Steve Neshyba

On the southernmost extension of the Peru-Chile undercurrent

Abstract The Peru-Chile undercurrent can now be traced from its origin at about 10°S off Peru to 48°S off Chile. The criterion for continuity is the association of a subsurface maximum in the southward geostrophic flow profile with the salinity maximum and oxygen minimum properties characteristic of Equatorial Subsurface Water.

On the size distribution of Antarctic icebergs

Abstract Composites are given of six reported Antarctic iceberg size distributions and theoretical Rayleigh distributions are fitted with reasonable small errors. A modal length of 0.4km is found for observations in East Antarctica; this increases to 0.7km when size data for icebergs observed by satellite in or near the pack ice in the Bellingshausen Sea are added. Results of model investigation of the equilibrium or standing population size distribution, based on the Gordienko observations of sizes near the Amery Ice Shelf as initial conditions and annual input together with the constraints of uniform sidewall wastage rate and that length/width ratios be maintained within the range 1.1 to 2.2 by iceberg fracturing into equal parts, show a distribution much unlike the Rayleigh in that the highest probability of occurrence is of icebergs of widths 0 – 0.2km. The model size distribution attains equilibrium after 21 years, at which time it is fitted very well to a Weibull distribution with parameters β = 0.5, η = 1.6 and γ = 0. A method is outlined by which the size distribution of freshly-calved icebergs might be assessed for use as the initial condition to such models.

Temperature profiles from Pos Crater Lake

In 1984, we took part in an expedition to measure the temperature field and bathymetry of the acid lake (Figure 1) that has formed in the crater of Poas volcano, Costa Rica, since its last eruption in 1953. Obtaining these data was the first step in a long-range study planned by researchers at the Center for Geophysical Research, University of Costa Rica (San Jose, Costa Rica), and the College of Oceanography, Oregon State University (Corvallis). The study will eventually consider all aspects of fluid behavior in a volcanic lake that is heated or otherwise convectively driven by energy injected at the lake bottom. Evidence of convection is clearly visible on the surface of the Poas lake most of the time. Fumarole activity has been continuous since 1953. Phreatic explosions are quite frequent, varying from weak to strong, and the height of the ejected column varies from 1 to more than 500 m. One immediately useful result of the research would be an estimate of the heat transfer from sources within the conduit to the overlying water column. As far as geophysical fluid behavior goes, we are interested in the turbulent and diffusive processes by which heat and chemical species are transferred. We are especially interested in the impact on the density stratification of the density changes that occur as particulates settle downward through the fluid column. The stratification would otherwise be controlled by the turbulent and diffusive processes driven by thermochemical factors.

Rapid alternating vertical temperature gradients in the East China Sea

Abstract Data from an array of current meters, thermistor chains, and meteorologically instrumented buoys deployed in the shallow water (depths of 97 to 115 m) of the East China Sea during AMTEX-75 (February 1985) show that the vertical temperature difference between the 20 and 70 m depths regularly alternated between∼0°C and a maximum of 0.65°C. The periodicity of change was predominantly that of the semidiurnal tide, 12.4 h, with some modulation at diurnal or near diurnal periodicity. The vertical gradient developed most often when currents were in a northwest direction, a direction parallel to the average northwest-southeast horizontal surface water temperature gradient; when the currents reversed 180°, to the southeast, the vertical gradients tended to decrease or disappear altogether. An explanation is given in terms of vertical shear in the semidiurnal tidal currents coupled with the slightly higher current speeds during the offshelf phase (southeast direction) of the current ellipse.

Phytoplankton community composition across the West Wind Drift off South America

Abstract The biogeography of phytoplankton sampled along 92°W and between 45 and 36°S in conjunction with a detailed XBT section corroborates the physical evidence of a counterflow to the West Wind Drift. Distinct communities exist to either side of an east-west tending front that separates the low-salinity surface water tongue off Chile from subantarctic waters to the south. A plankton group dominated by Ceratium trichoceros and other dinoflagellates exists south of the front in the subantarctic waters, while a distinct group dominated by Rhizosolenia bergonii and other diatoms occupies the low-salinity tongue; the northern limit of the lower salinity surface water is the Subtropical Convergence where a third, distinct plankton community is also dominated by dinoflagellates, mainly C. furca and C. gibberum . Factor analyses support the existence of the distinct groups; it also provides some evidence for the origin of the low-salinity surface water as a mixture of subantarctic surface and coastal runoff by identifying a unique factor axis for Nitzshcia longissima , a pennate diatom generally associated with shallow coastal waters and which comprises a major fraction of the phytoplankton assemblage near the frontal zone but within the low-salinity tongue.