On the circulation of the North Atlantic shadow zone

• Autores: J. Peña Izquierdo
• Directores de la Tesis: Xavi Gironella (dir. tes.), José Luis Pelegrí Llopart (dir. tes.)
• Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2015
• Idioma: inglés
• Tribunal Calificador de la Tesis: Fabrice Hernández (presid.), Vicente Gracía García (secret.), Evan Mason (voc.)
• Materias:
  • Ciencias de la tierra y del espacio
    • Oceanografía
      • Oceanografía física
• Enlaces
  • Tesis en acceso abierto en: TDX
• Resumen

  • Regions isolated from the wind-driven circulation are found in the eastern margins of the world's tropical oceans. The weak and stagnant circulation of these so-called “shadow zones”, in combination with the intense respiration of organic matter provided by the overlying highly productive waters, promotes the existence of large oceanic volumes with very low dissolved oxygen. These volumes are known as oxygen minimum zones (OMZs). The last decades of observations have revealed that OMZs are in general expanding, and thus potentially threatening the surrounding rich marine ecosystems. However, it is not clear whether this change is based on natural variability or it has an anthropogenic origin. Furthermore, under a global warming scenario, the future evolution of the OMZs is uncertain due to the complex interaction between the physical and biochemical processes that interplay in the OMZs dynamics. This dissertation seeks to unravel the key elements of the circulation in the North Atlantic shadow zone, aiming to provide a deeper understanding of the physical components that rule the dynamics of the North Atlantic OMZ (naOMZ) -this being the less intense OMZ of the world ocean but the one that has experienced the largest expansion. A comprehensive description of the North Atlantic shadow zone circulation is presented from novel (CANOA08 cruise) and historic observations, including numerical outputs from the assimilative ECCO2 circulation model. The main outcome of our work is that two markedly distinct regimes of circulation exist in the thermocline layers of the naOMZ, above and below ??=26.8 kg m-3. In the upper layer, within the upper Central Water (uCW), the circulation is governed by the cyclonic regime of the subtropical cell. This stratum is characterized by relatively high oxygenation, with a predominance of South Atlantic Central Water (SACW). The lower layer, within the lower Central Water (lCW), presents a drastic decrease in the oxygen content due to its sluggish circulation; in contrast with the uCW, a mean anticyclonic circulation leads to a marked increase of North Atlantic Central Water (NACW). This result implies an equatorward transfer of mass from the subtropical gyre to the shadow zone, providing a previously unaccounted supply of oxygen from the well-ventilated subtropical thermocline. In fact, at the core of the naOMZ, the contribution of NACW is 50%. Such a mixture of SACW and NACW, with very low oxygen levels, is defined as the regional water mass of the naOMZ, the Cape Verde SACW (SACWcv). A broad band of eastward flows is located between 10ºN and 20ºN, just south of the westward flowing North Equatorial Current. These flows, here referred as the Cape Verde Current system, emerge as the major contributor in the water mass supply to the naOMZ. Lagrangian simulation reveals that while in the uCW, most of the water supply occurs south of 10ºN, in the lCW, more than two thirds of the total water supply takes place north of 10ºN, through the Cape Verde Current system with a high contribution of water directly recirculated from the subtropical gyre. The accurate numerical reproduction of the water mass composition within the naOMZ thermocline, as directly deduced from particle-track Lagrangian simulations, supports the goodness of the ECCO2 velocity field. The uCW and lCW strata not only exhibit opposite circulation patterns, they also present opposite large-scale vertical motions with predominant upwelling in the uCW as part of the subtropical cell regime while the lCW presents a broad downwelling pattern. We propose the existence of an inverse subtropical cell within the lCW dynamically coupled to the uCW regime. The major role these cells play in the circulation of the North Atlantic shadow zone, together with their substantial natural interannual and decadal variability, makes them major players on the oxygen anomalies observed during the last decades in the world OMZs