Surface currents and significant wave height variability: a numerical investigation of the Agulhas current region

Abstract

Advances in the understanding and modelling of surface currents have revealed the importance of internal waves, mesoscale and submesoscale turbulence. Indeed all these features should have a large influence on wind waves, and in particular wave heights are expected to be modified by refraction. Still, the quantitative impact of currents on waves is not well known due to the complexity of the random wave fields and currents that are found in the ocean, and the lack of observations of both currents and waves at scales shorter than 150 km. Here we use novel satellite altimetry data and state of the art phaseaveraged numerical wave models forced by currents at different resolutions in the Agulhas region. We find that a numerical wave model that uses surface currents with resolutions of 30 km or less and a directional resolution of 7.5 • or less, can provide accurate representations of the significant wave height gradients found in the Agulhas current. Using smoother current fields, such as derived from satellite altimetry alone, generally underestimates gradients and extreme wave heights. This work suggests that high resolution satellite altimetry data can be combined with numerical wave models to provide a statistical validation of surface current gradients.

Current interaction in large-scale wave models with an application to Ireland

“…In this section the spatial variation induced by the currents and the scale at which they impact the wave propagation are analysed for the present application. It was shown in Ardhuin et al, 2017 andMarechal andArdhuin, 2020 that currents have an impact at short spatial scales, between 10 km and 100 km. These two studies focus on two strong boundary currents (Agulhas and Gulf stream respectively).…”

“…More recently, there has been some interest in the interaction with currents, which for the most part highlights the refraction induced by current gradients. The impact of large-scale currents like the Agulhas current or the Gulf Stream is well documented through numerical simulations cross-checked with satellite observations -see for example Marechal and Ardhuin, 2020;Barnes and Rautenbach, 2020 for recent studies on the Agulhas current. The impact of small scale currents has drawn less attention but in Ardhuin et al, 2017 it is shown that even submesoscale structures with shorter length scales (ranging from 10 km to 100 km) can explain the majority of the significant wave height spatial variability.…”

“…Large-scale wave models can have a more refined grid where the impact of current is more significant. In Marechal and Ardhuin, 2020, Ardhuin et al, 2017, and Barnes and Rautenbach, 2020 an offline coupling is done using an external current data-set. All these authors are directly interested in the wave height spatial variability caused by ocean currents.…”