Supergranule and Mesogranule Evolution

The MDI instrument on the SOHO satellite obtained a nearly continuous 45.5-hr run in high-resolution mode on 17–18 January 1997, collecting continuum, Dopplergram, and magnetogram images once per minute. This is one of the longest data sets yet obtained in this mode and shows significant evolution of the supergranulation pattern. After allowing for solar rotation within the fixed field of view, an area spanning 17° in latitude and 11° in longitude was extracted that covers the same area of the solar surface for the entire run. From the de-rotated continuum images, we computed flow maps of photospheric motions using local correlation techniques (LCT). Horizontal divergence maps constructed from the flow maps show local maxima of the size of mesogranules (5–10). We interpret these as mesogranules although the LCT flow map resolution (4.8 FWHM) may not completely resolve smaller mesogranules. Movies made from the divergence maps clearly show the outward convection (advection) of these mesogranules within each supergranule, and narrow boundaries of negative divergence outlining the supergranules. Several new supergranules are observed forming. These appear as areas of strong divergence that pop up between pre-existing supergranules and grow, pushing their neighbors apart. Others seem to perish between growing neighbors. We also computed the vertical component of vorticity from the flow maps. Movies of this vorticity do not show any obvious patterns.     

Estimating climatic temperature change in the ocean with syntheticacoustic apertures

An acoustic tomography simulation is carried out in the eastern North Pacific ocean to assess whether climate trends are better detected and mapped with mobile or fixed receivers. In both cases, acoustic signals from two stationary sources are transmitted to ten receivers. Natural variability of the sound-speed field is simulated with the Naval Research Laboratory (NRL) layered-ocean model. A sequential Kalman-Bucy filter is used to estimate the sound speed field, where the a priori error covariance matrix of the parameters is estimated from the NRL model. A spatially homogeneous climate trend is added to the NRL fluctuations of sound speed, but the trend is not parameterized in the Kalman filter. Acoustic travel times are computed between the sources and receivers by combining sound speeds from the NRL model with those from the unparameterized climate trend. The effects of the unparameterized climate trend are projected onto parameters which eventually drift beyond acceptable limits. At that time, the unparameterized trend is detected. Mobile and fixed receivers detect the trend at about the same time. At detection time, however, maps from fixed receivers are less accurate because some of the unparameterized climate trend is projected onto tile spatially varying harmonics of the sound-speed field. With mobile receivers, the synthetic apertures suppress the projection onto these harmonics. Instead, the unparametrized trend is correctly projected onto the spatially homogeneous portion of the parameterized sound-speed field     

Acoustic tomography at basin scales and clock errors

A basin-scale acoustic tomography simulation is carried out for the northeast Pacific Ocean to determine the accuracy with which time must be kept at the sources when clocks at the receivers are accurate. A sequential Kalman filter is used to estimate sound-speed fluctuations and clock errors. Sound-speed fluctuations in the simulated ocean are estimated from an eddy-resolving hydrodynamic model of the Pacific forced by realistic wind fields at daily resolution from 1981-1993. The model output resembles features associated with El Nino and the Southern Oscillation, as well as many other features of the ocean's circulation. Using a Rossby-wave resolving acoustic array of four fixed sources and twenty drifting receivers, the authors find that the percentage of the modeled ocean's sound-speed variance accounted for with tomography is 92% at 400-km resolution, regardless of the accuracy of the clocks. Clocks which drift up to hundreds of seconds of error or more for a year do not degrade tomographic images of the model ocean. Tomographic reconstructions of the sound-speed field are insensitive to clock error primarily because of the wide variety of distances between the receivers from each source. Every receiver “sees” the same clock error from each source, regardless of section length, but the sound-speed fluctuations in the modeled ocean cannot yield travel times which lead to systematic changes in travel time that are independent of section length. The Kalman filter is thus able to map the sound-speed field accurately in the presence of large errors at the source's clocks     

What controls the extreme flow through the Kerama Gap: a global HYbrid Coordinate Ocean Model reanalysis point of view

Ocean Dynamics - The temporal variability of volume transport from the North Pacific Ocean to the East China Sea (ECS) through the Kerama Gap (between Okinawa Island and Miyakojima Island—a...     

Watershed morphometric analysis of Wadi Baish Dam catchment area using integrated GIS-based approach

The integration of Remote Sensing (RS) and Geographic Information Systems (GIS) constitutes a powerful tool for the evaluation of watershed morphometric parameters. The benefits of this integration include saving time and effort as well as improving the accuracy of the analysis. Moreover, this technique is appropriate for describing the watershed and its streams. In this study, a detailed morphometric analysis of the Wadi Baish catchment area has been performed using the Shuttle Radar Topography Mission (SRTM). The performed morphometric analysis includes linear, areal, and relief aspects. The results of the morphometric analysis reveal that the catchment can be described as of eighth stream order and consists of an area of 4741.07 km². Additionally, the basin is characterized by a relatively high mean value of bifurcation (4.012), indicative of the scarcity of permeable rocks with high slope in the area. This value of bifurcation ratio is consistent with the high drainage density value of 2.064 km/km² and confirms the impermeability of the subsurface material and mountainous relief. The hypsometric integral of the catchment is 47.4%, and the erosion integral of the catchment is 52.6%, both were indications of a mature catchment area.     

Evaluating the use of 3'-(p-Aminophenyl) fluorescein for determining the formation of highly reactive oxygen species in particle suspensions

Background  

Given the importance of highly reactive oxygen species (hROS) as reactants in a wide range of biological, photochemical, and environmental systems there is an interest in detection and quantification of these species. The extreme reactivity of the hROS, which includes hydroxyl radicals, presents an analytical challenge. 3'-(p-Aminophenyl) fluorescein (APF) is a relatively new probe used for measuring hROS. Here, we further evaluate the use of APF as a method for the detection of hydroxyl radicals in particle suspensions.     

Impact of 1/8° to 1/64° resolution on Gulf Stream model–data comparisons in basin-scale subtropical Atlantic Ocean models

We investigate the impact of 1/8°, 1/16°, 1/32°, and 1/64° ocean model resolution on model–data comparisons for the Gulf Stream system mainly between the Florida Straits and the Grand Banks. This includes mean flow and variability, the Gulf Stream pathway, the associated nonlinear recirculation gyres, the large-scale C-shape of the subtropical gyre and the abyssal circulation. A nonlinear isopycnal, free surface model covering the Atlantic from 9°N to 47°N or 51°N, including the Caribbean and Gulf of Mexico, and a similar 1/16° global model are used. The models are forced by winds and by a global thermohaline component via ports in the model boundaries. When calculated using realistic wind forcing and Atlantic model boundaries, linear simulations with Munk western boundary layers and a Sverdrup interior show two unrealistic mean Gulf Stream pathways between Cape Hatteras and the Grand Banks, one proceeding due east from Cape Hatteras and a second one continuing northward along the western boundary until forced eastward by the regional northern boundary. The northern pathway is augmented when a linear version of the upper ocean global thermohaline contribution to the Gulf Stream is added as a Munk western boundary layer. A major change is required to obtain a realistic pathway in nonlinear models. Resolution of 1/8° is eddy-resolving but mainly gives a wiggly version of the linear model Gulf Stream pathway and weak abyssal flows except for the deep western boundary current (DWBC) forced by ports in the model boundaries. All of the higher resolution simulations show major improvement over the linear and 1/8° nonlinear simulations. Additional major improvement is seen with the increase from 1/16° to 1/32° resolution and modest improvement with a further increase to 1/64°. The improvements include (1) realistic separation of the Gulf Stream from the coast at Cape Hatteras and a realistic Gulf Stream pathway between Cape Hatteras and the Grand Banks based on comparisons with Gulf Stream pathways from satellite IR and from GEOSAT and TOPEX/Poseidon altimetry (but 1/32° resolution was required for robust results), (2) realistic eastern and western nonlinear recirculation gyres (which contribute to the large-scale C-shape of the subtropical gyre) based on comparisons with mean surface dynamic height from the generalized digital environmental model (GDEM) oceanic climatology and from the pattern and amplitude of sea surface height (SSH) variability surrounding the eastern gyre as seen in TOPEX/Poseidon altimetry, (3) realistic upper ocean and DWBC transports based on several types of measurements, (4) patterns and amplitude of SSH variability which are generally realistic compared to TOPEX/Poseidon altimetry, but which vary from simulation to simulation for specific features and which are most realistic overall in the 1/64° simulation, (5) a basin wide explosion in the number and strength of mesoscale eddies (with warm core rings (WCRs) north of the Gulf Stream, the regional eddy features best observed by satellite IR), (6) realistic statistics for WCRs north of the Gulf Stream based on comparison to IR analyses (low at 1/16° resolution and most realistic at 1/64° resolution for mean population and rings generated/year; realistic ring diameters at all resolutions), and (7) realistic patterns and amplitude of abyssal eddy kinetic energy (EKE) in comparison to historical measurements from current meters.     

Modeled Sverdrup flow in the North Atlantic from 11 different wind stress climatologies

In studies of large-scale ocean dynamics, often quoted values of Sverdrup transport are computed using the Hellerman–Rosenstein wind stress climatology. The Sverdrup solution varies, however, depending on the wind set used. We examine the differences in the large-scale upper ocean response to different surface momentum forcing fields for the North Atlantic Ocean by comparing the different Sverdrup interior/Munk western boundary layer solutions produced by a 1/16° linear numerical ocean model forced by 11 different wind stress climatologies. Significant differences in the results underscore the importance of careful selection of a wind set for Sverdrup transport calculation and for driving nonlinear models. This high-resolution modeling approach to solving the linear wind-driven ocean circulation problem is a convenient way to discern details of the Sverdrup flow and Munk western boundary layers in areas of complicated geometry such as the Caribbean and Bahamas. In addition, the linear solutions from a large number of wind sets provide a well-understood baseline oceanic response to wind stress forcing and thus, (1) insight into the dynamics of observed circulation features, by themselves and in conjunction with nonlinear models, and (2) insight into nonlinear model sensitivity to the choice of wind-forcing product.The wind stress products are evaluated and insight into the linear dynamics of specific ocean features is obtained by examining wind stress curl patterns in relation to the corresponding high-resolution linear solutions in conjunction with observational knowledge of the ocean circulation. In the Sverdrup/Munk solutions, the Gulf Stream pathway consists of two branches. One separates from the coast at the observed separation point, but penetrates due east in an unrealistic manner. The other, which overshoots the separation point at Cape Hatteras and continues to flow northward along the continental boundary, is required to balance the Sverdrup interior transport. A similar depiction of the Gulf Stream is commonly seen in the mean flow of nonlinear, eddy-resolving basin-scale models of the North Atlantic Ocean. An O(1) change from linear dynamics is required for realistic simulation of the Gulf Stream pathway. Nine of the eleven Sverdrup solutions have a C-shaped subtropical gyre, similar to what is seen in dynamic height contours derived from observations. Three mechanisms are identified that can contribute to this pattern in the Sverdrup transport contours. Along 27°N, several wind sets drive realistic total western boundary current transport (within 10% of observed) when a 14 Sv global thermohaline contribution is added (COADS, ECMWF 10 m re-analysis and operational, Hellerman–Rosenstein and National Centers for Environmental Prediction (NCEP) surface stress re-analysis), a few drive transport that is substantially too high (ECMWF 1000 mb re-analysis and operational and Isemer–Hasse) and Fleet Numerical Meteorology and Oceanography Center (FNMOC) surface stresses give linear transport that is slightly weaker than observed. However, higher order dynamics are required to explain the partitioning of this transport between the Florida Straits and just east of the Bahamas (minimal in the linear solutions vs. 5 Sv observed east of the Bahamas). Part of the Azores Current transport is explained by Sverdrup dynamics. So are the basic path of the North Atlantic Current (NAC) and the circulation features within the Intra-Americas Sea (IAS), when a linear rendition of the northward upper ocean return flow of the global thermohaline circulation is added in the form of a Munk western boundary layer.     

Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200°C), borosilicate septum bottles (70°C) and HDPE tubes (22°C) in the presence of Fe and Ni metal, awaruite (Ni₈₀Fe₂₀) and tetrataenite (Ni₅₀Fe₅₀). In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites) were found to contain up to 22 ppm N_tot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N^3-). This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic) ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts.