On the seasonality of waters below the seasonal thermocline in the Gulf of Cádiz

• Published in: Continental Shelf Research
• Main Authors: Sánchez-Leal, R.F. (Ricardo Félix), Bellanco, M.J. (María Jesús), Naranjo, C. (Cristina), García-Lafuente, J.M. (Jesús M.), González-Pola, C. (César)
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV 2020
• Subjects: North Atlantic
• Online Access: http://hdl.handle.net/10508/14617; https://doi.org/10.1016/j.csr.2020.104190

Eastern North Atlantic Central Water (ENACW, 50-400 m) and Mediterranean Over ow Water (MOW, 300 m to the bottom) occupy the water column as strati ed layers below surface waters (extending from 0-150 m depth) in the Northeastern part of the Gulf of Cádiz. We examined their seasonal variability using nine years of CTD observations distributed along two across-shelf transects sampled several times per year since 2009. To infer the physical processes that drive this seasonal variability, we decomposed thermohaline changes as the sum of changes along isopycnals (termed isopycnal change) and those exerted by the pure vertical displacement of isopycnals (termed heave). The decomposition was applied to the most o shore, deepest hydrographic station data (SP6 and GD6). Results show that ENACW properties exhibit a clear seasonal cycle featuring colder, less saline waters in summer and warmer, more saline conditions in autumn (the upper ENACW) and winter (the lower ENACW). This pattern is governed by the interplay between seasonal uctuations of the larger-scale poleward-equatorward ows (driving isopycnal changes) and intensi cation-relaxation of local wind-induced upwelling and Ekman pumping (driving heave-steered changes). Our data suggest that MOW exhibits a similar cycle approximately in phase with the ENACW changes featuring summer cooling and winter warming and late summer freshening and late winter salini cation, mostly driven by isopycnal change. This pattern appears controlled by the seasonal formation of Western Mediterranean Deep Water (WMDW) and seasonal oscillations of the MOW-ENACW interface depth (allowing a greater volume of MOW leave the Strait from October-April) interacting with mixing and entrainment along the MOW pathway with the (also colder and less saline in summer) ENACW. The question whether ENACW dynamics module the MOW thermohaline characteristics or vice-versa could not be unequivocally clari ed. Our results at SP6 and GD6 point a strong seasonal signal emanating from the ENACW core that is ...