Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin

Transport and mixing properties of surface currents can be detected from altimetric data by both Eulerian and Lagrangian diagnostics. In contrast with Eulerian diagnostics, Lagrangian tools like the local Lyapunov exponents have the advantage of exploiting both spatial and temporal variability of the velocity field and are in principle able to unveil subgrid filaments generated by chaotic stirring. However, one may wonder whether this theoretical advantage is of practical interest in real-data, mesoscale and submesoscale analysis, because of the uncertainties and resolution of altimetric products, and the non-passive nature of biogeochemical tracers. Here we compare the ability of standard Eulerian diagnostics and the finite-size Lyapunov exponent in detecting instantaneaous and climatological transport and mixing properties. By comparing with sea-surface temperature patterns, we find that the two diagnostics provide similar results for slowly evolving eddies like the first Alboran gyre. However, the Lyapunov exponent is also able to predict the (sub-)mesoscale filamentary process occuring along the Algerian current and above the Balearic Abyssal Plain. Such filaments are also observed, with some mismatch, in sea-surface temperature patterns. Climatologies of Lyapunov exponents do not show any compact relation with other Eulerian diagnostics, unveiling a different structure even at the basin scale. We conclude that filamentation dynamics can be detected by reprocessing available altimetric data with Lagrangian tools, giving insight into (sub-)mesoscale stirring processes relevant to tracer observations and complementing traditional Eulerian diagnostics

Seasonal modulation of mesoscale processes alters nutrient availability and plankton communities in the Red Sea

Hydrographic and atmospheric forcing set fundamental constraints on the biogeochemistry of aquatic ecosystems and manifest in the patterns of nutrient availability and recycling, species composition of communities, trophic dynamics, and ecosystem metabolism. In the Red Sea, latitudinal gradients in environmental conditions and primary production have been ascribed to fluctuations in Gulf of Aden Water inflow, upwelling/mixing, and regenerated nutrient utilization i.e. rapidly recycled nitrogen in upper layers. However, our understanding of upper layer dynamics and related changes in plankton communities, metabolism and carbon and nitrogen export is limited. We surmised that stratification and mesoscale eddies modulate the nutrient availability and taxonomic identity of plankton communities in the Red Sea. Based on remote-sensing data of sea level anomalies and high resolution in situ measurements (ScanFish) we selected stations for hydrographic CTD profiles, water sampling (nutrients, seawater oxygen stable isotopes [δ18OSW]), phytoplankton and zooplankton collections. In fall 2014, strong stratification subjected the plankton community to an overall nitrogen and phosphorus shortage. The nutrient deficiency increased numbers of heterotrophic dinoflagellates, microzooplankton, and diazotrophs (Trichodesmium, diatom-diazotroph associations [DDAs]), albeit largely decreased phytoplankton and mesozooplankton abundances. In spring 2015, mesoscale eddies increased the nutrient availability, and the thermohaline characteristics and low δ18OSW point to the interaction of eddies with Gulf of Aden Surface Water (GASW). Cyclonic eddies and, most likely, the availability of nutrients associated with the GASW, increased the abundances of autotrophs (diatoms, Prasinophytes) and supported larger numbers of zooplankton and their larvae. We demonstrate that the interplay of stratification, advection of Gulf of Aden water and mesoscale eddies are key elements to better understand changes in plankton community composition, ecosystem metabolism, and macronutrient export in the Red Sea in space and time

The Brazil-Malvinas Confluence : from local to global scales

The Southwest Atlantic Ocean is a key component of the global climate system. It holds the Confluence of the Brazil and Malvinas Currents (BMC), one of the highest frontal systems of the world ocean. The BMC is the encounter point of subtropical origin warm and salty waters, transported southward within the Brazil Current, and subantarctic water flowing equatorward along the Malvinas Current. The intense mixing and cross-frontal exchanges highly transform the water masses that will finally flow southward, eastward of be subducted into the subtropical thermocline. This PhD dissertation aims at extending our knowledge of the circulation and dynamics in the BMC, placing these results in perspective from the small to the global scales. The intense property contrast found in this region resemble the intense temperature gradients between low and high latitudes. We first analyse the changes in the heat content of the atmosphere and upper ocean for the last 450 kyr and analyse the sensitivity of the system to changes in albedo, cloud cover and atmospheric and oceanic heat transports. We then describe the hydrographic conditions at the BMC during and early fall cruise. These reveal the presence of brackish river water on top the frontal system. The salinity anomaly at the surface correlates with the presence of large ageostrophic velocities along the frontal jet. In addition, the rapid evolution of these waters impinges on the thermohaline variability in the proximity of the front. The comparison of surface overview during the cruise is well represented by the reanalysis although at depth, this misses the thermohaline intrusions developed both sides the front. On the other hand, climatological data misses the numerous mesoscale features. We also examine the circulation pattern in the upper 2000 m depth. We find a relatively weak MC near 41°S, 56°W followed by its cyclonic retroflection, an intense subtropical anticyclone replacing the BC-overshoot (BCO), a subantarctic inflow near 53°W maintained both by an upstream earlier diversion of the MC and the cyclonic recirculation of the flow leaving the east along the confluence front, and on the northern extreme of the Confluence the southward flow of BC. The results in this chapter also suggest the existence of dianeutral mixing and cross-frontal exchanges. We assess the high-frequency temperature variability at this region combining sea surface temperature images with novel high-resolution SeaSoar measurements. We found spatio-temporal scales between 1.5 and 6 days and between 20 and 50 km with the shortest scales along the shelf-break BC and over the Confluence front, with the largest ones along the MC and MRC. Variability increases at the subsurface due to submesoscale thermohaline intrusions. Finally, we analyse the relative role of dianeutral mixing by small scale turbulence and isoneutal mixing induced by mesoscale eddy stirring in setting the temperature-salinity relationship in the Argentine basin using microstructure measurements along the Subtropical Front (STF). Dieneutral mixing is enhanced in the upper 500 m of the water column, especially downstream the BCO, and at depth over the shelf-break and the eastern limit of the basin at the mid-Atlantic ridge. Isoneutral diffusivity dominates below 500 m in the centre of the basin. Moreover, we determine the cross-frontal eddy-advective mass flux. These induce the subduction of about 3 Sv of mode and intermediate waters into the subtropical thermocline and the poleward transport of Upper Circumpolar Deep Water.El suroeste del Océano Atlántico es clave del sistema climático global. En esta región se encuentran las corrientes de Brasil y Malvinas, dando lugar a uno de los sistemas frontales más intensos del océano mundial, la Confluencia de Brasil-Malvinas (BMC). Esta región frontal es el punto de encuentro de aguas de origen subtropical, cálidas y saladas, que son transportadas hacia el sur en la corriente de Brasil y aguas de origen subantárctico que viajan hacia el ecuador a lo largo de la corriente de Malvinas. La intensa mezcla y los intercambios frontales cruzados transforman en gran medida las masas de agua que aquí se encuentran y que finalmente fluirán hacia el sur, hacia el este o que serán subducidas hacia la termoclina subtropical. Esta tesis tiene como objetivo principal ampliar nuestro conocimiento sobre la circulación y dinámica en la BMC, colocando estos resultados en perspectiva, desde la pequeña escala a la escala global. El intenso contraste de propiedades de las aguas que se encuentran en esta región se asemeja a los intensos gradientes de temperatura que encontramos entre las altas y bajas latitudes. En un primer trabajo, analizamos los cambios en el contenido de calor de la atmósfera y el océano superior durante los últimos 450 millones de años. A continuación, describimos las condiciones hidrográficas encontradas en la BMC a principios de otoño de 2015 a partir de los datos recogidos en una campaña oceanográfica (TIC-MOC) y su comparación con datos de reanálisis y climatológicos. Estos revelan la presencia de aguas salobres del Río de la Plata en los primeros metros del sistema frontal. La anomalía de salinidad presenta una correlación positiva con las intensas velocidades ageostróficas a lo largo del jet frontal. La comparación de la visión general de las condiciones superficiales durante la campaña está bien representada por el reanálisis de alta resolución, aunque en profundidad su exactitud es menor debido a la presencia de intrusiones termohalinas desarrolladas a ambos lados del frente, no reproducidas por el modelo. Los datos climatológicos, sin embargo, al representar las condiciones medias del mes, no muestran las numerosas estructuras de mesoescala encontradas en la región. Asimismo, examinamos el patrón de circulación en los primeros 2000 metros de la columna de agua. Al sur del frente encontramos un transporte débil de la MC aproximadamente a 41oS, 56oW, seguido de su retroflexión ciclónica, un anticiclón subtropical intenso que reemplaza la retroflexión de la corriente de Brasil, un flujo de aguas subantárcticas a 53oW alimentado por un desvío aguas arriba de la MC y la recirculación ciclónica del flujo que sale al este a lo largo del frente de Confluencia. En el extremo norte, encontramos el transporte hacia el sur, dentro de los valores medios, de la corriente de Brasil. Los resultados en este capítulo sugieren también la existencia de mezcla dianeutra e intercambios frontales en la BMC. Evaluamos la variabilidad de alta frecuencia de la temperatura en esta región combinando imágenes de temperatura superficial del mar con nuevas mediciones de alta resolución recogidas con un SeaSoar (campaña RETRO-BMC). Encontramos escalas espacio-temporales características de entre 1.5 y 6 días y entre 20 y 50 km. Las escalas más cortas se encuentran la lo largo de la corriente de Brasil en su proximidad a la plataforma continental y sobre el frente de Confluencia, mientas que las mayores escalas corresponden a la MC y su retroflexión. La variabilidad aumenta en profundidad debido a la presencia de las intrusiones termohalinas submesoescalares. Finalmente, analizamos el papel relativo de la mezcla dianeutra debida a turbulencia de pequeña escala y de la mezcla isoneutra inducida por remolinos de mesoescala en establecer la relación temperatura-salinidad en la cuenca argentina utilizando para ello mediciones de microestructura. La mezcla dianeutra es máxima en los primeros 500 m de la columna de agua, especialmente aguas abajo de la BCO, y en profundidad sobre el borde de la plataforma continental y el límite oriental de la cuenca sobre la Dorsal Mesoatlántica. La difusividad isoneutral domina en profundidad (>500 m) en el centro de la cuenca. Además, determinamos el flujo de masa meridional debido a la advección inducida por los remolimos mesoscalares

The Brazil-Malvinas Confluence : from local to global scales

The Southwest Atlantic Ocean is a key component of the global climate system. It holds the Confluence of the Brazil and Malvinas Currents (BMC), one of the highest frontal systems of the world ocean. The BMC is the encounter point of subtropical origin warm and salty waters, transported southward within the Brazil Current, and subantarctic water flowing equatorward along the Malvinas Current. The intense mixing and cross-frontal exchanges highly transform the water masses that will finally flow southward, eastward of be subducted into the subtropical thermocline. This PhD dissertation aims at extending our knowledge of the circulation and dynamics in the BMC, placing these results in perspective from the small to the global scales. The intense property contrast found in this region resemble the intense temperature gradients between low and high latitudes. We first analyse the changes in the heat content of the atmosphere and upper ocean for the last 450 kyr and analyse the sensitivity of the system to changes in albedo, cloud cover and atmospheric and oceanic heat transports. We then describe the hydrographic conditions at the BMC during and early fall cruise. These reveal the presence of brackish river water on top the frontal system. The salinity anomaly at the surface correlates with the presence of large ageostrophic velocities along the frontal jet. In addition, the rapid evolution of these waters impinges on the thermohaline variability in the proximity of the front. The comparison of surface overview during the cruise is well represented by the reanalysis although at depth, this misses the thermohaline intrusions developed both sides the front. On the other hand, climatological data misses the numerous mesoscale features. We also examine the circulation pattern in the upper 2000 m depth. We find a relatively weak MC near 41°S, 56°W followed by its cyclonic retroflection, an intense subtropical anticyclone replacing the BC-overshoot (BCO), a subantarctic inflow near 53°W maintained both by an upstream earlier diversion of the MC and the cyclonic recirculation of the flow leaving the east along the confluence front, and on the northern extreme of the Confluence the southward flow of BC. The results in this chapter also suggest the existence of dianeutral mixing and cross-frontal exchanges. We assess the high-frequency temperature variability at this region combining sea surface temperature images with novel high-resolution SeaSoar measurements. We found spatio-temporal scales between 1.5 and 6 days and between 20 and 50 km with the shortest scales along the shelf-break BC and over the Confluence front, with the largest ones along the MC and MRC. Variability increases at the subsurface due to submesoscale thermohaline intrusions. Finally, we analyse the relative role of dianeutral mixing by small scale turbulence and isoneutal mixing induced by mesoscale eddy stirring in setting the temperature-salinity relationship in the Argentine basin using microstructure measurements along the Subtropical Front (STF). Dieneutral mixing is enhanced in the upper 500 m of the water column, especially downstream the BCO, and at depth over the shelf-break and the eastern limit of the basin at the mid-Atlantic ridge. Isoneutral diffusivity dominates below 500 m in the centre of the basin. Moreover, we determine the cross-frontal eddy-advective mass flux. These induce the subduction of about 3 Sv of mode and intermediate waters into the subtropical thermocline and the poleward transport of Upper Circumpolar Deep Water.El suroeste del Océano Atlántico es clave del sistema climático global. En esta región se encuentran las corrientes de Brasil y Malvinas, dando lugar a uno de los sistemas frontales más intensos del océano mundial, la Confluencia de Brasil-Malvinas (BMC). Esta región frontal es el punto de encuentro de aguas de origen subtropical, cálidas y saladas, que son transportadas hacia el sur en la corriente de Brasil y aguas de origen subantárctico que viajan hacia el ecuador a lo largo de la corriente de Malvinas. La intensa mezcla y los intercambios frontales cruzados transforman en gran medida las masas de agua que aquí se encuentran y que finalmente fluirán hacia el sur, hacia el este o que serán subducidas hacia la termoclina subtropical. Esta tesis tiene como objetivo principal ampliar nuestro conocimiento sobre la circulación y dinámica en la BMC, colocando estos resultados en perspectiva, desde la pequeña escala a la escala global. El intenso contraste de propiedades de las aguas que se encuentran en esta región se asemeja a los intensos gradientes de temperatura que encontramos entre las altas y bajas latitudes. En un primer trabajo, analizamos los cambios en el contenido de calor de la atmósfera y el océano superior durante los últimos 450 millones de años. A continuación, describimos las condiciones hidrográficas encontradas en la BMC a principios de otoño de 2015 a partir de los datos recogidos en una campaña oceanográfica (TIC-MOC) y su comparación con datos de reanálisis y climatológicos. Estos revelan la presencia de aguas salobres del Río de la Plata en los primeros metros del sistema frontal. La anomalía de salinidad presenta una correlación positiva con las intensas velocidades ageostróficas a lo largo del jet frontal. La comparación de la visión general de las condiciones superficiales durante la campaña está bien representada por el reanálisis de alta resolución, aunque en profundidad su exactitud es menor debido a la presencia de intrusiones termohalinas desarrolladas a ambos lados del frente, no reproducidas por el modelo. Los datos climatológicos, sin embargo, al representar las condiciones medias del mes, no muestran las numerosas estructuras de mesoescala encontradas en la región. Asimismo, examinamos el patrón de circulación en los primeros 2000 metros de la columna de agua. Al sur del frente encontramos un transporte débil de la MC aproximadamente a 41oS, 56oW, seguido de su retroflexión ciclónica, un anticiclón subtropical intenso que reemplaza la retroflexión de la corriente de Brasil, un flujo de aguas subantárcticas a 53oW alimentado por un desvío aguas arriba de la MC y la recirculación ciclónica del flujo que sale al este a lo largo del frente de Confluencia. En el extremo norte, encontramos el transporte hacia el sur, dentro de los valores medios, de la corriente de Brasil. Los resultados en este capítulo sugieren también la existencia de mezcla dianeutra e intercambios frontales en la BMC. Evaluamos la variabilidad de alta frecuencia de la temperatura en esta región combinando imágenes de temperatura superficial del mar con nuevas mediciones de alta resolución recogidas con un SeaSoar (campaña RETRO-BMC). Encontramos escalas espacio-temporales características de entre 1.5 y 6 días y entre 20 y 50 km. Las escalas más cortas se encuentran la lo largo de la corriente de Brasil en su proximidad a la plataforma continental y sobre el frente de Confluencia, mientas que las mayores escalas corresponden a la MC y su retroflexión. La variabilidad aumenta en profundidad debido a la presencia de las intrusiones termohalinas submesoescalares. Finalmente, analizamos el papel relativo de la mezcla dianeutra debida a turbulencia de pequeña escala y de la mezcla isoneutra inducida por remolinos de mesoescala en establecer la relación temperatura-salinidad en la cuenca argentina utilizando para ello mediciones de microestructura. La mezcla dianeutra es máxima en los primeros 500 m de la columna de agua, especialmente aguas abajo de la BCO, y en profundidad sobre el borde de la plataforma continental y el límite oriental de la cuenca sobre la Dorsal Mesoatlántica. La difusividad isoneutral domina en profundidad (>500 m) en el centro de la cuenca. Además, determinamos el flujo de masa meridional debido a la advección inducida por los remolimos mesoscalares.Postprint (published version

Exploiting coastal altimetry to improve the surface circulation scheme over the central Mediterranean Sea

This work is the first study exploiting along track altimetry data to observe and monitor coastal ocean features over the transition area between the western and eastern Mediterranean Basins. The relative performances of both the AVISO and the X‐TRACK research regional altimetric data sets are compared using in situ observations. Both products are cross validated with tide gauge records. The altimeter‐derived geostrophic velocities are also compared with observations from a moored Acoustic Doppler Current Profiler. Results indicate the good potential of satellite altimetry to retrieve dynamic features over the area. However, X‐TRACK shows a more homogenous data coverage than AVISO, with longer time series in the 50 km coastal band. The seasonal evolution of the surface circulation is therefore analyzed by conjointly using X‐TRACK data and remotely sensed sea surface temperature observations. This combined data set clearly depicts different current regimes and bifurcations, which allows us to propose a new seasonal circulation scheme for the central Mediterranean. The analysis shows variations of the path and temporal behavior of the main circulation features: the Atlantic Tunisian Current, the Atlantic Ionian Stream, the Atlantic Libyan Current, and the Sidra Gyre. The resulting bifurcating veins of these currents are also discussed, and a new current branch is observed for the first time

A descriptive analysis of the genesis and translation of a dipole vortex from the Agulhas retroflection region

Bibliography: leaves 111-126.An anomalous leakage of Agulhas Current water into the south-east Atlantic Ocean, exhibiting a mushroom-like shape, was observed during routine observations of A VHRR satellite imagery in early December 1996. The development of this anomaly was followed on the sea surface temperature (SST) imagery and it was tentatively identified as a consequence of filament interaction between the Agulhas retroflection and an occluding Agulhas ring. This interpretation prompted a cruise onboard the FR.S Africana with the objective of conducting a hydrographic survey of the Agulhas ring and the associated filament near Cape Town. A descriptive analysis, gleaned from A VHRR satellite imagery and in situ data, of the hydrographic characteristics of a vortex dipole, surveyed during this cruise, is presented in this thesis. An analysis of water mass properties and geostrophic flow patterns determined that an Agulhas ring and a cyclonic eddy, containing Benguela Current water in its core, constituted a dipole vortex in the south-eastern Atlantic Ocean. During the period of the hydrographic survey, the secondary vortex exhibited an anticlockwise rotation of 8. 6°/day around the Agulhas ring A warm filament, originating from the western Agulhas Bank, was entrained between the two counter-rotating eddies, thus resulting in the mixing of Agulhas Bank water into the South Atlantic Ocean. Satellite altimetry and A VHRR imagery were used to "backtrack" the vortex dipole to its origin at the Agulhas retroflectiOn. By combining interpretations from the altimetry and A VHRR imagery, it was possible to describe the complex interactions the dipole displayed with the retroflection and the Agulhas Bank as it franslated in a north-westerly direction. The mushroom configuration, identified earlier on SST imagery, betrayed the presence of an adjacent pair of circulatory features of opposing spm. As the dipole translated northward, it interacted with the Agulhas Bank and the cyclone was strained, becoming a filament as it was forced between the Agulhas ring and the Agulhas Bank. West of Cape Town the dipole was re-established when the cyclone redeveloped, changing the orientation of the dipole so that a filament was drawn directly from the Agulhas Bank

Multicore structures and the splitting and merging of eddies in global oceans from satellite altimeter data

This study investigated the statistics of eddy splitting and merging in the global oceans based on 23 years of altimetry data. Multicore structures were identified using an improved geometric closed-contour algorithm of sea surface height. Splitting and merging events were discerned from continuous time series maps of sea level anomalies. Multicore structures represent an intermediate stage in the process of eddy evolution, similar to the generation of multiple nuclei in a cell as a preparatory phase for cell division. Generally, splitting or merging events can substantially change (by a factor of 2 or more) the eddy scale, amplitude, and eddy kinetic energy. Specifically, merging (splitting) generally causes an increase (decrease) of eddy properties. Multicore eddies were found to tend to split into two eddies with different intensities. Similarly, eddy merging is not an interaction of two equal-intensity eddies, and it tends to manifest as a strong eddy merging with a weaker one. A hybrid tracking strategy based on the eddy overlap ratio, considering both multicore and single-core eddies, was used to confirm splitting and merging events globally. The census revealed that eddy splitting and merging do not always occur most frequently in eddy-rich regions; e.g., their frequencies of occurrence in the Antarctic Circumpolar Current and western boundary currents were found to be greater than in midlatitude regions (20–35∘) to the north and south. Eddy splitting and merging are caused primarily by an unstable configuration of multicore structures due to obvious current– or eddy–topography interaction, strong current variation, and eddy–mean flow interaction.</p

Mean and eddy induced transport in the ocean region adjacent to the Greenland-Scotland Ridge

The circulation and transport in the ocean region adjacent to the Greenland- Scotland Ridge (GSR) are crucial for maintaining heat, freshwater, and sea-ice exchange between the Nordic Seas and Subpolar North Atlantic Ocean. The Nordic Seas receive low-density Atlantic Water and transform it into dense water. The dense water overflow contributes to the North Atlantic Deep Water mass formation, which feeds the lower limb of the Atlantic Meridional Overturning Circulation. The thesis presents results from a study of the temperature and salinity transport in the ocean region adjacent to the GSR consisting of three sub-projects. The first sub-project is focused on model simulations of interannual to decadal variability of the exchange through the GSR and its impact on the variability of the temperature and salinity in the Nordic Seas. The model results demonstrate that the increase in the transport of fresh and cold waters through Fram Strait in the 1960s was concurrent with a reduction in the exchange over the GSR. The resulting imbalance in salinity and heat fluxes through the strait and over the ridge contributed to the freshening of the water masses of the Nordic Seas and intensified the Great Salinity Anomaly in the 1960s. In the late 1980s the AW transport over the GSR was stronger than normal while the exchange through Fram Strait was close to normal. The related imbalance in the lateral heat fluxes through the strait and over the ridge warmed the Nordic Seas and caused an increase in the temperature of the AW inflow to the Arctic Ocean in the late 1980s (i.e., about a decade earlier than the warming of the subpolar North Atlantic Ocean in the mid-1990s). The second sub-project is focused on observational estimates of the heat flux convergence of the ocean region adjacent to the GSR. Along-track altimeter and sea surface temperature satellite observations and ARGO in-situ measurements of temperature and salinity are used to investigate the heat transport by mean currents and eddies in the ocean region adjacent to the GSR from 2003 to 2008. The results from the analysis show that the heat advection by the mean flow in the surface layer is zonally asymmetric with a higher magnitude in the western part of the region. This asymmetry is associated with an excessive mean heat advection in an area adjacent to the Denmark Strait. The advection of heat is high and positive south of the strait and low and negative north of it. We suggest that this heat advection impacts the local budgets of heat and potential energy of the mean flow in the surface layer. The third sub-project studies the vertical structure of the convergence of advective fluxes of heat (HFC) and salt (SFC) in the ocean region adjacent to the GSR. The study is based on SODA (Simple Ocean Data Assimilation) ocean reanalysis for the period between 1965 and 2010. The SODA based estimates show that the high values found in satellite based estimates of HFC over the Denmark Strait are part of a pattern of high HFC and SFC in the whole water column. In this region, the HFC has a maximum at the surface. The highest values of SFC there are found in the subsurface layer at depths between 500 and 1500 m. A similar structure of high positive SFC and HFC are observed at intermediate depths over the Iceland-Faroe Ridge. The EOF analysis of the HFC and SFC shows that the variability of the HFC and SFC in these two regions are dominated by modes of coherent variations in the SFC and HFC. During warming period in the late 1980s these modes drove strong variations of the HFC and SFC over Denmark Strait and Iceland-Faroe Ridge which correlated well with the variations in the surface wind stress curl