ALBERT

Description: Historical data from the region between the Greenwich meridian and the African continental shelf are used to compute the offshore geostrophic transport of the Benguela Current. At 32°S, the Benguela Current is located near the African coast, transporting about 21 Sv (1 Sv = 106 m3 s−1) of surface water toward the north relative to a potential density surface lying between the upper branch of Circumpolar Deep Water and the North Atlantic Deep Watar. Two warm core eddies of probable Agulhas Current origin an observed west of the Benguela Current at 32°S. Near 30°S, the Benguela Current turns toward the northwest and begins to separate from the eastern boundary. It carries about 18 Sv of surface water across 28°S. The current then turns mainly toward the west to flow over a relatively deep segment of the Walvis Ridge south of the Valdivia Bank. A surface current with northward surface of about 10 cm s−1 flows along the western side of the Valdivia Bank, while another northward surface current flows at about 20 cm s−1 some 300 km west of the bank. About 3 Sv of surface now do not leave the Cape Basin south of the Vaidivia Bank, but instead drift northward as a wide. sluggish flow out of the northern end of the Cape Basin. Because of the more southerly seaward extensions of most of the Benguela Current, there are no deep-reaching interactions observed between this current and the cyclonic gyre in the Angola Basin east of the Greenwich meridian. Beneath the surface layer, about 4–5 Sv of Antarctic Intermediate Water are carried northward across 32° and 28°S by the Benguela Current, essentially all of which turns westward to cross the Greenwich meridian south of 24°S.

Description: Maximal growth rates of 15 Antarctic phytoplankton species at 0 degree C ranged from 0.32 to 0.72/d, showing only a weak dependence on cell size. Comparisons were made with two models for size dependence of temperature-corrected rates of maximal growth. Schlesinger's general phytoplankton model predicts a strong size dependence of growth rates and grossly underestimates the maximal growth rates of the larger species, but gives reasonable estimates for the smallest ones. Banse's marine diatom model assumes a weak size dependence of growth rates and gives generally better predictions.

Description: The nutritional status of phytoplankton in a shallow, hypertrophic lake was analyzed by stoichiometry of seston and by enrichment bioassays during a 6-month period. Both methods suggested moderate and temporally interrupted nutrient limitation of reproductive rates. Nitrogen was the most frequently limiting nutrient, phosphorus was next, and silicate limitation of three diatom species occurred only once. The nutritional status of the most abundant individual species could be described by the Monod equation. The nutritional status of the entire phytoplankton assemblage could be described by a modified version of the Droop equation. In accordance with competition theory, phytoplankton species were arrayed along resource ratio gradients. These results are consistent with ecophysiological models derived from culture experiments.

Description: We examine the closure of the current plate motion circuit between the African, North American, and Eurasian plates to test whether these plates are rigid and whether the Gloria fault is an active transform fault. We also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries, and compare the predicted direction of motion along the African‐Eurasian plate boundary in the Mediterranean with the direction of slip observed in earthquakes. From marine geophysical data we obtain 13 transform fault azimuths and 40 3‐m.y.‐average spreading rates, 34 of which are determined from comparison of synthetic magnetic anomaly profiles to ∼140 observed profiles. Slip vectors from 32 earthquake focal mechanisms further describe plate motion. Detailed magnetic surveys north of Iceland provide 11 rates in a region where prior plate motion models had few data. Magnetic profiles north of the Azores triple junction record a rate of 24 mm/yr, 4 mm/yr slower than used by prior models. Gloria and Sea Beam surveys accurately measure the azimuths of seven transform faults; our plate motion model fits six of the seven within 2°. Two transform faults surveyed by Gloria side scan sonar lie near FAMOUS area transform faults A and B and give azimuths 13° clockwise of them. Because recent studies show that short‐offset transforms, such as transforms A and B, are in many places oblique to the direction of plate motion, we exclude azimuths from transforms with less than 35‐km offset. The best fitting and closure‐enforced vectors fit the data well, except for a small systematic misfit to the slip vectors: On right‐lateral slipping transforms, slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths, whereas on left‐lateral slipping transforms, slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths. We search the long Eurasia‐North America boundary for evidence of an additional plate, but find no systematic misfits to the data. In particular, if a Spitsbergen plate exists and moves relative to Eurasia, its motion is less than 3 mm/yr. An Africa‐Eurasia Euler vector determined by adding the Eurasia‐North America and Africa‐North America Euler vectors is consistent with the Gloria fault trend and with slip vectors from eastern Azores‐Gibraltar Ridge focal mechanisms. A small circle, centered at the Africa‐Eurasia closure‐enforced pole, fits the trace of the Gloria fault. The model in which closure was enforced predicts ∼4 mm/yr slip across the Azores‐Gibraltar Ridge, and west‐northwest convergence near Gibraltar, ∼45° more oblique than suggested by a recent model based on compressive axes of focal mechanisms. Moreover, our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between Gibraltar and Sicily. Because closure‐enforced vectors fit the data nearly as well as the best fitting vectors, we conclude that the data are consistent with a rigid plate model and with the Gloria fault being a transform fault.

Description: We report a study of a coastal frontal zone of the southeastern United States based on a field experiment and numerical modeling. The study was conducted in the spring of 1985 during weak to moderate wind stress and strong input of buoyancy from solar radiation and river discharge. The study confirms that the structure and slope of the frontal zone depends on a combination of wind stress and cross-shelf advection of buoyancy. A cross-shelf/depth two-dimensional (x, y), time-dependent numerical model illustrated the response of the frontal zone to the local wind stress regimes. A comparison of model results with field data showed that the model successfully predicted onsets of stratification and mixing. When alongshore wind stress was negative (southward), isopycnals in the frontal zone steepened due to a combination of horizontal advection and vertical convection. When stress was positive (northward), the offshore advection of low density water flattened the isopycnals and potential energy decreased, demonstrating that horizontal advection terms are important in the equation of conservation of buoyancy. The model predicts die offshore advection of lenses of less dense water during upwelling-favorable wind stress. These lenses are of the order of 20 km in cross-shelf scale and represent an efficient mechanism to export nearshore water. The lenses consist of a mixture of low-salinity coastal water and continental shelf water originating further offshore and advected onshore along the bottom. The mean flow inside the frontal zone opposed the mean alongshore wind stress. Part of the alongshore flow was in geostrophy with the cross-shore pressure gradient; the other part was due to an alongshore pressure gradient force (kinematic) of about 1 × 10−6 m s−2 (equivalent sea surface slope = 1 × 10−7), which was trapped along the coast with an offshore width scale of O(10 km). It is likely that the alongshore extent of this pressure gradient was governed by the scale at which freshwater is injected to the continental shelf, i.e., 20–30 km. The pressure gradient force immediately outside of the frontal zone was about −5 × 10−7 m s−2 in the direction of the mean alongshore wind stress. It is hypothesized that, as a result of wind setup and freshwater influx, the northward pressure gradient forced over outer shelf/slope by the Gulf Stream decreases in magnitude onshore, and can even change sign across a nearshore frontal zone of O(10 km). The implied flow field near the frontal zone is therefore highly three-dimensional with |∂v/∂y|≈|∂u/∂x|, where (u, v) are velocities in the cross-shore (x) and alongshore (y) directions, respectively.

Description: The sensitivity of the global climate system to interannual variability of he Eurasian snow cover has been investigated with numerical models. It was found that heavier than normal Eurasian snow cover in spring leads to a “poor” monsoon over Southeast Asia thereby verifying an idea over 100 years old. The poor monsoon was characterized by reduced rainfall over India and Burma, reduced wind stress over the Indian Ocean, lower than normal temperatures on the Asian land mass and in the overlying atmospheric column, reduced tropical jet, increased soil moisture, and other features associated with poor monsoons. Lighter than normal snow cover led to a “good” monsoon with atmospheric anomalies like those described above but of opposite sign. Remote responses from the snow field perturbation include readjustment of the Northern Hemispheric mass field in midlatitude, an equatorially symmetric response of the tropical geopotential height and temperature field and weak, but significant, perturbations in the surface wind stress and heat flux in the tropical Pacific. The physics responsible for the regional response involves all elements of both the surface heat budget and heat budget of the full atmospheric column. In essence, the snow, soil and atmospheric moisture all act to keep the land and overlying atmospheric column colder than normal during a heavy snow simulation thus reducing the land–ocean temperature contrast needed to initiate the monsoon. The remote responses are driven by heating anomalies associated with both large scale air-sea interactions and precipitation events. The model winds from the heavy snow experiment were used to drive an ocean model. The SST field in that model developed a weak El Niño in the equatorial Pacific. A coupled ocean-atmosphere model simulation perturbed only by anomalous Eurasian snow cover was also run and it developed a much stranger El Niño in the Pacific. The coupled system clearly amplified the wind stress anomaly associated with the poor monsoon. These results show the important role of an evolving (not specified) sea surface temperature in numerical experiments and the real climate system. Our general results also demonstrate the importance of land processes in global climate dynamics and their possible role as one of the factors that could trigger ENSO events.