Thermohaline variability and mixing dynamics in the Western Mediterranean deep waters within the Western Mediterranean Transition
Resumen Abstract Índice Conclusiones
Piñeiro Rodríguez, Olav Safo
2023-A
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Resumen
Durante el invierno de 2005, una gran producción de aguas densas anómalas desencadenó la formación de una compleja estructura termohalina profunda que causó un cambio abrupto en la evolución histórica de las características del Mediterráneo occidental. Esta nueva situación, denominada la Transición del Mediterráneo Occidental (WMT por sus siglas en inglés), ha sido monitorizada desde su formación por el Instituto Español de Oceanografía a través de una estación hidrográfica profunda visitada regularmente, localizada en el talud continental nororiental de la isla de Menorca. En esta tesis doctoral, a partir de esta serie temporal hidrográfica de larga duración, se analiza la evolución termohalina de la señal de la WMT entre 2005 y 2017. Por medio de un modelo numérico unidimensional de difusión que incluye parametrizaciones de fenómenos de mezcla doble-difusiva, se reprodujo la evolución difusiva de la estructura de la WMT y se evaluó la contribución al contenido de calor y sal de las aguas profundas en términos de ventilación por advección lateral de aguas densas y de transferencia difusiva desde las capas intermedias. Los resultados muestran distintos estadios en la evolución de las aguas profundas, dominados por constante mezcla diapicna y renovaciones de agua interanuales. En general, las capas profundas del Mediterráneo occidental sufrieron importantes ganancias de calor y sal durante este periodo, debido principalmente a la producción de aguas profundas durante los periodos 2005-2006 y 2011-2013. La tasa de absorción de calor durante la WMT excede las estimaciones para las capas intermedias del océano global. El análisis de la evolución a largo plazo remarcó la rápida erosión de firmas termohalinas de la parte más profunda de la estructura hidrográfica de la WMT durante sus estadios iniciales. Permitiendo que el coeficiente de mezcla variase verticalmente en el modelo unidimensional e incluyendo un término fuente localizado de calor y sal para representar la advección lateral de aguas densas desde la zona de formación, se reprodujo satisfactoriamente la evolución observada. Los resultados indican robustamente que no solo los niveles más densos, si no que casi toda la columna de agua ocupada por la estructura de la WMT frente a Menorca estuvo sujeta a mezcla turbulenta intensificada y persistente durante el periodo 2005-2007, muy por encima de las estimaciones regionales realizadas anteriormente y de los valores comunes en el interior del Mediterráneo occidental profundo. El talud continental de Menorca fue considerado una fuente plausible de la mezcla intensificada necesaria para reproducir las observaciones con el modelo durante el periodo contemplado. Por medio de nuevas y exhaustivas observaciones oceanográficas obtenidas en años recientes, se evidenció el desarrollo de turbulencia de pequeña escala originada sobre el fondo del talud continental de Menorca durante periodos de intensificación de la corriente de contorno profunda a lo largo del talud. Las observaciones son compatibles con un mecanismo de mezcla de contorno, recientemente documentado en el océano Antártico, que favorece tasas elevadas de mezcla diapicna sobre la topografía e intercambio lateral entre aguas bien mezcladas cercanas al talud y aguas interiores estratificadas. Esta tesis doctoral complementa y avanza nuestro conocimiento del evento climático de la WMT, su evolución temporal, los efectos de larga escala de los diferentes regímenes de mezcla que operan a lo largo de la columna de agua frente a Menorca, la contribución de los transportes por difusión vertical y advección lateral al balance de calor y sal en las profundidades y aporta información sobre el ambiente de mezcla profunda inexplorado en la región.
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.
Agradecimientos ……..i
Abstract ……..iii
Resumen ……..v
Resum ……..vii
Funding ……..ix
Publications ……..xi
1. General introduction ……..1
1.1. The Mediterranean Sea ……..3
1.1.1. Surface layer ……..4
1.1.2. Intermediate layer ……..6
1.1.3. Deep layer ……..6
1.2. The Western Mediterranean Deep Water ……..7
1.3. The Western Mediterranean Transition ……..8
1.4. Aims of the thesis ……..10
2. General methodology ……..13
2.1. Long-term oceanographic records ……..15
2.2. The RADMED monitoring program ……..15
2.2.1. The Minorca deep station ……..17
2.3. The ATHAPOC project ……..17
2.3.1. The ATHAPOC mooring ……..18
2.3.2. The ATHAPOC oceanographic section ……..20
3. Thermohaline evolution of the Western Mediterranean deep waters since 2005 diffusive stages and interannual renewal injections ……..21
3.1. Introduction ……..23
3.2. Dataset ……..24
3.3. Methodology ……..25
3.3.1. Smoothed evolution of the hydrography ……..26
3.3.2. Turbulent diffusion and double-diffusive mixing parameterization ……..26
3.3.3. Numerical integration of the diffusion equation ……..28
3.3.4. Kinf local estimation ……..30
3.4. Results ……..31
3.4.1. Theta-S shape evolution ……..31
3.4.2. Best estimate of Kinf ……..32
3.4.3. The 2005-2017 simulations. Long-term single runs ……..34
3.4.4. The 2005-2017 simulations. Re-starting runs by observations ……..37
3.5. Discussion ……..39
3.5.1. Interannual contribution of lateral advection versus vertical diffusion ……..39
3.5.2. Bulk changes in the deep Western Mediterranean: heat, salt and density ……..41
3.5.3. Diffusive mixing of the WMT structure ……..43
3.6. Conclusions ……..44
4. Persistent, depth-intensified mixing during the Western Mediterranean Transition’s initial stages ……..47
4.1. Introduction ……..49
4.2. Observational record and initial hypotheses ……..50
4.2.1. The Minorca time series ……..50
4.2.2. The ATHAPOC CTD section and mooring ……..51
4.2.3. Bottom-intensified mixing in 2005-2007: direct evidence and working hypotheses ……..52
4.3. Methodology ……..53
4.3.1. Lateral advection of heat and salt ……..54
4.3.2. Diapycnal mixing parameterization ……..54
4.3.3. Kinf local optimization ……..56
4.4. Simulations setup ……..56
4.4.1. Source water term ……..57
4.4.2. Vertical structure of the mixing rate ……..59
4.5. Results ……..60
4.5.1. Simulated 2005-2007 theta-S profile evolution ……..60
4.5.2. Slope density structure and circulation during winter 2017-2018 ……..64
4.6. Discussion ……..66
4.6.1. Simulation of the 2005-2007 theta-S evolution in the deep waters ……..67
4.6.1.1. Source term properties ……..67
4.6.1.2. Biases associated with cascading water sources ……..67
4.6.1.3. The post-convective third year: the heave issue ……..68
4.6.1.4. Bulk heat and salt content: advection vs diffusion ……..69
4.6.1.5. Advective inputs of newly-formed convective waters ……..69
4.6.2. Bottom-enhanced mixing during the post-convective stages ……..70
4.7. Conclusions ……..72
5. Boundary mixing of the Western Mediterranean deep waters ……..75
5.1. Introduction ……..77
5.2. Dataset ……..78
5.2.1. The ATHAPOC and HC-IEO-M moorings ……..79
5.2.2. The ATHAPOC oceanographic section ……..80
5.3. Methodology ……..81
5.3.1. Diagnosis of potential flow instabilities of the boundary current ……..81
5.4. Results ……..83
5.4.1. Spatio-temporal variability and stability of the deep boundary flow ……..83
5.4.2. Observations over the continental slope during 2018 post-convective stages ……..86
5.5. Discussion ……..90
5.5.1. Near-boundary mixing during intensification periods of the boundary circulation ……..90
5.5.2. Winter 2018: evidence of intensified boundary mixing and plausible lateral export of near-bottom waters from the continental slope ……..92
5.6. Conclusions ……..94
6. Summary and final remarks ……..97
7. Conclusions ……..103
Appendix ……..107
A. Data assimilation in the model upper boundary ……..109
B. Differential Evolution global optimization algorithm ……..109
Bibliography ……..113
Terms and Abbreviations ……..129
– A 1-D diffusion numerical model was set up in order to analyze in detail the long-term thermohaline evolution of the WMT since its formation in 2005, as observed in the outer continental slope of Minorca Island. Such a model has proven to be a useful tool to study several aspects of the anomaly under simple conceptual premises.
– Since the onset of the WMT, the evolution of the deep waters off Minorca Island shows several stages, driven by diapycnal mixing and interannual deep-water renewals. After 12 years of evolution, its original interleaved structure had been effectively eroded.
– The deep waters off Minorca underwent substantial warming (0.059 ºC)and salt increase (0.021) between 2004 and 2017. In 2017, deep waters were notably denser and more stratified than those prior to 2005, potentially with important implications for the future deep-ventilation rate in the basin. Heat uptake rate of the deep layers within the WMT exceeds that estimated for the upper 2000 dbar in the global ocean during the same period.
– Two intense deep-convective periods, 2005-2006 and 2011-2013, account for most of the heat and salt gains. Continuous downward diffusion from the intermediate layers played a modest role in the overall evolution of the heat and salt content of the deep waters.
– Background diapycnal mixing, the driver of the regional deep thermohaline evolution during non-deep-convective periods, is also an important agent during deep-convective ones, since it promotes rapid vertical propagation of the intruding thermohaline signatures and erosion of the deep stratification.
– Analysis of the local evolution of the WMT hydrographic structure during its initial stages was satisfactorily reproduced through a combination of localized lateral advection of heat and salt and a depth-dependent background turbulent mixing coefficient. Results indicate that almost the whole deep water column disrupted by the WMT structure underwent persistent, intensified turbulent mixing during the 2005-2007 period at a rate exceeding previous regional estimates.
– Strong deep boundary mixing over the regional continental slope and subsequent lateral transport of deep waters toward the Minorca deep station was considered as a plausible source of the local, intense deep-water transformations inferred for the 2005-2007 period.
– Hydrographic and current measurements across the Minorca oceanographic section evidence the development of bottom-generated turbulent mixing over the deep topographic slope, concurrent with the intensification of the along-slope deep boundary transport. The observations present a wide range of features of a deep-ocean boundary mixing mechanism not previously described in the WMED, which generates stress-induced, intense near-boundary turbulent mixing, and lateral exchange of well-mixed waters and stratified interior waters.