Hummels, Rebecca (2012) On the variability of turbulent mixing within the upper layers of the Atlantic Cold Tongue region. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 155 pp.

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Abstract

Sea surface temperature (SST) variability within the equatorial Atlantic is of climatic relevance for the surrounding continents. A striking feature of this SST variability is the annual appearance of the Atlantic cold tongue (ACT). The respective contributions to the ML heat budget forming the seasonal cycle of SSTs within the ACT could up to date not be clarified. Especially the role of the diapycnal heat flux due to turbulence in cooling SSTs is still controversially discussed. The main focus of this study is to infer regional and seasonal variability of upper ocean turbulent mixing and the inferred diapycnal heat flux within the ACT region using a multi cruise data set of microstructure observations. The assessed varibility of the diapycnal heat flux is then integrated into the ML heat budget within different regions and seasons of the ACT. In addition, the variability in mixing intensity is related to the variability in large scale background conditions, which were additionally observed during the cruises. The observations indicate fundamental differences in background conditions in terms of shear and stratification below the mixed layer (ML) for the western and eastern equatorial as well as the southern ACT region. This leads to the occurrence of critical Froude numbers (Fr), which points towards elevated mixing intensity, most frequently in the western equatorial ACT. The distribution of critical Fr below the ML reflects the regional and seasonal variability of mixing intensity. Turbulent dissipation rates (ε) at the equator (2°N-2°S) are strongly increased in the upper thermocline compared to off-equatorial locations. In addition, ε is elevated in the western equatorial ACT compared to the east from May to November, whereas boreal summer appears as the season of highest mixing intensities throughout the equatorial ACT region, coinciding with ACT development. Diapycnal heat fluxes at the base of the ML in the western equatorial ACT region inferred from ǫ and stratification range from a maximum of 90 W/m² in boreal summer to 40 W/m² in November. In the eastern equatorial ACT region maximum values of about 25 W/m² were estimated during boreal summer. Outside the equatorial region, inferred diapycnal heat fluxes are comparably low rarely exceeding 10 W/m². Critical to the enhanced diapycnal heat flux in the western equatorial ACT region during boreal summer and autumn is elevated meridional velocity shear in the upper thermocline. It is thus suggested that TIWs are crucial contributors to mixing within this region during this time period. Integrating the obtained heat flux estimates in the ML heat budget accentuates the diapycnal heat flux as the largest ML cooling term during boreal summer and early autumn in the entire equatorial ACT region and crucial for decreasing SSTs for ACT development. Within the southern ACT region SST cooling is dominated by atmospheric forcing. Additionally, it is shown that most of the existing parametrization schemes for the equatorial thermocline, which are supposed to estimate the general magnitude of mixing related parameters without cost-intensive observations, tend to overestimate turbulent mixing intensity and the inferred diapycnal heat fluxes within the equatorial ACT region.

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