South Equatorial Current Region Research Articles

Seasonal variability of aragonite saturation state in the Western Pacific

The oceanic uptake of atmospheric carbon dioxide (CO2) since pre-industrial times has increased acidity levels, resulting in a decrease in the pH and aragonite saturation state (Ωar) of surface waters. Aragonite is the predominant biogenic form of calcium carbonate precipitated by calcifying organisms in tropical reef ecosystems. Values of Ωar are often used as a proxy for estimating calcification rates in corals and other calcifying species. We quantify the regional and seasonal variability of Ωar and its main drivers for the Pacific island region (120°E:140°W, 35°S:30°N). The calculation of Ωar uses a seasonal climatology of the surface-water partial pressure of CO2, combined with values of total alkalinity (TA) estimated from a relationship between surface water salinity and total alkalinity that is derived from measurements in the region. This relationship is valid for all phases of El Niño/Southern Oscillation and for mixed layer waters with less than 15μmolkg−1 dissolved nitrate.The influence of seasonal changes in sea surface temperature (SST) on Ωar is small except in the subtropical waters on the northern and southern boundaries of the study region. Here, SST seasonal variability is large (>5°C), causing a change in Ωar of greater than 0.1. Seasonal changes in mixed layer depth and net evaporation (precipitation) do influence the seasonality in TCO2 and TA. However, these processes tend to increase (decrease) the TCO2 (TA) in unison, resulting in a small net effect on seasonal change in Ωar for most of the region. Net biological production and sea–air gas exchange were also found to have only a small impact on the seasonal change in Ωar through the region. Changes in Ωar of between 0.1 and 0.2 occurred where variations in wind-driven upwelling in the Central Equatorial Pacific and the transport of Eastern Pacific waters into the South Equatorial Current region caused a change in the TCO2/TA ratio of the surface waters. In contrast to these two regions, the combined effect of biological and physical processes in the West Pacific Warm Pool and North Equatorial Counter Current subregions resulted in Ωar variability of less than 0.1.

Distribution and early life-history characteristics of anguillid leptocephali in the western South Pacific

Freshwater eels are important fisheries species in parts of the western South Pacific, but little is known about their oceanic early life history or spawning areas. The age, growth, morphology and geographic distribution of five species of genetically identified anguillid leptocephali collected in 1995, 2000 and 2005 were compared. The sizes and ages of the leptocephali collected, Anguilla australis (n = 18), Anguilla marmorata (n = 15), Anguilla reinhardtii (n = 12), Anguilla megastoma (n = 2) and Anguilla obscura (n = 1), ranged from 19.0 to 50.9 mm and from 25 to 155 days, respectively. Leptocephali were mostly collected in the South Equatorial Current region. The total myomere ranges overlapped among species, but anodorsal myomere numbers clearly divided shortfinned and longfinned eels. The myomere ranges of the leptocephali were similar to the reported ranges of the numbers of vertebrae in adults. Larval growth rates suggested that the temperate species A. australis had slightly slower growth than the tropical species A. reinhardtii. The present study suggests that both temperate and tropical anguillid eels use the South Equatorial Current region for spawning and larval development, although some species might have different early life parameters and migration routes to their recruitment areas.

Evidence for Shallow Zonal Jets in the South Equatorial Current Region of the Southwest Pacific

At depths above 1000 m, the currents of the southwestern Pacific are traditionally considered to be dominated by the broad westward flow of the South Equatorial Current, part of the South Pacific subtropical gyre. Results from the OCCAM global ocean model, reported here, indicate that in reality the South Equatorial Current is broken up into a series of zonal jets by the extensive shallow topography associated with islands and reefs. This paper discusses the large-scale structure of the jets and their effect on the temperature and salinity fields. It is shown that some of the jets advect cores of high or low salinity water, which may be used experimentally to identify the jets and their pathways. Comparisons with hydrographic observations show some agreement with the model results but a more detailed experimental program is required.

Geostrophic transport of the South Equatorial Current in the Atlantic

Geostrophic transport calculations from historical data of the equatorial South Atlantic are presented for the investigation of the flow field in the South Equatorial Current region. On the basis of water mass distribution, the potential density surface of sigma-1 = 32.15 kg m-3 is used as a reference for geostrophic shears. This reference surface is located at a depth of 1000 to 1200 m and represents the boundary between the upper branch of the Circumpolar Deep Water and the Upper North Atlantic Deep Water. The southern band of the South Equatorial Current (SSEC) is fed by the Benguela Current, which crosses the Greenwich Meridian south of 20S. West of the Greenwich Meridian the subtropical gyre has its northermost current band as the westward flowing SSEC. The SSEC was found to be a broad sluggish flow between 10S and 25S. The transport of the SSEC in the upper 500 m is in the order of 20 Sv, with surface velocities of around 10 cm s-1. At 30W the SSEC turns northward. A small part of the water turns poleward south of 10S to form the Brazil Current, whereas the bulk of the flow contributes to the North Brazil Current and the South Equatorial Countercurrent (SECC). The SECC seems to cross the entire South Atlantic eastward to at least the Greenwich Meridian, but part of the flow might contribute to the middle branch of the South Equatorial Current flowing westward. The northernmost current band sampled is the eastward flowing South Equatorial Undercurrent. From this data no seasonality in the geostrophic field can be proven.

Late Quaternary paleoceanography of the tropical Atlantic, 2: The seasonal cycle of sea surface temperatures, 0–20,000 years B.P.

Foraminiferal species abundances are used to estimate seasonal variability of late Quaternary sea surface temperatures in the tropical Atlantic Ocean. Empirical orthogonal functions analysis of 28 time series isolates two patterns (modes) of variation. The dominant mode 1 reflects seasonal temperature contrast of the South Equatorial Current region 5°–6°C higher at the glacial maximum than at present, probably indicating larger seasonal variations of the southern trade winds. Forcing for this pattern may have come from equatorward compression of glacial climate zones due to high‐latitude cooling in both hemispheres and/or suppression of meridional monsoonal circulation via feedback from northern hemisphere ice cover. When compared with CLIMAP estimates of lower seasonal variations of glacial age sea ice fronts in Antarctica, mode 1 suggests decoupling of low‐ and high‐ latitude seasonal cycles in the southern hemisphere on a glacial‐interglacial scale. Mode 2 variations reflect seasonal contrast in sea surface temperature of 2°–3°C less 9000 to 14,000 years B.P. than at present along the equator and in the eastern subtropical Atlantic, perhaps related to an equatorial position of the Intertropical Convergence Zone, weakened trade winds, and/or strengthened monsoonal circulation during deglaciation. Our findings emphasize the need to isolate spatially coherent (and thus dynamically linked) patterns of climate change from a complex record of multiple climatic effects. Only then can effective tests of hypotheses be made with a coupled strategy of data acquisition and climate modeling.

Cadmium in Halobates micans from the central and south Atlantic Ocean

The concentrations of cadmium in 428 samples of the sea-skater Halobates micans from 57 locations in the central and south Atlantic Ocean ranged from 1.7 to 122 μg Cd g-1 dry wt. The concentrations in females, males and nymphs were not significantly different. The data suggest that the cadmium concentrations in these insects are somehow related to surface currents. The highest values (50 to 70 μg g-1, maximum value 122 μg g-1) were recorded from the South Equatorial Current region; intermediate values (10 to 20 μg g-1) from the Equatorial Counter Current and the North Equatorial Current; and relatively low concentrations (>10 μg g-1) from the Canary Current in the north and the Benguela Current in the south. A remarkable feature of these data lies in the fact that they reveal areas of high Cd content which correspond closely with those indicated by data obtained by Bull et al. (1977) from a different set of samples, analyzed by different methods and obtained in different years.