High-Frequency Asymptotics for Backscattering Cross Sections from the Statistically Rough Sea Surface
High-frequency radio wave diffraction by perfectly conducting arbitrary shaped smooth surface is considered analytically. The diffraction corrections to the geometric optics (GO) surface currents are obtained as solutions of the integral equation, followed from the Stratton-Chu equation for scattered field, if the observation point is taken to approach the surface. The first two iterations of the surface integral equation allow obtaining the asymptotic high-frequency expansions of the surface currents up to terms of order of 1/tk2, where kis the wave number of the incident field. These diffraction corrections to the surface currents depend on the first three derivatives of the surface normal cross sections at every point. These general results are applied to the specific problem of backscattering from the statistically rough surface. The diffraction corrections to the GO statistically averaged backscattering cross sections and HH/VV polarization ratio (which is equal to unity in the GO limit) are obtained as functions of the 2nd (surface slope variances) and 4th (surface curvature variances) moments of the surface roughness power spectra. It is shown that diffraction results in appearance the cross-polarized component in the backscattered field, which is equal to zero in the GO limit. The cross polarized backscattering is caused entirely by the orthogonal to GO components of surface currents, which are caused by surface roughness diffraction. In the specific case of microwave radio wave backscattering by the sea surface with the Elfouhaily power spectrum of roughness, the wind-speed dependence of cross-polarization coefficients and diffraction corrections to backscattering cross sections were obtained. It was shown by direct calculations of diffraction corrections that the GO can be applied directly to backscattering from a wind-driven sea surface only for radio wave lengths less that 1 cm. For longer wavelengths, the GO can be applicable for a surface, the original spectrum of which is truncated at a wavelength equal to the tripled wavelength of the incident wave. This would ensure the smallness of the corresponding diffraction corrections.
Currents, Water Budget and Turn-Over Time Within A Man-Made Coastal Mega-Structure: Palm Jumeirah, Southern Arabian Gulf, United Arab Emirates
Large-scale land reclamation projects along the coastline of Dubai, United Arab Emirates (UAE), are set to expand from 45 km of natural shoreline to more than 1,500 km by 2010. The increasing scale of coastal development in Dubai has large-scale effects on water movement throughout the project area. Palm Jumeirah is the oldest, most well-developed of several man-made coastal mega-constructions in Dubai. This construction project started 6 years ago, has an overall surface area of 7.9 km2 and a footprint of 23 km2, and is connected to the coast via a 5-km long spine from the mainland to the crescent tip. We made time series observations of hydrographic properties and currents within the Palm Jumeirah Lagoon (PJL) during 29 days in April/May 2008 to examine the current flows, water budget, salt transport, and turn-over time. The currents and water flow within the PJL were quite variable, with stronger currents, greater material transports, lower water temperatures, and slightly lower salinities in areas close to the entrances where flushing is ample. Due to the shallow water depths, we found only weak vertical stratification during a tidal cycle. The lagoon system is comprised of 18 blind channels surrounded by a semi-circular open channel, which in turn is linked to the coastal waters by 4 gaps (one each on the East and West sides and two at the base of the development) in the rocky breakwater that surrounds the development. We found substantial differences in water discharge between the east and west gaps, with high discharge on average exiting the east gap and extremely low water discharge exiting the west gap. Our results indicate that the PJL is flushed unequally between the east and west sides with residence times of 1.2 and 42 days for the east and west sides of the development, respectively. Previous studies of water residence time using the DELFT3D hydrodynamics simulation modeling software did not capture this difference accurately, estimating residence times for the two sides of PJL to be 14 and 28 days, respectively. Stratification remains absent during the spring-neap tidal cycle with well mixed conditions throughout the lagoon system. PJL tides are mixed mainly semidiurnal, with the spring and neap tidal ranges measuring 116 and 56 cm, respectively, with no measureable change in either amplitude or phase within the man-made lagoon.
Seismic Reflection Along the Path of the Mediterranean Undercurrent
Seismic reflection profiling is applied to the study of large scale physical oceanographic processes in the Gulf of Cadiz and western Iberian coast, coinciding with the path of the Mediterranean Undercurrent. The multi-channel seismic reflection method provides clear images of thermohaline finestructure with a horizontal resolution approximately two orders of magnitude higher than CTD casting. The seismic data are corroborated against co-located historical oceanographic data. Seismic images show that different water masses are stratified and coherent over horizontal distances of some tens of kilometers. However, both the intensity of the reflectors and their horizontal coherence decreases downstream. This decrease in seismic reflectivity is probably the result of diminished vertical thermohaline contrasts while the reduced horizontal coherence suggests that mixing processes become dominant. Three identified seismic reflectivity zones are identified, North Atlantic Central Water, Mediterranean Water and North Atlantic Deep Water. Seismic evidence for the Mediterranean Undercurrent is found in the reflectivity patterns seen between about 500-1500 m. Lateral thermohaline stratification decreases downstream especially within the nearest 80 km from the coast of Portugal, where the Mediterranean Undercurrent flows attached to the continental shelf of Iberia. Analysis of root mean square (rms) seismic amplitudes in comparison with average temperature and salinity for the Mediterranean Undercurrent reveals similarly decreasing trends indicating that there is a causal relationship between observed thermohaline fine structure and true seismic amplitudes. To this end, the evolution of the MU may be studied seismically.
Internal wave induced boundary layer instabilities: global instability versus isolated events
Over the past few years the role of internal wave induced instabilities has been elucidated by observational, experimental and numerical work. The focus of much of the experimental and numerical work has been on so- called global instabilities that propagate with the wave. In this talk I will contrast numerical simulations of global instabilities in the footprint of internal solitary waves with instability events that occur either due to wave- wave or wave-boundary interactions. These events do not always propagate with the wave. However, they do prove to be vigorous, coherent and common in simulations. They thus considerably expand the size of parameter space (e.g. wave amplitude, geographical extent ) over which vertically trapped internal waves may contribute to sediment resuspension and transport.
Observations of a Stationary Tidal Intrusion Front in the Coastal Lagoon of Chacahua, México
The Chacahua lagoon is located in southern México on the Pacific coast. It is part of the Chacahua- Pastoría lagoon system. Both lagoons were connected to the ocean, however, actually just the Chacahua lagoon is connected to the ocean by a 2 km channel with an approximately width of 150 m and a mean depth of 3 m. During the end of the rainy season (November 2008), currents (ADP), tides and CTD measurements were carried out in the Chacahua channel. The channel has particular features since the first one km is mostly uniform but suddenly there is a hollow of 7 to 8 m which decreases gradually until the end of the channel. These topographic features play an important role in the circulation and seem, during this period, to induce a stationary front at the vicinity of the hollow induced by the sudden increase in depth. The current vertical structure was measured with an ADP along and across the channel and the effect of the hollow on the frontal intrusion is quite evident. Finally, the modeling (ELCOM) of the circulation of the channel tidal intrusion allows to gain an insight into the effect of the hollow on the movement of the frontal intrusion.
What Controls Seasonal Variation of Phytoplankton Growth in the East China Sea?
The seasonal variation of phytoplankton growth in the East China Sea (ECS) is simulated with a three- dimensional coupled physical-biogeochemical model, which includes discharges from Changjiang (aka the Yangtze River). The purpose is to determine the main control on the seasonality of primary productivity in the ECS shelf, which nurtures rich biological resources. The model has a horizontal resolution of 1/6 degree in the domain from 23N to 41N and from 116E to 134E, excluding the Japan/East Sea, and 33 layers in the vertical. The nitrogen-based biogeochemical model has four compartments: dissolved inorganic nitrogen (DIN), phytoplankton, zooplankton and detritus. The chlorophyll to phytoplankton ratio depends on light and DIN availability. The model is driven by monthly climatological winds with the sea surface temperature, salinity and DIN relaxed towards the climatological mean values. It successfully reproduces the observed seasonal variation of phytoplankton growth over the ECS shelf with a strong peak in later spring and summer. The modeled annual mean primary production over the entire ECS shelf is 439 mg C m-2 d-1, which falls within the reported range of 390-529 mg C m-2 d-1. It also reproduces the marked gradient of DIN across the shelf decreasing away from the Changjiang River plume. An alternative model run, Free-N, which deviates from the standard run by essentially removing nudging on DIN, generates the same seasonal pattern of primary productivity but somewhat reduced productivity. In yet another alternative run, Fix-PAR, which deviates from Free-N by removing the seasonal cycle of photosynthetically active radiation, the seasonality of primary productivity almost vanishes. These model results demonstrate that light availability is the major control on the seasonality of primary productivity. However, nutrient supply from vertical nutrient pumping and from Changjiang discharges is still important. It is the insufficient nutrient pumping that leads to the lowered primary production predicted by the Free-N experiment.
Extreme Algal Bloom Detection with MERIS
Harmful Algal Blooms (HAB's) are a major concern all over the world due to their negative impacts on the marine environment, human health, and the economy. Their detection from space still remains a challenge particularly in turbid coastal waters. In this study we propose a simple reflectance band difference approach for use with Medium Resolution Imaging Spectrometer (MERIS) data to detect intense plankton blooms. For convenience we label this approach as the Extreme Bloom Index (EBI) which is defined as EBI = Rrs (709) - Rrs (665). Our initial analysis shows that this band difference approach has some advantages over the band ratio approaches, particularly in reducing errors due to imperfect atmospheric corrections. We also do a comparison between the proposed EBI technique and the Maximum Chlorophyll Index (MCI) Gower technique. Our preliminary result shows that both the EBI and MCI indeces detect intense plankton blooms, however, MCI is more vulnerable in highly scattering waters, giving more positive false alarms than EBI.
Winkler Method Overestimates Dissolved Oxygen In Seawater: Oceanographic Implications
When the Winkler's reaction scheme is used, naturally occurring iodate interferes in the determination of dissolved oxygen in seawater. Each mole of iodate results in an apparent presence of 1.5 moles of dissolved oxygen. At the concentrations of iodate in the surface and deep open ocean, it can lead to an overestimation of 0.53±0.15 and 0.65±0.05 μM of oxygen in these waters respectively. In coastal and inshore waters, the effect is less predictable as the concentration of iodate is more variable. The error was carried into the solubility equations of oxygen in seawater in which the data sources were based on the Winkler's reaction scheme. The solubility equation of García and Gordon (1992) derived from the results of Benson and Krause (1984) is free from this source of error and is recommended for general use. By neglecting the presence of iodate, the average global super-saturation of oxygen in the surface oceans and the corresponding efflux of oxygen to the atmosphere may both have been over-estimated by about 8%. Regionally, in areas where the degree of super-saturation or under-saturation of oxygen in the surface water is small, such as in the tropical oceans, the net air-sea exchange flux can be grossly under- or over-estimated. Even the estimated direction of the exchange can be reversed. Furthermore, the presence of iodate can lead to an over-estimation of the saturation anomaly of oxygen in the upper ocean attributed to biological production by 0.23±0.07%. At the increasing level of sophistication in the interpretation of oxygen data, this source of error should now be taken into account.