Salinity Depth Profiles Research Articles

Multi-scale assimilation of simulated SWOT observations

We use an Observing System Simulation Experiment (OSSE) to quantify improvements in ocean state estimation due to the assimilation of simulated Surface Water Ocean Topography (SWOT) observations using a multi-scale 3DVAR approach. The sequential multi-scale assimilation first generates a large-scale analysis and then updates that analysis with smaller scale corrections. Since we use temperature and salinity depth profiles as proxies for sea surface height (SSH) observations, the results are idealized. Skill metrics consistently show that the multi-scale analysis is superior to the single-scale analysis, specifically because it improves small-scale skill without sacrificing skill at larger scales. The analysis skill over a range of spatial scales is determined using wavenumber spectral analysis of 100 m temperature, SSH, and mixed layer depth (MLD). For MLD, the multi-scale assimilation of SWOT data reduces the minimum constrained wavelength from 158 km to 122 km, a 36 km reduction, compared to a single-scale assimilation of the same data. For SSH, the multi-scale approach only reduces constrained scales from 73 km to 72 km, a 1 km reduction. This small increase in skill is caused by the steep wavenumber spectral slope associated with SSH, which suggests that SSH variability is concentrated at long wavelengths. Ultimately, the small-scale update in the multi-scale assimilation has less to correct for SSH. In contrast, MLD has a relatively flat spectral slope. The multi-scale solution can make a more substantial update to the MLD field because it has more small-scale variability. Thus, our results suggest that the magnitude of the skill improvement provided by the multi-scale solution is negatively correlated with the spectral slope of the ocean variable.

CTD data profiling to assess the natural hazard of active submarine vent fields: the case of Santorini Island

Almost three quarters of known volcanic activity on Earth occurs in underwater locations. The presence of active hydrothermal vent fields in such environments is a potential natural hazard for the environment, the society, and the economy. Despite its importance for risk assessment and risk mitigation, monitoring of the activity is impeded by the remoteness and the extreme conditions of underwater volcanoes. The large difference of population present on Santorini between the winter and summer seasons, all within a partially enclosed system, make the Santorini-Kolumbo volcanic field, an ideal place for detailed exploration. In 2017, GEOMAR in collaboration with the National and Kapodistrian University of Athens (mission: POS-510 ANYDROS), used an Autonomous Underwater Vehicle (AUV) to map the NE–trending Santorini–Kolumbo line, where it also collected CTD data. Here we present the preliminary results from the 15-hour survey held on the 25th March 2017, during the POS-510 expedition targeting the vent field which is located in the North Basin of Santorini Caldera. Detailed CTD 3D profiles have been reconstructed from the raw data of Santorini’s vent field. An anomaly emerges at the depth of 350 m in the Conductivity and Salinity depth profiles, as the CTD sensor is placed directly above the vent sources. Anomalies were evident in the 3D maps reconstructed, showing for the first time a rather weak, but underlying hydrothermal vent activity at various locations. As the present results are the first ones produced from this expedition, further investigation is required incorporating the full dataset. Based on those results, the impact of developing appropriate mechanisms and policies to avoid the associated natural hazard is expected to be immense.

Occurrence, fate, and fluxes of perfluorochemicals (PFCs) in an urban catchment: Marina Reservoir, Singapore

A study was carried out to characterize the occurrence, sources and sinks of perfluorochemicals (PFCs) in the Marina Catchment and Reservoir, Singapore. Salinity depth profiles indicated the reservoir was stratified with lower layers consisting of sea water (salinity ranging from 32 to 35 g L(-1)) and a brackish surface layer containing approximately 14-65% seawater. The PFC mixture detected in catchment waters contained perfluoroalkyl carboxylates (PFCAs), particularly perfluorooctanoate (PFOA), perfluorohexanoate (PFHpA), perfluorooctane sulfonate (PFOS) and PFC transformation products. PFC concentrations in storm runoff were generally higher than those in dry weather flow of canals and rivers. PFC concentration profiles measured during storm events indicated 'first flush' behavior, probably because storm water is leaching PFC compounds from non-point sources present in the catchment area. Storm runoff carries high concentrations of suspended solids (SS), which suggests that PFC transport is via SS. In Marina Bay, PFCs are deposited in the sediments along with the SS. In sediments, the total PFC concentration was 4,700 ng kg(-1), approximately 200 times higher than in the bottom water layers. Total perfluoroalkyl sulfonates (PFSAs), particularly PFOS and 6:2 fluoro telomer sulfonate (6:2 FtS) were dominant PFCs in the sediments. PFC sorption by sediments varied with perfluorocarbon chain length, type of functional group and sediment characteristics. A first approximation analysis based on SS transport suggested that the annual PFC input into the reservoir was approximately 35 ± 12 kg y(-1). Contributions of SS, dry weather flow of river/canals, and rainfall were approximately 70, 25 and 5%, respectively. This information will be useful for improving strategies to protect the reservoir from PFC contamination.

Salinity and suspended matter variations in the Tay estuary

The concept of salinity-induced density layering in estuaries was first demonstrated from the upper reaches of the Tay. In this study the nature of the layering and its variation through the tidal cycle is demonstrated from time series of observations taken at many locations within this estuary. Thorough mixing of the waters on the rising tide, as defined by depth profiles of salinity, is commonly replaced by salinity layering during the high water slack period. This condition continues into the falling tide. Frequently the mixed waters are abruptly replaced by stratified waters within the half-hourly sampling interval. This is attributed to the activity of longitudinal fronts, manifest as surficial foam bands, along which water masses shear past each other on both the flood and ebb tides. Offsetting of transverse salinity coutours along the fronts is introduced to explain apparent complexities in surface water salinity distributions measured at high water slack. Suspended particulate matter concentrations increase towards the limit of the saline water intrusion before decreasing headwards into the freshwater zone of the estuary. The suspensions in the water column may also be displaced by the lateral offsetting of the waters along the fronts. The recognition of the presence of fronts, and a knowledge of their impact on the estuarine waters may provide an alternative means of understanding the flow characteristics of these challenging water bodies. The techniques of spatial and temporal averaging normally widely used today may not be the most realistic approach to analysis of the flows.

The annual cycle of temperature in a temperate estuary and associated heat fluxes to the coastal zone

Longitudinal and depth profiles of salinity and temperature were measured in the Tamar Estuary, southwest England, during 1985. During late autumn, winter and early spring, the observed temperature-salinity scatter plots illustrated either near-conservative behaviour or heat loss from the estuary to the atmosphere, whereas the rest of the year they illustrated heat gain from the atmosphere. Observed maximum and minimum freshwater temperatures occurred during late July (19.6°C) and mid January (3.1°C). Maximum and minimum observed coastal-water temperatures occurred during early October (15.1°C) and late February (6.2°C). Similar coastal (15.1°C) and offshore (15.4°C) surface temperatures were observed in summer as a consequence of stratified conditions offshore and the existence of a near-surface, mixed layer there. Air temperature, freshwater (zero salinity) temperature and coastal water temperature showed a strong correlation with coastal soil temperatures measured at 0.05, 0.3 and 1 m beneath ground, respectively. Simulated temperatures were derived from a model that solved the heat budget equations for 1985 on an hourly time-scale, using hourly measurements of local meteorological variables. The yearly average and standard deviation of temperatures in mid estuary were 11.6°C and 4.3°C, respectively. Seasonal temperature amplitude exceeded tidal and diurnal amplitudes by almost an order of magnitude. Temperatures exhibited strongly tidal, predominantly semidiurnal signals, as well as non-tidal, diurnal signals. The daily averaged rates of horizontal heat transport to the coastal waters from the estuary and its rivers during 1985 were typically between 1–2 GW during the spring and summer months. Heat transport was generally reversed during late autumn, winter and spring. The estuary then generally received heat from the coastal zone, with maximum rates of heat transport that were typically 3 GW. Because estuary residence times varied from less than a few days in winter to less than two weeks in summer, there was a close coupling (on seasonal time-scales) between estuarine heat fluxes and coastal seawater and terrestrial freshwater temperature differences.

Factors controlling trophic conditions in the North-West Adriatic basin: Seasonal variability

Abstract The North-Adriatic basin shows typical shallow water mass characteristics which in a first approach, can be considered independent of the Middle and the Southern basins, being more affected by seasonal temperature and salinity variability. Primary production estimates represent the main quantitative assessments of the trophic conditions of a marine system, resulting from the combined effect of a large number of oceanographic factors. In this paper the results from three EUROMARGE AS (EEC-MAST II-MTP project) field trips carried out in 1994 are presented as a contribution to the better understanding of the factors controlling the trophic balance in the Northern Adriatic basin. These results include: depth profiles of salinity, nutrients and chlorophyll a concentrations, oxygen saturation, phytoplankton taxonomy and abundance, estimated biomass and primary production measurements by the 14 C in-situ incubation method. The field trips were carried out in three seasons (February, July, September 1994) and the results reported belong to three stations in the northern basin, 5 miles off Ravenna, Cesenatico and Ancona, respectively. As expected, the physical situation of the water column was different in the three periods: the water was mixed in February and stratified in July and September. Nutrient concentrations were higher in winter, whilst the maximum of primary production was measured in September. The phytoplankton was composed predominantly of diatoms. The correlations between primary production and salinity reflect a difference in the factors controlling primary production. During February and September nutrients coming from rivers play an important role, although with a decreasing influence from station 1, nearest to the Po delta, towards station 3. Depth profiles of nutrient concentrations and O 2 saturation measured during summer in the water column suggest that regeneration of nutrients in the water column down to the bottom boundary layer must play an important role in the nutrient cycling and dynamics in the basin.

Groundwater regime of a tidally influenced coastal pond

Hydraulic gradients between the water level of Little Pond, a small, tidally influenced coastal pond located on Cape Cod, Massachusetts, and 41 mini-piezometers located along the shores and in the sand layer underlying soft bottom sediments indicate that the flow of groundwater to the Pond is mainly concentrated along the western shore, within a nearshore sandy strip. Similar results are obtained from the distribution of salinity in shallow bottom sediments. The point measurements of groundwater head are translated to long-term values by approximating tidal movements to a sinusoid and using a simple mathematical model. Salinity depth profiles in the shore and bottom sandy sediments indicate at least occasional outflow of salt water to the ground, and suggest that a density-driven convection may play an important role in determining the direction of flow and the distribution of solutes. Salinity depth profiles in the soft sediments compared with the results of an advection-diffusion model indicate a locally strong upward flow of fresh groundwater. The model is also used to estimate long-term average velocities of groundwater flow within soft bottom sediments. This flow makes up only about 1% of total groundwater inflow, but may be important for the exchange of solutes across the sediment-water interface. In 1989, groundwater contributed about 60% of total freshwater input to the Pond.

Spectra of North Pacific Temperature and Salinity Perturbations in the Depth Domain

In the central North Pacific, temperature and salinity depth profiles show considerable fine structure, the features of which are most pronounced in the thermocline and halocline, as well as near water mass boundaries. By partitioning the observed series into mean and perturbation values, it is possible to obtain some statistical measures for the perturbations. Wavenumber spectra of temperature, salinity, sound velocity and density were computed for the wavenumber range between 0 and 167 cycles per kilometer. The perturbation spectra for the different geographical regions of the transition zone are essentially the same, for a given variable. In all cases the power density of the perturbations decreases with increasing wavenumber. The power law decay coefficient at low wavenumbers is about −8/3. It decreases gradually with increasing wavenumber. Superimposed upon the smooth decrease in power density are random fluctuations, most of them insignificant at the 95% confidence level. The perturbations are generally incoherent over distances of the order of 50 km. For a given station, temperature and salinity perturbations are coherent at low wavenumbers, but incoherent at high wavenumbers.