OC4331-Mesoscale Oceanography
Final Project Summary

Topic Area

Sensitivity of Bottom Topography on the Dynamics and Sound Speed Structure in the Northern Canary Current System

Project Team Member

LT Alicia A. Hopkins, USN

Major Findings

This study will determine if different types of topographic smoothing, e.g., Gaussian and direct iterative methods, and the addition of a volume constraint to the Princeton Ocean Model can significantly influence the generation, evolution, and maintenance of currents, upwelling, meanders, eddies, filaments, MO, and Meddies in the Northern Canary Current System.

The NCCS is unmatched amongst the subtropical eastern boundary currents of the world in that it contains the unique influence of Mediterranean Outflow (MO). After leaving the Strait of Gibraltar, the MO descends close to the continental slope in the Gulf of Cadiz as a narrow gravity current. This salty MO plume is deflected to the right due to the Coriolis acceleration (Bower et al., 1997) and dilutes, thickens, and becomes vertically differentiated into two distinct cores as it follows the northern rim of the Gulf of Cadiz (Iorga and Lozier, 1999; Bower et al., 1997). At ~7°W in the Gulf of Cadiz, the two cores exist at depths of ~600-900 m and ~1100-1200 m. Both cores continue to flow westward along the southern coast of Spain and turn poleward around Cabo de Sao Vicente (Amber and Howe, 1979; Iorga and Lozier, 1999). A third, shallower, poleward core of MO has also been traced from the Strait of Gibraltar northward to ~38.5°N off western Portugal (Amber, 1983). Additionally, climatological cyclonic circulation in the southwestern Gulf of Cadiz spreads the salty MO south to ~34°N (Iorga and Lozier, 1999).

Unique to the NCCS is the generation of subsurface, anticyclonic, submesoscale eddies, called Meddies, which contribute to the maintenance of the Mid-Atlantic Salt Tongue (e.g., Armi et al., 1989). Meddies form from the complex flow of waters entering and exiting through the Straits of Gibraltar; cool North Atlantic water flows into the Mediterranean, while at the same time warmer, denser and more saline water exits beneath the incoming flow. The high-salinity water MO flowing out of the Mediterranean Sea descends to mid depths in the density-stratified ocean, continues as a narrow jet along the Iberian continental slope, and intermittently detaches large-scale eddies, called Meddies. This is an important process because it maintains the relatively high mean salinity of a major water mass in the North Atlantic Ocean.

The primary generation region of Meddies is near Cabo de Sao Vicente off southwest Portugal. Several different trajectories of Meddies have been observed, including a southwestward movement into the Canary Basin and westward translations south of the Azores (Richardson and Tychensky, 1998). It has been shown that bathymetry impacts the evolution of Meddies (Price and Baringer, 1994). The degree to which the MO descends along the continental slope is sensitive to the steepness of the bottom slope it travels along. As the MO transits along the slope, Meddies can also spin off to form isolated eddies at abrupt changes in topography, as observed off Cabo de Sao Vicente (Serra et al., 2002; Bower et al., 1997).

Four numerical experiments were run, all on a beta-plane, with a sigma coordinate numerical model, i.e., the Princeton Ocean Model. The first experiment studied the effect of annual wind forcing on a flat bottom. The second experiment investigated the additional effect of topography. The third experiment examined the additional role of the full annual climatology. The fourth experiment incorporates a new iterative topography that has been shown to have the unique advantage of maintaining coastline irregularities, continental shelves, and relative maxima such as seamounts and islands.

Experiment 1 produced classical features of the NCCS, an offshore surface equatorward meandering jet, realistic surface and subsurface poleward currents, upwelling, meanders, eddies and filaments. In addition, these experiments depicted unique NCCS features, including the geographical separation of the Gulf of Cadiz region from the west coast upwelling regimes, poleward spreading of the MO, and the development and propagation of Meddies from the Cabo de Sao Vicente and Cabo da Roca regions.

A comparison between Experiments 1 and 2 showed that bottom topography plays an important role in trapping and intensifying the equatorward current near the coast, in weakening and deepening the poleward undercurrent and in producing eddies off Figueira da Foz. Stronger eddies occurred off Cabo da Roca and off Figueira da Foz.

Unlike Experiment 1, no formation of Meddies off Cabo Sao Vicente in Experiment 3 occurred. It was shown that the lack of formation was primarily due to both vortex stretching and increased radius of curvature of the smoothed topography, which inhibited boundary current separation.

In Experiment 3, the additional effect of the full annual climatology produced the tightening of currents near the coast and slightly weaker currents due to the opposing effects of thermohaline gradients and wind forcing. As in Experiment 2, there was no development of Meddies of Cabo de Sao Vicente.

A Meddy develops to the west of Cabo de Sao Vicente in Experiment 4, which is consistent with available observations of Meddies (e.g., Batteen et al., 2000).  This Meddy was also produced in the flat bottom case, but not in both Gaussian smoothed topography cases, Experiments 2 and 3. Just as in Experiment 2, a Meddy forms off Figueira da Foz, although much weaker in intensity.  As in previous experiments, a Meddy off Cabo da Roca forms and subsequently propagates to the west.

Overall, the results of these experiments show that while wind forcing is the primary mechanism for generating classical EBC features, bottom topography and thermohaline gradients also play important roles in the generation, evolution, and maintenance of classical as well as unique features in the NCCS.


Figure 1. Experiment 1. Salinity (psu) and velocity (cm/s) at 1250 m depth on day 60.

Figure 2 Experiment 2. Salinity (psu) and velocity (cm/s) at 1250 m depth on day 60.


Figure 3. Experiment 3. Salinity (psu) and velocity (cm/s) at 1250 m depth on day 90.

Figure 4. Experiment 4. Salinity (psu) and velocity (cm/s) at 1250 m depth on day 30.

 

 

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