Mesoscale air-sea coupled interactions in the South-East Pacific

Cette thèse s’intéresse aux interactions entre l’océan et l’atmosphère dans le Pacifique Sud-Est, à des échelles comprises entre 10 et 300 km ("meso-échelle" océanique). Des observations satellites et un modèle couplé à haute résolution (1/12°) sont utilisés pour mettre en évidence et caractériser la relation entre la meso-échelle de température de surface de la mer (SST) et celle de l’intensité de la tension de vent (TV). Les observations montrent qu’environ un tiers de la meso-échelle de l’intensité de la TV est expliquée par les anomalies de la SST. L’intensité de la réponse de la TV aux anomalies de SST présente des variations spatiales et un cycle saisonnier marqué, également reproduits par le modèle. Une analyse de l’ajustement de la couche limite atmosphérique aux anomalies de meso-échelle de la SST dans les simulations permet d’expliquer ce cycle saisonnier et de comprendre l'origine des variations de la TV et de la vitesse du vent. Le modèle permet également d'étudier les conséquence de la modulation des flux à l’interface air-mer par la meso-échelle de SST et de courant de surface sur la dynamique océanique du Pacifique Sud-Est. D’une part, près de la côte, la réponse de la TV à la présence du front de SST diminue l’intensité de l’upwelling et la génération d’énergie cinétique turbulente (EKE) par instabilité barocline. La réponse de l’atmosphère à la meso-échelle de SST a également une rétroaction négative sur les anomalies de SST. D’autre part, la modulation de la TV par les courants de surface diminue la génération d’EKE par le travail des anomalies de TV, et créé un pompage d’Ekman qui atténue les anomalies de meso-échelle de la hauteur du niveau de la mer.This PhD thesis studies the air/sea interactions at the oceanic mesoscale (10-300 km) in the South-East Pacific and their consequences. Satellite observations and a high-resolution regional ocean-atmosphere coupled model are used to evidence and characterize the mesoscale Sea Surface Temperature (SST)-wind stress (WS) interactions. Offshore from 150km, observations show that one third of the WS mesoscale intensity is explained by the SST mesoscale anomalies. The intensity of the WS response intensity to the SST displays similar spatial and seasonal variability in both the model and the observations. The simulation is further analyzed to study this variations and to understand the boundary layer adjustment mechanisms. A momentum balance evidenced that the near surface wind anomalies are created by the anomalies of the turbulent mixing term. It is shown that WS intensity anomalies due to SST anomalies are are mainly forced by mixing coefficient anomalies and partially compensated by wind shear anomalies. The consequences on the oceanic dynamics of the air-sea momentum, heat and fresh water fluxes by mesoscale SST and surface current are investigated in the simulations. On one hand, near the coast, the WS response to the upwelling SST front decreases both the upwelling and the eddy kinetic energy (EKE) generation by baroclinic conversion. A negative feedback of the atmospheric response on the SST anomalies amplitude is also evidenced. On the other hand, the WS modulation by oceanic surface currents decreases the EKE generation by the mesoscale wind work. It also creates an Ekman pumping centered above the eddies and attenuating sea surface height anomalies

Coastal Upwelling Front Detection off Central Chile (36.5–37°S) and Spatio-Temporal Variability of Frontal Characteristics

In Eastern Boundary Upwelling Systems, cold coastal waters are separated from offshore by a strong cross-shore Sea Surface Temperature (SST) gradient zone. This upwelling front plays a major role for the coastal ecosystem. This paper proposes a method to automatically identify the front and define its main characteristics (position, width, and intensity) from high resolution data. The spatio-temporal variability of the front characteristics is then analyzed in a region off Central Chile (37°S), from 2003 to 2016. The front is defined on daily 1 km-resolution SST maps by isotherm T0 with T0 computed from mean SST with respect to the distance from the coast. The probability of detecting a front, as well as the front width and intensity are driven by coastal wind conditions and increased over the 2007–2016 period compared to the 2003–2006 period. The front position, highly variable, is related to the coastal jet configuration and does not depend on the atmospheric forcing. This study shows an increase by 14% in the probability of detecting a front and also an intensification by 17% of the cross-front SST difference over the last 14 years. No trend was found in the front position

The 2016 red tide crisis in southern Chile: Possible influence of the mass oceanic dumping of dead salmons

In 2016, a massive harmful algal bloom (HAB) of Alexandrium catenella around Chiloé island caused one of the major socio-ecological crisis in Chilean history. This red tide occurred in two distinct pulses, the second, most anomalous, bursting with extreme toxicity on the Pacific coast, weeks after the highly controversial dumping off Chiloé of 4,700 t of rotting salmons, killed by a previous HAB of Pseudochattonella verruculosa. We study the transport of this pollution, analyzing the physical oceanographic conditions during and after the dumping. We find that a cyclonic gyre was present between the dumping site and the coast, visible in satellite altimetry and sea surface temperature data. Using Lagrangian simulations, we confirm that near-surface currents could have brought part of the pollution to the coast, and fueled the bloom. This scenario explains also the anomalous later finding of ammonium near Chiloé. Finally we discuss the mismanagement of risk throughout the events

Ocean Front Detection with Glider and Satellite-Derived SST Data in the Southern California Current System

This study proposes a method to detect ocean fronts from in situ temperature and density glider measurements. This method is applied to data collected along the CalCOFI Line 90, south of the California Current System (CCS), over the 2006–2013 period. It is based on image-processing techniques commonly applied to sea surface temperature (SST) satellite data. Front detection results using glider data are consistent with those obtained in other studies carried out in the CCS. SST images of the Multi-scale Ultra-high Resolution (MUR) dataset were also used to compare the probability of occurrence or front frequency (FF) obtained with the two datasets. Glider and MUR temperatures are highly correlated. Along Line 90, frontal frequency exhibited the same maxima near the transition zone (~130 km offshore) as derived from MUR and glider datasets. However, marked differences were found in the bimonthly FF probability with high (low) front frequency in spring-summer for glider (MUR) data. Methodological differences explaining these contrasting results are investigated. Thermohaline-compensated fronts are more abundant towards the oceanic zone, although most fronts are detected using both temperature and density criteria, indicating a significant contribution of temperature to density in this region

Ocean Front Detection with Glider and Satellite-Derived SST Data in the Southern California Current System

This study proposes a method to detect ocean fronts from in situ temperature and density glider measurements. This method is applied to data collected along the CalCOFI Line 90, south of the California Current System (CCS), over the 2006–2013 period. It is based on image-processing techniques commonly applied to sea surface temperature (SST) satellite data. Front detection results using glider data are consistent with those obtained in other studies carried out in the CCS. SST images of the Multi-scale Ultra-high Resolution (MUR) dataset were also used to compare the probability of occurrence or front frequency (FF) obtained with the two datasets. Glider and MUR temperatures are highly correlated. Along Line 90, frontal frequency exhibited the same maxima near the transition zone (~130 km offshore) as derived from MUR and glider datasets. However, marked differences were found in the bimonthly FF probability with high (low) front frequency in spring-summer for glider (MUR) data. Methodological differences explaining these contrasting results are investigated. Thermohaline-compensated fronts are more abundant towards the oceanic zone, although most fronts are detected using both temperature and density criteria, indicating a significant contribution of temperature to density in this region

The lateral transport of zooplankton explains trophic and taxonomic similarities over the zonal gradient of central Chile

The highly productive upwelling zone in the Southeast Pacific provides inputs of C and N to the oceanic deep-water system via lateral transport, although the quality of this organic matter, the organisms being transported, and the controlling mechanisms are unclear. Here, we assessed whether the taxonomic and trophic structure of zooplankton over more oceanic offshore waters differ from that of zooplankton located in the upwelling zone, along with the characterization of the oceanographic processes and variables associated with the lateral transport. For this, epipelagic, mesopelagic, and bathypelagic zooplankton along with oceanographic variables and particulate organic matter (POM) were analyzed at the upwelling zone and adjacent oceanic stations off Central Chile (27–33°S), some of which were located over the Atacama trench, during September 2016. The community structure of size-fractioned zooplankton (2000 μm size classes) was assessed by an automated method, along with the analysis of the isotopic niche computed from their δ15N and δ13C. The isotope signatures of POM indicated that diatoms contributed mostly to production of new organic C, being the main food source for small-sized zooplankton in coastal waters. The zooplankton community structure differed between the upwelling and oceanic areas, but their isotopic signature showed a large overlap (78%). Satellite-derived geostrophic flow and the depth of the mixed layer appeared as the main factors explaining the homogeneity in the isotopic and biogeochemical signatures of zooplankton between the upwelling zone and offshore waters. Cross-shelf advection is thus suggested as a key process promoting zooplankton export to the deep-water ecosystem, including the ultra-deep Atacama Trench.MOPEX Cruise was funded by the Chilean National Agency of Research and Development (ANID) Grant AUB 160003. Funding was also provided by the Millennium Institute of Oceanography (Grant ICN12_019) and ANID-FONDECYT Postdoctoral (3220468) and Initiation (11221079) Projects. We are grateful to D. Toledo, M. Cornejo and K. Aniñir for sampling assistance. The work is a contribution to TROPHONET Project REDES 1039 funded by ANID of Chile and SCOR WG155 on Eastern Boundary Upwelling Systems (EBUS).Peer reviewe