Thermohaline variability and mixing dynamics in the Western Mediterranean deep waters within the Western Mediterranean Transition
During winter 2005, a large production of anomalous dense waters triggered the formation of a complex deep thermohaline structure, which led to a basin-scale abrupt shift in the historical evolution of the Western Mediterranean characteristics. This new situation, termed the Western Mediterranean Tr...
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| Tipo de recurso: | tesis doctoral |
| Fecha de publicación: | 2021 |
| País: | España |
| Institución: | Consejo Superior de Investigaciones Científicas (CSIC) |
| Repositorio: | DIGITAL.CSIC. Repositorio Institucional del CSIC |
| OAI Identifier: | oai:digital.csic.es:10261/318514 |
| Acceso en línea: | http://hdl.handle.net/10261/318514 |
| Access Level: | acceso abierto |
| Palabra clave: | Western Mediterranean Transition Centro Oceanográfico de Baleares Medio Marino Western Mediterranean Deep Waters Thermohaline variability Deep-ocean mixing |
| Sumario: | During winter 2005, a large production of anomalous dense waters triggered the formation of a complex deep thermohaline structure, which led to a basin-scale abrupt shift in the historical evolution of the Western Mediterranean characteristics. This new situation, termed the Western Mediterranean Transition (WMT), has been traced since its formation by the Instituto Español de Oceanografía throughout a regularly sampled deep hydrographic station located in the northeastern continental slope of Minorca Island. From this long-term hydrographic time series, the thermohaline evolution of the WMT signal between 2005 and 2017 is analyzed in this doctoral thesis. By means of a 1-D diffusion numerical model that includes parameterizations of double-diffusive mixing phenomena, the diffusive evolution of the WMT structure was reproduced and the contribution to the heat and salt budgets of the deep waters in terms of ventilation due to lateral advection of dense waters and downward diffusive transference from the intermediate layers was evaluated. Results show distinct stages in the deep waters evolution, dominated by permanent diapycnal mixing and interannual water renewals. Overall, the deep layers of the Western Mediterranean underwent remarkable heat and salt gains during this period, mostly due to the production of deep waters during the 2005-2006 and 2011-2013 periods. Heat uptake rate within the WMT was higher than estimations for the intermediate layers of the global ocean. The analysis of the long-term evolution highlighted the rapid consumption of near-bottom signatures of the hydrographic structure of the WMT during its initial stages. By permitting vertical variation of the background mixing coefficient in the 1-D model and including a localized heat and salt source term to represent the lateral advection of dense waters from the formation area, the observed evolution was successfully reproduced. The results robustly indicate that not only the densest levels, but almost the whole portion of the deep waters disturbed by the WMT structure off Minorca, was subjected to persistent, enhanced turbulent mixing during the 2005-2007 period, well above previous regional estimates and common values of the deep Western Mediterranean interior. The regional continental slope was regarded as a plausible source of the intensified deep mixing diagnosed by the modeling approach necessary to reproduce the observations during the considered period. By means of novel, extensive oceanographic observations gathered in recent years, the occurrence of bottom-generated small-scale turbulence over the continental slope of Minorca is evidenced during intensification periods of the along-slope deep boundary current. The observations are compatible with a deep-ocean boundary mixing mechanism recently documented in the Southern Ocean, which promotes enhanced diapycnal mixing rates over the topography and lateral exchange of near-boundary well-mixed waters and stratified interior waters. This doctoral thesis complements and advances our knowledge on the WMT climatic event, its temporal evolution, the large-scale effects of the distinct mixing regimes operating throughout the water column off Minorca, the contribution of vertical diffusive and lateral advective heat-salt transports to the deep budgets, and provides insights into the unexplored regional deep mixing environment. |
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