The introduction of Observation-Based Physics into the WAM Wave Model and Wave-Coupled Interactions

Abstract

Ocean surface waves are an unparalleled ocean phenomenon, wreaking havoc on ocean-going vessels and spreading their tendrils into effects on many aspects of weather and climate. To understand and predict these waves, the best tool available is the third-generation spectral wave model. These models predict the growth, decay, and transformation of wind-generated waves. The modern wave physics package for such models is the observation-based approach developed by A. V. Babanin, I. R. Young, M. A. Donelan, M. L. Banner and W. E. Rogers (hereafter BYDBR). Its source functions were measured and therefore are not subject to tuning (within confidence limits of the measurements). Furthermore, the observations revealed and/or quantified physical phenomena missing or neglected in other source-term packages for spectral wave models such as non-linear airflow separation (hence relative reduction of wind input at strong winds), steepness-dependent wave growth (making the wind input a weakly non-linear function of the wave spectrum), negative wind input under adverse winds, two-part wave dissipation consisting of a local in wavenumber space term and a cumulative term (which is an integral of the spectrum), a wave breaking threshold below which no breaking occurs, and swell decay due to interaction with and production of oceanic turbulence. This observation-based physics is available in two of the three dominant third- generation spectral wave models, WAVEWATCH-III and SWAN, but not yet WAM, the introduction of which (coding, testing and validation for both the ECMWF- and Hereon-managed versions of WAM) is a key outcome of this thesis, and enables its use by the remaining third of the research and practical oceanographic community.

This observation-based physics is primarily concerned with the open ocean however, and neglects regions which are dominated by different physical processes, such as the polar regions. These regions are amongst those changing most rapidly in response to climate change, and contribute the large uncertainty as to how the climate will continue to evolve over the next century. In these regions - or more specifically, what's known as the Marginal Ice Zone - waves and sea ice form a closely coupled system: waves govern sea ice through stress, floe break-up and wave-induced currents, whilst sea ice affects waves through attenuation and reflection. The break-up of sea ice by waves is particularly important as it can regulate air-sea interaction and consequently also regulate the growth and melt of sea ice. This coupled nature between waves and sea ice is complex and generally neglected in modelling of the polar climate system (especially at the large scale). Here we extend the observation-based physics approach to the Marginal Ice Zone to formulate a new model for wave-sea ice interactions. Our approach builds upon previous investigations of wave-sea ice interaction and can be summarised as follows: 1) sea ice takes a binary form, either 'broken' or 'unbroken', 2) waves may break sea ice, transitioning it from unbroken to broken, 3) a threshold separating breaking and non-breaking wave fields is used to identify when this occurs, 4) two modes of attenuation for waves in ice (dependent upon the ice state), representing the observed on/off switch in wave attenuation. By characterising wave attenuation and sea ice break-up as described above, we achieve a two-way wave-sea ice coupling, thereby allowing wave-sea ice feedbacks. This model is then applied to an Antarctic case study for summer of 2019/2020. We demonstrate that our model can very accurately simulate both the wave field within, and the evolution of, the Marginal Ice Zone. These results further demonstrate the validity of the observation-based approach, as well as substantiate the various observation-based elements drawn on here and their suitability to operate in combination. Furthermore, the results substantiate that waves have a critical influence on the morphology of the Marginal Ice Zone.

As shown above, waves are important not only in their own right, but also because they regulate other domains of the Earth system. The ocean is no exception. Next we consider the effect of waves in the ocean, consistent with the BYDBR observation-based physics: waves, through the process of swell decay, can produce turbulence and mixing deep within the ocean. Ocean models typically neglect this, and we establish here that this is likely a source of considerable bias in such models. This is done through the introduction of this effect into the NEMO ocean model and investigation of its role. We find that this wave-coupled process is a vital source of mixing throughout the upper ocean, and its inclusion can improve oceanic predictions in a physically consistent manner.