Near-surface Dataset Research Articles

The influence of surface gravity waves on the injection of turbulence in the upper ocean

Abstract. Observations were made in the near-surface layer, at about 8 m depth in 132 m deep water off the coast of Ålesund in Norway, for a duration of 2.5 months in late 2011. The measurement period covers the passage of two low pressure systems with substantial wind and wave forcing. The time series of the dissipation rate of turbulent kinetic energy, ε, and the estimates of surface gravity waves are analysed. Dissipation rates varied by 5 orders of magnitude and reached 10-5–10-4 W kg−1 in conditions when wind speed exceeded 15 m s−1 and the significant wave height was of the order of 10 m. The data set suggests substantial injection of turbulence from breaking surface gravity waves and Langmuir turbulence. To support and interpret the observations, numerical calculations are conducted using a second-order turbulence closure scheme based on the Mellor–Yamada level 2.5 scheme, modified to incorporate the near-surface processes such as Langmuir circulation and wave breaking. The results from a run forced by observed wind and wave fields compare favourably with the observations. Comparisons with other near-surface data sets available from the literature lend confidence on our dissipation measurements and the wave-forced simulations.

Impacting Factors and Cumulative Impacts by Midcentury on Wetlands in the Louisiana Coastal Area

Natural and anthropogenic impacting factors simultaneously cause wetland loss in the Louisiana coastal area. Ongoing natural processes are integrated with anthropogenic impacting factors to rationalize the contribution of each impact type within a total system. From baseline conditions of today, two natural processes account for approximately 85% of the estimated cumulative land loss by 2050: (1) subsidence by sediment compaction and tectonism that is driven by the weight of the Mississippi Delta's sedimentary pile and (2) absolute sea-level rise. Hurricanes are sudden natural processes that act as impact accelerators. After the Mississippi River became hydrologically isolated from the delta it built, anthropogenic impacts coinciding with the ramp up to peak oil production in the Louisiana coastal area caused many direct impacts, such as the construction of access canals and pipelines, but indirect impacts are largely symptomatic of natural delta platform submergence. Formation extraction is the only significant anthropogenic activity influencing projected land losses: about 6%. Other anthropogenic activities, such as onshore development, maintenance dredging, and oil spills, are not significant impacting factors. Additive processes, such as delta outbuilding and coastal restoration, are also not significant. A growing body of Louisiana coastal area subsidence studies is proceeding in absence of insight as to how conclusions might be ordered by the subsurface structural fabric that has influenced coastal geomorphology and that continues to influence surface processes. Subsidence assessments have been heavily weighted with inputs based on surface and near-surface data sets that are easy to see and touch. Approaches using basin analysis techniques are a critical omission. Such outputs integrate subsurface geologic logs and seismic data sets into products able to inform decisions about locating coastal restoration or flood protection projects and avoid areas of geologically persistent subsidence.