Seasonal and regional variability of upper ocean diapycnal heat flux in the Atlantic cold tongue

Abstract

SST variability within the Atlantic cold tongue (ACT) region is of climatic relevance for the surrounding continents. A multi cruise data set of microstructure observations is used to infer regional as well as seasonal variability of upper ocean mixing and diapycnal heat flux within the ACT region. 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 ACT region causing critical Froude numbers (Fr) to be more frequently observed in the western equatorial ACT. The distribution of critical Fr occurrence below the ML reflects the regional and seasonal variability of mixing intensity. Turbulent dissipation rates (ɛ) at the equator (2°N–2°S) are strongly increased in the upper thermocline compared to off-equatorial locations. In addition, ɛ 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 ε and stratification range from a maximum of 90Wm−2 in boreal summer to 55Wm−2 in September and 40Wm−2 in November. In the eastern equatorial ACT region maximum values of about 25Wm−2 were estimated during boreal summer reducing to about 5Wm−2 towards the end of the year. Outside the equatorial region, inferred diapycnal heat fluxes are comparably low and rarely exceeding 10Wm−2. Integrating the obtained heat flux estimates in the ML heat budget at 10°W on the equator accentuates the diapycnal heat flux as the largest ML cooling term during boreal summer and early autumn. In the western equatorial ACT elevated meridional velocity shear in the upper thermocline contributes to the enhanced diapycnal heat flux within this region during boreal summer and autumn. The elevated meridional velocity shear appears to be associated with intra-seasonal wave activity.