Mississippi River and Sea Surface Height Effects on Oil Slick Migration

Millions of barrels of oil escaped into the Gulf of Mexico (GoM) after the 20 April, 2010 explosion of Deepwater Horizon (DH). Ocean circulation models were used to forecast oil slick migration in the GoM, however such models do not explicitly treat the effects of secondary eddy-slopes or Mississippi River (MR) hydrodynamics. Here we report oil front migration that appears to be driven by sea surface level (SSL) slopes, and identify a previously unreported effect of the MR plume: under conditions of relatively high river discharge and weak winds, a freshwater mound can form around the MR Delta. We performed temporal oil slick position and altimeter analysis, employing both interpolated altimetry data and along-track measurements for coastal applications. The observed freshwater mound appears to have pushed the DH oil slick seaward from the Delta coastline. We provide a physical mechanism for this novel effect of the MR, using a two-layer pressure-driven flow model. Results show how SSL variations can drive a cross-slope migration of surface oil slicks that may reach velocities of order km/day, and confirm a lag time of order 5–10 days between mound formation and slick migration, as observed form the satellite analysis. Incorporating these effects into more complex ocean models will improve forecasts of slick migration for future spills. More generally, large SSL variations at the MR mouth may also affect the dispersal of freshwater, nutrients and sediment associated with the MR plume

Hydrology in the Sea of Marmara during the last 23 ka : implications for timing of Black Sea connections and sapropel deposition

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 25 (2010): PA1205, doi:10.1029/2009PA001735.Sediments deposited under lacustrine and marine conditions in the Sea of Marmara hold a Late Quaternary record for water exchange between the Black Sea and the Mediterranean Sea. Here we report a multiproxy data set based on oxygen and strontium isotope results obtained from carbonate shells, major and trace elements, and specific organic biomarker measurements, as well as a micropaleontological study from a 14C-dated sediment core retrieved from the Sea of Marmara. Pronounced changes occurred in δ18O and 87Sr/86Sr values at the fresh and marine water transition, providing additional information in relation to micropaleontological data. Organic biomarker concentrations documented the marine origin of the sapropelic layer while changes in n-alkane concentrations clearly indicated an enhanced contribution for organic matter of terrestrial origin before and after the event. When compared with the Black Sea record, the results suggest that the Black Sea was outflowing to the Sea of Marmara from the Last Glacial Maximum until the warmer Bølling-Allerød. The first marine incursion in the Sea of Marmara occurred at 14.7 cal ka B.P. However, salinification of the basin was gradual, indicating that Black Sea freshwaters were still contributing to the Marmara seawater budget. After the Younger Dryas (which is associated with a high input of organic matter of terrestrial origin) both basins were disconnected, resulting in a salinity increase in the Sea of Marmara. The deposition of organic-rich sapropel that followed was mainly related to enhanced primary productivity characterized by a reorganization of the phytoplankton population.We acknowledge support from INSU and the French Polar Institute IPEV

Emptying non-adiabatic filling boxes: The effects of heat transfers on the fluid dynamics of natural ventilation

AbstractA model for steady flow in a ventilated space containing a heat source is developed, taking account of the main heat transfers at the upper and lower boundaries. The space has an opening at low level, allowing cool ambient air to enter the space, and an opening near the ceiling, allowing warm air to leave the space. The flow is driven by the temperature contrast between the air inside and outside the space (natural ventilation). Conductive heat transfer through the ceiling and radiant heat transfer from the ceiling to the floor are incorporated into the model, to investigate how these heat transports affect the flow and temperature distribution within the space. In the steady state, a layer of warm air occupies the upper part of the space, with the lower part of the space filled with cooler air (although this is warmer than the ambient air when the radiant transfer from ceiling to floor is included). Suitable scales are derived for the heat transfers, so that their relative importance can be characterized. Explicit relationships are found between the height of the interface, the opening area and the relative size of the heat transfers. Increasing heat conduction leads to a lowering of the interface height, while the inclusion of the radiant transfer tends to increase the interface height. Both of these effects are relatively small, but the effect on the temperatures of the layers is significant. Conductive heat transfer through the upper boundary leads to a significant lowering of the temperature in the space as a proportion of the injected heat flux is taken out of the space by conduction rather than advection. Radiative transfer from the ceiling to floor results in the lower layer becoming warmer than the ambient air. The results of the model are compared with full-scale laboratory results and a more complex unsteady model, and are shown to give results that are much more accurate than models which ignore the heat transfers.</jats:p

Observations of mixed layer deepening during an Antarctic gale

Observations of mixed layer deepening made during a gale in February 2005 near an ice shelf, Fimbulisen, Antarctica, are reported. The observations were made from the RRS James Clark Ross in the lee of the ice shelf, using repeated downcasts (“yo-yo”) of a conductivity-temperature-depth package, together with shipboard meteorological and other measurements. The mixed layer deepened from less than 40?m to over 120?m over the course of 27?h, with a very rapid deepening from 80?m to 120?m over a period of under 11?h. The mixed layer became both colder and fresher, with the change in salinity and heat content likely to be caused by melting ice. Oxygen isotope results suggest the source of the fresh water was melting sea ice rather than precipitation or ice shelf melt. The input of melt water at the surface stabilizes the mixed layer, so extra energy is required to deepen the mixed layer. The observations suggest that approximately 1.8% of the available “wind-work” energy was used to mix the upper water column, while the stabilizing surface buoyancy flux inhibits the turbulence in the mixed layer, limiting the mixing length to 1.6?m. The eventual depth of the mixed layer is in line with estimates based on the planetary length scale u*/f. The rate of mixed layer deepening is given by Ue/u*?=?0.035. The apparent peak ice melting rate was approximately 60?mm?hr?1, although this is likely to be exaggerated by convergence and downwelling