Alberto R. Piola

Subtropical Shelf Front off eastern South America

Historical hydrographic data from the continental shelf off eastern South America are used to examine the thermohaline properties of the water masses in the region between 20°S and 40°S. The continental shelf water masses are originated by dilution of open ocean waters of the western boundary currents of the South Atlantic Ocean. On the basis of temperature-salinity relation, two distinct water masses are identified, namely, the Subantarctic Shelf Water and the Subtropical Shelf Water. Subantarctic Shelf Water originates by dilution of Subantarctic Water, primarily in the southeast Pacific, due to excess precipitation and continental runoff and enters the continental shelf near 55°S. The Subtropical Shelf Water is modified South Atlantic Central Water diluted by continental runoff from the coast of Brazil. In addition, substantial dilution of the upper shelf waters takes place at the mouth of Rio de la Plata (approximately located at 36°S) and, in a lesser extent, at the Patos-Mirim Lagoon (at 32°S). The Rio de la Plata and the Patos outflows form a low-salinity tongue that caps the shelf water leading to a salinity decrease to values <30. The low-salinity tongue extends northward over the shelf penetrating farther north in winter than in summer. The extent of the low-salinity water has a strong impact on the vertical stratification and acts to limit winter convection to the layer above the halocline. There is little or no indication of mixing between Subantarctic Shelf Water and Subtropical Shelf Water. An intense temperature, salinity, and nutrient front separates these water masses. The front is oriented along the north-south direction, located on average near the 50 m isobath at 32°S and extends southward toward the shelf break near 36°S. Between 32° and 34°S the Subtropical Shelf Front follows the 100 to 200 m isobaths and separates Subantarctic Shelf Water from the oceanic South Atlantic Central Water. On the basis of the temperature and salinity distributions, beneath the low-salinity surface layer, the Subtropical Shelf Front appears as an extension of the Brazil-Malvinas Confluence over the continental shelf of South America. Thus the location of the Subtropical Shelf Front may be linked to the migrations of the separation point of the Brazil-Malvinas Confluence from the continental slope.

Intermediate waters in the southwest South Atlantic

Abstract The density (sigma-theta, σ θ ) interval 27.05–27.20 in the Subantarctic Zone of the northern Drake Passage is characterized by two water types with potential temperatures of 3.7 and 4.8°C, respectively, both with salinity of approximately 34.2. These major contributors to the low salinity intermediate water mass are advected northward along the continental slope of South America. The lower density water type enters the Argentine Basin both east and west of Burdwood Bank. Its thermohaline characteristics are modified by winter sea-air interaction near Burdwood Bank and mixing with the surrounding waters further north. The denser water type flows east of Burdwood Bank, undergoing salinity decrease, primarily by isopycnal processes. Low salinity water, derived from the Polar Front, is introduced into a still denser horizon (27.25 σ θ ), from along the axis of the cyclonic circulation feature described by the Malvinas Current and its return to the south. The thermohaline structure across the Malvinas Current is similar to the water mass zonation observed in the northern Drake Passage. In the vicinity of 38°S, the northward-flowing modified subantarctic water converges with subtropical thermocline water at the Brazil-Malvinas Confluence. The less dense subantarctic water spreads under the subtropical thermocline; however, the denser water (27.25 σ θ ) of Polar Front origin is not found under the subtropical thermocline in the western South Atlantic. In general the salinity at the salinity minimum increases rapidly across the Brazil-Malvinas Confluence, suggesting that the bulk of the subantarctic water advected into the region by the Malvinas Current turns towards the interior, spreading under the subtropical thermocline along a broader expanse of the South Atlantic.

A numerical study of the Southwestern Atlantic Shelf circulation : Stratified ocean response to local and offshore forcing

[1] This article discusses the results of a suite of numerical simulations of the oceanic circulation in the Southwestern Atlantic Shelf region that are aimed to characterize its mean circulation and seasonal variability and to determine the dynamical mechanisms controlling them. Our experiments indicate that south of 40S the mean circulation is dominated by a general northeastward flow in the southern portion of the shelf, which is controlled by the discharges from the Magellan Straits, tidal mixing, wind forcing, and the offshore influence of the Malvinas Current farther north. The region from 40 St o 33S presents the highest seasonal variability, with intrusions of cold sub-Antarctic waters

Circumpolar properties of Antarctic intermediate water and Subantarctic Mode Water

Antarctic Intermediate Water and Subantarctic Mode Water are studied by examining their modal density and modal salinity characteristics around Antarctic. The study focuses on 15 regions between 40 and 45°S. Circumpolar distributions for the cores of Antarctic Intermediate and Subantarctic Mode Water and potential temperature-salinity diagrams provide a detailed description of the water masses. The modal salinity and density change gradually across each of the three oceans, presumably due to the alteration of a single water mass. Large changes occur in the transition regions in the Drake Passage, south of Africa, and south of New Zealand. These large changes are due to the presence of distinctly different water masses. Thermohaline finestructure is associated with the water mass transitions. The finestructure intensity between the 27.10 and 27.40 σθ isopycnals is quantified. Although continuous temperature-depth data are not available for the entire region the study indicates that the most intense finestructure occurs in regions of large water mass contrast and decreases by an order of magnitude away from them. Increased vertical temperature gradient variances were observed near a 100-km diameter eddy south of New Zealand.

Interannual variability of the sea surface temperature in the South Brazil Bight

In the austral winter of 1993, during a COROAS hydrographic cruise in the South Brazil Bight (SBB), an unexpected mass of cold and fresh water was observed on the continental shelf near 23°S. Subsequent analyses of different data sets suggested that the origin of that water was probably the Argentine continental shelf, near the Rio de la Plata mouth. In this article, a 13-year time series of AVHRR Sea Surface Temperature anomalies is analyzed to investigate the occurrence of this phenomenon in other periods. The results of these analyses and a good correlation with the Southern Oscillation Index suggest that the penetration of these waters into the SBB occurs in a frequency that may be associated with ENSO events.

Atlantic Climate Variability and Predictability: A CLIVAR Perspective

Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.

Pacific and Indian Ocean Upper-Layer Salinity Budget

Abstract The freshwater balance in the upper layer of the Pacific and Indian Oceans is investigated by means of mass and salinity conservation arguments in simple advective box models. The model uses estimates of atmospheric freshwater input to the ocean and upwelling of deep water into the upper layer at a rate required to balance North Atlantic deep water formation, proportioned by the areas of each ocean. The salinity of the upper layer outflow relative to observed salinity is too low for the Pacific and too high for the Indian Oman. Either the upwelling rates am 5 to 20 times higher than estimated or the freshwater input is grossly exaggerated. The problem is alleviated by taking account of the Pacific-Indian tropical link within the Indonesian Passages of the Southeast Asian Seas. The role of the Pacific-Indian Ocean equatorial connection (through the Southeast Asian Seas) is tested by dividing the Pacific Ocean basin into three zones. Meridional mass transports between zones are estimated from the m...

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

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.

Temporal variability of the meridional overturning circulation at 34.5 S: Results from two pilot boundary arrays in the South Atlantic

Data from two boundary arrays deployed along 34.5°S are combined to produce the first continuous in situ time series observations of the basin-wide meridional overturning circulation (MOC) in the South Atlantic. Daily estimates of the MOC between March 2009 and December 2010 range between 3 Sv and 39 Sv (1 Sv = 106 m3 s−1) after a 10 day low-pass filter is applied. Much of the variability in this ∼20 month record occurs at periods shorter than 100 days. Approximately two-thirds of the MOC variability is due to changes in the geostrophic (baroclinic plus barotropic) volume transport, with the remainder associated with the direct wind-forced Ekman transport. When low-pass filtered to match previously published analyses in the North Atlantic, the observed temporal standard deviation at 34.5°S matches or somewhat exceeds that observed by time series observations at 16°N, 26.5°N, and 41°N. For periods shorter than 20 days the basin-wide MOC variations are most strongly influenced by Ekman flows, while at periods between 20 and 90 days the geostrophic flows tend to exert slightly more control over the total transport variability of the MOC. The geostrophic shear variations are roughly equally controlled by density variations on the western and eastern boundaries at all time scales captured in the record. The observed time-mean MOC vertical structure and temporal variability agree well with the limited independent observations available for confirmation.

Mixing in the Brazil-Malvinas confluence

Conductwity-temperature-depth profile data from the Western Argentine Basin collected from 1984 to 1989 are used to quantify the cross-front heat and salt transfers associated with the vertical finestructure across the Brazil-Malvmas Confluence The fluxes are estimated following the statistical model of JoYcE (Journal of Physical Oceanography, 7,626-629, 1977). The data indicate that the upper ocean cross-front structure of the large-scale temperature and sahnlty fields is constant The medium-scale finestructure mtenslty is quantified by the variance of the vertical temperature and sahmty gra&ents m the 10-100 m wavelength band Due to the abundance of intrusions, the upper layer (0--1000 m) variances increase by a factor of four at &stances <20 km from the front Heat and salt flux estimates associated with medium-scale mixing in the upper ocean are of the order 10-2°C m s -I and 10 -3 m s -1 respectively. These fluxes are an order of magnitude greater than available estimates for other frontal regions. The medium-scale finestructure may therefore play a key role in the &ssipatlon of eddies and intrusive lenses in the region. Heat and salt fluxes between North Atlantic Deep Water and Circumpolar Deep Water are 6.5 x 10 -4 °C m s -1 and 1.8 × 10 -4 m s -i, and agree w~th ex~stmg estimates Extrapolation of upper layer Brazil-Malwnas Confluence cross-frontal fluxes to the Subtropical Convergence across the South Atlantic suggests that the medmm-scale southward heat flux is about 20% of the oceamc northward heat flux at 30°S Similarly, the freshwater flux balances 20% of the excess evaporation north of 30°S.