Clear skies, subsequent to Hurricane Ivan's passage across the Gulf of Mexico in September 2004, provided a unique opportunity to investigate upper ocean responses to a major hurricane. Oceanic cyclonic circulation was rapidly intensified by the hurricane's wind field (59–62 m s−1), maximizing upwelling and surface cooling (3–7°C) in two large areas along Ivan's track. Upward isothermal displacements of 50–65 m, computed from wind stress and sea surface height changes, caused rapid ventilation of thermoclines and nutriclines, leading to phytoplankton blooms with peak concentrations 3–4 days later. Wind speed changes along Ivan's track demonstrated that the cool waters (20–26°C) provided immediate negative feedback to the hurricane's intensity. Although our study focused on a relatively small ocean area, it revealed that mesoscale cyclones, in addition to warm anticyclones, may play an important role in producing along‐track hurricane intensity changes.

[1] High-resolution numerical simulations of the northern Gulf of Mexico region using the Hybrid Coordinate Ocean Model (HYCOM) were employed to investigate the dynamical processes controlling the fate of the Mississippi River plume, in particular the conditions that favor cross-marginal transport. The study focuses on the effects of topography, wind-driven and eddy-driven circulation on the offshore removal of plume waters. A realistically forced simulation (nested in a data-assimilative regional Gulf of Mexico HYCOM model) reveals that the offshore removal is a frequent plume pathway. Eastward wind-driven currents promote large freshwater transport toward the shelf break and the DeSoto Canyon, where eddies with diameters ranging from 50 to 130 km interact with the buoyant plume and effectively entrain the riverine waters. Our estimates show that the offshore removal by eddies can be as large as the wind-driven shelf transport. The proximity of eddies to the shelf break is a sufficient condition for offshore removal, and shelf-to-offshore interaction is facilitated by the steep bottom topography near the delta. Strong eddy-plume interactions were observed when the Loop Current System impinged against the shelf break, causing the formation of coherent, narrow low-salinity bands that extended toward the gulf interior. The offshore pathways depend on the position of the eddies near the shelf edge, their life span and the formation of eddy pairs that generate coherent cross-shelf flows. This study elucidates the dynamics that initiate a unique cross-marginal removal mechanism of riverine low-salinity, nutrient-rich waters, allowing their export along connectivity pathways, induced by a large-scale current system. Citation: Schiller, R. V., V. H. Kourafalou, P. Hogan, and N. D. Walker (2011), The dynamics of the Mississippi River plume: Impact of topography, wind and offshore forcing on the fate of plume waters,

The biological consequences of the Deepwater Horizon oil spill are unknown, especially for resident organisms. Here, we report results from a field study tracking the effects of contaminating oil across space and time in resident killifish during the first 4 mo of the spill event. Remote sensing and analytical chemistry identified exposures, which were linked to effects in fish characterized by genome expression and associated gill immunohistochemistry, despite very low concentrations of hydrocarbons remaining in water and tissues. Divergence in genome expression coincides with contaminating oil and is consistent with genome responses that are predictive of exposure to hydrocarbon-like chemicals and indicative of physiological and reproductive impairment. Oil-contaminated waters are also associated with aberrant protein expression in gill tissues of larval and adult fish. These data suggest that heavily weathered crude oil from the spill imparts significant biological impacts in sensitive Louisiana marshes, some of which remain for over 2 mo following initial exposures.

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Close associations are identified between interannual sea surface temperature (SST) variability within the Agulhas Current system, the Agulhas Retroflection region, and the Benguela Current system and summer rainfall behavior of South Africa. The relationships between local ocean temperatures and summer rainfall are strongest when extreme global‐scale atmospheric events, i.e., Southern Oscillation high‐ and low‐index phases, are excluded from the time series. The results indicate that rainfall prediction efforts may benefit from a consideration of SST anomalies within the western portion of the south Indian Ocean, in combination with other meteorological indices. Surface winds, heat fluxes, and atmospheric boundary layer characteristics are investigated as ocean‐atmosphere coupling mechanisms. Easterly wind anomalies across the southwest Indian Ocean and over source regions of the Agulhas Current accompany and precede local oceanic “warm events” which co‐occur with increased rainfall. Whereas previous climatic research has revealed that intensification of the South Atlantic Anticyclone co‐occurs with wet periods over the summer rainfall region, this research has revealed an additional mode of climate variability involving enhanced atmospheric circulation within the southwest Indian Ocean. The simultaneous occurrence of strong easterly winds and a moister atmospheric boundary layer produces increased moisture flux convergence over tropical and subtropical regions of the eastern subcontinent. South of Africa, altered surface heat flux distributions enhance low‐level baroclinicity and instability within the marine boundary layer, optimizing conditions for the intensification of mid‐latitude frontal systems. These processes provide mechanisms which increase the likelihood of tropical‐temperate trough formation across southern Africa, the major rain‐producing system in summer.