Carlos Alberto Eiras Garcia

Ocean surface processes on the southern Brazilian shelf: characterization and seasonal variability

Abstract The oceanographic environment in the southern Brazilian shelf was investigated using maritime weather reports from the U.S. National Climatic Center and data from three hydrographic cruises. The circulation in the area shows a clear seasonal pattern in which the interaction between wind forcing, geostrophic circulation and meso-scale variability of the Brazil Current produce southward and offshore flow over the shelf during summer, and northward and onshore flow during winter. The interaction between eddy-induced upwelling and wind-generated transport in summer and winter results in enhanced upwelling and a strong bottom intrusion of SACW on the shelf in summer, whereas in winter the upwelling is restricted to the shelf break. Thus, the combination of the aforementioned processes produces a complex and highly dynamic hydrographic system with a high degree of seasonal variation in both direction of flow and water mass composition.

Shifts in the dominance between diatoms and cryptophytes during three late summers in the Bransfield Strait (Antarctic Peninsula)

Recent global warming reduces surface water salinity around the Antarctic Peninsula as a result of the glacial meltwater runoff, which increases the occurrence and abundance of certain phytoplankton groups, such as cryptophytes. The dominance of this particular group over diatoms affects grazers, such as Antarctic krill, which preferentially feed on diatoms. Using three late summer data sets from the Bransfield Strait (2008–2010), we observed variations in the dominant phytoplankton groups determined by HPLC/CHEMTAX pigment analysis and confirmed by microscopy. Results indicate that the dominance of diatoms, particularly in 2008 and 2009, was associated with a deeper upper mixed layer (UML), higher salinity and warmer sea surface temperature. In contrast, cryptophytes, which were dominant in 2010, were associated with a shallower UML, lower salinity and colder sea surface temperatures. The low diatom biomass observed in the summer of 2010 was associated with high nutrient concentration, particularly silicate, and low chlorophyll a (summer monthly average calculated from satellite images). The interannual variability here observed suggests a delayed seasonal succession cycle of phytoplankton in the summer of 2010 associated with a cold summer and a late ice retreat process in the region. This successional delay resulted in a notable decrease of primary producers’ biomass, which is likely to have impacted regional food web interactions. This study demonstrates the susceptibility of the Antarctic phytoplankton community structure to air temperature, which directly influences the timing of ice melting and consequently the magnitude of primary production and succession pattern of phytoplankton groups.

Empirical and semi‐analytical chlorophyll algorithms in the south‐western Atlantic coastal region (25–40°S and 60–45°W)

Global ocean colour algorithms, used to extract chlorophyll concentration in the ocean surface, normally overestimate pigment values in coastal regions, due to optical interference of water components. The objective of the present investigation was to test the performance of both empirical (SeaWiFS OC4v4) and semi‐analytical (GSM01 and CARDER) algorithms in the south‐western Atlantic. In situ pigment and optical data have been collected in waters influenced by continental discharge from La Plata River and Patos Lagoon. The data was used to develop a regional version of the empirical SeaWiFS OC2v4 algorithm (termed OC2‐LP). The relative percentage difference (RPD) between in situ and algorithm‐derived chlorophyll was 11% in the regional version as compared to the global OC4v4 (RPD = 27%). The GSM01 and CARDER showed RPD of 14% and 31%, respectively. We have also tested the accuracy of the four algorithms (OC4v4, OC2‐LP, GSM01 and CARDER) on SeaWiFS images taken over two cruise periods in the study region (winter of 2003 and summer of 2004). A seasonal difference was observed, where both OC4v4 and OC2‐LP overestimate chlorophyll in summer at a higher magnitude than in winter, and the GSM01 algorithm showed a marked underestimation of chlorophyll in winter. The CARDER model showed a good performance both in winter and summer, when applied to satellite‐retrieved radiances. Our results show that the use of semi‐analytical models does not improve significantly the accuracy of chlorophyll retrievals in coastal areas when not properly tuned with regional inherent optical properties measurements.

Advective Surface Velocities Derived from Sequential Infrared Images in the Southwestern Atlantic Ocean

Abstract Sequential infrared satellite images between November 1995 and August 1996 have been used to derive sea surface advective velocities in the southwestern Atlantic Ocean using the Maximum Cross-Correlation (MCC) technique. The infrared images, geometrically corrected and accurately registered to a map grid, are masked for land and cloud cover. The linear displacements of the sea surface thermal features are determined by locating the maximum cross-correlation in windowed portions of successive images. The advective velocities are estimated for each pair of images based on regular grids. Incoherent vectors are eliminated from these fields using a statistical method. After this, a visual inspection by the operator is completed to remove the remaining fictitious vectors. The region studied includes the southern Brazilian, Uruguayan, and northern Argentinian shelves and the neighboring oceanic area known as the Brazil-Malvinas Confluence. Despite the rather complex circulation of this region, the advective surface currents calculated by the MCC method are consistent with the shapes of the sea surface temperature features. The general patterns of the surface circulation estimated by the MCC agree well with previous studies that used direct measurements, hydrographic data, and/or modeling efforts. In the shallower areas of the study region, the MCC technique is less applicable due to the predominance, in violation of the assumptions of the method, of nonadvective processes.

Large-scale ichthyoplankton and water mass distribution along the South Brazil Shelf.

Ichthyoplankton is an essential component of pelagic ecosystems, and environmental factors play an important role in determining its distribution. We have investigated simultaneous latitudinal and cross-shelf gradients in ichthyoplankton abundance to test the hypothesis that the large-scale distribution of fish larvae in the South Brazil Shelf is associated with water mass composition. Vertical plankton tows were collected between 21°27′ and 34°51′S at 107 stations, in austral late spring and early summer seasons. Samples were taken with a conical-cylindrical plankton net from the depth of chlorophyll maxima to the surface in deep stations, or from 10 m from the bottom to the surface in shallow waters. Salinity and temperature were obtained with a CTD/rosette system, which provided seawater for chlorophyll-a and nutrient concentrations. The influence of water mass on larval fish species was studied using Indicator Species Analysis, whereas environmental effects on the distribution of larval fish species were analyzed by Distance-based Redundancy Analysis. Larval fish species were associated with specific water masses: in the north, Sardinella brasiliensis was found in Shelf Water; whereas in the south, Engraulis anchoita inhabited the Plata Plume Water. At the slope, Tropical Water was characterized by the bristlemouth Cyclothone acclinidens. The concurrent analysis showed the importance of both cross-shelf and latitudinal gradients on the large-scale distribution of larval fish species. Our findings reveal that ichthyoplankton composition and large-scale spatial distribution are determined by water mass composition in both latitudinal and cross-shelf gradients.

On the temporal variability of the Weddell Sea Deep Water masses

Abstract The Weddell Sea is one of the key regions of the Southern Ocean with respect to climate as most of the Antarctic Bottom Water (AABW) that occupies the world ocean deepest layers is likely to originate from this region. This study applies the Optimum Multiparameter water mass analysis to the Weddell deep waters in order to investigate their distribution and variability. The dataset used is based on the WOCE repeat sections in the area (SR04 and A12) from 1984 to 1998. The mean water mass distribution is consistent with previous knowledge of the region, along with high interannual variability. Regarding the temporal variability, it seems that the years of maximum Weddell Sea Deep Water (WSDW) contribution correspond to the lowest levels of Weddell Sea Bottom Water (WSBW), and vice versa. In order to identify possible forcing mechanisms for such variability, the water mass temporal anomalies were compared with oceanic and atmospheric modes of variability in that region such as the Southern Annular Mode (SAM). An apparent correlation between the SAM index temporal gradients and WSBW anomalies indicate that the Weddell Sea export of dense waters to the world ocean may be linked to that index on several time scales.

BoCluSt: Bootstrap Clustering Stability Algorithm for Community Detection.

The identification of modules or communities in sets of related variables is a key step in the analysis and modeling of biological systems. Procedures for this identification are usually designed to allow fast analyses of very large datasets and may produce suboptimal results when these sets are of a small to moderate size. This article introduces BoCluSt, a new, somewhat more computationally intensive, community detection procedure that is based on combining a clustering algorithm with a measure of stability under bootstrap resampling. Both computer simulation and analyses of experimental data showed that BoCluSt can outperform current procedures in the identification of multiple modules in data sets with a moderate number of variables. In addition, the procedure provides users with a null distribution of results to evaluate the support for the existence of community structure in the data. BoCluSt takes individual measures for a set of variables as input, and may be a valuable and robust exploratory tool of network analysis, as it provides 1) an estimation of the best partition of variables into modules, 2) a measure of the support for the existence of modular structures, and 3) an overall description of the whole structure, which may reveal hierarchical modular situations, in which modules are composed of smaller sub-modules.

Potential source regions of biogenic aerosol number concentration apportioning at King George Island, Antarctic Peninsula

Abstract Nowadays it is well accepted that background aerosols in the boundary layer over remote oceans are of marine origin and not aged continental. Particularly in the Atlantic sector of the Southern Ocean at least four main important regions exhibit significant ocean primary productivity. They are the Bellingshausen–Amundsen Sea, the Weddell Sea, the southern Argentinean shelf and the southern Chilean coast. In this work, we have combined ground-based continuous atmospheric sampling of aerosol number concentration (ANC), over-sea dimethyl sulphide (DMS) measurements, chlorophyll a (chl a) concentration provided by Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite images, in situ meteorological data and monthly regional NCEP-NCAR re-analysis wind fields in order to investigate the relative contribution of each of the above regions to the apportionment of the ANC at King George Island (KGI), South Shetland Islands. Our results suggest that, at least during the period from September 1998–December 1999, the southern Argentinean shelf acted as the main contributor to the ANC measured in KGI.

The ANTARES observation network

The ANTARES network seeks to understand the variability of the coastal environment on a continental scale and the local, regional, and global factors and processes that effect this change. The focus are coastal zones of South America and the Caribbean Sea. The initial approach includes developing time series of in situ and satellite-based environmental observations in coastal and oceanic regions. The network is constituted by experts that seek to exchange ideas, develop an infrastructure for mutual logistical and knowledge support, and link in situ time series of observations located around the Americas with real-time and historical satellite-derived time series of relevant products. A major objective is to generate information that will be distributed publicly and openly in the service of coastal ocean research, resource management, science-based policy making and education in the Americas. As a first stage, the network has linked oceanographic time series located in Argentina, Brazil, Chile and Venezuela. The group has also developed an online tool to examine satellite data collected with sensors such as NASAs MODIS. Specifically, continental-scale high-resolution (1 km) maps of chlorophyll and of sea surface temperature are generated and served daily over the web according to specifications of users within the ANTARES network. Other satellite-derived variables will be added as support for the network is solidified. ANTARES serves data and offers simple analysis tools that anyone can use with the ultimate goal of improving coastal assessments, management and policies.