Retroflections in the tropical Atlantic Ocean
The upper ocean circulation is the area of confluence of two major vertical cells: the large-scale overturning Atlantic Meridional Overturning Circulation (AMOC) and the regional relatively shallow Subtropical Cells (STCs). Both meridional cells interact in a complex system of zonal currents driven...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2021 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/673241 |
| Acceso en línea: | http://hdl.handle.net/10803/673241 https://dx.doi.org/10.5821/dissertation-2117-360913 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria civil i ambiental 55 |
| Sumario: | The upper ocean circulation is the area of confluence of two major vertical cells: the large-scale overturning Atlantic Meridional Overturning Circulation (AMOC) and the regional relatively shallow Subtropical Cells (STCs). Both meridional cells interact in a complex system of zonal currents driven by fluctuations of the trade winds. The thermocline waters that subduct in the subtropical South Atlantic join the northward AMOC limb in the western boundary North Brazil Current and Undercurrent system (NBC/NBUC). When the NBC crosses the equator the Coriolis force vanishes and it becomes unstable, therefore it retroflects forced by fluctuations in the easterly winds and associated zonal sea surface height (SSH) gradient . These changes are part of coupled ocean-atmosphere variations that cover a wide range of timescales. A total net transport of 20.6 Sv and 1.0 PW are effectively exported equatorward through the NBC/NBUC system as a result transfer pathways between tropical and subtropical gyres in the South Atlantic. Once at the equator, 11.4 Sv of South Atlantic waters retroflect from the NBC into the eastward Equatorial Undercurrent (EUC) at thermocline layer which join 2.8 Sv arriving from the northern hemisphere. The NBC-EUC retroflection takes place at different latitudinal bands between 3 degrees south and 12 degrees north. A boreal fall maximum is only present in the northernmost retroflection coinciding with the seasonal cycle of the North Equatorial Countercurrent (NECC). At interannual scales the EUC reflects a major increase in the contribution from the South Atlantic tropical waters during 2008-2016 as compared with 1997-2007. The NBC-EUC retroflection occurs within an area where changes in SSH anomalies respond to the two main modes of tropical Atlantic variability: the Meridional Mode and Equatorial Mode. During the development of both events the zonal SSH gradient is modified by the activation of the Rosbby wave (RW) reflected mechanism. Consequently, the zonal current system is altered. Concretely, during a positive Meridional Mode NECC, EUC and north South Equatorial Current (nSEC) intensify, or weaken during negative phase. The associated RW-reflected mechanism results to a Kelvin wave (KW) which reverses the equatorial zonal gradients developing an Equatorial Mode-like pattern. The ocean currents response is not than robust compared with the Meridional Mode. Concretely, the EUC displays a non-linear response probably as a result of external forcings and/or the intrinsic diversity of the Equatorial Mode. The diversity of Equatorial Mode has been studied by applying an empirical orthogonal function (EOF) analysis of a sample of 22 time series of warm Equatorial Mode (or Atlantic Niño) events. This technique reveals four different types of spatio-temporal distributions of SST anomalies (SSTA) in the equatorial Atlantic in the period 1982-2019. These four different types are preceded by different local and remote forcings (i.e: ENSO). Moreover, they are associated with different climatic response on areas in the surrounding continents: northeast region in South America and Western Africa. |
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