Cycling the representer algorithm for variational data assimilation with a nonlinear reduced gravity ocean model

The representer method was used by [Ngodock, H.E., Jacobs, G.A., Chen, M., 2006. The representer method, the ensemble Kalman filter and the ensemble Kalman smoother: a comparison study using a nonlinear reduced gravity ocean model. Ocean Modelling 12, 378–400] in a comparison study with the ensemble Kalman filter and smoother involving a 1.5 nonlinear reduced gravity idealized ocean model simulating the Loop Current (LC) and the Loop Current eddies (LCE) in the Gulf of Mexico. It was reported that the representer method was more accurate than its ensemble counterparts, yet it had difficulties fitting the data in the last month of the 4-month assimilation window when the data density was significantly decreased. The authors attributed this failure to increased advective nonlinearities in the presence of an eddy shedding causing the tangent linear model (TLM) to become inaccurate. In a separate study [Ngodock, H.E., Smith, S.R., Jacobs, G.A., 2007. Cycling the representer algorithm for variational data assimilation with the Lorenz attractor. Monthly Weather Review 135 (2), 373–386] applied the cycling representer algorithm to the Lorenz attractor and demonstrated that the cycling solution was able to accurately fit the data within each cycle and beyond the range of accuracy of the TLM, once adjustments were made in the early cycles, thus overcoming the difficulties of the non-cycling solution. The cycling algorithm is used here in assimilation experiments with the nonlinear reduced gravity model. It is shown that the cycling solution overcomes the difficulties encountered by the non-cycling solution due to a limited time range of accuracy of the TLM. Thus, for variational assimilation applications where the TLM accuracy is limited in time, the cycling representer becomes a very powerful and attractive alternative, given that its computational cost is significantly lower than that of the non-cycling algorithm.     

Cross-shelf exchange in a model of the Ross Sea circulation and biogeochemistry

Transport of warm, nutrient-rich Circumpolar Deep Water (CDW) onto Antarctic continental shelves and coastal seas has important effects on physical and biological processes. The present study investigates the locations of this transport and its dynamics in the Ross Sea with a high-resolution three-dimensional numerical model. The model circulation is forced by daily wind stress along with heat and salt fluxes calculated from atmospheric climatologies by bulk formulae. All surface fluxes are modified by an imposed climatological ice cover. Waters under the Ross Ice Shelf are not included explicitly, but their effect on temperature and salinity is imposed in a buffer zone at the southern end of the model domain. A simple nutrient uptake is calculated based on the climatological chlorophyll distribution and Monod uptake kinetics.Model circulation is strongly affected by bottom topography, due to weak stratification, and agrees with schematics of the general flow and long-term current measurements except near the southern boundary. The sea-surface temperature is similar to satellite estimates except that the warmest simulated temperatures are slightly higher than observations. There is a significant correlation between the curvature of the shelf break and the transport across the shelf break. A momentum term balance shows that momentum advection helps to force flow across the shelf break in specific locations due to the curvature of the bathymetry (that is, the isobaths curve in front of the flow). For the model to create a strong intrusion of CDW onto the shelf, it appears two mechanisms are necessary. First, CDW is driven onto the shelf at least partially due to momentum advection and the curvature of the shelf break; then, the general circulation on the shelf takes the CDW into the interior.     

Reproductive Cycle of the Caribbean Feather Star Nemaster rubiginosa (Echinodermata: Crinoidea)

Abstract. The reproductive cycle of the comatulid crinoid Nemaster rubiginosa from Discovery Bay, Jamaica is described histologically. The cycle is annual and may be divided into (1) a "resting" phase (summer) in which most adults possess immature, unsexable gonads, (2) a recovery phase (early fall) marked by the re-initiation of gametogenesis and rapid gonadal growth, (3) a breeding phase (late fall and winter) during which the gonads are mature and repeated spawning likely takes place, and (4) a post-spawning phase (spring) during which relict gametes are removed from the shrinking gonads. Although unsexable individuals predominate during the "resting" phase, a small proportion of the adult population is unsexable at all times of the year. The reproductive condition of animals in the same month in two successive years was very similar, suggesting that the timing of reproduction is quite predictable from year to year. The re-initiation of gametogenesis in the early fall is correlated with both rising sea temperature and shortening daylength, and the October to March breeding season corresponds to the period of short daylengths at the study site. The well-defined and synchronized annual reproductive cycle of Nemaster rubiginosa contrasts with the more prolonged and variable reproductive cycles reported for other tropical crinoids and points to a diversity of breeding patterns among tropical crinoids.     

Usefulness of classification and index properties of clay-rich sediments in evaluating sea-bed environmental conditions

A comprehensive study of undisturbed, mostly silty clay samples, taken from large-diameter surface cores collected in the North Atlantic and its bordering seas, shows that measurement of the Atterberg limits, water content, specific gravity of grains and grain-size distribution can provide the basis for predicting many seabed geotechnical and geophysical properties which can lead to a quick assessment of environmental conditions. Thus, the sediment compression curve can be reconstructed, acoustic properties assessed, electrical and thermal resistivities deduced and an indication of the shearing forces, which have affected the sediment, inferred.     

The European coastal zone: characterization and first assessment of ecosystem metabolism

The geomorphic, oceanographic, terrestrial and anthropogenic attributes of the European coastal zone are described and published data on ecosystem function (primary production and respiration) are reviewed. Four regions are considered: the Baltic Sea, Mediterranean Sea, Black Sea and the European Atlantic coast including the North Sea. The metabolic database (194 papers) suffers from a non-homogeneous geographical coverage with no usable data for the Black Sea which was therefore excluded from this part of our study. Pelagic gross primary production in European open shelves is, by far, the most documented parameter with an estimated mean of 41 mmol C m⁻² d⁻¹, the lowest value is reported in the Mediterranean Sea (21 mmol C m⁻² d⁻¹) and the highest one in the Atlantic/North Sea area (51 mmol C m⁻² d⁻¹). Microphytobenthic primary production, mostly measured in shallow areas, is extrapolated to the entire 0–200 m depth range. Its contribution to total primary production is low in all regions (mean: 1.5 mmol C m⁻² d⁻¹). Although macrophyte beds are very productive, a regional production estimate is not provided in this study because their geographical distribution along the European coastline remains unknown. Measurements of pelagic community respiration are clearly too sparse, especially below the euphotic zone, to yield an accurate picture of the fate of organic matter produced in the water column. With a mean value of 17 mmol C m⁻² d⁻¹, benthic community respiration consumes approximately 40% of the pelagic organic matter production. Estuaries generally exhibit high metabolic rates and a large range of variation in all parameters, except microphytobenthic primary production. Finally, the problem of eutrophication in Europe is discussed and the metabolic data obtained in the framework of the Land–Ocean Interactions in the Coastal Zone (LOICZ) project are compared with available direct measurements of net ecosystem production.     

PCDD/F and PCB in harbour seals (Phoca vitulina) from British Columbia: response to exposure to pulp mill effluents

Blubber of harbour seals (Phoca vitulina) sampled from the Strait of Georgia, BC, in 1991 and 1992 contained higher concentrations of polychlorinated dibenzo-p-dioxins and furans (PCDD/F) than did samples from Quatsino Sound on western Vancouver Island. The higher proportions of 1,2,3,6,7,8-hexachlorodibenzodioxin and 2,3,7,8-tetrachlorodibenzofuran in the Strait of Georgia samples probably reflect contamination arising from the discharge of effluents from bleached kraft mills. Higher concentrations of non-ortho- and mono-ortho substituted polychlorinated biphenyls were also found in the Strait of Georgia samples, probably as a result of general industrial activity in the region. Principal components analysis showed that residues were distributed differently between males and females. Within males, concentrations of PCDD, PCDF and PCBs appeared to increase with age. Toxic equivalents (TEQ) to 2,3,7,8-tetrachlorodibenzodioxin in the Quatsino Sd. samples were close to the working guidelines for edible fish used by Health Canada; TEQ calculated for the Strait of Georgia samples exceeded these guidelines by up to a factor of five.     

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Nutrient Mass Flux between Florida Bay and the Florida Keys National Marine Sanctuary

There is a net discharge of water and nutrients through Long Key Channel from Florida Bay to the Florida Keys National Marine Sanctuary (FKNMS). There has been speculation that this water and its constituents may be contributing to the loss of coral cover on the Florida Keys Reef tract over the past few decades, as well as speculation that changes in freshwater flow in the upstream Everglades ecosystem associated with the Comprehensive Everglades Restoration Plan may exacerbate this phenomenon. The results of this study indicate that although there is a net export of approximately 3,850 (±404) ton N year^?1 and 63 (±7) ton P year^?1, the concentrations of these nutrients flowing out of Florida Bay are the same as those flowing in. This implies that no significant nutrient enrichment is occurring in the waters of the FKNMS in the vicinity of Long Key Channel. Because of the effect of restricted southwestward water flow through Florida Bay by shallow banks and small islands, the volume of relatively high-nutrient water from central and eastern portions of the bay exiting through the channel is small compared to the average tidal exchange. Nutrient loading of relatively enriched bay waters is mediated by tidal exchange and mixing with more ambient concentrations of the western Florida Bay and Hawk Channel. System-wide budgets indicate that the contribution of Florida Bay waters to the inorganic nitrogen pool of the Keys coral reef is small relative to offshore inputs.     

Sediment Organic Carbon in Todos Santos Bay, Baja California, Mexico

Sediment grain size and organic carbon (OC) data collected over the past 50 years, together with δ¹³C values of OC in recently collected samples, were analyzed to improve understanding of sediment OC distribution and abundance in Todos Santos Bay. Sediments in the submarine canyon at the mouth of the bay and in quiet-water locations along the shore are fine grained, high in OC, and have generally low δ¹³C values; sediments in high-energy environments are low in OC and have high δ¹³C values. A bivariate isotopic mixing model indicates that none of the sediments contain >50% terrigenous OC (average ~30%), and that the terrigenous OC content of the sediments is a small proportion of the OC content of local soils. Sediment OC composition is apparently controlled by energy-related sorting and deposition, oxidation of much of the original terrigenous OC, and replacement of some terrigenous OC by marine OC.