Donald R. Johnson

Developing jellyfish strategy hypotheses using circulation models

Little information exists relating life histories of jellyfish species to ocean currents. Successful cycling from sessile polyp to mature jellyfish and back must doubtlessly rely on circulation patterns that serve to retain the species in an optimum environment or disperse the species for other adaptive advantages. In this study, current vectors from a high resolution numerical model of the Gulf of Mexico are applied to a simple advection scheme to develop estimates of time and distance scales from probable polyp habitats to areas in which mature scyphomedusae are observed in the northern Gulf of Mexico. Although seasonal patterns of wind stress form the basis for circulation processes that favour shoreward distribution of medusae of oceanic origin, this dynamic may be altered by deep basin events that occur during critical life history stages. Inter-annual differences in distributional patterns of the sea nettle, Chrysaora quinquecirrha (Desor 1848), in Mississippi coastal waters could be explained by Loop Current processes that alter shelf circulation in the Mississippi Bight.

Red Snapper Larval Transport in the Northern Gulf of Mexico

Abstract This study examines the advection of red snapper Lutjanus campechanus larvae in the northern Gulf of Mexico. The potential for repopulating the eastern Gulf stock through larval transport from the more populous western stock is addressed. Transport pathways across topographic features that inhibit alongshelf flow (e.g., the Mississippi River delta, DeSoto Canyon, and the Apalachicola peninsula) and interregional larval transport are considered. An advective field of currents is developed from a large database of drifter and moored currents, augmented by an operational model to fill gaps. The starting points for larval transport are the locations and day of the year of larval captures from the Southeast Area Monitoring and Assessment Program ichthyoplankton surveys. Because the field of currents is derived from near-surface observations and the depth distribution of larvae is uncertain, findings are expressed in terms of maximal transport pathways. Transport pathways were principally vectored towa...

Connections between Campeche Bank and Red Snapper Populations in the Gulf of Mexico via Modeled Larval Transport

Abstract The potential for Red Snapper Lutjanus campechanus on Campeche Bank to contribute to regional fisheries in the Gulf of Mexico through larval transport was studied using numerical circulation model data. A tracking algorithm was applied at an array of starting locations over Campeche Bank and simulated larval propagules launched every 3xa0d during the spawning seasons of four model years within the period 2003–2010. Successful recruitment was defined as arrival in water depths less than 200xa0m after 31 d of planktonic drift, regional recruitment being defined as a percentage of propagules launched. It was found that successful natal retention to Campeche Bank was high, varying between 67% and 73% of all launched propagules. However, successful recruitment to other regions around the Gulf of Mexico (GOM) was sporadic and extremely low. Robustness of the methodology was examined in a set of experiments involving larval depth and subgrid scale diffusion. The results suggest that larvae from Campeche Ban...

A study of the Iceland-Faeroe Front using GEOSAT altimetry and current-following drifters

Abstract In this study we investigate the utility of GEOSAT altimetry for monitoring the Iceland-Faeroe frontal zone. Since an expected dynamic topography relief of 10–20 cm over the Iceland-Faeroe Front (IFF) was not much above the 10 cm uncertainty in GEOSAT observations, validation by AVHRR imagery and satellite-tracked drifters constituted an important part of the experiment. Sea Surface Height (SSH) relief of greater than 20 cm occurred a long the western side of the IFF and along the eastern side, north of the Shetland Islands. However, with SSH relief of only 10–15 cm in the central region of the IFF, substantial difficulties were encountered in the ability to unambiguously monitor the location of the front. In contrast, frontal meanders with 20–30 cm SSH relief, current speeds up to 50 cm s−1 and radii of curvature of 25 km, were clearly observed on three occasions during the 2 year study. These meanders first appeared north of the Faeroe Islands, in the region from 6 to 8°W, and propagated southeastward at speeds of about 3.3 km day−1, being lost from view in the Faeroe-Shetland Channel. Their strong signals and lifetimes of 2–3 months would appear to make them important constituents of IFF dynamics.

A comparison of Geosat altimeter and synthetic aperture radar measurements over east Greenland pack ice

Abstract Altimetry over sea ice has attracted attention as a possible source of geophysical information. However, the interpretation of altimeter data records over ice has been hampered by a lack of high-resolution imagery for comparison. In this paper we compare Geosat altimeter data along a track over pack ice east of Greenland with coincident synthetic aperture radar data obtained as part of the marginal ice zone experiment (MIZEX-87). Fuzzy-cluster analysis of altimeter pulse echo parameters delineates the marginal ice zone, pack ice and fast ice. Results demonstrate the possibility of operational altimetry products for monitoring large-scale pack ice structure.