A Rossby whistle: a resonant basin mode observed in the Caribbean Sea

We show that an important source of coastal sea level variability around the Caribbean Sea is a resonant basin mode. The mode consists of a baroclinic Rossby wave which propagates westward across the basin and is rapidly returned to the east along the southern boundary as coastal shelf waves. Almost two wavelengths of the Rossby wave fit across the basin, and it has a period of 120 days. The porous boundary of the Caribbean Sea results in this mode exciting a mass exchange with the wider ocean, leading to a dominant mode of bottom pressure variability which is almost uniform over the Grenada, Venezuela, and Colombia basins and has a sharp spectral peak at 120 day period. As the Rossby waves have been shown to be excited by instability of the Caribbean Current, this resonant mode is dynamically equivalent to the operation of a whistle

Identification of the master sex determining gene in Northern pike (Esox lucius) reveals restricted sex chromosome differentiation.

Teleost fishes, thanks to their rapid evolution of sex determination mechanisms, provide remarkable opportunities to study the formation of sex chromosomes and the mechanisms driving the birth of new master sex determining (MSD) genes. However, the evolutionary interplay between the sex chromosomes and the MSD genes they harbor is rather unexplored. We characterized a male-specific duplicate of the anti-Müllerian hormone (amh) as the MSD gene in Northern Pike (Esox lucius), using genomic and expression evidence as well as by loss-of-function and gain-of-function experiments. Using RAD-Sequencing from a family panel, we identified Linkage Group (LG) 24 as the sex chromosome and positioned the sex locus in its sub-telomeric region. Furthermore, we demonstrated that this MSD originated from an ancient duplication of the autosomal amh gene, which was subsequently translocated to LG24. Using sex-specific pooled genome sequencing and a new male genome sequence assembled using Nanopore long reads, we also characterized the differentiation of the X and Y chromosomes, revealing a small male-specific insertion containing the MSD gene and a limited region with reduced recombination. Our study reveals an unexpectedly low level of differentiation between a pair of sex chromosomes harboring an old MSD gene in a wild teleost fish population, and highlights both the pivotal role of genes from the amh pathway in sex determination, as well as the importance of gene duplication as a mechanism driving the turnover of sex chromosomes in this clade

Variability and dynamics of the Yucatan upwelling : high-resolution simulations

The Yucatan shelf in the southern Gulf of Mexico is under the influence of an upwelling that uplifts cool and nutrient rich waters over the continental shelf. The analysis of a set of high-resolution (x=y approximate to 2.8 km) simulations of the Gulf of Mexico shows two dominant modes of variability of the Yucatan upwelling system: (1) a low-frequency mode related to variations in position and intensity of the Loop Current along the shelf, with upwelling intensified when the Loop Current is strong and approaches to the Yucatan shelf break and (2) a high-frequency mode with peak frequency in the 6-10 days band related to wind-forced coastal waves that force vertical velocities along the eastern Yucatan shelf break. To first order, the strength and position of the Loop Current are found to control the intensity of the upwelling, but we show that high-frequency winds also contribute (approximate to 17%) to a net input of cool waters (<22.5 degrees C) on the Yucatan shelf. Finally, although more observational studies are needed to corroborate the topographic character of the Yucatan upwelling system, this study reveals the key role played by a notch along the Yucatan shelf break: a sensitivity simulation without the notch shows a 55% reduction of the upwelling

Equatorial Atlantic interannual variability and its relation to dynamic and thermodynamic processes

The contributions of the dynamic and thermodynamic forcing to the interannual variability of the equatorial Atlantic sea surface temperature (SST) are investigated using a set of interannual regional simulations of the tropical Atlantic Ocean. The ocean model is forced with an interactive atmospheric boundary layer, avoiding damping toward prescribed air temperature as is usually the case in forced ocean models. The model successfully reproduces a large fraction (R2  =  0.55) of the observed interannual variability in the equatorial Atlantic. In agreement with leading theories, our results confirm that the interannual variations of the dynamical forcing largely contributes to this variability. We show that mean and seasonal upper ocean temperature biases, commonly found in fully coupled models, strongly favor an unrealistic thermodynamic control of the equatorial Atlantic interannual variability

International conference ICAWA 2017 and 2018 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters

A consistent Sea Surface Salinity (SSS) signature of the tropical Atlantic meridional and equatorial interannual modes is extracted from in situ observations and a regional numerical simulation, by a statistical analysis on the 1980-2012 period. Oceanic and/or atmospheric processes responsible for the signature of each mode are identified through a mixed-layer salt budget in the validated model. The meridional mode is associated in spring with a meridional SSS dipole in the equatorial band, due to changes in fresh water flux related to a meridional shift of the Inter-Tropical Convergence Zone (ITCZ). It is also associated with large SSS anomalies in the north and south west tropical Atlantic, due to advection of relatively fresh equatorial waters by strengthened western boundary currents, and off the Congo River where both meridional and vertical advection are involved. The equatorial mode is associated in summer with 3 zonal bands of alternating SSS anomalies between 5°S and 10°N. The southernmost band is due to vertical advection and diffusion at the mixed layer base, the two others to a shift of the ITCZ-related rainfall maximum, with additional contribution of meridional advection in the northernmost band. The equatorial mode also leads to large SSS anomalies in the North Brazil Current retroflection region, mainly due to horizontal advection of equatorial SSS anomalies. The SSS signatures of the meridional and equatorial modes are well captured by the SMOS satellite during particular events

Erosion of the Subsurface Salinity Maximum of the Loop Current Eddies From Glider Observations and a Numerical Model

The erosion of the subsurface salinity maximum, signature of the Caribbean Subtropical UnderWater (SUW), within the Loop Current Eddy (LCE) Poseidon (August 2016 to July 2017) in the Gulf of Mexico (GoM) and the formation of the Gulf Common Water (GCW) during its journey westward, was observed using glider data. Most of the dilution of the SUW high-salinity core within Poseidon occurs during late autumn and winter associated withNorthernwinds and mixed-layer deepening. The physical processes that contribute to salt dilution of the SUW inside the LCEs' core are investigated using a numerical regional model. The analysis of the salt budget in a long-lasting numerical LCE and a composite analysis of sixteen LCEs reveal that the salinity trend is mostly explained by the vertical salinity diffusion. Cold and dryNorthernwinds during the first winter drive strong negative net heat fluxes that trigger turbulent flux of salt into the LCEs' thermostad and dilution of the SUW high-salinity core below. The vertical salinity diffusion continues homogenizing the salinity in the upper ocean until the vertical gradient of salinity is negligible. As a result, SUW is transformed to precursor of GCW that is ultimately diffused to surrounding waters in the western GoM. Although the contribution of advection to the salinity trend above the isopycnal of 1,026 kg m(-3)is of second order, below is the most important process driving loss of salinity, presumably due to, eddy-eddy interactions during LCEs' westward propagation and eddy pumping (upwelling) during the LCEs' decaying phase along the western slope

Sea Surface Salinity signature of the tropical Atlantic interannual climatic modes [résumé de poster]

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018A consistent Sea Surface Salinity (SSS) signature of the tropical Atlantic meridional and equatorial interannual modes is extracted from in situ observations and a regional numerical simulation, by a statistical analysis on the 1980-2012 period. Oceanic and/or atmospheric processes responsible for the signature of each mode are identified through a mixed-layer salt budget in the validated model. The meridional mode is associated in spring with a meridional SSS dipole in the equatorial band, due to changes in fresh water flux related to a meridional shift of the Inter-Tropical Convergence Zone (ITCZ). It is also associated with large SSS anomalies in the north and south west tropical Atlantic, due to advection of relatively fresh equatorial waters by strengthened western boundary currents, and off the Congo River where both meridional and vertical advection are involved. The equatorial mode is associated in summer with 3 zonal bands of alternating SSS anomalies between 5°S and 10°N. The southernmost band is due to vertical advection and diffusion at the mixed layer base, the two others to a shift of the ITCZ-related rainfall maximum, with additional contribution of meridional advection in the northernmost band. The equatorial mode also leads to large SSS anomalies in the North Brazil Current retroflection region, mainly due to horizontal advection of equatorial SSS anomalies. The SSS signatures of the meridional and equatorial modes are well captured by the SMOS satellite during particular events

Erosion of the subsurface salinity maximum of the loop current eddies from glider observations and a numerical model

The erosion of the subsurface salinity maximum, signature of the Caribbean Subtropical UnderWater (SUW), within the Loop Current Eddy (LCE) Poseidon (August 2016 to July 2017) in the Gulf of Mexico (GoM) and the formation of the Gulf Common Water (GCW) during its journey westward, was observed using glider data. Most of the dilution of the SUW high-salinity core within Poseidon occurs during late autumn and winter associated withNorthernwinds and mixed-layer deepening. The physical processes that contribute to salt dilution of the SUW inside the LCEs' core are investigated using a numerical regional model. The analysis of the salt budget in a long-lasting numerical LCE and a composite analysis of sixteen LCEs reveal that the salinity trend is mostly explained by the vertical salinity diffusion. Cold and dryNorthernwinds during the first winter drive strong negative net heat fluxes that trigger turbulent flux of salt into the LCEs' thermostad and dilution of the SUW high-salinity core below. The vertical salinity diffusion continues homogenizing the salinity in the upper ocean until the vertical gradient of salinity is negligible. As a result, SUW is transformed to precursor of GCW that is ultimately diffused to surrounding waters in the western GoM. Although the contribution of advection to the salinity trend above the isopycnal of 1,026 kg m(-3)is of second order, below is the most important process driving loss of salinity, presumably due to, eddy-eddy interactions during LCEs' westward propagation and eddy pumping (upwelling) during the LCEs' decaying phase along the western slope

Sea Surface Salinity Signature of the Tropical Atlantic Interannual Climatic Modes

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. Plain Language Summary 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

Causes of the Northern Gulf of Guinea Cold Event in 2012

International audienceParticularly 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