David L. Eslinger

Airborne laser study quantifies El Ni˜o‐induced Coastal Change

Winter storms during the 1997–1998 El Nino caused extensive changes to the beaches and cliffs of the west coast of the United States, a NASA-NOAA-USGS investigation using a scanning airborne laser has found. For example, near Pacifica in central California, the cliff eroded locally as much as 10–13 m landward during the El Nino winter, at least 40 times the long term average erosion rate. However, only several hundred meters away the cliff was stable. This variability in cliff response may be related to differences in local beach changes where an accreting beach protected part of the cliff and an eroding beach exposed another part to attack by waves.

The effects of convective and wind-driven mixing on spring phytoplankton dynamics in the Southeastern Bering Sea middle shelf domain

Abstract Spring phytoplankton bloom conditions for the southeastern Bering Sea shelf were simulated with a coupled phytoplankton-nutrient-detritus model that received input from a physical mixed-layer model. The models captured the essential features of chlorophyll, dissolved inorganic nitrogen concentration, and temperature fields during the spring bloom onset and progression in 1980 and 1981. In contrast to critical depth theory, the occurrence of a shallow mixed-layer depth and a period of low wind speed were not sufficient to trigger the spring bloom. In both years, the spring bloom onset occurred in response to the cessation of convective mixing during a period of increasing atmospheric temperature and decreasing wind speed. Differences between 1980 and 1981 post-spring-bloom characteristics, however, resulted from differences in water column stability, and wind speed variability and magnitude through time. Those factors affected the vertical distributions of nitrogen and chlorophyll, and, therefore, phytoplankton growth rate. A high degree of model accuracy was indicated by low average RMSE values for euphotic zone model variable values compared to data. This was a consequence of the dominant role that meteorological forcing had on variable fields and processes during spring 1980 and 1981, and the application of a physical model that was specifically designed to model vertical mixing processes.

Modeling of dimethyl sulfide ocean mixing, biological production, and sea-to-air flux for high latitudes

To explore the extent to which ocean mixed-layer dynamics influences dimethyl sulfide (DMS) sea-to-air flux at high latitudes, a model of DMS ocean mixing, biological production, and sea-to-air flux was developed. This biophysical one-dimensional model is driven by meteorology. The model simulates DMS seawater concentrations and vertical distributions, and DMS sea-to-air flux for Prince William Sound and the Gulf of Alaska, from early March through December. Sensitivity analyses revealed that DMS sea-to-air flux is most affected by the rates of flagellate production, Zooplankton grazing, photooxidation, and microbial consumption of DMS. Model results show that under conditions of substantial vertical mixing, such as high wind stress or convective mixing, DMS sea-to-air flux increases significantly. At high latitudes these events may coincide with wind-driven mixing or the overturning of surface seawater due to decreasing sea surface temperatures in the autumn. Parameterizations used to estimate emissions of such a highly variable gas as dimethyl sulfide need to include ocean mixed-layer dynamics. The current model is limited by the small number of DMS loss and production rate measurements available. The measurements that do exist have large ranges and come almost exclusively from low and midlatitude regions, mostly during the summer months under calm conditions. Field measurements are needed from high-latitude systems to refine this model, making it an effective tool for designing field campaigns, improving the accuracy of DMS sea-to-air flux estimations, and assessing the contribution of northern oceans to the atmospheric sulfur budget.