Brazil-Malvinas Confluence Zone Research Articles

A novel filtering method for geodetically determined ocean surface currents using deep learning

Abstract Determining an accurate picture of ocean currents is an important societal challenge for oceanographers, aiding our understanding of the vital role currents play in regulating Earth’s climate, and in the dispersal of marine species and pollutants, including microplastics. The geodetic approach, which combines satellite observations of sea level and Earth’s gravity, offers the only means to estimate the dominant geostrophic component of these currents globally. Unfortunately, however, geodetically-determined geostrophic currents suffer from high levels of contamination in the form of geodetic noise. Conventional approaches use isotropic spatial filters to improve the signal-to-noise ratio, though this results in high levels of attenuation. Hence, the use of deep learning to improve the geodetic determination of the ocean currents is investigated. Supervised machine learning typically requires clean targets from which to learn. However, such targets do not exist in this case. Therefore, a training dataset is generated by substituting clean targets with naturally smooth climate model data and generative machine learning networks are employed to replicate geodetic noise, providing noisy input and clean target pairs. Prior knowledge of the geodetic noise is exploited to develop a more realistic training dataset. A convolutional denoising autoencoder (CDAE) is then trained on these pairs. The trained CDAE model is then applied to unseen real geodetic ocean currents. It is demonstrated that our method outperforms conventional isotropic filtering in a case study of four key regions: the Gulf Stream, the Kuroshio Current, the Agulhas Current, and the Brazil-Malvinas Confluence Zone.

Evaluation of the ABI/GOES-16 SST Product in the Tropical and Southwestern Atlantic Ocean

Sea surface temperature (SST) is an essential climate variable used for ocean and weather monitoring and forecasting. The NOAA’s next generation geostationary satellite GOES-16 was declared operational at the east position (75°W) in December 2017, carrying onboard an Advanced Baseline Imager (ABI). The hyperspectral ABI sensor now allows SST estimates every 10–15 min at both day and nighttime, with advanced options for cloud screening and water vapor correction. In the present work, we compare the first operational ABI SST product (OSI SAF, 2018) with an in situ match-up database (MDB) across the Tropical and Southwestern Atlantic Ocean, off the Brazilian coast, throughout the year of 2018. The MDB was obtained from two long-term programs, i.e., PIRATA moored buoys (FOLTZ et al., 2016) and PNBoia moored and drifting buoys (MARINHA DO BRASIL, 2017). Separate comparisons were made for each data set, analyzing the uncertainties according to the program (i.e., buoy type and region), satellite SST quality level and influence of diurnal heating. We also compare the ABI product with the OSTIA analysis L4 SST (DONLON et al., 2012) to increment our analyses on the spatio-temporal biases within the study region. The results show that the OSI SAF ABI SST L3C has a mean bias (0.1 °C) and error (RMSE, 0.5 °C) within the GHRSST standards, with an exception being coastal waters off the southeast Brazilian coast (RMSE, 0.65 °C), which are subjected to sharp thermal fronts. The highest biases are for regions/seasons subjected to persistent cloud coverage and high water-vapor content, i.e., the Intertropical and South Atlantic Convergence Zones, as well as highly dynamic frontal zones, i.e., the Brazil Malvinas Confluence Zone, the Subtropical Front and coastal waters. The ABI SST product is suitable for operational use, and applications should explore more deeply the new set of information provided.

Narrowband shortwave minima of multispectral reflectance as indication of algal blooms associated with the mesoscale variability in the Brazil-Malvinas Confluence

Summary We examine the narrowband shortwave minima (NSM) of multispectral reflectance as indication of mesoscale algal blooms. They are frequent in the Brazil-Malvinas confluence zone (BMCZ) where our testing site (TS) belongs. Its MODIS A images of December 2008 and 2014 were the source of initial data. Classification of reflectance spectra in these images revealed that the TS look from space was determined by the most populated cluster of pixels having the only NSM at 443 nm. We divided this cluster into sub-clusters by maximum wavelengths λmax from 412 to 555 nm and retrieved the estimates of λmax (proxy for abundance of colored dissolved organic matter (CDOM)), chl_a (MODIS chlorophyll), Rrs (555) (turbidity proxy), and CALH (NSM-based chlorophyll) on a pixel-by-pixel basis. This allowed us to demonstrate: (1) the NSM magnitude at 443 nm peaked in mesoscale structures, (2) CALH was consistent with chlorophyll in the BMCZ waters samples, (3) positive linear correlation of Rrs (555) and CALH was characteristic of the TS waters at any λmax, (4) the MODIS chl_a was overestimated when λmax > 488 nm, (5) localization and outlines of mesoscale structures agreed well in the fields of pairs Rrs(555) – CALH and λmax – chl_a, but not in the CALH – chl_a pair. The NSM-based chlorophyll CALH outperformed the standard chl_a determinations in exactness because the CALH is insensitive to CDOM. This is advantageous when studying the Case 1 waters of intensive mesoscale variability where chlorophyll co-exists with the CDOM from eddy-induced blooms.

On the influence of Subtropical Mode Water on the South Atlantic Ocean

Abstract We investigate the spatiotemporal variability of the source water masses (i.e., varieties of Subtropical Mode Water – STMW) that contribute to the South Atlantic Central Water (SACW) in the South Atlantic Ocean. Thus, the composition of the SACW layer is updated. For this investigation, we applied an optimum multiparameter (OMP) analysis and used the conservative and semi-conservative parameters available from the World Ocean Database and Argo floats for the South Atlantic Ocean. The STMW18 (at upper levels) sourced in the central and eastern regions of the South Atlantic and the STMW12 (at lower levels) sourced at the boundaries of the South Atlantic Subtropical Front are the main contributors to the SACW. Although also important, the contribution of STMW14 (sourced in the Brazil-Malvinas Confluence zone) is regionally confined by the Brazil Current recirculation gyre. The contributions from Subtropical Indian Mode Water (SIMW) increased westward along the Agulhas Corridor, while the contribution from STMW12 decreased. The relatively low contribution from SIMW matches the results of previous studies regarding the influence of these waters in the climatology of the South Atlantic Ocean. However, it cannot be ignored, since the results bring new light to further investigations of the mixing processes in the ocean interior of the South Atlantic Ocean.

Spatio-temporal variability in the Brazil-Malvinas Confluence Zone (BMCZ), based on spectroradiometric MODIS-AQUA chlorophyll-a observations

Summary The Brazil–Malvinas Confluence Zone (BMCZ) is characterized by high environmental variation, which could be reflected in several optical types of water, from one containing only phytoplankton and sea water to other optically more complex. In this paper, we analyze the spatio-temporal variability of the Chlorophyll- a detected by the ocean color sensor (CHLA sat ) in BMCZ in order to understand its environmental variability. We use the MODIS-Aqua CHLA sat monthly composites imagery from 2002 to 2015, and applied two statistical methods: the correlogram-based robust periodogram to identify, over a broad spectrum of temporal, the most significant periodicities, and the pixel gradient distribution to study the spatial-temporal gradients within the BMCZ and variations over the continental shelf and coastal waters. Our results point out to the predominance of the annual cycle over most of the investigated area, although an area from latitude 37°S in direction NE, alongshore of Uruguay to Brazil, evidences interannual periodicities, possibly related to variations in the discharge of the Rio de la Plata associated with the El Nino phenomena. The ocean color spectroradiometric signature in terms of pixel gradient presents a relatively high variability (∼0.0 to 0.65 mg m −3 ); in particular the high values of the pixel gradient correspond to saline front of the estuarine system of Rio de la Plata, and to the strip of the platform that extends along the isobaths of 80 m (coast of Uruguay), especially in the center and south of the study area.

Fisher–Shannon analysis of the time variability of remotely sensed sea surface temperature at the Brazil–Malvinas Confluence

Summary The collision of the warm and salty southward flowing Brazil Current and the cold and relatively fresh northward flowing Malvinas Current produces a strong frontal zone known as the Brazil–Malvinas Confluence Zone (BMCZ). This is featured by intense presence of eddies and meanders and is one of the most energetic areas of the world oceans. We apply the statistical method of Fisher–Shannon (FS) to the time series of sea surface temperature, derived from the satellite Advanced Very High Resolution Radiometer (AVHRR) imagery, acquired from 1984 to 1999. The FS method consists of the joint application of Fisher information measure (FIM) and Shannon entropy (SE), measuring respectively the degree of organization and the disorder of a system. Our findings indicate that the FS method is able to locate very clearly the BMCZ, which corresponds to the less organized and more disordered area within the area of confluence between the Brazil and Malvinas Currents.

Latitudinal diversity patterns of diatoms in the Southwestern Atlantic and Antarctic waters

Latitudinal changes in the species richness of planktonic diatoms in Southwestern Atlantic and Antarctic waters (29 degrees-76 degrees S) were analyzed from local to regional scale in three spring and one summer seasons. A total of 220 diatom species (16-87 per station) were recorded. Despite the noticeable interannual variability, species richness showed a maximum in the northern sector of the Brazil-Malvinas Confluence Zone and a highly significant decrease with latitude.

Analysis of marine mammal bycatch in the Uruguayan pelagic longline fishery operating in the Southwestern Atlantic Ocean

Abstract Bycatch is one of the main causes of human-caused mortality and population decline of many marine mammals. Monitoring bycatch is the first step to understand the impact of the fisheries on the species affected. Understanding how the interaction between marine mammals and fishing operations varies in space and time, and how it is influenced by environmental variables, is essential for designing mitigation strategies to reduce bycatch mortality. In this paper, we use data gathered by scientific observers and a fishing skipper to analyse marine mammals bycatch by the Uruguayan pelagic longline fishery operating in the Southwestern Atlantic Ocean from 1996 to 2007. The total bycatch per unit effort (Bcpue) was 0.0150 marine mammals/1000 hooks and the highest values (∼0.2) were recorded between 37°–38°S and 49°–51°W. Total cetacean Bcpue during the study period was low (0.0051 cetacean/1000 hooks) and occurred between 32°–37°S and 46°–54°W. Generalized additive models showed that cetaceans' bycatch was mainly affected by the depth, sea surface temperature, and season. Although cetaceans were captured year-round, the highest values were registered in spring months, most bycatch events occurred over the continental slope (median = 619 m) and in waters with a median temperature of 19.7°C. The bycatch of pinnipeds was influenced by depth, location, and season. Pinniped bycatch occurred mainly in winter, in waters ranging from 80 to 5000 m of depth (median = 2366 m) between 34°–37°S (median = 35.9°S) and 54°–49°W (median = 51.8°W). The spatial analysis showed that most bycatch events occurred within the Brazil–Malvinas Confluence zone, an area of high productivity where the pelagic longline fleet concentrates its fishing effort and where marine mammals probably concentrate to feed.

New insights in Southwestern Atlantic Ocean Oegopsid squid distribution based on juvenile green turtle (Chelonia mydas) diet analysis

A total of 52 cephalopod beaks were found in the stomachs and intestines of 17 out of 54 green turtles, Chelonia mydas, stranded on the Uruguayan coast between 2009 and 2013 (frequency of occurrence = 31.5 %). Upper and lower beaks were assigned to at least six Oegopsid species of four different genera, Chiroteuthis, Histioteuthis, Onykia (= Moroteuthis) and Mastigoteuthis. Although the presence of cephalopods in the diet of green turtles has been reported previously, it has been quoted as a sporadic or less important diet category. Our findings suggest that this intake of cephalopods by juvenile green turtles during their oceanic stages may be more common in the Southwestern Atlantic Ocean waters than previously thought. According to our records, two of the six Oegopsid squid species found would have a more extended distribution than previously reported, ranging from circumpolar sub-Antarctic areas to the Brazil-Malvinas confluence zone. We highlight the potential of diet analysis of pelagic predators as a tool to enhance our knowledge of the diversity and distribution of poorly known cephalopod species.

Hydrodynamic and geomorphological controls on surface sedimentation at the Subtropical Shelf Front / Brazil–Malvinas Confluence transition off Uruguay (Southwestern Atlantic Continental Margin)

Abstract The hydrology of the Southwestern Atlantic margin is dominated by the confluence of water masses with contrasting thermohaline properties, generating a frontal zone that extends from the Brazil-Malvinas Confluence zone (BMC) in the open ocean to the Subtropical Shelf Front (STSF) on the continental shelf. However, the hydrodynamics of the transition between these hydrographic features is still not fully understood. High-resolution morphological (multibeam) and hydrological (CTD) data were integrated with sedimentological data (textural and productivity proxies) in order to develop a sound framework to understand surface sedimentation of the slope extension of the STSF (subsurface, outer shelf and upper slope) and of the BMC (intermediate level, middle slope) on the Uruguayan continental margin (34° to 36°S). Since the detailed morphology of the study area is presented for the first time, related geological processes are briefly discussed. On the outer shelf and upper slope, and north of the STSF, the current direction of erosive mound scarps indicates the dominance of the southward-flowing Brazil Current (BC). This suggests a northernmost boundary for the distribution of the STSF. A strong flux of the BC favored by a steeper slope as well as by the occurrence of canyons incised on the upper slope is evidenced by the occurrence of coarser sediments and low values of productivity proxies, but also by the presence of deep sea coral reefs. Southwards, the effect of a less energetic Malvinas Current (MC) and the highly productive STSF is indicated by the deposition of fine sediments with high organic matter content, as well as by the absence of deep sea coral reefs. This depositional scenario is enhanced by a smooth slope and the occurrence of canyons incised deeper in the middle slope. Off-shelf sediment transport along the STSF is inferred by the similar texture registered between the outer shelf and shallow upper slope, the occurrence of biogenic shelly reworked sands and gravel, and by the observed decrease in grain size with depth. Glacial iceberg transport northwards and/or gravity processes are suggested by the occurrence of igneous/metamorphic gravel in lag deposits on the upper slope. On the middle slope, the northernmost influence of the erosive Antarctic Intermediate Water is evidenced by the vanishing of morphologic contouritic structures. This is also imprinted in a pronounced northward fining in grain size. This contribution increase our understanding of this highly dynamic and complex area, providing the first detailed analysis of the regional sediment patterns and oceanographic and morphological controls on surface sedimentation.

Shifts of the Subtropical Shelf Front controlled by atmospheric variations

In the western South Atlantic, off the coast of South America, a band of cold, fresh, nutrient‐rich Sub‐Antarctic Shelf Water (SASW) meets warm, salty, nutrient‐poor Subtropical Shelf Water (STSW) to form the Subtropical Shelf Front (STSF). This front is the shallow‐water expression of the major Brazil‐Malvinas Confluence Zone and has moved northward and southward during the Holocene (the past ~12,000 years). Bender et al. reconstruct the latitudinal shifts of the STSF over the past 11,000 calendar‐equivalent years using records of oxygen and carbon stable isotope compositions of benthic foraminifera and total organic carbon and calcium carbonate content from a sediment record collected off Uruguay. These measurements serve as proxies for ocean water temperature and nutrient content, which can be used to distinguish the SASW and STSW.

Influence of environmental and longline fishing operational variables on the presence of killer whales (Orcinus orca) in south-western Atlantic

Killer whale (Orcinus orca) is frequently encountered in coastal and high productive pelagic waters, near the shelf break. In the south-western Atlantic Ocean, spatial and temporal occurrence patterns are poorly known. However, the monitoring of the interaction between killer whales and longline fishery suggests that the species is frequent in this region. We analysed the killer whale presence within the Uruguayan pelagic longline fishing zone. Data were collected from 1996 to 2007, during 2189 fishing events, by vessel skippers and on-board observers. We estimated the sighting rate (SR = sightings days/fishing days * 100) for different time scales and in 1 × 1 degree grids. Generalized linear models were used to evaluate the effect of spatial, temporal, environmental and operational variables on the species presence. Killer whales were sighted in 100 fishing days (SR = 4.5%), this occurrence being explained by distance from shore and sea surface temperature, varying among months and fishing boats. Although sightings occurred year round, they were more frequent in autumn and winter, at 150–400 nautical miles (nm) from shore (mean = 250 nm) and in waters with temperatures ranging from 19 to 24°C (mean = 22°C). Sets took place between 19°–40°S and 21°–54°W, while killer whales occurred mostly from 34°–37°S and 48°–53°W. In this region, the high productive Brazil—Malvinas Confluence Zone is located, and concentrates fishing effort and also killer whales.

Comparison between three implementations of automatic identification algorithms for the quantification and characterization of mesoscale eddies in the South Atlantic Ocean

Abstract. Three methods for automatic detection of mesoscale coherent structures are applied to Sea Level Anomaly (SLA) fields in the South Atlantic. The first method is based on the wavelet packet decomposition of the SLA data, the second on the estimation of the Okubo-Weiss parameter and the third on a geometric criterion using the winding-angle approach. The results provide a comprehensive picture of the mesoscale eddies over the South Atlantic Ocean, emphasizing their main characteristics: amplitude, diameter, duration and propagation velocity. Five areas of particular eddy dynamics were selected: the Brazil Current, the Agulhas eddies propagation corridor, the Agulhas Current retroflexion, the Brazil-Malvinas confluence zone and the northern branch of the Antarctic Circumpolar Current (ACC). For these areas, mean propagation velocities and amplitudes were calculated. Two regions with long duration eddies were observed, corresponding to the propagation of Agulhas and ACC eddies. Through the comparison between the identification methods, their main advantages and shortcomings were detailed. The geometric criterion presents the best performance, mainly in terms of number of detections, duration of the eddies and propagation velocities. The results are particularly good for the Agulhas Rings, which have the longest lifetimes of all South Atlantic eddies.

Middle Pleistocene sea surface temperature in the Brazil-Malvinas Confluence Zone: Paleoceanographic implications based on planktonic foraminifera

We reconstructed Middle Pleistocene surface hydrography in the western South Atlantic based on planktonic foraminiferal assemblages, modern analog technique and Globorotalia truncatulinoides isotopic ratios of core SP1251 (38o29.7´S / 53o40.7´W / 3400 m water depth). Biostratigraphic analysis suggests that sediments were deposited between 0.3 and 0.12 Ma and therefore correlate to Marine Isotopic Stage 6 or 8. Faunal assemblage-based winter and summer SST estimates suggest that the western South Atlantic at 38oS was 4-6oC colder than at present, within the expected range for a glacial interval. High relative abundances of subantarctic species, particularly the dominance of Neogloboquadrina pachyderma (left), support lower than present SSTs throughout the recorded period. The oxygen isotopic composition of G. truncatulinoides suggests a northward shift of the Brazil-Malvinas Confluence Zone and of the associated mid-latitude frontal system during this Middle Pleistocene cold period, and a stronger than present influence of superficial subantarctic waters and lowering in SSTs at our core site during the recorded Middle Pleistocene glacial.

Seasonal trends in phytoplankton biomass over the Uruguayan Shelf

Vertical and horizontal distributions of chlorophyll were analyzed during six cruises (three in autumn and three in spring), from 1991 to 1998, over the Uruguayan shelf. Differences in biomass were found between and within seasons, displaying great variability. Phytoplankton biomass was high even during the spring (max: 19.25 mg m - 3 ; mean ± SD: 2.82 ± mg m - 3 ); autumn values were also relatively high (max: 7.62 mg m - 3 ; mean ± SD: 1.69 ± 1.45 mg m - 3 ) compared with other frontal regions. Maximum values were associated with Coastal Water (CW), Subtropical Water (STW) and the result of them mixing together in autumn. Maximum values were also observed in CW, Subantartic Water (SAW) and their mixing together during the spring. These features highlight the role of CW in fertilization of oligotrophic water masses (i.e. tropical water) and its action in adding stability to unstable SAW, thereby enhancing productivity. The highest values in both seasons were recorded in 1995 (ENSO cold year), emphasizing the high influence of SAW and defining it as a major nutrient source. Two well-defined zones with high chlorophyll concentration were observed in both seasons; one in the southern extreme of the study area and the other in the northern coastal region. Both zones were associated with frontal zones. This shelf convergence system is similar to the Brazil–Malvinas Confluence Zone, but with the CW as another important component, responsible for the surface density fronts and the main source of silicates.

Circumpolar Deep Water Circulation and Variability in a Coupled Climate Model

Abstract The natural variability of Circumpolar Deep Water (CDW) is analyzed using a long-term integration of a coupled climate model. The variability is decomposed using a standard EOF analysis into three separate modes accounting for 68% and 82% of the total variance in the upper and lower CDW layers, respectively. The first mode exhibits an interbasin-scale variability on multicentennial time scales, originating in the North Atlantic and flowing southward into the Southern Ocean via North Atlantic Deep Water (NADW). Salinity dipole anomalies appear to propagate around the Atlantic meridional overturning circulation on these time scales with the strengthening and weakening of NADW formation. The anomaly propagates northward from the midlatitude subsurface of the South Atlantic and sinks in the North Atlantic before flowing southward along the CDW isopycnal layers. This suggests an interhemispheric connection in the generation of the first CDW variability mode. The second mode shows a localized θ−S variability in the Brazil–Malvinas confluence zone on multidecadal to centennial time scales. Heat and salt budget analyses reveal that this variability is controlled by meridional advection driven by fluctuations in the strength of the Deep Western Boundary and the Malvinas Currents. The third mode suggests an Antarctic Intermediate Water source in the South Pacific contributing to variability in upper CDW. It is further found that NADW formation is mainly buoyancy driven on the time scales resolved, with only a weak connection with Southern Hemisphere winds. On the other hand, Southern Hemisphere winds have a more direct influence on the rate of NADW outflow into the Southern Ocean. The model’s spatial pattern of θ−S variability is consistent with the limited observational record in the Southern Hemisphere. However, some observations of decadal CDW θ−S changes are beyond that seen in the model in its unperturbed state.

Dominant Modes of Variability in the South Atlantic: A Study with a Hierarchy of Ocean–Atmosphere Models

Abstract Using an atmosphere model of intermediate complexity and a hierarchy of ocean models, the dominant modes of interannual and decadal variability in the South Atlantic Ocean are studied. The atmosphere Simplified Parameterizations Primitive Equation Dynamics (SPEEDY) model has T30L7 resolution. The physical package consists of a set of simplified physical parameterization schemes, based on the same principles adopted in the schemes of state-of-the-art AGCMs. It is at least an order of magnitude faster, whereas the quality of the simulated climate compares well with those models. The hierarchy of ocean models consists of simple mixed layer models with an increasing number of physical processes involved such as Ekman transport, wind-induced mixing, and wind-driven barotropic transport. Finally, the atmosphere model is coupled to a regional version of the Miami Isopycnal Coordinate Ocean Model (MICOM) covering the South Atlantic with a horizontal resolution of 1° and 16 vertical layers. The coupled modes of mean sea level pressure and sea surface temperature simulated by SPEEDY–MICOM strongly resemble the modes as analyzed from the NCEP–NCAR reanalysis, indicating that this model configuration possesses the required physical mechanisms for generating these modes of variability. Using the ocean model hierarchy the authors were able to show that turbulent heat fluxes, Ekman transport, and wind-induced mixing contribute to the generation of the dominant modes of coupled SST variability. The different roles of these terms in generating these modes are analyzed. Variations in the wind-driven barotropic transport mainly seem to affect the SST variability in the Brazil–Malvinas confluence zone. The spectra of the mixed layer models appeared to be too red in comparison with the fully coupled SPEEDY–MICOM model due to the too strong coupling between SST and surface air temperatures (SATs), resulting from the inability to advect and subduct SST anomalies by the mixed layer models. In SPEEDY–MICOM anomalies in the southeastern corner of the South Atlantic are subducted and advected toward the north Brazilian coast on a time scale of about 6 yr.

Tintinnid diversity trends in the southwestern Atlantic Ocean (29 to 60°S)

A latitudinal profile of tintinnid specific richness and diversity in the southwestern Atlantic Ocean during spring was evaluated in relation to environmental parameters obtained during 4 oceanographic cruises (TABIA series). The area surveyed included different biogeographic zones and comprised waters of the Brazil-Malvinas Confluence zone, the Argentine shelf-slope, the Malvinas Current and the Antarctic Circumpolar Current. Parameters of tintinnid diversity decreased as latitude of the oceanic cruises increased (TABIA I to III). Latitude explained 53 to 60 and 28 to 60% of specific richness and diversity variation, respectively. Comparisons between cruises indicated that the rate of change in diversity parameters was constant with latitude, but diversity parameters did not change with latitude during TABIA V cruise over the shelf-break front. To explain this difference, environmental (physical and biological) factors that could affect and determine diversity parameters in the Confluence and Subantarctic Zones were analyzed. The conclusion drawn is that the determination and maintenance of tintinnid diversity were mainly caused by various biological factors (available food, predators, 'coocurrence', resilience of dominant tintinnid species) and by particular physical and hydrological characteristics of each biogeographic zone (ingress and egress of water masses in the Confluence, oscillations in the flow of the Malvinas and Brazil Currents, and the presence of a large spatial and temporal scale event: the shelf-break front).

Formation of Southern Hemisphere Thermocline Waters: Water Mass Conversion and Subduction*

The ventilation of the permanent thermocline of the Southern Hemisphere gyres is quantified using climatological and synoptic observational data. Ventilation is estimated with three independent methods: the kinematic method provides subduction rates from the vertical and horizontal fluxes through the base of the mixed layer, the water age uses in situ age distribution of thermocline waters, and the annual-mean water mass formation through air–sea interaction is calculated. All three independent estimates agree within their error bars, which are admittedly large. The subduction rates are mainly controlled through their vertical and lateral components with only minor transient eddy contributions. The vertical transfer, derived from Ekman pumping, ventilates over most of the areas of the subtropical gyres, while lateral transfer occurs mainly along the Subtropical and Subantarctic Fronts, where it injects mode and intermediate waters. For the permanent thermocline the overall ventilation of the South Atlantic is about 21 Sv (Sv ≡ 106 m3 s−1). Of this, lateral transfer contributes 10 Sv, mainly in the Brazil–Malvinas confluence zone and to the northeast of Drake Passage. The effective vertical transfer at the bottom of the mixed layer is only two-thirds of the Ekman pumping due to strong northward forcing of the mixed layer itself. The Indian Ocean is ventilated at a rate of 35 Sv with equal lateral and vertical contributions. The South Pacific's overall ventilation is 44 Sv of which the lateral input contributes little more than half. West of 130°W, the South Pacific is ventilated through Ekman pumping and with only minor lateral transfer. In the east lateral transfer dominates between 10° and 20°S and along the Subantarctic Front in a narrow density range. Combining overall transports with earlier estimates for the Northern Hemisphere gives a ventilation of the World Ocean's permanent thermocline of about 160 Sv. Analysis of atmospheric reanalysis air–sea flux data reveals an overall increase in the formation of thermocline waters for all three Southern Hemisphere oceans.

South Atlantic Multidecadal Variability in the Climate System Model

Abstract Strong multidecadal variability is detected in a 300-yr integration of the NCAR Climate System Model in the South Atlantic region, through the application of two signal recognition techniques: the multitaper method and singular spectrum analysis. Significant oscillations of a 25–30-yr period are found in the sea surface temperature, sea level pressure, and barotropic transport fields. A similar-scale signal is also captured in about one century-long observational records. A composite analysis of several model variables is performed based on the extremes of the sea surface temperature oscillation. The proposed mechanism for this basin-scale multidecadal signal involves changes in the intensity of the westerlies, associated with variability in the southward extension of the subtropical anticyclone, which drives changes in the ocean mass transport. This results in variability in the intensity of the Malvinas western boundary current and in the position of the Brazil–Malvinas confluence zone. Anomalo...