Search

We study mesoscale eddy characteristics in the tropical Atlantic Ocean, using 23 years of daily altimetry sea level anomalies. Eddies are mainly generated in the eastern boundary upwelling systems and in the Brazil Current region. Their westward propagation speed reaches 20 cm/s in equatorial areas, decreasing with latitudes. They present typical amplitudes of 1‐5 cm. The largest and most energetic eddies are observed in the equatorial region, especially in the North Brazil Current (NBC) retroflection. The seasonal cycle of the eddy characteristics shows higher amplitudes along the NBC retroflection and the western part of the North Equatorial Countercurrent. A new criterion, based on altimetry data, determines the probability that barotropic instability of mean surface currents is responsible for eddy generation. We find a strong likelihood that the latter plays a key role along the North Equatorial Countercurrent, whereas other mechanisms must be invoked for the NBC region. The kinetic energy in the ocean is dominated by mesoscale eddies that are quasi‐circular structures having typical spatial scales of 10 to 100 km. Eddies have a clear signature in sea surface height measured by satellite altimeters and play a significant role in the transport and redistribution of water masses, heat, and biogeochemical properties throughout the oceans. In this work, we use 23 years of satellite altimetry data in the tropical Atlantic Ocean to analyze eddy properties (size, amplitude, kinetic energy, and vorticity) at the basin scale and their seasonal variability. Although they are more numerous on the borders of the subtropical gyres, the most energetic and largest eddies are found in the equatorial region. Eddy characteristics exhibit the strongest seasonal cycle in the North Brazil Current retroflection and the western part of the North Equatorial Countercurrent. In this latter region, eddy genesis is due to barotropic instabilities resulting from the horizontal shear of the large‐scale currents, whereas in the North Brazil Current retroflection other processes have to be considered. Mesoscale eddies in the tropical Atlantic ocean have amplitudes, radii, and kinetic energies of 1–5 cm, 30–100 km, and 50–100 cm2/s 2, respectivelySeasonal cycle of eddy properties has the highest amplitude in the NBC retroflection and in the western part of the NECCBarotropic instability is the main mechanism for eddy generation in the western part of the NECC

The characteristic sea surface salinity (SSS) patterns associated with the tropical Atlantic meridional and equatorial interannual modes are extracted from in situ observations, by a statistical analysis performed on the 1980–2012 period. These SSS signatures of the interannual climatic modes are reproduced in a regional numerical simulation. For each mode, oceanic and/or atmospheric processes driving the SSS signature are identified through a mixed‐layer salt budget in the validated model. During a positive meridional mode in spring, a northward shift of the Intertropical Convergence Zone and related precipitation maximum creates a south‐north dipole of positive‐negative SSS anomalies around the equator. Western boundary currents strengthen and advect relatively fresh equatorial waters, which creates negative SSS anomalies in the north and south west tropical Atlantic. Meridional and vertical advection create positive SSS anomalies off the Congo River. During a positive equatorial mode in summer, a southward shift of the Intertropical Convergence Zone‐related rainfall maximum creates a south‐north dipole of negative‐positive SSS anomalies between the equator and 10°N. Meridional advection also contributes to the positive SSS anomalies between 5°N and 10°N. Vertical advection and diffusion at the mixed‐layer base create positive SSS anomalies between 5°S and the equator. Horizontal advection creates large SSS anomalies in the North Brazil Current retroflection region, negative along the coast and positive further offshore. The SSS signatures of the meridional and equatorial modes described above are well captured by the Soil Moisture–Ocean Salinity satellite during the 2010 and 2012 events. This study shows that both meridional and equatorial interannual climatic modes impact the sea surface salinity (SSS) in tropical Atlantic through atmospheric and/or oceanic processes. The atmospheric forcing, related to Intertropical Convergence Zone migration, controls the equatorial region, while the advection, due to modulation of current dynamics, vertical SSS gradient, and mixing at the base of mixed layer, drives SSS in the region under the influence of river plumes. The SSS signature of the tropical Atlantic meridional and equatorial interannual modes is extracted from in situ observationsOceanic/atmospheric processes driving the signature of each mode are identified through a mixed‐layer salt budget in a validated modelThe SSS signatures of the meridional and equatorial modes are well captured by SMOS satellite during particular events

In the easternmost portion of the Gulf of Guinea, Bioko Island marks the beginning of an island chain that stretches NE–SW to the Equator, where São Tomé Island sits in the path of the Equatorial Undercurrent (EUC). In this study, we explore the meso‐to‐large‐scale effects of local flow‐topography interactions that escalate from the EUC encounter with the Gulf of Guinea islands. A mean shipboard Acoustic Doppler Current Profiler section captures the EUC as a strong subsurface jet that ultimately hits São Tomé. Motivated by these observations, we ran two ocean general circulation model simulations that differ by the presence versus absence of the Gulf of Guinea islands. Diagnostics of salinity and potential vorticity (PV) from these simulations show that the EUC bifurcates at 6°E, triggering mesoscale activity that spreads the EUC waters. On both sides of São Tomé, the EUC branches roll up into eddies that propagate westward. These low‐PV anticyclones carry high salinity through the tropical Atlantic, introducing PV anomalies along eddy corridors mirrored by the Equator. The formation of such eddies is affected by diapycnal and isopycnal mixing and friction, and their distribution between hemispheres is intrinsically related to the location of São Tomé. The eddies are modulated at both seasonal and interannual scales. A strong EUC generates numerous and stronger eddies, while seasonal equatorial upwelling indirectly hinders their formation. Convergence of the EUC upstream of São Tomé reveals intense downwelling and freshening, whereas divergence and upwelling are associated with the EUC negotiating the island. In the eastern portion of the tropical Atlantic, in the Gulf of Guinea, Bioko Island marks the beginning of an island chain that stretches NE–SW to the Equator, where São Tomé Island sits in the path of the strongest current in the tropical Atlantic, the Equatorial Undercurrent (EUC). The EUC connects the tropical Atlantic, transporting high‐salinity waters eastward. In this study we explore the effects of flow‐topography interactions on the EUC circulation. Velocity observations capture the EUC as a strong current that hits São Tomé Island. Motivated by these observations, we ran two model simulations that differ by the presence versus absence of the Gulf of Guinea islands. Diagnostics of salinity and potential vorticity (a dynamical quantity that relates to the circulation) show that the EUC bifurcates at 6°E, spreading the EUC waters. On both sides of São Tomé Island, the EUC branches form eddies that propagate westward through the tropical Atlantic. The formation and distribution of such eddies present an asymmetry between hemispheres due to the EUC interaction with São Tomé. On regional scales, a particle‐tracking experiment reveals sinking before the particles reach São Tomé, with mixing and freshening of the EUC waters as they flow around the island. Velocity observations show that the Equatorial Undercurrent (EUC) hits São Tomé Island at 6°E, forcing the jet to bifurcate earlier than if the island did not existThe EUC dynamics and the location of São Tomé impose an asymmetry to the bifurcation, affecting net zonal fluxes in the tropical AtlanticThe EUC‐island encounter triggers vertical advection and transformation of the EUC water via diapycnal and isopycnal mixing and friction Velocity observations show that the Equatorial Undercurrent (EUC) hits São Tomé Island at 6°E, forcing the jet to bifurcate earlier than if the island did not exist The EUC dynamics and the location of São Tomé impose an asymmetry to the bifurcation, affecting net zonal fluxes in the tropical Atlantic The EUC‐island encounter triggers vertical advection and transformation of the EUC water via diapycnal and isopycnal mixing and friction

Particularly cool sea surface temperatures (SSTs) were observed in 2012 along the Northern Gulf of Guinea coast. This strong cooling event was seen from February to June and reached maxima in the coastal upwelling areas: SST anomalies of −1°C were observed in Sassandra Upwelling area in Côte d'Ivoire (SUC, situated east of Cape Palmas) and SST anomalies of −0.5°C were observed in Takoradi Upwelling area in Ghana (TUG, located east of Cape Three Points). In SUC and TUG regions, the 2012 decrease in SST was the coldest event recorded over the 1990–2018 period (29 years). From the analysis of regional simulations, we show that the mechanisms behind this SST decrease differ in the two regions. In the SUC region, we identify changes in both zonal advection (related to zonal SST gradient changes) and increased vertical mixing as the main drivers of the anomalous cooling. The anomalous vertical mixing is linked to increased vertical shear of the zonal current in response to the Guinea Current strengthening. In the TUG region, acceleration of the southward advection of the surface water, due to the intensification of the meridional Ekman current generated by the strengthening of the zonal wind stress, was identified as the major cause of the SST anomalous cooling. We investigate the causes of the anomalous cooling that happened in the northern Gulf of Guinea coast between February and June 2012 using observations and model data. The abnormal sea surface temperature decrease was pronounced at the upwelling regions of Sassandra in Côte d'Ivoire and Takoradi in Ghana. Results reveal that the anomalous cooling, in the Sassandra upwelling area, was mainly due to changes in both the vertical mixing at the base of mixed layer and zonal advection, whereas only changes in meridional advection explained the anomalous cooling seen in the Takoradi upwelling area. Causes of the anomalous cooling seen in the northern Gulf of Guinea coast between February and June 2012 are investigatedSea Surface Temperature anomalies were important in the coastal upwelling regions of Sassandra in Côte d'Ivoire and Takoradi in GhanaSassandra cooling was due to vertical mixing and zonal advection changes, whereas only meridional advection changes caused Takoradi cooling Causes of the anomalous cooling seen in the northern Gulf of Guinea coast between February and June 2012 are investigated Sea Surface Temperature anomalies were important in the coastal upwelling regions of Sassandra in Côte d'Ivoire and Takoradi in Ghana Sassandra cooling was due to vertical mixing and zonal advection changes, whereas only meridional advection changes caused Takoradi cooling

SummarySodium cromoglycate formulated as a pressurized aerosol (2 mg 4-times daily) and placebo were compared in 31 adult asthmatic patients in a double-blind crossover trial lasting 12 weeks. Sodium cromoglycate was superior to placebo in improving breathlessness at rest (p<0.001), breathlessness on exertion (p<0.05) and the quality of sleep (p<0.001), and also in improving the morning peak expiratory flow rate value (p<0.05). Both the patients’ (p<0.05) and the clinicians’ (p<0.01) treatment opinions, and their treatment preferences (p<0.05), favoured sodium cromoglycate. Moreover, usage of bronchodilators (theophylline and aerosol beta-stimulants combined) declined significantly (p<0.001) during sodium cromoglycate treatment compared to placebo. No side-effects were reported. The results of this study show that sodium cromoglycate delivered by pressurized aerosol was significantly superior to placebo, not only in improving asthmatic symptoms, but also in reducing the amount of concomitant bronchodilator therapy required.