Links between climate and the upper ocean structure : the Canary current upwelling system case

• Autor(a) principal: Georg, Tina
• Data de Publicação: 2020
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.1/15276
• topic: Canary current upwelling system (ccus); Upwelling indices; Isothermal depth; Layer (ild); North Atlantic oscillation (nao); Upper ocean structure; Domínio/Área Científica::Ciências Naturais::Outras Ciências Naturais

Upwelling in the Eastern Boundary Currents is wind-driven. Ekman transport off the coast of the Canary Current Upwelling System (CCUS) is induced by equatorward, alongshore trade winds leading to persistent upwelling of cooler subsurface water throughout the year. This study uses Upwelling Indices (UI) between 25 and 35°N to assess upwelling in the CCUS and relates it to the changes in the upper ocean structure as well as climate indices. Upwelling was determined using different approaches such as differences in the Sea Surface Temperature (SST) at the coast and offshore (UISST), and the prevailing wind conditions with the resulting Ekman transport (UIw). Within the study area, the detection of upwelling varies on a temporal and spatial scale depending on the different determination methods of the UI’s. These results imply that the existing indices need an adaptation to be more reliable. Nevertheless, all indices revealed strong upwelling events along a wider shelf between 25 and 33°N and some downwelling events along a narrow shelf between 33 and 35°N. A significant correlation was found between the UI’s and the vertical structure of the upper ocean which is especially represented by the isothermal layer depth (ILD). In regards to the prevailing climate, the North Atlantic Oscillation (NAO) influences the location and strength of the westerly and trade winds in the study area. Its strong signal during winter (December to March) is reflected in significant correlations between the NAO and all parameters, however, when considering the correlations for the whole year, its signal is ambiguous. The NAO correlates well with the UI’s and the ILD. During its positive phase, cooler temperatures can be observed at the coast indicating upwelling. Simultaneously, the ILD deepens at the coast and becomes shallower offshore which is reversed in NAO- years and enhanced during coupled, opposite phases with the East Atlantic pattern. Still, upwelling occurs in years of a neutral NAO so it cannot be ascribed as the main driver for upwelling processes in the area. The interplay of upwelling, vertical structure and climate is rather due to a complex system of different forcings and cause-effect relationships. Ocean dynamics related to the kinetic and potential energy as well as the thermohaline circulation in its long-term variability may have an impact on these variables on different temporal and spatial scales.