Wave forecast investigations on downscaling, source terms, and tides for Aotearoa New Zealand

Abstract. This study evaluates the effects of downscaling, source terms, and tidal interactions on numerical wave forecasts in Aotearoa New Zealand. We utilised a set of three nested domains (from global to regional scale) to examine significant wave height (Hs), first-order mean period (Tm01), and peak wave direction at two coastal locations, Banks Peninsula and Baring Head. Downscaling markedly improved forecast accuracy at Baring Head, a tidally constricted region, reducing Hs forecast error by 28 %. However, improvements at Banks Peninsula were minimal, likely due to its open-coast characteristics, which are adequately represented even by lower-resolution models. This variability was also evident in the Tm01 predictions, with notable improvements in bias reduction through model downscaling, particularly at Baring Head. Using default source term 6 (ST6) parameters generally improved Hs predictions on the west coast but worsened them on the east, indicating a geographical dependency in model performance. Tidal forcing had a small impact on the overall forecast skill, and its impact was mostly noticed at Baring Head, where tides force large variability. However, the tidally driven wave model showed smaller 12 h variability compared to observations. The study underscores the importance of tailored modelling approaches that consider local geographical and hydrodynamic conditions to optimise wave forecasting.