Description: A relaxation technique applied to the ECMWF model is used to analyse 11, 21 and 31 year trends in the boreal winter mean 500 hPa North Atlantic Oscillation (NAO), Pacific North America pattern (PNA) and Southern Annular Mode (SAM) indices. For the PNA, the results indicate a strong influence from the tropics on all time scales, whereas for the NAO, the stratosphere is important on time scales of 11 and 21 but with an indication of feedback from extratropical sea surface temperature and sea-ice (SSTSI) anomalies on the 11 year time scale. For the SAM, the tropics emerge as the most important influence. We find an influence from the stratosphere consistent with expectations based on ozone depletion, although no clear role for stratospheric forcing of the SAM is found in these experiments.

Description: In austral winter, biological productivity at the Angolan shelf reaches its maximum. The alongshore winds, however, reach their seasonal minimum suggesting that processes other than local wind‐driven upwelling contribute to near‐coastal cooling and upward nutrient supply, one possibility being mixing induced by internal tides (ITs). Here, we apply a three‐dimensional ocean model to simulate the generation, propagation, and dissipation of ITs at the Angolan continental slope and shelf. Model results are validated against moored acoustic Doppler current profiler and other observations. Simulated ITs are mainly generated in regions with a critical/supercritical slope typically between the 200‐ and 500‐m isobaths. Mixing induced by ITs is found to be strongest close to the coast and gradually decreases offshore thereby contributing to the establishment of cross‐shore temperature gradients. The available seasonal coverage of hydrographic data is used to design simulations to investigate the influence of seasonally varying stratification characterized by low stratification in austral winter and high stratification in austral summer. The results show that IT characteristics, such as their wavelengths, sea surface convergence patterns, and baroclinic structure, have substantial seasonal variations and additionally strong spatial inhomogeneities. However, seasonal variations in the spatially averaged generation, onshore flux, and dissipation of IT energy are weak. By evaluating the change of potential energy, it is shown, nevertheless, that mixing due to ITs is more effective during austral winter. We argue that this is because the weaker background stratification in austral winter than in austral summer acts as a preconditioning for IT mixing.