Abstract
Abstract. Operational ocean forecasts are typically produced by modelling systems run using a forced mode approach. The evolution of the ocean state is not directly influenced by surface waves, and the ocean dynamics are driven by an external source of meteorological data which are independent of the ocean state. Model coupling provides one approach to increase the extent to which ocean forecast systems can represent the interactions and feedbacks between ocean, waves, and the atmosphere seen in nature. This paper demonstrates the impact of improving how the effect of waves on the momentum exchange across the ocean–atmosphere interface is represented through ocean–wave coupling on the performance of an operational regional ocean prediction system. This study focuses on the eddy-resolving (1.5 km resolution) Atlantic Margin Model (AMM15) ocean model configuration for the north-west European Shelf (NWS) region. A series of 2-year duration forecast trials of the Copernicus Marine Environment Monitoring Service (CMEMS) north-west European Shelf regional ocean prediction system are analysed. The impact of including ocean–wave feedbacks via dynamic coupling on the simulated ocean is discussed. The main interactions included are the modification of surface stress by wave growth and dissipation, Stokes–Coriolis forcing, and wave-height-dependent ocean surface roughness. Given the relevance to operational forecasting, trials with and without ocean data assimilation are considered. Summary forecast metrics demonstrate that the ocean–wave coupled system is a viable evolution for future operational implementation. When results are considered in more depth, wave coupling was found to result in an annual cycle of relatively warmer winter and cooler summer sea surface temperatures for seasonally stratified regions of the NWS. This is driven by enhanced mixing due to waves, and a deepening of the ocean mixed layer during summer. The impact of wave coupling is shown to be reduced within the mixed layer with assimilation of ocean observations. Evaluation of salinity and ocean currents against profile measurements in the German Bight demonstrates improved simulation with wave coupling relative to control simulations. Further, evidence is provided of improvement to simulation of extremes of sea surface height anomalies relative to coastal tide gauges.
Highlights
There is a growing understanding of the role that ocean surface waves play in the Earth system, modulating the exchange of momentum, energy, and other fluxes between the atmosphere and oceans (Cavaleri et al, 2012)
Both CPL_DA and DA are clearly too fresh with substantial biases in sea surface salinity (SSS) and seabed salinity (SBS). This can be partly attributed to the use of a climatological freshwater boundary condition in the operational AMM15 configuration considered in this study
This might suggest that accounting for the effects of seabed–wave coupling in shallow seas could lead to further improvement (e.g. Soulsby et al, 1995)
Summary
There is a growing understanding of the role that ocean surface waves play in the Earth system, modulating the exchange of momentum, energy, and other fluxes between the atmosphere and oceans (Cavaleri et al, 2012). A key effect is in absorbing momentum and energy from the atmosphere as waves grow, and releasing it to the ocean when they break. Prediction systems of the ocean, waves, or atmosphere have typically been developed in relative isolation and with little or no interaction between each component. The development of coupled prediction approaches is increasingly enabling research on the sensitivity of the Earth system to wave impacts Pullen et al, 2017). Lewis et al.: Can wave coupling improve operational regional ocean forecasts?
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