Subtropical North Research Articles

On the structure, paths, and fluxes associated with Agulhas rings

In January 1993, two Agulhas rings were sampled in the eastern South Atlantic during the World Ocean Circulation Experiment one‐time hydrographic section A11. The first of these, Ring 1, was sampled at 36°S, 4°E in the Subtropical Frontal Zone and was smaller and less energetic than other, more typical Agulhas rings, owing partly to its interactions with the North Subtropical Front. The other, Ring 2, was sampled at 33°S, 10°E in the subtropics. This ring was average in size and reasonably representative of the rings observed in the southeast Atlantic during the early 1990s. The path of the rings is found to be dominated by advection in the mean flow rather than self‐induced velocities of eddies under the β effect. We observe significant differences in reduced gravity and available heat and salt anomalies in the rings; from these we infer temporal variability in the stratification of rings upon their formation at the Agulhas Retroflection. This temporal variability in the structure of rings will affect the fluxes associated with them. Data collected within Ring 2 show that neglecting consideration of the water beneath the 10°C isotherm underestimates the volume flux by half and the potential temperature and salinity flux by in excess of one third and one half, respectively. Using a base of the 3.5°C isotherm and including the high‐salinity intermediate water from the Indian Ocean is found to be more appropriate. With these conditions, six Agulhas rings per year entering the subtropical South Atlantic would equate to a volume flux of 9 Sv and an absolute potential temperature and salinity flux of 84 Sv°C and 322 Sv psu (1 Sv psu ≈ 106 kg s−1). The significant flux of intermediate water from the Indian Ocean may be as important in the return path of the thermohaline circulation as the intermediate water that enters the South Atlantic through Drake Passage. Further, the water associated with Agulhas rings and cooler than 10°C potentially contributes a larger volume flux to the warm water return path than the corresponding surface and upper thermocline water.

Agaristoxylon garennicum Gerrienne et al., gen. et sp. nov., an arborescent Ericaceae from the Belgian Upper Paleocene: palaeoenvironmental implications

This paper describes a silicified trunk fragment, Agaristoxylon garennicum Gerrienne et al., gen. et sp. nov., collected from a new locality at Péruwelz (Belgium) in the marine Thanetian (Upper Paleocene), and discusses the fossilisation conditions of the specimen. The anatomical structure of Agaristoxylon is similar to that of the modern ericaceous genus Agarista ( Agauria), encountered in the tropical and subtropical North and South America, and in the tropical regions of Africa, Madagascar and the Mascarene Islands. Together with other palaeontological data, the presence of Agaristoxylon supports the hypothesis that a climate of subtropical type with high atmospheric humidity could have prevailed in Belgium, at least locally, during the Late Paleocene. The occurrence of indistinct growth rings suggests the existence of seasonal drier periods.

Surface oceanic fronts between Africa and Antarctica

A study of the location of oceanic fronts during austral summers 1993–1995 in the Southern Ocean south and east of South Africa was carried out. Continuously sampled surface temperature and salinity data enable the locations and structures of oceanic fronts to be examined in greater detail than do discrete measurements. Nine fronts, including those of the Antarctic Circumpolar Current (ACC), are identified and their average positions and characteristic temperatures and salinities from 61 crossings during 19 legs of four UK WOCE (World Ocean Circulation Experiment) cruises are presented and discussed. The Agulhas Return Front is identified in surface water as far east as 56°E. The South Subtropical Front is distinguished from the North Subtropical Front, and its characteristic stepped structure with small bands of homogeneous water between steps is described. Two separate surface features associated with the Polar Front are identified northwest and southeast of Kerguelen. Surface manifestations of the southern ACC front and southern boundary of the ACC are located south of the Kerguelen Plateau. It is suggested that the so-called “Crozet Front”, where several fronts apparently merge, is a result of widely spaced sampling, and such features can be resolved into the individual fronts with continuous surface sampling. The fine scale features common in this region are discussed, including temperature inversions south of fronts and the presence of meanders and cyclonic and anticyclonic eddies.

Arrival of an Australian anguillid eel in New Zealand: an example of transoceanic dispersal

Anguilla reinhardtii, hitherto known from eastern Australia, New Caledonia, Norfolk Island, and Lord Howe Island, has recently been discovered in rivers of northern New Zealand. Identification, based on morphological and genetic characteristics, is unequivocal; eight consecutive year classes have been found. The only reasonable explanation of this occurrence is transoceanic dispersal to New Zealand, probably from subtropical oceanic spawning grounds north of New Zealand. This corroborates past hypotheses that the strongly diadromous freshwater fish fauna of New Zealand is derived by transoceanic dispersal of known marine life intervals.

Stratospheric warming due to Agung, El Chichón, and Pinatubo taking into account the quasi‐biennial oscillation

Radiosonde data are used to estimate the impact on stratospheric temperature of Agung, El Chichon, and Pinatubo volcanic eruptions, taking into account the quasi‐biennial oscillation (QBO). This is done by comparing the temperature variations following the three eruptions with the average quasi‐biennial variation in temperature obtained by applying the superposed epoch method to the 13 QBO cycles (centered on Singapore 50 mbar east wind maxima) between 1958 and 1996 which do not include these three eruptions. Confidence estimates are based on the 2‐standard deviation value of the 13 comparisons. Shown are estimates of the volcanic impact and approximate 95% confidence intervals in 300–100 and 100–50 mbar layers of polar, temperate, subtropical, and equatorial zones (and the globe as a whole), as well as in north subtropical and equatorial zones at pressure surfaces of 50, 30, 20, and 10 mbar. Based on this procedure, and ignoring other possible influences on stratospheric temperature, all three eruptions warmed the tropical low stratosphere by nearly 2°C, with the largest warming 3°C at 50 mbar in the equatorial zone following Agung and Pinatubo. Above 50 mbar, the volcanic warming usually decreases with height in the tropics, an exception being the 2°C warming of the 10 mbar surface in the north subtropics following El Chichon. While the volcanic warming is most frequently a maximum 2 seasons after the eruption, the warming duration can be 2 years. In the polar zones there is evidence of cooling of stratospheric 300–100 and 100–50 mbar layers following the eruptions. Comparisons are made with other volcanic warming estimates.

Orbital forcing of climate over South Africa: A 200,000-year rainfall record from the pretoria saltpan

Late Pleistocene variations in rainfall in subtropical southern African are estimated from sediments preserved in the Pretoria Saltpan, a 200000 year-old closed-basin crater lake on the interior plateau of South Africa. We show that South African summer rainfall covaried with changes in southern hemisphere summer insolation resulting from orbital precession. As predicted by orbital precession geometry ( Berger, 1978), this South African record is out of phase with North African palaeomonsoon indices ( Street and Grove, 1979; Rossignol-Strick, 1983; McIntyre et al., 1989) ; the amplitude of the rainfall response to insolation forcing agrees with climate model estimates ( Prell and Kutzbach, 1987). These results document the importance of direct orbital insolation forcing on both subtropical North and South African climate as well as the predicted antiphase sensitivity to precessional insolation forcing.

Southern Ocean fronts from the Greenwich meridian to Tasmania

All available meridional sections have been analyzed to investigate the evolution of main fronts between 0° and 150°E. The central South Atlantic is featured by the Subtropical Frontal Zone (STFZ), bordered by the North and South Subtropical Fronts (NSTF and SSTF, respectively), and by the Polar Frontal Zone (PFZ), bordered by the Subantarctic and Polar Fronts (SAF and PF, respectively). This structure becomes more complex in the African sector as the Agulhas Retroflection and the bottom topography force a more convoluted pattern. The Retroflection and associated Agulhas Front (AF) press the SSTF from 38° to 42°–43°S. Strong interactions of the AF, SSTF, and SAF with topography shift the fronts but do not obliterate them. The AF can be traced reliably up to 52°E, sometimes up to 75°E. The SAF is deflected from 45° to 43°S by the Mid‐Ocean Ridge and converges with the SSTF north of the Prince Edward Islands to form a combined SSTF/SAF. This front intensifies east of 50°–52°E as a result of the confluence with the AF, and between 52° and 65°E a triple AF/SSTF/SAF (“the Crozet Front”) is observed. The PF continues along 49° and 50°S between the Crozet Plateau and the Ob‐Lena (Conrad) Rise, passing north of Kerguelen, nearly joining the triple Crozet Front. Downstream of the Kerguelen‐Amsterdam Passage, the canonical structure is being restored (SSTF, SAF, PF); however, the front parameters in the Australian sector are different from the African sector, largely because of strong air‐sea interaction and cross‐frontal exchanges in the Crozet‐Kerguelen region. The SSTF, squeezed between the AF and SAF, loses characteristics to both. The SSTF/SAF interaction results in the Australian SAF being warmer and saltier downstream, while the SSTF becomes shallower and weaker. The Australian STF derives its characteristics mostly from the AF, thus bringing the modified Agulhas waters to the Pacific Ocean. The newly defined North Subtropical Front (NSTF) was distinguished in the Indian Ocean between 31° and 38°S. The front marks the southern boundary of the subtropical salty, warm water pool of the central South Indian Ocean. The NSTF location is coincident with the position of the wind convergence between westerlies and easterlies, suggesting the possible wind‐driven frontogenesis.

Neobenedenia pargueraensis n. sp. (Monogenea: Capsalidae) from the Red Hind, Epinephelus guttatus, and Comments about Neobenedenia melleni

Neobenedenia pargueraensis n. sp. from Epinephelus guttatus of Puerto Rico differs from all known species in possessing scoop-shaped accessory sclerites with pointed tips. It is most similar to Neobenedenia melleni and Neobenedenia longiprostata but differs from the former in having a fenestrate ovary and from the latter in that the ducts of the accessory glands are short, terminating just anterior to the vitelline reservoir rather than reaching the caudal end of the body. It differs from both species in having smooth rather than lobate testes. Neobenedenia melleni occurs throughout the tropical and subtropical western North Atlantic including Bermuda. It kills aquarium and aquaculture fishes with massive infestations and is a severe restraint on the culture of tilapia in sea water in the Caribbean.

Arbovirus Investigations in Argentina, 1977–1980

Serologic surveys of wild and domestic birds, wild mammals, and horses were conducted during arbovirus field studies in Argentina from 1977 through 1980, a non-epizootic interval. The prevalence of neutralizing antibodies to eastern equine encephalitis (EEE) was consistently higher than to western equine encephalitis (WEE) virus in all species and all areas. The presence of antibodies in short-lived avian species and in young unvaccinated horses and the demonstration of seroconversions in horses during the period, indicated that these viruses are either enzootic in, or annually reintroduced into, Argentina. Antibodies to AG80-646, a new subtype of WEE virus isolated in the subtropical north (Chaco Province) from Culex (Melanoconion) mosquitoes, were found in horses and rodents in that region. Antibodies to the TC-83 strain of Venezuelan equine encephalitis (VEE) virus were found in all areas studied. The presence of antibodies in some horses was probably related to vaccination, but the demonstration of seroconversions in sentinel horses and of antibodies in birds and wild mammals indicates active transmission of VEE virus. In 1980 a new enzootic subtype of VEE virus (AG80-663) was isolated from mosquitoes in Chaco; neutralizing antibodies to this virus were prevalent in horses and rodents in this area. Infections with Aura and Una viruses were most common in the subtropical northern provinces. Infection with St. Louis encephalitis was prevalent and widespread, and birds, principally passerine and columbiform species, appear to be the principal hosts. An interesting and unexplained finding was the absence of arbovirus antibodies, in particular SLE antibodies in house sparrows (Passer domesticus). Antibody prevalences in horses exceeded 50% in all areas, and 12% of horses surveyed in Santa Fe Province developed antibody in a 17-month period. Antibodies to other flaviviruses were rare. A high prevalence of immunity to Maguari virus was found in horses; this agent is considered to be a potential equine pathogen. Antibodies to 2 new viruses, Barranqueras and Resistencia, which had been isolated from Cx. (Melanoconion) in Chaco Province, were found in rodents there. Immunity to Gamboa group viruses was prevalent, and birds were implicated as principal hosts.

Comparison of Stratospheric Warmings Following Agung and Chichon

Temperatures at 50, 30, 20 and 10 mb in the western quadrant of the Northern Hemisphere are used to estimate the stratospheric warming resulting from the volcanic eruptions of Agung in 1963 and El Chichon in 1982. After allowing for quasi-biennial temperature variations, a warming of about 4°C is fond at 50 mb in the equatorial zone following Agung, and at 30 mb in the north subtropics following Chichon. In both cases the warming was at a maximum two seasons after the eruption. The Chichon-induced warming in the American sector of the north subtropics is indicated to be slightly greater than in the Pacific sector.

Late neogene biostratigraphy and stable isotope stratigraphy of a drilled core from the Gulf of Mexico

Late Neogene stable isotope stratigraphy and planktonic foraminiferal biostratigraphy have been examined in a high sedimentation rate core (E67-135, Shell Oil Co.) drilled at 725 m water depth in the De Soto Canyon, Gulf of Mexico. The 305 m core contains sections that are Late Miocene, Early Pliocene, Late Pliocene, and Quaternary in age, and is rich in well-preserved assemblages of planktonic foraminifera. A biostratigraphy has been established based on the ranges of 34 selected species of foraminifera. The core 3orrelates with sections from the Gulf of Mexico, the Caribbean Sea, and the subtropical North and South Atlantic Oceans using, as datums, the evolutionary appearances of Globorotalia miocenica Palmer and Globorotalia margaritae evoluta Cita, the extinction of Globorotalia miocenica and the first appearance of Globorotalia truncatulinoides (d'Orbigny). Oxygen and carbon isotope stratigraphy is based on analysis of the benthonic foraminifer, Uvigerina d'Orbigny. Isotopic trends are similar to those observed in the Pacific and Atlantic Oceans. From Early Pliocene to Late Pleistocene time, average δ 18O values increase (2.42‰ to 3.36‰) and exhibit a wider range of values (0.71‰ in Early Pliocene compared to 1.65‰ in Late Pleistocene sediments), probably reflecting Late Neogene climatic deterioration. The ratio 13 C 12 C decreases significantly by −0.21‰ from the Late Miocene to the Early Pliocene. A decrease in δ 13C is observed in other cores and is probably related to changing oceanic circulation patterns in Late Miocene time.

On the Subtropical Frontal Zone North of Hawaii During Winter

Abstract Oceanic fronts in the subtropical frontal zone north of Hawaii are investigated and related to atmospheric forcing. Particular attention is paid to the winter of 1974 when a detailed study was made of the thermohaline structure aboard the R.V. Thomas G. Thompson. In that winter, well-defined fronts occurred at 34, 31 and 28°N. In the upper 100 m, these fronts are nearly vertical and are characterized by temperature, salinity and sound velocity gradients of up to 2°C (27 km)−1, 0.3‰ (27 km)−1 and 12 m s−1 (27 km)−1, respectively. Horizontal density gradients across the northern two fronts are small because of compensating horizontal temperature and salinity gradients. A thin layer of increased stability is encountered between 100 and 125 m. Below this layer, there are prominent lateral intrusions of cool and low-salinity subsurface water under warmer and higher salinity surface water, at latitudes north of 31°N and longitudes east of 155°W. The 0/1500 db dynamic height topography bears no similari...

Nitrate metabolism in abyssal waters

Activities of nitrate assimilation and nitrate reduction were measured 50 cm above the ocean floor (5,845 m and 5,207 m) by an in situ 15N tracer technique at stations in the subtropical (28°29.8′N; 144°58′E) and subarctic (44°10.2′N; 154°03′E) western North Pacific Ocean. Nitrate assimilation ranged from 0.009 to 0.11 μg-at N/1/day, and nitrate reduction from 0 to 0.42 μg-at N/1/day in the presence of added peptone and yeast extract. Nitrate assimilation was higher than nitrite formation at the southern station, but the reverse was the case at the subarctic station. No correlation was observed between bacterial growth and nitrate metabolizing activities. Data are also presented on the effect of hydrostatic pressures upon nitrate metabolism by microbial populations in the surface waters.

TRADE STATISTICS IN ASIA

By Asian countries we mean here countries between the Republic of Korea in the east and Pakistan in the west. In a general way we regard t~ese countries as falling under one and the same category. However, each country has characteristics of its own so that certain reservations are necessary in order to treat these countries as a whole. It can be said that many ofthem are situated near the equator, or in the tropical or subtropical areas north of it, and that the peoples inhabiting these countries are neither Caucasians nor Negroes. Further, many of them were colonies of Western countries from the 18th century or earlier and were liberated and became independent after~ World War II. Viewed from the standpoint of industrial structure, they were mostly monocultural, sending their products to the United States, European countries, and Japan and depending on irnports for the supply of their foodstuffs, daily necessities and machinery. However, after World War 11 and the attainment of independence, these countries have followed a national policy of industrialization and of increasing self-sufficiency in the supply of industrial products. This is the result of increased consciousness of independence and the major changes that have been wrought since the termination of the last World War. These are the common features that characterize almost all these countries.