The western boundary current system off Brazil is a key region for diagnosing variations of the Atlantic meridional overturning circulation (AMOC) and the southern subtropical cell. In July 2013 a mooring array was installed off the coast at 11°S similar to an array installed between 2000 and 2004 at the same location. Here we present results from two research cruises and the first 10.5 months of moored observations in comparison to the observations a decade ago. Average transports of the North Brazil Undercurrent and the Deep Western Boundary Current (DWBC) have not changed between the observational periods. DWBC eddies that are predicted to disappear with a weakening AMOC are still present. Upper layer changes in salinity and oxygen within the last decade are consistent with an increased Agulhas leakage, while at depths water mass changes are likely related to changes in the North Atlantic as well as tropical circulation changes. HUMMELS ET AL. BOUNDARY CIRCULATION CHANGES AT 11°S 7615

The cross-wavelet transform (XWT) is a powerful tool for testing the proposed connections between two time series. Because of XWT’s skeletal structure, which is based on the wavelet transform, it is suitable for the analysis of nonstationary periodic signals. Recent work has shown that the power spectrum based on the wavelet transform can produce a deviation, which can be corrected by choosing a proper rectification scale. In this study, it is shown that the standard application of the XWT can also lead to a biased result. A corrected version of the standard XWT was constructed using the scale of each series as normalizing factors. This correction was first tested with an artificial example involving two series built from combinations of two harmonic series with different amplitudes and frequencies. The standard XWT applied to this example produces a biased result, whereas the correct result is obtained with the use of the proposed normalization. This analysis was then applied to a real geophysical situation with important implications to climate modulation on the northwestern Brazilian coast. The linkage between the relative humidity and the shortwave radiation measurements, obtained from the 8°S, 30°W Autonomous Temperature Line Acquisition System (ATLAS) buoy of the Southwestern Extension of the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA-SWE), was explored. The analysis revealed the importance of including the correction in order to not overlook any possible connections. The requirements of incorporating this correction in the XWT calculations are emphasized.

Tropical Atlantic (TA) Ocean-atmosphere interactions and their contributions to strong variability of rainfall along the Northeast Brazilian (NEB) coast were investigated for the years 1974–2008. The core rainy seasons of March-April and June-July were identified for Fortaleza (northern NEB; NNEB) and Recife (eastern NEB; ENEB), respectively. Lagged linear regressions between sea surface temperature (SST) and pseudo wind stress (PWS) anomalies over the entire TA and strong rainfall anomalies at Fortaleza and Recife show that the rainfall variability of these regions is differentially influenced by the dynamics of the TA. When the Intertropical Convergence Zone is abnormally displaced southward a few months prior to the NNEB rainy season, the associated meridional mode increases humidity and precipitation during the rainy season. Additionally, this study shows predictive effect of SST, meridional PWS, and barrier layer thickness, in the Northwestern equatorial Atlantic, on the NNEB rainfall. The dynamical influence of the TA on the June-July ENEB rainfall variability shows a northwestward-propagating area of strong, positively correlated SST from the southeastern TA to the southwestern Atlantic warm pool (SAWP) offshore of Brazil. Our results also show predictive effect of SST, zonal PWS, and mixed layer depth, in the SAWP, on the ENEB rainfall.

No abstract

The North Brazil Current (NBC) is an intense western boundary current, which is the dominant feature of the surface circulation in the Western Tropical North Atlantic (WTNA). It flows predominantly northwestward along Brazilian northern coast and, around 6°N-8°N and 45°W, the current separates from the coast and retroflects to the east, feeding the North Equatorial Countercurrent (NECC; Garzoli et al., 2003; Johns

This study focuses on analysing the potential impact of the Amazon and Pará Rivers on the salinity, temperature and hydrodynamics of the Western Tropical North Atlantic (WTNA) region between 60.5°-24 °W and 5 °S-16 °N.The Regional Ocean Model System (ROMS) was used to simulate ocean circulation with 0.25° horizontal resolution and 32 vertical levels. Two numerical experiments were performed considering river discharge and river input. Temperature and salinity distributions obtained numerically were compared with Simple Ocean Data Assimilation (SODA) and in situ observations from the Prediction Research Moored Array in the Tropical Atlantic (PIRATA) buoys located at 38 °W8 °N and 38 °W12 °N. Surface currents were compared with Surface Currents from Diagnostic model (SCUD). Once we verified that model results agreed with observations, scenarios with and without river discharges were compared. The difference between both simulations in the Sea Surface Temperature distribution was smaller than 2 °C, whereas the Sea Surface Salinity(SSS) changed by approximately 8 psu in the plume area close to the coast from August to December and reaching SSS differences of approximately 4 psu in the region of the North Equatorial Counter Current (NECC). The surface current velocities are stronger in the experiment with river discharge, mainly in the NECC area from September to December and close to the coast in June to August. The results show that river discharges also cause a phase shift in the zonal currents, anticipating the retroflection of the North Brazil Current by two months and enhancing eastward NECC transport, which is in agreement with observations. The Mixed Layer Depth and Isothermal Layer Depth in the presence of river discharge is 20-50 m shallower over the entire extension of the Amazon plume compared with the situation without continental inflows. As a consequence, stronger Barrier Layers develop in the river plumes, reducing the Oceanic Heat Content in the WTNA.

The Amazon generates the world's largest offshore river plume, which covers extensive areas of the tropical Atlantic. The data and samples in this study were obtained during the oceanographic cruise Camadas Finas III in October 2012 along the Amazon River-Ocean Continuum (AROC). The cruise occurred during boreal autumn, when the river plume reaches its maximum eastward extent. In this study, we examine the links between physics, biogeochemistry and plankton community structure along the AROC. Hydrographic results showed very different conditions, ranging from shallow well-mixed coastal waters to offshore areas, where low salinity Amazonian waters mix with open ocean waters. Nutrients, mainly NO3− and SiO2−, were highly depleted in coastal regions, and the magnitude of primary production was greater than that of respiration (negative apparent oxygen utilization). In terms of phytoplankton groups, diatoms dominated the region from the river mouth to the edge of the area affected by the North Brazil Current (NBC) retroflection (with chlorophyll a concentrations ranging from 0.02 to 0.94 mg m−3). The North Equatorial Counter Current (NECC) region, east of retroflection, is fully oligotrophic and the most representative groups are Cyanobacteria and dinoflagellates. Additionally, in this region, blooms of cyanophyte species were associated with diatoms and Mesozooplankton (copepods). A total of 178 zooplankton taxa were observed in this area, with Copepoda being the most diverse and abundant group. Two different zooplankton communities were identified: a low-diversity, high-abundance coastal community and a high-diversity, low-abundance oceanic community offshore. The CO2 fugacity (fCO2sw), calculated from total alkalinity (1,450 < TA < 2,394 μmol kg−1) and dissolved inorganic carbon (1,303 < DIC < 2,062 μmol kg−1) measurements, confirms that the Amazon River plume is a sink of atmospheric CO2 in areas with salinities <35 psu, whereas, in regions with salinities >35 and higher-intensity winds, the CO2 flux is reversed. Lower fCO2sw values were observed in the NECC area. The ΔfCO2 in this region was less than 5 μatm (−0.3 mmol m−2 d−1), while the ΔfCO2 in the coastal region was approximately 50 μatm (+3.7 mmol m−2 d−1). During the cruise, heterotrophic and autotrophic processes were observed and are indicative of the influences of terrestrial material and biological activity, respectively.

At the mouth of the Amazon River, a widespread carbonate ecosystem exists below the river plume, generating a hard-bottom reef (∼9500 km2) that includes mainly large sponges but also rhodolith beds. The mesozooplankton associated with the pelagic realm over the reef formation was characterized, considering the estuarine plume and oceanic influence. Vertical hauls were carried out using a standard plankton net with 200 μm mesh size during September 2014. An indicator index was applied to express species importance as ecological indicators in community. Information on functional traits was gathered for the most abundant copepod species. Overall, 179 zooplankton taxa were recorded. Copepods were the richest (92 species), most diverse and most abundant group, whereas meroplankton were rare and less abundant. Species diversity (>3.0 bits.ind-1) and evenness (>0.6) were high, indicating a complex community. Small holoplanktonic species dominated the zooplankton, and the total density varied from 107.98 ind. m-3 over the reef area to 2,609.24 ind. m-3 in the estuarine plume, with a significant difference between coastal and oceanic areas. The most abundant copepods were the coastal species ithona plumifera and Clausocalanus furcatus and early stages copepodites of Paracalanidae. The holoplanktonic Oikopleura, an important producer of mucous houses, was very abundant on the reefs. The indicator species index revealed three groups: (1) indicative of coastal waters under the influence of the estuarine plume [Euterpina acutifrons, Parvocalanus crassirostris, Oikopleura (Vexillaria) dioica and Hydromedusae]; (2) characterized coastal and oceanic conditions (Clausocalanus); (3) characterized the reef system (O. plumifera). Two major copepods functional groups were identified and sorted according to their trophic strategy and coastal-oceanic distribution. The species that dominated the coastal area and the area over the rhodolith beds are indicators of the estuarine plume and are mixed with species of the North Brazil Current. These species practically disappear offshore, where occur oceanic species commonly found in other oligotrophic tropical areas. This ecosystem shows a mixture of estuarine, coastal and oceanic communities coexisting in the waters over the Amazon reefs, with no significant differences among these areas. However, the MDS clearly separated the communities along the salinity gradient in the plume.