Thinning ice in West Antarctica

No abstract

[1] For almost half a century, it has been suspected that hydraulic jumps, which consist of a sudden decrease in downstream velocity and increase in flow thickness, are an important feature of submarine density currents such as turbidity currents and debris flows. Hydraulic jumps are implicated in major seafloor processes, including changes from channel erosion to fan deposition, flow transformations from debris flow to turbidity current, and large-scale seafloor scouring. We provide the first direct evidence of hydraulic jumps in a submarine density current and show that the observed hydraulic jumps are in phase with seafloor scours. Our measurements reveal strong vertical velocities across the jumps and smaller than predicted decreases in downstream velocity. Thus, we demonstrate that hydraulic jumps need not cause instantaneous and catastrophic deposition from the flow as previously suspected. Furthermore, our unique data set highlights problems in using depth-averaged velocities to calculate densimetric Froude numbers for gravity currents.

Submarine channel systems transport vast amounts of terrestrial sediment into the deep sea.Understanding the dynamics of the gravity currents that create these systems, and in particular, how these flows interact with and form bends, is fundamental to predicting system architecture and evolution. Bend flow is characterized by a helical structure and in rivers typically comprises inwardly directed near-bed flow and outwardly directed near-surface flow. Following a decade of debate, it is now accepted that helical flow in submarine channel bends can exhibit a variety of structures including being opposed to that observed in rivers. The new challenge is to understand what controls the orientation of helical flow cells within submarine flows and determines the conditions for reversal. We present data from the Black Sea showing, for the first time, the three-dimensional velocity and density structure of an active submarine gravity current. By calculating the forces acting on the flow, we evaluate what controls the orientation of helical flow cells. We demonstrate that radial pressure gradients caused by across-channel stratification of the flow are more important than centrifugal acceleration in controlling the orientation of helical flow. We also demonstrate that nonlocal acceleration of the flow due to topographic forcing and downstream advection of the cross-stream flow are significant terms in the momentum balance. These findings have major implications for conceptual and numerical models of submarine channel dynamics, because they show that three-dimensional models that incorporate across-channel flow stratification are required to accurately represent curvature-induced helical flow in such systems.

Horizontal transport is a key factor in air pollution meteorology. In several recent air quality field campaigns, networks of wind profiling Doppler radars have been deployed to help characterize this important phenomenon. This paper describes a Lagrangian particle trajectory tool developed to take advantage of the hourly wind observations provided by these special profiler networks. The tool uses only the observed wind profiles to calculate trajectory positions and does not involve any model physics or parameterizations. An interpolation scheme is used to determine the wind speed and direction at any given location and altitude along the trajectory. Only the horizontal winds measured by the profilers are included because the type of profiling radars used in this study are unable to resolve synoptic‐scale vertical motions. The trajectory tool is applied to a case study from the International Consortium for Research on Transport and Transformation air quality experiment conducted during the summer of 2004 (ICARTT‐04). During this international field study, air chemistry observations were collected at Chebogue Point, a coastal station in southwestern Nova Scotia, and factor analysis was used to identify time periods when air pollution from the United States arrived at the site. The profiler trajectories are compared to trajectories produced from numerical model initialization fields. The profiler‐based trajectories more accurately reflect changes in the synoptic weather pattern that occurred between operational upper air soundings, thereby providing a more accurate depiction of the horizontal transport responsible for air pollution arriving in Nova Scotia.