ALBERT

Description: Using recently developed techniques we estimate snow and sea ice thickness distributions for the Arctic basin through the combination of freeboard data from the Ice, Cloud, and land Elevation Satellite (ICESat) and a snow depth model. These data are used with meteorological data and a thermodynamic sea ice model to calculate ocean-atmosphere heat exchange and ice volume production during the 2003–2008 fall and winter seasons. The calculated heat fluxes and ice growth rates are in agreement with previous observations over multiyear ice. In this study, we calculate heat fluxes and ice growth rates for the full distribution of ice thicknesses covering the Arctic basin and determine the impact of ice thickness change on the calculated values. Thinning of the sea ice is observed which greatly increases the 2005–2007 fall period ocean-atmosphere heat fluxes compared to those observed in 2003. Although there was also a decline in sea ice thickness for the winter periods, the winter time heat flux was found to be less impacted by the observed changes in ice thickness. A large increase in the net Arctic ocean-atmosphere heat output is also observed in the fall periods due to changes in the areal coverage of sea ice. The anomalously low sea ice coverage in 2007 led to a net ocean-atmosphere heat output approximately 3 times greater than was observed in previous years and suggests that sea ice losses are now playing a role in increasing surface air temperatures in the Arctic.

Description: Within the Subantarctic Mode Water (SAMW) density level, we study temporal changes in salinity, nutrients, oxygen and TTD (Transit Time Distribution) ages in the western (W) and eastern (E) subtropical gyre of the Indian Ocean (IO) from 1987 to 2002. Additionally, changes in Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC) are evaluated between 1995 and 2002. The mechanisms behind the detected changes are discussed along with the results from a hindcast model run (Community Climate System Model). The increasing salinity and decreasing oxygen trends from 1960 to 1987 reversed from 1987 to 2002 along the gyre. In the W-IO a decreasing trend in TTD ages points to a faster delivery of SAMW, thus less biogenic matter remineralization, explaining the oxygen increase and noisier nutrients decrease. In the E-IO SAMW, no change in TTD ages was detected, therefore the trends in oxygen and inorganic nutrients relate to changes in the Antarctic Surface Water transported into the E-IO SAMW formation area. In the W-IO between 1995 and 2002, the DIC increase is equal or even less than the anthropogenic input as the reduction in remineralization contributes to mask the increasing trend. In the E-IO between 1995 and 2002, DIC decreases slightly despite the increase in the anthropogenic input. Differences in the preformed E-IO SAMW conditions would explain this behavior. Trends in the W and E IO SAMW are decoupled and related to different forcing mechanisms in the two main sites of SAMW formation in the IO, at 40°S–70°E and 45°S–90°E, respectively.

Description: Recently, independent concerns about declining oxygen and pH conditions in the coastal ocean have emerged. In coastal upwelling regions, hypoxia can be driven by onshore advection of oxygen-depleted offshore waters as well as by local biological consumption triggered by high productivity. As both mechanisms can also decrease pH and carbonate saturation states, coupled studies of oxygen and carbon are imperative. A quasi two-dimensional model coupling carbon, oxygen, and nitrogen was developed for the summer wind-driven upwelling region off southern Vancouver Island, using the Regional Ocean Modeling System. The physical model is coupled to an ecosystem module that tracks 11 state variables and allows nonfixed C:N ratios for detritus and dissolved organic matter. Given uncertainties in sediment parameterizations in biophysical models, three sediment models are compared and discussed. Results demonstrate that sediment-associated processes play a dominant role in consuming oxygen from, and releasing inorganic carbon to, the bottom waters over the shelf. This study also examines the unique characteristics of the southern Vancouver Island shelf. Two key features distinguish this region from other shelves in the California Current System and protect inner shelf waters from severe hypoxia and corrosive (i.e., undersaturated in aragonite) conditions. First, the near-shore Vancouver Island Coastal Current provides a source of oxygen and nutrients and forms a barrier that prevents upwelled waters (depleted in oxygen and rich in carbon) from penetrating the inner shelf. Second, the greater width of the shelf dilutes these upwelled offshore waters and reduces their penetration onto the shallower shelf region.

Description: Four years of beach elevation surveys at Ocean Beach, San Francisco, California, are used to extend an existing equilibrium shoreline change model, previously calibrated with fine sand and moderate energy waves, to medium sand and higher-energy waves. The shoreline, characterized as the cross-shore location of the mean high water contour, varied seasonally by between 30 and 60 m, depending on the alongshore location. The equilibrium shoreline change model relates the rate of horizontal shoreline displacement to the hourly wave energy E and the wave energy disequilibrium, the difference between E and the equilibrium wave energy that would cause no change in the present shoreline location. Values for the model shoreline response coefficients are tuned to fit the observations in 500 m alongshore segments and averaged over segments where the model has good skill and the estimated effects of neglected alongshore sediment transport are relatively small. Using these representative response coefficients for 0.3 mm sand from Ocean Beach and driving the model with much lower-energy winter waves observed at San Onofre Beach (also 0.3 mm sand) in southern California, qualitatively reproduces the small seasonal shoreline fluctuations at San Onofre. This consistency suggests that the shoreline model response coefficients depend on grain size and may be constant, and thus transportable, between sites with similar grain size and different wave climates. The calibrated model response coefficients predict that for equal fluctuations in wave energy, changes in shoreline location on a medium-grained (0.3 mm) beach are much smaller than on a previously studied fine-grained (0.2 mm) beach.

Description: The baroclinic response to barotropic tidal forcing in the Camarinal Sill area, within the Strait of Gibraltar, is investigated with a three-dimensional, fully nonlinear, nonhydrostatic numerical model. The aim of numerical efforts was the assessment of three-dimensional effects, which are potentially significant in the area because of rather irregular bottom topography, variable background stratification, and complex structure of barotropic tides. Model results reveal a complex baroclinic response under relatively moderate flood tidal currents, which includes the formation of internal hydraulic jumps upstream of the sill, internal cross waves close to the channel walls, and a plunging pycnocline at the lee side of the sill crest. These structures exhibit significant cross-channel spatial dependence and may appear to be aligned together across the channel. This fact makes their identification difficult from the surface pattern captured by remote sensing images. Under strong barotropic forcing (spring tides) the upstream hydraulic jumps are shifted to the lee side of Camarinal Sill, where a single internal hydraulic jump is formed. Significant first- and second-mode hydraulic jumps are also generated near smaller secondary sills in Tangier basin, thus extending the occurrence of intense water mixing and energy dissipation to other zones of the strait.

Description: The global ocean biogeochemical models that are used in order to assess the ocean role in the global carbon cycle and estimate the impact of the climate change on marine ecosystems are getting more and more sophisticated. They now often account for several phytoplankton functional types that play particular roles in marine food webs and the ocean carbon cycle. These phytoplankton functional types have specific physiological characteristics, which are usually poorly known and therefore add uncertainties to model results. Indeed, this evolution in model complexity is not accompanied by a similar increase in the number and diversity of in situ data sets necessary for model calibration and evaluation. Thus, it is of primary importance to develop new methods to improve model performance using existing biogeochemical data sets, despite their current limitations. In this paper, we have optimized 45 physiological parameters of the PISCES global model, using a variational optimal control method. In order to bypass a global 3-D ocean variational assimilation, which would require enormous computation and memory storage, we have simplified the estimation procedure by assimilating monthly climatological in situ observations at five contrasted oceanographic stations of the JGOFS program in a 1-D version of the PISCES model. We began by estimating the weight matrix in the cost function by using heuristic considerations. Then we used this matrix to estimate the 45 parameters of the 1-D version of the PISCES model by assimilating the different monthly profiles (observed profiles at the five stations) in the same variational procedure on a time window of 1 year. This set of optimized parameters was then used in the standard 3-D global PISCES version to perform a 500 year global simulation. The results of both the standard and the optimized versions of the model were compared to satellite-derived chlorophyll-a images, which are an independent and global data set, showing that our approach leads to significant improvements in simulated surface chlorophyll-a in most of the regions of the world ocean. Besides demonstrating that we have improved the accuracy of the PISCES model, this study proposes a sound methodology that could be used to efficiently account for in situ data in biogeochemical ocean models.

Description: The marginal ice zone (MIZ) is the boundary between the open ocean and ice-covered seas, where sea ice is significantly affected by the onslaught of ocean waves. Waves are responsible for the breakup of ice floes and determine the extent of the MIZ and floe size distribution. When the ice cover is highly fragmented, its behavior is qualitatively different from that of pack ice with large floes. Therefore, it is important to incorporate wave-ice interactions into sea ice–ocean models. In order to achieve this goal, two effects are considered: the role of sea ice as a dampener of wave energy and the wave-induced breakup of ice floes. These two processes act in concert to modify the incident wave spectrum and determine the main properties of the MIZ. A simple but novel parameterization for floe breaking is derived by considering alternatively ice as a flexible and rigid material and by using current estimates of ice critical flexural strain and strength. This parameterization is combined with a wave scattering model in a one-dimensional numerical framework to evaluate the floe size distribution and the extent of the MIZ. The model predicts a sharp transition between fragmented sea ice and the central pack, thus providing a natural definition for the MIZ. Reasonable values are found for the extent of the MIZ given realistic initial and boundary conditions. The numerical setting is commensurate with typical ice-ocean models, with the future implementation into two-dimensional sea ice models in mind.

Description: The validity and accuracy of approaches used to determine hurricane surge hazard risk received much attention following the hurricane seasons in mid- to late-2000, which caused record surge-related damage along the Gulf of Mexico coastline. Following Hurricane Katrina in 2005, research showed that most extreme-value statistics approaches underestimated the risk associated with this surge event. In this paper, two of the most popular methods for determining hurricane surge extreme-value statistics are reviewed: the historical surge population approach and the joint probability method. Here, it is demonstrated that both limited historical record length and random along-coast variability in hurricane landfall location can introduce significant errors into surge estimates. For example, the historical surge population approach gives errors of 9% to 17% for return periods between 50 and 1000 years when a surge record of 100 years is considered. In contrast, it is shown that the joint probability method yields significantly more reliable surge estimates, with errors of 2% to 3% for return periods between 50 and 1000 years when a storm record of 100 years is considered. Finally, we show that both methods remain robust when decadal-scale climate variability in the storm rate of occurrence is considered, so long as the hurricane history is long enough to capture the full decadal cycle. When used in conjunction with continuous surge response information, it can be concluded that the joint probability method is a practical and reliable approach for determining extreme-value hurricane surge statistics.

Description: The dynamics of near-inertial motions, and their relation to mixing, is investigated here with an extensive data set, including turbulence and high-resolution velocity observations from two cruises conducted in 2008 (summer) and 2010 (winter) in the Bornholm Basin of the Baltic Sea. In the absence of tides, it is found that the basin-scale energetics are governed by inertial oscillations and low-mode near-inertial wave motions that are generated near the lateral slopes of the basin. These motions are shown to be associated with persistent narrow shear-bands, strongly correlated with bands of enhanced dissipation rates that are the major source of mixing inside the permanent halocline of the basin. In spite of different stratification, near-inertial wave structure, and atmospheric forcing during summer and winter conditions, respectively, the observed dissipation rates were found to scale with local shear and stratification in a nearly identical way. This scaling was different from the Gregg-Henyey-type models used for the open ocean, but largely consistent with the MacKinnon-Gregg scaling developed for the continental shelf.

Description: The Southern Ocean Gas Exchange Experiment (SO GasEx) is the third in a series of U.S.-led open ocean process studies aimed at improving the quantification of gas transfer velocities and air-sea CO2 fluxes. Two deliberate 3He/SF6 tracer releases into relatively stable water masses selected for large ΔpCO2 took place in the southwest Atlantic sector of the Southern Ocean in austral fall of 2008. The tracer patches were sampled in a Lagrangian manner, using observations from discrete CTD/Rosette casts, continuous surface ocean and atmospheric monitoring, and autonomous drifting instruments to study the evolution of chemical and biological properties over the course of the experiment. CO2 and DMS fluxes were directly measured in the marine air boundary layer with micrometeorological techniques, and physical, chemical, and biological processes controlling air-sea fluxes were quantified with measurements in the upper ocean and marine air. Average wind speeds of 9 m s−1 to a maximum of 16 m s−1 were encountered during the tracer patch observations, providing additional data to constrain wind speed/gas exchange parameterizations. In this paper, we set the stage for the experiment by detailing the hydrographic observations during the site surveys and tracer patch occupations that form the underpinning of observations presented in the SO GasEx special section. Particular consideration is given to the mixed layer depth as this is a critical variable for estimates of fluxes and biogeochemical transformations based on mixed layer budgets.