Alboran sea shelf hydrodynamic processes

Resumo

The Alborán Sea is a back-arc region located in the western Mediterranean Sea. This region is connected to the Atlantic Ocean through the Gibraltar Strait and opens to the east into the Balearic Basin through the Alborán Trough. In the Alborán Sea, oceanic motions involve a great variety of both spatial and temporal scales. The influence of tides, subinertial flow induced by atmospheric pressure gradients and wind stress, and the wind waves near the coast has been intensively studied. However, a detailed study of long-period waves, as Continental Shelf Waves (CSWs), propagating along the continental shelves of the Alborán Sea is still lacking. This type of waves propagates in one direction with the coast on the right (Northern Hemisphere). The presence of two narrow, opposite, and approximately parallel continental shelves provides an additional interest to the study site, because of the possible interaction of these kind of waves travelling in opposite directions. Then, this Thesis studies the existence of these waves in the continental shelves of the Alborán Sea. Another special feature of the Alborán Sea is that it is indented by transverse incisions. Some of these incisions are large enough to be considered submarine canyons. Several studies suggest that wind modifies the circulation in submarine canyons and drives cross-shore transports and vertical motions. However, little is known on the role of local atmospheric pressure gradients on the circulation of a submarine canyon. This Thesis also explores the role of this forcing in the circulation of a submarine canyon of the Alborán Sea, the Jolúcar canyon. Thus, the general aim of this Thesis is to study hydrodynamic processes over the continental shelves (northern and southern) of Alborán Sea (regional scale) and the local circulation in a submarine canyon indenting in the northern shelf. The objective is addressed using data analysis of field observations, and semi-analytical models. Firstly, the study of characterization and propagation of CSWs on the Alborán Sea was performed. A statistical approach was applied to determine both propagation direction and phase speed of free continental shelf waves. The presence of a westward-travelling signal on the north continental shelf and a west-to-east travelling signal on the south continental shelf of the Albor\'an Sea was confirmed. Moreover, the cross spectra between residual sea level signals along Alborán Sea have showed significant coherence at long periods. Phase speeds of free CSWs were calculated from significant periods. There is evidence of a seasonal variation in the phase speed of the waves, which may be related to variations in the speeds of ambient currents in the Alborán Sea. The linearized long-waves equations governing hydrostatic motion of an inviscid, barotropic, coastal ocean on a Northern Hemisphere f-plane, including bottom friction and the bottom topography of Alborán Sea continental shelf, were used in the theoretical study. The main results were that the presence of bottom friction, alongshore wind stress, and atmospheric pressure gradient alter the structure of free Continental Shelf Waves. On the other hand, the hydrodynamic processes inside a submarine canyon in the Alborán Sea were also studied. Currents data of two field surveys, and measurements of atmospheric pressure and wind stress were analyzed. Spectral and Empirical Orthogonal Functions analyses were applied to identify the most energetic frequencies related to the main forcings and to obtain the relationship between the main dominant modes that drive the Jolúcar canyon circulation, respectively. Results indicated that local atmospheric pressure gradients induced 1/3 of the horizontal circulation in the submarine canyon. Internal waves with a period of about 25 minutes were identified, and were only observed during the daytime. These observations suggest the relationship between physical water motions by internal waves and zooplankton behaviour. A positive vorticity was observed at 30 m depth, which indicates the existence of the cyclonic circulation. Moreover, although two distinct layers were observed in the canyon water column, which indicates both down/ upcanyon flows, the net direction of the canyon flow was downcanyon. It might suggest that canyon might acts as an escape mechanism to any type of substances. The analytical model developed related the canyon circulation with both local wind stress and atmospheric pressure gradient. Thus, the classical analytical solution of wind-induced current profile was extended here by considering the superposition of bottom and surface Ekman layers, and including the influence of barotropic pressure gradients in a non-homogeneous water column. The analytical solutions proposed in this Thesis reproduce well the main features of currents spatial structure associated with the wind and the local barometric pressure gradient.