Wind-current feedback is an energy sink for oceanic internal waves.

Internal waves contain a large amount of energy in the ocean and are an important source of turbulent mixing. Ocean mixing is relevant for climate because it drives vertical transport of water, heat, carbon and other tracers. Understanding the life cycle of internal waves, from generation to dissipation, is therefore important for improving the representation of ocean mixing in climate models. Here, we provide evidence from a regional realistic numerical simulation in the northeastern Pacific that the wind can play an important role in damping internal waves through current feedback. This results in a reduction of 67% of wind power input at near-inertial frequencies in the region of study. Wind-current feedback also provides a net energy sink for internal tides, removing energy at a rate of 0.2 mW/m[Formula: see text] on average, corresponding to 8% of the local internal tide generation at the Mendocino ridge. The temporal variability and modal distribution of this energy sink are also investigated.

Deep-ocean mixing driven by small-scale internal tides.

Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth's climate. In the deep ocean, tides supply much of the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fate-the relative importance of their local versus remote breaking into turbulence-remains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models.

Response of a vulnerable barrier island to multi-year storm impacts: LiDAR-data-inferred morphodynamic changes on Ship Island, Mississippi, USA.

Barrier systems around the world are experiencing accelerated sea-level rise, reduced sediment supply, and frequent hurricane impacts. However, detailed quantitative field-based studies concerning the response to these external forcing mechanisms are scarce, particularly on the scale of entire islands. The Mississippi - Alabama barrier island chain, located along the U.S. Gulf of Mexico coastline has lost land on the order of hectares per year since records began in the 1840s, putting mainland coastal communities and important ecosystems at risk. Here we present an analysis of Light Detection and Ranging (LiDAR) digital elevation models, revealing erosional/depositional patterns and geomorphologic changes around the most vulnerable of these islands, Ship Island. Four LiDAR datasets (2004, 2007, 2010, and 2012), capturing the complete topography of the island and some bathymetry in the inlet and surrounding shallows to depths of up to 8 m, are used to investigate subaerial and subaqueous sediment volume changes between these years. The impact of Hurricane Katrina, which produced the highest storm surge ever recorded in the United States, is captured in the 2004-2007 dataset. During this time, sediment comparable to 1.5 times the 2004 subaerial island volume was lost from the area included in the topographic/bathymetric dataset. Only 1/5 of this volume was recovered to this area between 2007 and 2010. The island returned to a state of sediment loss between 2010 and 2012, albeit within the error bounds, while the areal extent of the islands continued to increase. This study examines the impact severe storm events can have on vulnerable barrier islands. It highlights the importance of utilizing 3D datasets that include both topographic and bathymetric data for morphodynamic analyses of barrier island systems.

Climate Process Team on Internal Wave-Driven Ocean Mixing.

Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.

The formation and fate of internal waves in the South China Sea.

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.