Subtropical Convergence Zone Research Articles

Shipboard measurements of dimethyl sulfide and SO2 southwest of Tasmania during the First Aerosol Characterization Experiment (ACE 1)

Measurements of seawater dimethylsulfide (DMS), atmospheric dimethylsulfide, and sulfur dioxide (SO2) were made on board the R/V Discoverer in the Southern Ocean, southeast of Australia, as part of the First Aerosol Characterization Experiment (ACE 1). The measurements covered a latitude range of 40°S–55°S during November‐December 1995. Seawater DMS concentrations ranged from 0.4 to 6.8 nM, with a mean of 1.7±1.1 nM (1σ). The highest DMS concentrations were found in subtropical convergence zone waters north of 44°S, and the lowest were found in polar waters south of 49°S. In general, seawater DMS concentrations increased during the course of the study, presumably due to the onset of austral spring warming. Atmospheric DMS concentrations ranged from 24 to 350 parts per trillion by volume (pptv), with a mean of 112±61 pptv (1σ). Atmospheric SO2 was predominantly of marine origin with occasional anthropogenic input, as evidenced by correlation with elevated 222Rn and air mass trajectories. Concentrations ranged from 3 to 1000 pptv with a mean of 48.8± 49 pptv (1σ) and a median 15.8 pptv. The mean SO2 concentration observed in undisturbed marine air was 11.9±7.6 pptv (1σ), and the mean DMS to SO2 ratio in these conditions was 13±9 (1σ). Diurnal variations in SO2 were observed, with a daytime maximum and early morning minimum in agreement with model simulations of DMS oxidation in the marine boundary layer. Steady state calculations and photochemical box model simulations suggest that the DMS to SO2 conversion efficiency in this region is 30–50%. Comparison of these results with results from warmer regions suggests that the DMS to SO2 conversion efficiency has a positive temperature dependence.

Variation of the phytoplankton community across the subtropical convergence zone in the west pacific sector of the southern ocean during early austral Summer 1995/6

Phytoplankton of the Southern Ocean, 140–148°E and 40–53°S, was sampled from early austral summer Nov. 1995 to Dec. 1995 to examine cell abundance, cell volume and biomass (cell carbon) distribution across the fronts. A total of 90 phytoplankton taxa were identified. They were 50 diatoms, 37 dinoflagellates, 2 silicoflagellates, and 1 prymnesiophyte. 73 species were observed from north of the subtropical convergence zone and 71 species from south of the subtropical convergence zone.Pseudonitzschia spp. was the most widely distributed species. Nanoplankton predominated cell number of phytoplankton throughout the stations. The abundance of diatoms was higher than that of dinofiagellates. Total biomass profiles were dependent to microphytoplankton biomass. Maximum cell number and biomass were observed from subsurface layer. Phytoplankton community changed across the subtropical convergence zone and 50–53°S (antarctic convergence zone), and physicoehemical factors seem to controll the distribution.

The Dynamical Simulation of the NCAR Community Climate Model Version 3 (CCM3)*

The dynamical simulation of the standard configuration of the latest version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3) is examined, including the seasonal variation of its mean state and its intraseasonal and interannual variability. A 15-yr integration in which the model is forced with observed monthly varying sea surface temperatures (SSTs) since 1979 is compared to coexisting observations. Results show that the most serious systematic errors in previous NCAR CCM versions have either been eliminated or substantially reduced. At sea level, CCM3 reproduces the basic observed patterns of the pressure field very well. Simulated surface pressures are higher than observed over the subtropics, however, an error consistent with an easterly bias in the simulated trade winds and low-latitude surface wind stress. Amplitude errors and phase shifts of the subpolar low pressure centers over both hemispheres during winter produce the largest regional errors, which are on the order of 5 mb. In the upper troposphere, both the amplitude and location of the major circulation centers are very well captured by the model, in agreement with relatively small regional biases in the simulated winds. Errors in the zonal wind component at 200 mb are most notable between 40° and 50° lat of both hemispheres, where the modeled westerlies are stronger than observed especially over the Southern Hemisphere during winter. A ∼50% reduction in the magnitude of the zonally averaged westerly bias in the equatorial upper troposphere that plagued previous CCM versions can be attributed to a significantly improved tropical hydrologic cycle and reduced Walker circulation. Over middle latitudes, the CCM3 realistically depicts the main storm tracks, although the transient kinetic energy is generally underestimated, especially over the summer hemispheres. Over lower latitudes, the model simulates tropical intraseasonal oscillations with marked seasonality in their occurrence. Typical periodicities, however, are near 20–30 days, which are shorter than observed, and the simulated amplitudes are weaker than in both observations and previous versions of the model. The simulated response to interannual variations in tropical SSTs is also realistic in CCM3. A simulated index of the Southern Oscillation agrees well with the observed, and the model captures the overall structure and magnitude of observed shifts in tropical and subtropical convergence zones and monthly rainfall anomalies associated with the tropical SST changes.

Anthropogenic CO2 in the Atlantic Ocean

The anthropogenic CO2 in the Atlantic Ocean is separated from the large natural variability of dissolved inorganic carbon using the method developed by Gruber et al. [1996]. Surface concentrations of anthropogenic CO2 are found to be highest in the tropical to subtropical regions and to decrease toward the high latitudes. They are very close to what is expected from thermodynamic considerations assuming that the surface ocean followed the atmospheric CO2 perturbation. Highest specific inventories (inventory per square meter) of anthropogenic CO2 occur in the subtropical convergence zones. Large differences exist between the North and South Atlantic high latitudes: In the North Atlantic, anthropogenic CO2 has already invaded deeply into the interior; north of 50°N it has even reached the bottom. By contrast, waters south of 50°S contain relatively little anthropogenic CO2, and hence specific inventories are very low. An anthropogenic CO2 inventory of about 22 ± 5 Gt C is estimated for the Atlantic north of the equator for 1982, and 18 ± 4 Gt C is estimated for the Atlantic south of the equator for 1989. The Princeton ocean biogeochemistry model predicts anthropogenic CO2 inventories of 20.0 Gt C (North Atlantic, 1982) and 17.7 Gt C (South Atlantic, 1989) for the same regions in good agreement with the observed inventories. Important differences exist on a more regional scale, associated with known deficiencies of the model.

Cd/Ca in Planktonic Foraminifera of the Glacial Southern Ocean

Since the industrial revolution, 200 years ago, atmospheric carbon dioxide levels have risen by 90 ppmV, an amount equalling the natural variation between the last glacial maximum and pre-industrial values. To predict the fate and influence of anthropogenic carbon dioxide, it is vital to understand the natural controls on the glacial-interglacial variations in atmospheric composition. The biological pump is known to be an important draw-down of carbon dioxide today due to the complete removal of nutrients from the euphotic zone of most surface waters and regeneration at depth. An exception exists in the Southern Ocean today, where the surface waters have high phosphate concentrations indicating incomplete removal of the nutrients either due to very rapid upwelling rates or iron limitation. There is huge potential for glacial carbon dioxide draw-down if the efficiency of the biological pump had been enhanced in the glacial Southern Ocean. Particle proxies suggest that export productivity increased in the glacial subantarctic Southern Ocean (1) but there has been no clear evidence of a lowering of the glacial surface-water nutrients (2). fil3c in planktonic foraminifera has been found to be unreliable as a nutrient tracer in surface waters due to a number of factors such as susceptibility to isotopic fractionation during air-sea exchange (3). Cd/Ca in foraminifera, a reliable deep-water nutrient tracer, has been uncalibrated for surface waters and is often present at levels too low for determination by existing methods. We have developed a new isotope dilution, mass-spectrometric method for low-level determination of Cd/Ca in foraminifera which has allowed reliable and accurate planktonic analyses to be made. We have investigated the application of Cd/ Ca in planktonic foraminifera for the reconstruction of past surface water phosphate levels. Down-core records of Cd/Ca in G.Bulloides covering the last complete glacial cycle have been obtained from a latitudinal transect of cores spanning the present day Subtropical Convergence Zone (SCZ), at approximately 40~ and the Antarctic Polar Front (APF), at approximately 50~ in the Indian Ocean sector of the Southern Ocean. In each core there is a striking correlation between the Cd/Ca and the 5180 of G.Bulloides over the last glacial cycle indicating low surface water nutrient levels during the glacials and high nutrient levels during the interglacials. The greatest reduction in nutrient levels occurs in the subantarctic zone of the Southern Ocean, with little change south of the APF, consistent with the export productivity conclusions. There are two possible explanations for this glacial lowering of surface water nutrients. Firstly the rate of upwelling remains constant but the efficiency of nutrient removal is increased, possibly by iron fertilization due to an increased atmospheric dust fallout in glacial times. Secondly, the glacial Southern Ocean water column became stratified with a stable thermocline altowing the surface water productivity to deplete the nutrients without constant replenishment from upwelling. It is important to distinguish between these two mechanisms as each has a differing potential for glacial draw-down of atmospheric carbon dioxide. A simple box-model following some of the concepts in (4) has been run to resolve which of these two mechanisms is more consistent with the new surface water nutrient data and the existing export productivity conclusions.

Optical properties of ocean waters in the vicinity of the Chatham Rise, South Pacific Ocean

Apparent optical properties of the water at 21 ocean stations in the South Pacific, east of New Zealand in the vicinity of the Chatham Rise, have been studied. Spectral irradiance attenuation and reflectance, and PAR irradiance attenuation, were measured in Sub-tropical and Sub-antarctic water masses, in the Sub-tropical Convergence zone between them, and in coastal waters. The waters studied are considered to be predominantly Jerlov optical types II and III. Anomalous features at wavelengths above 590 nm in the spectra of reflectance and of the vertical attenuation coefficient for upward irradiance were attributed predominantly to Raman emission with a contribution from chlorophyll fluorescence at longer wavelengths.

Response of nannoplankton to major changes in sea-surface temperature and movements of hydrological fronts over Site DSDP 594 (south Chatham Rise, southeastern New Zealand), during the last 130 kyr

A high-resolution history of paleoceanographic changes in the subpolar waters of the southern margin of the Subtropical Convergence Zone during the last 130 kyr, is present in foraminiferal assemblages of DSDP Site 594. The foraminifera indicate that sea-surface temperatures during the Last Interglacial Climax were warmer than today, and that between substage 5d through to the end of isotope stage 2, temperatures were mostly cooler than Holocene temperatures. The paleotemperatures suggest that (1) the Subtropical Convergence was located over the site during substage 5e, later moving further north, then moving southwards to near the site during the Holocene, and (2) the Polar Front was positioned over the Site during glacial stages 6, 4, 2 and possibly parts of stage 3. Several major events are indicated by the nannofloral assemblages during these large changes in sea-surface temperature and associated reorganization of ocean circulation. First, the time-progressive trends between E. huxleyi and medium to large Gephyrocapsa are unique to this site, with E. huxleyi dominating over medium Gephyrocapsa during stages 5c-a, middle part of stage 4 and after the middle point of stage 3. This unusual trend may (at least partly) be caused by the shift of the Polar Front across the site. Second, upwelling flora ( E. huxleyi and small placoliths) increase in abundance during stages 1, 3 and 5, suggesting that upwelling or disturbance of water stratification took place during the interglacials. Thirdly, there are no significant differences between the distribution patterns of the various morphotypes of medium to large Gephyrocapsa, and the combined value of all medium Gephyrocapsa increases in abundance during glacials (stages 2 and 4 and the end of stage 6), similar to the abundance trends in benthic foraminifera. Finally, subordinate nannofossil taxa also show distinctive. climatic trends during the last glacial cycle: (1) Syracosphaera spp. are present in increased abundance during warmer extremes in climate (substages 5e, 5a, and stage 1); (2) Coccolithus pelagicus and Calcidiscus leptoporus dominate the subordinate nannofossil taxa, and their relative proportions seem to provide a useful paleoceanographic index, with C. pelagicus dominating when the Polar Front Zone is over the site (stages 6, 4 and 2), whilst C. leptoporus is relatively more abundant when the STC is positioned over the site (stages 1 and 5e). Increased abundance of C. pelagicus also can indicate intensified coastal upwelling.

An improved method for detecting anthropogenic CO2 in the oceans

An improved method has been developed for the separation of the anthropogenic CO2 from the large natural background variability of dissolved inorganic carbon (C) in the ocean. This technique employs a new quasi‐conservative carbon tracer ΔC*, which reflects the uptake of anthropogenic CO2 and the air‐sea disequilibrium when a water parcel loses contact with the atmosphere. The air‐sea disequilibrium component can be discriminated from the anthropogenic signal using either information about the water age or the distribution of ΔC* in regions not affected by the anthropogenic transient. This technique has been applied to data from the North Atlantic sampled during the Transient Tracers in the Ocean North Atlantic (TTO NAS) and Tropical Atlantic study (TTO TAS) cruises in 1981–1983. The highest anthropogenic CO2 concentrations and specific inventories (inventory per square meter) are found in the subtropical convergence zone. In the North Atlantic, anthropogenic CO2 has already invaded deeply into the interior of the ocean, north of 50°N it has even reached the bottom. Only waters below 3000 m and south of 30°N are not yet affected. We estimate an anthropogenic CO2 inventory of 20 ± 4 Gt C in the North Atlantic between 10°N and 80°N. The 2.5‐dimensional ocean circulation model of Stocker et al. [1994] and the three‐dimensional ocean general circulation biogeochemistry model of Sarmiento et al. [1995] predict anthropogenic CO2 inventories of 18.7 Gt C and 18.4 Gt C, respectively, in good agreement with the observed inventory. Important differences exist on a more regional scale, associated with known deficiencies of the models.

Holocene and Pleistocene glacial palaeoceanography off southeastern Australia, based on foraminifers and nannofossils in Vema cored hole V18–222∗

Sea‐surface temperature trends in planktonic foraminifers, isotopic and population abundance trends in benthic foraminifers, and abundance trends in calcareous nannofossils of V18–222 (38°34'S, 140°37'E, southeast Australia: 1904 m water depth) reveal an interglacial interval (oxygen isotope stage 1) distinctly different from underlying glacial intervals (isotope stages 10 and 12). During the glacial times mean sea‐surface temperatures (FI‐2 planktonic foraminiferal transfer function) ranged between 12°C and 9°C, indicating the close proximity of the Subtropical Convergence Zone. The cooler waters were unfavourable for the existence of the nannofossil Florisphaera profunda, while cooler water flora like Coccolithus pelagicus and Gephyrocapsa caribbeanica flourished. Absence of tropical and subtropical nannoflora suggest that the Leeuwin Current did not flow across the area at this time. High surface productivity, associated with strong upwelling during this time is indicated by Bulimina aculeata‐dominated benthic foraminiferal populations, and by Uvigerina spp. depleted in δ13C. At depth it is likely that the core site was immersed in Circumpolar Deep Water. In contrast, during interglacial stage 1 and the end of stage 2 deglaciation, mean sea‐surface temperatures at the site ranged between 14°C and 18°C, similar to modern temperatures. High summer sea‐surface palaeotemperatures indicate that the Subtropical Convergence was positioned well south of the site during the last ∼14 000 years. The persistent presence of a warm‐water nannoflora indicates that the Leeuwin Current flowed over the area throughout warm extremes in climate through the last ∼14 000 years. Anomalously low winter sea‐surface palaeotemperatures during the same time, however, coupled with the persistent presence of Florisphaera profunda in low abundance, indicate a persistence of mild upwelling activity in the area, similar to that which exists today, since ca 14 ka. Stable deep‐water conditions during the interglacial are indicated by benthic foraminiferal population trends with a fauna dominated by Bulimina striata and uniform, but relatively high, δ13C signals in Uvigerina spp., with the site immersed in a mixture of Antarctic Intermediate Water and Antarctic Circumpolar Water.

The eastern boundary of the Gulf Stream recirculation

A meridionally aligned thermocline front near 60W in the subtropical North Atlantic is revealed by the 1992 Trident data set. The front separates saltier thermocline water to the east from less salty water to the west. The eastern water is subjected to excess evaporation of the subtropics, while the western water is fed by lower salinity Gulf Stream water, which derives water from the wet tropical Atlantic. It is suggested that the front marks the eastern edge of the Gulf Stream recirculation cell, hence refer to it as the recirculation front. The surface layer displays a fan-like T/S scatter above the 18°C Subtropical Mode Water, with the fresher surface water located west of the recirculation front, and a subsurface salinity maximum to the east. In the lower thermocline (8 to 12°C) there is a step-like salinity increase of about 0.04 toward the east as measured along isotherms, producing two modes in the T/S scatter. At the intermediate water level (approximately in the 4 to 8°C range) the extent of the low salinity Antarctic Intermediate Water and salty Mediterranean outflow water are also reflected in the position of the recirculation front. That the front marks the easternmost extent of the Gulf Stream recirculation is supported by the potential vorticity, which reveals a region of high homogeneous values within the recirculation cell. East of the front, the potential vorticity field is sloped along isopycnals indicating the meridional flow of the Sverdrup interior. Mapping of the recirculation front using archived data reveals that it extends deep into the subtropical convergence zone (STCZ), a region whose fronts have all been previously attributed to Ekman convergence.

Distribution of calcareous nannoplankton near the Subtropical Convergence, south of Tasmania, Australia

The floral composition of living calcareous nannoplankton collected from surface waters near the Subtropical Convergence (STC), offshore south-eastern Tasmania, in January 1994 is presented. Emiliania huxleyi represented more than 90% of all coccolithophorids. Scanning electron microscopy showed a substantial increase in intraspecific variation of E. huxleyi, and in variation in coccolith size of Calcidiscus leptoporus, E. huxleyi type A and C. leptoporus (small), north of the STC. It is postulated that the percentage abundance of these taxa may be used to determine the position andlor shift of the STC zone through time, and therefore may be of use for palaeoceanographic studies.

Optical properties of the coastal and oceanic waters off South Island, New Zealand: Regional variation

Abstract A study of the optical properties of the varied water masses in inshore and oceanic waters around the South Island of New Zealand was made. This was a contribution to a larger effort to model the productivity of the southern sector of New Zealand's Exclusive Economic Zone. A combination of spectroradiometry, PAR sensor deployments, transmissometry, and yellow‐substance measurement was used at 30 stations to characterise the optical properties of the region. The waters were classified according to Jerlov's system for ocean waters. Inshore sites on the west and east coasts and Foveaux Strait fell into optical types Coastal I and Oceanic III. Oceanic II waters occurred in the Subtropical Convergence Zone (STCZ). Subantarctic waters and those south of the STCZ were in the clear categories Oceanic I and IB. Attenuation coefficients (K) varied more than three‐fold from 0.05 to 0.16 m−1. Reflectance was lowest off the Fiordland coast where light‐absorbing yellow substance was maximal. An index of the average lighting of the mixed layer (Eo ) for phytoplankton photosynthesis was calculated from the light‐depth profile. Highly varying mixed‐layer depths, combined with a 3‐fold difference in K, caused Eo to vary by as much as 11, suggesting that the growth of oceanic phytoplankton in the region may vary far more than predicted by present models.

An Integrated Latest Quaternary (Stage 3 to Present) Paleoclimatic and Paleoceanographic Record from Offshore Northern New Zealand

Paleoceanographic and onshore paleoclimatic changes during the last 59,000 yr are established from three deep-sea sediment cores off northeast New Zealand using an integrated log of sediment texture, CaCO3 content, palynology, and planktonic and benthic foraminiferal δ18O and δ13C data, together with dated silicic tephras. These records from the isotopic stage 4-3 boundary to the present record northern New Zealand vegetation history, changes in a subsidiary equatorward flow of Circumpolar Deep Water, and sea-surface temperatures (SSTs) for subtropical water (STW) between latitudes 36°42′ and 35°51′S. Relative to the Holocene, isotopically derived SSTs record average changes of +2°C, -2°C, and -2°C for the 59,000-43,000, 43,000-24,000, and 24,000-12,000 yr time slices, respectively. The apparent +2°C warming for the 59,000-43,000 yr period is interpreted to reflect changes in the dominant depth habitat of Globigerina bulloides in response to upwelling. A -2°C cooling of SSTs during isotope stage 2 is interpreted, in part, to reflect upwelling of cool subsurface water resulting from strong and persistent westerly airflow across New Zealand, with the concomitant enhanced surface-water production of CaCO3. Onshore, vegetation consistent with these changes are recorded, with full conifer-hardwood forest prior to 43,000 yr, followed by a change to vegetation implying cooler and drier conditions between 43,000 and 12,000 yr, and a subsequent return to full forest during the Holocene. The sequence of biopelagic and hemipelagic sedimentation observed within these cores reflect oscillation of sea level about a threshold eustatic level that controls the transport of terrigenous detritus offshore. Local variations and interplay of the regional oceanography and morphology and tectonism of the continental shelf will dictate that, relative to present sea level, this threshold eustatic sea level will vary in depth, and hence age, along a continental margin. Data from the New Zealand region reveal an extremely steep meridional thermal gradient across the southern and central New Zealand region during the last glaciation with minor cooling of STW to the north, apart from localized nearshore upwelling zones, but pronounced cooling of subantarctic water to the south of the subtropical convergence zone.

Evolution of the instability of a mixed‐layer front

Observations from the Frontal Air‐Sea Interaction Experiment (FASINEX), indicating the presence of small‐scale cold‐core features in the North Atlantic Subtropical Convergence Zone, motivated a recent linear analysis of the instability of a geostrophically balanced mixed‐layer front. The results of that analysis suggested that the instability would preferentially form small‐scale cold‐core eddies at finite amplitude. In the present study, the finite‐amplitude evolution of the fastest growing mode of this nongeostrophic baroclinic instability is investigated numerically. The linear prediction of cold‐core eddy formation is confirmed by the nonlinear calculation. There are large horizontal and vertical heat and potential vorticity fluxes associated with the developing disturbance. The heat flux is confined above the thermocline, in the region of sloping frontal isotherms that provide the energy source for the instability, but the potential vorticity fluxes are maximum 50–75 m deeper and reach into the thermocline. A tongue of low‐potential‐vorticity fluid is advected 50–75 m downward along isopycnal surfaces from the cold side of the front into the thermocline at the mixed‐layer base. The small‐scale potential vorticity structure has similarities to estimates of the upper ocean potential vorticity field obtained previously from FASINEX observations. The calculations illustrate the role that frontal instabilities may play in the flux of heat and potential vorticity from the mixed layer into the thermocline. The evolution of the disturbance resembles baroclinic wave life cycles obtained in atmospheric models.

Inversion for heat anomaly transport from sea surface temperature time series in the northwest Pacific

We describe a heat anomaly transport in the upper ocean mixed layer in the Kuroshio extension region and the subtropical gyre of the northwest Pacific. Emphasis is on behavior in the cool season (December‐March) during the Asian Winter Monsoon. The heat anomaly transport is estimated by applying an inversion technique to the stochastic partial differential equation for the heat anomaly balance of advection, diffusion, stabilizing feedback, and atmospheric forcing. The inversion consists of (1) derivation of statistical parametric model from the heat anomaly balance equation; (2) fitting the derived statistical model to the sea surface temperature (SST) anomaly covariances; and (3) calculation of the heat anomaly net advection velocity, horizontal diffusion coefficient, feedback factor and atmospheric forcing correlation from the parameters of the evaluated statistical model. The inversion was applied to the Comprehensive Ocean‐Atmosphere Data Set Compressed Marine Reports SST data averaged at 1° latitude × 2° longitude boxes on a 10‐day mean basis from 1965 to 1990. The estimates of the net advection velocity are consistent in magnitude and direction with the general circulation in the surface layer of the Northwest Pacific in winter. SST anomalies are transported to the west at ∼0.15 m s−1 in the northern part of the North Equatorial Current. Between 21° and 29°N in the recirculating region, SST anomalies propagate westward with the mean velocity less than 0.1 m s−1. South and east of Honshu the observed pattern of the SST anomaly transport agrees broadly with the circulations of the Kuroshio current and its extension and the Oyashio current. South of Honshu, the eastward transport is about 200–300 km wide; its absolute velocity is up to 0.2 m s−1. One branch of the transport separates from the coast near the large meander path of the Kuroshio current and follows the east‐southeast direction. The second separation from the coast occurs south of Hokkaido. Over the analysis domain the estimates of the diffusion coefficient are in the range of 3×103 to 6×103 m2 s−1. The higher values of the diffusion coefficient confirm the enhancement of the mesoscale eddy processes near the subtropical convergence zone. The analysis supports Hasselmann's (1976) theory in which generation of midlatitude SST anomalies lasting the dominant timescale of atmospheric processes is primarily attributed to the short period stochastic weather forcing. However, the analysis indicates that the inertia of SST anomalies to their “memory” of earlier winds can not be neglected in the vicinity of the western boundary and in the tropics.

Frontogenesis, subduction, and cross‐front exchange at upper ocean fronts

The subduction of water at upper ocean fronts driven by internal instabilities is investigated. A simple analytic model which assumes conservation of potential density and potential vorticity is used to characterize the expected structure of subducted parcels as a function of the surface layer depth and the length scale of the parcel. Parcels subducted from a deep surface layer will be characterized by anomalously low potential vorticity and anticyclonic circulation, while shallow surface layers result in subducted parcels with high potential vorticity and cyclonic circulation. A nonlinear isopycnal primitive equation model is used to demonstrate how baroclinic instability and the resulting frontogenesis force the subduction of parcels below and across the front. Frontogenesis generates vertical velocities of 𝒪(30 m d−1) and ageostrophic cross front flows of 3 to 5 cm s−1. The subduction is achieved by the deep ageostrophic flow which carries water from the surface layer below and across the front. The subducted parcels quickly develop a circulation through balanced adjustment, anticyclonic for deep surface layers and cyclonic for shallow surface layers. The characteristics of parcels subducted in winter conditions closely resemble a class of eddies called submesoscale coherent vortices. The horizontal and vertical scales of the vortices are determined by the frontogenetic mechanism and the depth of the surface layer. The total permanent subduction rate is estimated to be 20 m yr−1 for frontal subduction driven by frontogenesis, although local (and temporary) subduction can be much larger. The subducted anticyclonic vortices are often coupled to upper ocean cyclonic vortices, thus forming a baroclinic dipole that propagates at approximately 3.5 cm s−1 and is capable of transporting the anomalous water properties far from the region of formation. Similarities with observations in the North Atlantic Subtropical Convergence Zone and near the Gulf Stream are discussed.

Species assemblages of leptocephali in the Sargasso Sea and Florida Current

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 121:11-26 (1995) - doi:10.3354/meps121011 Species assemblages of leptocephali in the Sargasso Sea and Florida Current Miller MJ Regional assemblages of leptocephali of 5 families of shelf eels (Chlopsidae, Congridae, Moringuidae, Muraenidae and Ophichthidae) from the Sargasso Sea and Florida Current were compared with hydrographic features and adult distributions. There were 2 major patterns in the distributions of the &GT37 species of leptocephali that were collected. First, more species and greater abundances were found at or south of fronts in the Subtropical Convergence Zone (STCZ) of the Sargasso Sea, with the most diverse assemblages at stations closest to fronts in the west. Second, the smallest leptocephali of all species were located close to the Bahama Banks, in the Florida Current and in stations close to southerly fronts in the western STCZ. The most distinct discontinuities in numbers of species occurred at fronts at the boundary between southern Sargasso Sea surface water and mixed convergence zone water. Impoverished assemblages were found north of these fronts and in the eastern STCZ. Species richness was highest in the Florida Current and at the westernmost frontal station in the STCZ. Anticyclonic circulation northeast of the northern Bahamas may facilitate entrainment of leptocephali from the Bahamas and Florida Current into fronts in the STCZ, which appear to transport leptocephali eastward. The circulation patterns of the region are hypothesized to influence both the regional assemblage structure and the diversity of life history strategies that may be used by eels inhabiting the region. Leptocephali . Anguilliformes . Distribution . Species assemblages . Sargasso Sea . Florida Current . Fronts . Bahamas . Life history . Spawning . Larval transport Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 121. Publication date: May 25, 1995 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1995 Inter-Research.

Numerical simulation of a response of an oceanic front to an atmospheric frontal passage

The influence of middle‐latitude cyclones and their associated cold fronts on open‐ocean fronts is studied with an embedded mixed layer ocean circulation model similar to that of Adamec et al. (1981). Finite differences with horizontal resolution of 5 km in a staggered grid are used to solve the model equations within a domain of total meridional extension of 300 km and 500 m depth. Three numerical experiments were performed to investigate the response of an oceanic front, similar to the South Atlantic subtropical convergence zone, to an atmospheric frontal passage. Under the effect of the cold air outbreak the thermal and dynamic structures of the upper layers are substantially modified with a higher sinking/cooling rate of the mixed layer. The sinking/cooling rate depends strongly on the magnitude of the thermal gradient at the top of the thermocline and on the mixed layer depth. Due to the differences in sinking/cooling rates, a single storm can move the oceanic front up to 20 km from its initial position. For low‐intensity winds, a change in the wind direction does not affect the thermal structure of the upper layer ocean, even in the frontal region. The mixed layer oscillations are entirely a function of the daily scale of the solar heating. For a sudden change in the wind direction, the response of the current velocity field in the mixed layer is almost instantaneous.

Species assemblages of leptocephali in the Subtropical Convergence Zone of the Sargasso Sea

Species assemblages of leptocephali are described in relation to density in the Subtropical Convergence Zone (STCZ) of the Sargasso Sea. Seven transects across fronts were made during four cruises in the late winter or early spring of 1983, 1985 and 1989. About 50 species from 13 families of eels were collected at 66 stations, but fewer than 10 species were abundant in all transects. Four oceanic species, two Anguilla species and Conger oceanicus appeared to be spawning in the STCZ. Leptocephali of most species whose adults inhabit the continental shelf were consistently large in size and were more abundant at or south of fronts and in the western transects. Leptocephali of the two most common oceanic species, Nemichthys scolopaceus and Serrivomer beanii, and the most common shelf species, Ariosoma baleaticum, were also more abundant in the western transects, but were abundant at some stations on both sides of fronts. Discontinuities in the assemblages of Anguilla and most shelf species occurred at the location of fronts that formed at the northernmost extent of southern Sargasso Sea surface water (defined as sigma-t < 25.6 kg m-j). These species were rare or absent in mixed convergence zone water (defined as sigma-t 25.2-25.6) north of the fronts. Cluster analysis and ordination of assemblages at 31 night stations reflected the greater species richness and abundance in the west and in the southern water mass. Patterns of assemblage structure within and among transects suggest that convergence of surface water toward fronts in the STCZ may concentrate leptocephali close to fronts and that frontal jets may transport leptocephali eastward.

Interannual scales of variation of pigment concentrations from coastal zone color scanner data in the Benguela Upwelling system and the Subtropical Convergence zone south of Africa

South African oceanographers were engaged in collecting hydrographic and biological sea truth data in order to calibrate the coastal zone color scanner (CZCS) measurements from the Nimbus 7 satellite over the Benguela Upwelling region near Cape Town, South Africa, during the period 1978 to 1981 (Shannon, 1985). We give a brief overview of this research and then concentrate on a more recent analysis of level III CZCS data obtained from NASA for the region 10–60°S, 10–100°E. This area includes the Benguela Upwelling system on the continental shelf and the southern ocean with the Subtropical Convergence zone south of Africa. High annual concentrations of chlorophyll (5 mg m−3) typical of other upwelling systems in the world ocean occurred in the Benguela shelf region, and the data show a strong interannual signal in the 7 years of composited data from 1978 to 1985. Two distinct regimes were found in the Benguela Upwelling system, the seasonal variations of pigment concentration in the northern and southern Benguete regions being out of phase. In the southern ocean, levels of chlorophyll were generally low (0.15 mg m−3), with the strongest signal (1.5 mg m−3) found at the southern border of the Agulhas retroflection region and its frontal boundary with the colder Subantarctic water to the south. The high levels of chlorophyll found in this region are 10 times those of the typical open southern ocean. There is a very clear interannual signal in the CZCS data for this Subtropical Convergence region, which has a low value in 1979, rises to a maximum in 1981, and then decreases to another low value in 1985. There appears to be no clear seasonal variation in the Subtropical Convergence data. Reasons for the strong signal in the surface chlorophyll concentrations at the front between the Agulhas Return Current and the southern ocean are discussed, and it is shown that the Agulhas Plateau sets up a topographic Rossby wave in the Agulhas Return Current, which can be clearly identified in the CZCS signal.