ECMWF held the 5th workshop on ocean waves and wave-coupled processes in Reading (UK) from 10 to 12 April 2024. It was attended by 80 scientists from across the world and followed workshops in Melbourne (2016), Qingdao (2017), Hangzhou (2018) and Uppsala (2023).

Ocean surface waves play a critical role in the Earth system, modulating many surface exchanges and acting in both atmospheric and oceanic boundary layers.

Accounting for their impact in weather and climate systems has recently attracted renewed interest, but the modelling, analysis and observation of ocean waves and wave-coupled processes still faces many challenges.

Forty-one presentations were given at the event, including eight keynote ones. Oral and poster contributions supplemented the talks, all of which are available on the ECMWF website.

“The workshop was a great opportunity to discuss the topics at the centre of recent research, and ample time was provided to foster discussion on the issues presented,” says Jean Bidlot, one of the ECMWF scientists who organised the event.

Keynote talks

The impact of waves on the exchange of momentum between the atmosphere and the ocean has long been recognised. The first keynote speaker, Nobuhito Mori from Kyoto University (Japan), gave a talk on ‘Wave coupling effects in regional and global atmosphere–ocean modeling’.

He noted that coupled models have been developed, but that the physics of what happens at the interface needs further work. Sea-surface parametrizations, including wave effects, are old and need to be updated, he said.

Peter Janssen, who is an advisor at ECMWF, presented an update on his work on the interaction of wind and ocean waves. He explained that the drag coefficient, while generally reaching maximum value for storm winds, should level or even decrease for particularly strong winds.

Hendrik Tolman, from the National Weather Service (USA), gave a keynote presentation on ‘Coupling strategies’. His talk was centred on the Unified Forecast System (UFS), which is a research tool and serves as the basis for US National Oceanic and Atmospheric Administration (NOAA) operational forecasts.

Sophia Brumer’s keynote presentation highlighted the role of waves in many other processes. For instance, waves break and entrain air into the water that forms bubbles. This greatly modifies surface exchanges and creates visible whitecaps at the sea surface.

Sophia, who is from the Laboratoire d’Aérologie (France), explained that wave breaking and shearing of the wave crests by strong winds generate sea spray, which greatly modifies heat and moisture fluxes and impacts sea salt aerosol fluxes.

Shuyi Chen from the University of Washington (USA) gave a keynote talk about ‘Atmosphere–wave–ocean coupling in extreme conditions and high-impact weather prediction’. She picked up on the role of sea spray as a key coupled process in her review of the role of wave effects in predicting extreme weather conditions.

Øyvind Breivik from the Norwegian Meteorological Institute started a discussion on the level of complexity that should be required in coupled systems. In his keynote presentation, he said that representing wave-driven processes does not necessarily require a full wave model.

According to him, simplified wave models or wave-aware parametrizations might be quite suited for applications which do not require a full and accurate description of the sea state.

Erick Rogers (US Naval Research Laboratory) gave a keynote presentation on ‘Ocean waves in sea ice: dependence of dissipation on ice thickness for coupled wave modeling’. His review of recent developments supported ECMWF’s recent work on the role of wave–sea-ice interactions. Discussions centred on the importance of detailed observations.

Trygve Halsne from the Norwegian Meteorological Institute gave another keynote presentation on ‘Wave–current interaction in operational wave forecasting’. His talk centred on adding the impact of surface currents to achieve improved high-resolution wave forecasts for the Norwegian coast.

There was considerable discussion on the complexity of wave models. How complex should they be? What does this complexity provide? How much does it cost?

Complex models have been very effective at enabling very high accuracy in wave forecasting. If, however, the target is representing waves within the coupled climate system, perhaps the complexity could be dialled back a little. This would make wave models more affordable, and therefore more likely to be integrated within the climate modelling community.

To decrease model complexity, there was some discussion about the potential role of machine learning. For example, the physics and nonlinear wave-wave interaction could be replaced with machine-learned components.

Within the context of operational weather forecasting, the wave model costs about seven per cent of the overall cost of the forecast model. There was general agreement that high-complexity wave models are required for use in weather forecasting and achieving the best possible forecasts for marine safety. Indeed, there was significant emphasis on wave coupling with the boundary layer of the ocean and the atmosphere and how this can be improved.

“This workshop was an absolute success in terms of the professionalism of those who made sure that the three days went extremely smoothly, with an excellent level of engagement from many of the participants,” Jean Bidlot says. “I am very proud to have showcased ECMWF as a world-leading institution for the science, but also for leadership in bringing the community together to recognise the importance of ocean waves in the further development of Earth system models.”

The workshop also provided an opportunity to celebrate the careers of Saleh Abdalla, who recently retired from ECMWF, and Jean, who will retire in June.

The 6th workshop on waves and wave-coupled processes is due to be held in Melbourne, Australia, from 13 to 17 January 2025.