On the dynamics of strait flows: an ocean model study of the Aleutian passages and the Bering Strait

A high-resolution numerical ocean circulation model of the Bering Sea (BS) is used to study the natural variability of the BS straits. Three distinct categories of strait dynamics have been identified: (1) Shallow passages such as the Bering Strait and the Unimak Passage have northward, near barotropic flow with periodic pulses of larger transports; (2) wide passages such as Near Straits, Amukta Pass, and Buldir Pass have complex flow patterns driven by the passage of mesoscale eddies across the strait; and (3) deep passages such as Amchitka Pass and Kamchatka Strait have persistent deep return flows opposite in direction to major surface currents; the deep flows persist independent of the local wind. Empirical orthogonal function analyses reveal the spatial structure and the temporal variability of strait flows and demonstrate how mesoscale variations in the Aleutian passages influence the Bering Strait flow toward the Arctic Ocean. The study suggests a general relation between the barotropic and baroclinic Rossby radii of deformations in each strait, and the level of flow variability through the strait, independent of geographical location. The mesoscale variability in the BS seems to originate from two different sources: a remote origin from variability in the Alaskan Stream that enters the BS through the Aleutian passages and a local origin from the interaction of currents with the Bowers Ridge in the Aleutian Basin. Comparisons between the flow in the Aleutian passages and flow in other straits, such as the Yucatan Channel and the Faroe Bank Channel, suggest some universal topographically induced dynamics in strait flows.     

Interactive Effects of UV-B Radiation and Chemical Contamination on Physiological Parameters in the Lichen Ramalina lacera

To assess the impact of ultraviolet B (UV-B) radiation under laboratory conditions on lichens from different biomonitoring sites, thalli of the lichen Ramalina lacera were transplanted from a clean air site to nine different polluted sites in Israel. Our objective was to determine the probability of synergistic/antagonistic effects on physiological parameters by considering two stressors: chemical contamination and UV-B. Exposure to both airborne pollutants in the field and subsequent UV-B radiation in the laboratory which led to a severe disintegration of cell membranes, followed by electrolyte leakage, indicate the probability of a synergistic effect. The impairment of the potential quantum yield of electron transfer through photosystem II (PSII) of photosynthesis indicated by a significant decrease of the ratio F_v/F_m, further demonstrated the synergistic effect of both chemical contamination and UV-B radiation. Besides enlarged amounts of stress-ethylene in lichens at contaminated sites, the reduced levels of ethylene production upon UV-B radiation in the present study suggest the likelihood of an antagonistic effect of xenobiotic elements and UV-B radiation as a result of an impaired ethylene-producing system.     

On the dynamics of low latitude,wide and shallow coastal system: numerical simulations of the Upper Gulf of Thailand

A high-resolution (～1 km horizontal grid and 21 vertical layers) numerical model based on the Princeton Ocean Model (POM) has been used to study the 3D dynamics of the Upper Gulf of Thailand (UGOT). While influenced by tides and rivers like other estuarine systems, the UGOT is unique because it is wide (～100 km?×?100 km), it is shallow (average depth of only ～15 m), it is located in low latitudes (～12.5°N–13.5°N), and it is influenced by the seasonal monsoon. Sensitivity studies were thus conducted to evaluate the impact that surface heat fluxes, monsoonal winds, river runoffs, and the low latitude may have on the dynamics; the latter has been evaluated by modifying the Coriolis parameter and comparing simulations representing low and mid latitudes. The circulation in the UGOT changes seasonally from counter-clockwise during the northeast monsoon (dry season) to clockwise during the southwest monsoon (wet season). River discharges generate coastal jets, whereas river plumes tend to be more symmetric near the river mouth and remain closer to the coast in low latitudes, compared with mid-latitude simulations. River plumes are also dispersed along the coast in different directions during different stages of the monsoonal winds. The model results are compared favorably with a simple wind-driven analytical estuarine model. Comparisons between an El Niño year (1998) and a La Niña year (2000) suggest that water temperatures, warmer by as much as 2 °C in 1998 relative to 2000, are largely driven by decrease cloudiness during the El Niño year. The developed model of the UGOT could be used in the future to address various environmental problems affecting the region.     

Particle transport in gravel-bed rivers: Revisiting passive tracer data

Data from tracer experiments were compiled and analysed in order to explore the role of geomorphological, hydrological and sedimentological constraints on fluvial gravel transport in gravel-bed rivers. A large data set from 217 transport episodes of tagged stones were compiled from 33 scientific papers. Our analyses showed that while magnitude of peak discharge is a major control on gravel transport and mobility, tracer travel distances show some scale dependence on the morphological configuration of the channel. Our results also highlight differences in the way tracers are displaced between step–pool and riffle and pool channels. The riffle–pool sequence seems to be a more efficient trap for travelling gravels than the step–pool pair. In addition, in step–pool channels there are clear differences in tracer transport between observations of first displacements after tracer seeding (unconstrained-stone conditions), and second and subsequent observations of tracer displacements (constrained-stone conditions). The comparison between tracer experiments under constrained conditions and those under unconstrained conditions also highlights the importance of bed state and structures in gravel mobility. The results of this study confirm that sediment transport in gravel-bed rivers is a complex process, whereby sedimentological and geomorphological controls are superimposed on the hydraulic forcing. © 2018 John Wiley & Sons, Ltd.     

A modeling study of the role that bottom topography plays in Gulf Stream dynamics and in influencing the tilt of mean sea level along the US East Coast

Two aspects of the interactions between the Gulf Stream (GS) and the bottom topography are investigated: 1. the spatial variations associated with the north-south tilt of mean sea level along the US East Coast and 2. the high-frequency temporal variations of coastal sea level (CSL) that are related to Gulf Stream dynamics. A regional ocean circulation model is used to assess the role of topography; this is done by conducting numerical simulations of the GS with two different topographies–one case with a realistic topography and another case with an idealized smooth topography that neglects the details of the coastline and the very deep ocean. High-frequency oscillations (with a 5-day period) in the zonal wind and in the GS transport are imposed on the model; the source of the GS variability is either the Florida Current (FC) in the south or the Slope Current (SC) in the north. The results demonstrate that the abrupt change of topography at Cape Hatteras, near the point where the GS separates from the coast, amplifies the northward downward mean sea level tilt along the coast there. The results suggest that idealized or coarse resolution models that do not resolve the details of the coastline may underestimate the difference between the higher mean sea level in the South Atlantic Bight (SAB) and the lower mean sea level in the Mid-Atlantic Bight (MAB). Imposed variations in the model’s GS transport can generate coherent sea level variability along the coast, similar to the observations. However, when the bottom topography in the model is modified (or not well resolved), the shape of the coastline and the continental shelf influence the propagation of coastal-trapped waves and impact the CSL variability. The results can explain the different characteristics of sea level variability in the SAB and in the MAB and help understand unexpected water level anomalies and flooding related to remote influence of the GS.     

Seasonal variability of soil phosphate stable oxygen isotopes in rainfall manipulation experiments

Phosphorus (P) availability limits productivity in many ecosystems worldwide. As a result, improved understanding of P cycling through soil and plants is much desirable. The use of the oxygen isotopes associated to phosphate can be used to study the cycle of P in terrestrial systems. However, changes with time in the oxygen isotopes associated to available P have not yet been evaluated under field conditions. Here we present the variations in available-P oxygen isotopes, based on resin extractions, in a semi-arid site that included plots in which the amount of rainfall reaching the soil was modified. In addition, the oxygen isotopes in the less dynamic fraction which is extractable by HCl, were also measured. The δ¹⁸O of the HCl-extractable phosphate shows no seasonal pattern and corresponds to the average value of the available phosphate of 16.5‰. This value is in the expected range for equilibration with soil water at the prevailing temperatures in the site. The δ¹⁸O values of resin-extractable P showed a range of 14.5-19.1‰ (SMOW), and evidence of seasonal variability, as well as variability induced by rainfall manipulation experiments. We present a framework for analyzing the isotopic ratios in soil phosphate and explain the variability as mainly driven by phosphate equilibration with soil water, and by the isotopic effects associated with extracellular mineralization. Additional isotopic effects result from fractionation in uptake, and the input to the soil of phosphate equilibrated in leaves. These results suggest that the δ¹⁸O of resin-extractable P is an interesting marker for the rate of biological P transformations in soil systems.