason, numerical modeling of the AF and its as- poorly represented in general circulation models, which sociated variability has been dif®cult. The front is either may be the cause for a nonexistent AC in many cases. absent or very weak in general circulation models. For The AC in the eastern basin and the GoC occupy a instance, the high resolution model of the North Atlantic similar latitudinal extent and this may not be coinci- developed under the Community Modelling Effort dental. The following is an argument to explain how (CME) does not produce the separation of the GS into this may operate. the NAC and the AC (Bryan and Holland 1989). The At the Strait of Gibraltar dense Mediterranean water model AC develops only in the eastern basin (Spall spills over the sills into the North Atlantic. The transport 1990). The EKE at the AC latitudes is barely above the of the Mediterranean out¯ow water at the western end background level of variability (Treguier 1992). The of the Strait of Gibraltar is typically 1 Sv (Lacombe and meridional density gradient associated with the model Richez 1982; Bryden et al. 1994; Baringer and Price AC is too weak and the necessary condition for baro- 1997). Intense mixing in the GoC increases this trans- clinic instability is not satis®ed (Beckmann et al. 1994a). port by a factor of about 3 by entraining the overlying No signi®cant improvement is found with increased hor- North Atlantic Central Water (NACW) (Ambar and izontal resolution (Beckmann et al. 1994b). Howe 1979; Ochoa and Bray 1991; Baringer and Price This paper reports the occurrence of an AC in a gen- 1997). This entrainment process in the GoC introduces eral circulation model of the North Atlantic. A mech- a sink at the eastern boundary to the ocean upper layer anism for the formation of the AC is ®rst proposed. The in addition to the in¯ow into the Mediterranean. Such characteristics of the AC in the model are then presented a sink is capable of inducing strong zonal ¯ows such and compared with what we know from available ob- as the AC. servations. Further sensitivity experiments are per- There have been earlier laboratory experiments and formed to verify the proposed mechanism. theoretical studies to suggest that horizontal circulation can be deduced from a given distribution of sources and sinks. Such a source (sink) could be a direct injection 2. A mechanism for the formation of the (extraction) of ¯ow into (out of) the system, or through Azores Current vertical ¯ux of mass across density surfaces. In a lab- The presence of the AC, a coherent zonal ¯ow in the oratory experiment, Stommel et al. (1958) showed the inner subtropical gyre that stretches across a large extent induction of basin-scale zonal ¯ows by a point source of the basin, is a major feature of the circulation of the and sink placed near the eastern boundary in a rotating North Atlantic. It is situated well to the south of the system. In a theoretical analysis, Pedlosky (1996) mean zero wind stress curl where Ekman pumping im- showed that, for a localized source or sink of ®nite plies southward transport in the ocean, thus its zonal extent situated some distance away from the lateral orientation cannot be fully explained by Sverdrup dy- boundaries, zonal ¯ows form west of and within the namics.
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