Search

Consequences of the mesoscale Thermal FeedBack (TFB) on the ocean dynamics are studied in the South‐East Pacific (SEP) using a high‐resolution regional ocean–atmosphere coupled model. Three simulations are compared: the first one is a fully coupled simulation. In the second one, the TFB has been removed with an online smoothing of the Sea Surface Temperature (SST) conditions used by the atmosphere. In the third one, to disentangle the impact of the nearshore and the offshore TFB, the smoothing is only applied in the offshore region. In the SEP, the coastal upwelling cold tongue constitutes a permanent mesoscale SST pattern. We show that this SST pattern alters the coastal wind structure, reducing the coastal upwelling‐favorable wind intensity. So, the nearshore TFB reduces the coastal surface current and the vertical velocities. As a result, the Eddy Kinetic Energy (EKE) generation by baroclinic conversion is also weakened. In the offshore region, on the contrary, the oceanic mean state is not affected by the TFB and only the EKE is weakened. Composites above the coherent eddies show that the heat flux response to the mesoscale SST anomalies is responsible for the mesoscale activity weakening over the whole studied area. Although the wind response to the SST anomalies has a very weak mean impact on the EKE generation through wind work, we show that it strongly modifies the mean oceanic vertical velocity anomalies over the coherent eddies. The ocean dynamics is usually seen as driven by the atmosphere. However, the influence between the ocean and the atmosphere is mutual and the oceanic surface conditions feedback on the atmosphere. In this paper, we focus on the response of the atmosphere to the sea surface temperature mesoscale structures (structures that ranges from a few tens of km to a few hundred km). This response, called Thermal Feedback (TFB), is modeled with a high‐resolution ocean‐atmosphere coupled model to investigate its consequences on the oceanic dynamics in the South‐East Pacific. This region is characterized by a wind‐driven upwelling of sub‐surface waters near the coast, forming a sea surface coastal cold tongue. We show that this structure alters in return the coastal wind structure, which contributes to reduce the upwelling and the surface current. The generation of mesoscale structures, such as coherent eddies and filaments, is also reduced. The mesoscale structures are also weakened in the offshore region. We analyze the atmospheric response above the coherent eddies and show that the heat flux response to the SST anomalies is responsible for the eddy damping. Vertical velocities are also created in the eddy by the wind response to the SST anomalies. Mesoscale thermal feedback decreases the mean coastal upwelling‐favorable wind, surface current and nearshore eddy generationMesoscale thermal feedback on the heat fluxes decreases the coherent eddy intensity through baroclinic conversionCoherent eddy mean vertical velocity is strongly modified by the mesoscale thermal feedback Mesoscale thermal feedback decreases the mean coastal upwelling‐favorable wind, surface current and nearshore eddy generation Mesoscale thermal feedback on the heat fluxes decreases the coherent eddy intensity through baroclinic conversion Coherent eddy mean vertical velocity is strongly modified by the mesoscale thermal feedback

Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Recent modeling work suggested that ocean warming increases methylmercury (MeHg) levels in fish. Here, we studied the influence of El Niño Southern Oscillations (ENSO) on Hg concentrations and stable isotopes in time series of seabird blood from the Peruvian upwelling and oxygen minimum zone. Between 2009 and 2016, La Niña (2011) and El Niño conditions (2015–2016) were accompanied by sea surface temperature anomalies up to 3 °C, oxycline depth change (20–100 m), and strong primary production gradients. Seabird Hg levels were stable and did not co-vary significantly with oceanographic parameters, nor with anchovy biomass, the primary dietary source to seabirds (90%). In contrast, seabird Δ199Hg, proxy for marine photochemical MeHg breakdown, and δ15N showed strong interannual variability (up to 0.8 and 3‰, respectively) and sharply decreased during El Niño. We suggest that lower Δ199Hg during El Niño represents reduced MeHg photodegradation due to the deepening of the oxycline. This process was balanced by equally reduced Hg methylation due to reduced productivity, carbon export, and remineralization. The non-dependence of seabird MeHg levels on strong ENSO variability suggests that marine predator MeHg levels may not be as sensitive to climate change as is currently thought.

In recent years, persistent quasi‐zonal jets or striations have been ubiquitously detected in the world ocean using satellite and in situ data as well as numerical models. This study aims at determining the role of mesoscale eddies in the generation and persistence of striations off Chile in the eastern South Pacific. A 50 year climatological integration of an eddy‐resolving numerical ocean model is used to assess the long‐term persistence of striations. Automated eddy tracking algorithms are applied to the model outputs and altimetry data. Results reveal that striations coincide with both polarized eddy tracks and the offshore formation of new eddies in the subtropical front and coastal transition zone, without any significant decay over time that discards random eddies as a primary driver of the striations. Localized patches of vortex stretching and relative vorticity advection, alternating meridionally near the eastern edge of the subtropical front, are associated with topographic steering of the background flow in the presence of steep topography, and with baroclinically and barotropically unstable meridional flow. These sinks and sources of vorticity are suggested to generate the banded structure further west, consistently with a β‐plume mechanism. On the other hand, zonal/meridional eddy advection of relative vorticity and the associated Reynolds stress covariance are consistent with eddy deformation over rough topography and participate to sustain the striations in the far field. Shear instability of mean striations is proposed to feedback onto the eddy field, acting to maintain the subtropical front eddy streets and thus the striations. Striations coincide with polarized eddy tracks and the offshore formation of new eddiesVorticity produced by topographically steered unstable meridional flow triggers striations as β‐plumesEddy vorticity advection and striation shear instability participate to maintain eddy streets and striations

Coastal winds and upwelling of deep nutrient-rich water along subtropical eastern boundaries yield some of the ocean’s most productive ecosystems. Simple indices of coastal wind strength have been extensively used to estimate the timing and magnitude of biological productivity on seasonal and interannual timescales and underlie the prediction that anthropogenic climate warming will increase the productivity by making coastal winds stronger. The effect of wind patterns on regional net primary productivity is not captured by such indices and is poorly understood. Here we present evidence, using a realistic model of the California Current system and satellite measurements, that the observed slackening of the winds near the coast has little effect on near-shore phytoplankton productivity despite a large reduction in upwelling velocity. On the regional scale the wind drop-off leads to substantially higher production even when the total upwelling rate remains the same. This partial decoupling of productivity from upwelling results from the impact of wind patterns on alongshore currents and the eddies they generate. Our results imply that productivity in eastern boundary upwelling systems will be better predicted from indices of the coastal wind that account for its offshore structure.

Description de Leiperenia moreli n. sp., parasite de Loxodonta africana du Parc National de Gona-Re-Zhou au Zimbabwe. L'espèce diffère de L. leiperi Khalil, 1922 parasite d'un éléphant d'Ouganda par ses dimensions (le corps est plus petit, mais les pièces génitales sont plus longues). Les deux espèces différent de L. galebi Khalil, 1922, parasite de l'éléphant Indien par le queue des femelles beaucoup plus longue. Le genre a de nombreux caractères très originaux: papilles labiales et céphaliques à l'extrémité d'un pédoncule, oesophage avec portion antérieure subsphérique et portion postérieure claviforme, portion pharyngée de l'oesophage avec une dent dorsale, pore excréteur en fente longitudinale, femelle monodelphe pondant par endotokie matricide, mâle avec plusieurs paires de papilles piriformes hypertrophiées.

We present a two year study of the evolution of SL9 impact aerosol debris we observed between 0.4 and 0.9 micrometers with continuous high temporal coverage from July 1994 through September 1996 and at 1.7 and 2.3 micrometers during three observing runs in July 1994 and March and August 1995. Temporal cylindrical map projections at red continuum wavelengths in the region covered by the impact debris show the contributions of different mechanisms in producing the complicated morphological evolution of the sites during the first month. Long-term horizontal aerosol transport was mainly due to the zonal jets in the upper troposphere with extreme measured velocities of −10 and 20 m s−1. A comparison of the zonal drift of the core sites in the red continuum and in the 890-nm methane band (sensitive to higher levels) during the first month do not show significant velocity differences between these filters, indicating a low vertical wind shear in the upper troposphere. The spread of the aerosols resulted from the meridional and vertical shears of the zonal winds. Rapid initial outward expansions (speeds of ∼30 to 60 m s−1) and interactions with nearby vortices (speeds of ∼10 to 25 m s−1) also contributed to the dispersion of particulates. Using methane band images we have measured a steady poleward and equatorward meridional transport of the particulates with velocities of ∼−6 and 40 cm s−1, respectively. Particulates were detected up to ∼−20° by August 1995. Limb brightening in the 890-nm methane band was observed up to ∼−30° during the last observation (September 1996) reported here, indicating that a small population of aerosols was still present two years after impact. Photometric observations in the 890-nm band, together with a radiative transfer model, allowed us to calculate the evolution of the aerosol optical depth in the main impact core areas and in the subsequent SL9 band. We found a rapid decrease in optical depth in the largest impacts during July and August 1994 (from ≈3.2 to 2.1), followed by a gradual decrease during the next two years to ≈0.3 (June 1996). This behavior can be explained by simple models of debris horizontal dispersion by the wind shear and by sedimentation. Calculations of the characteristic times related to the microphysical processes in the aerosols (sedimentation, coagulation, and coalescence) together with their observed residence times (≥2 years) indicates that this persistent population of particles had sizes ≤0.1 micrometers during 1995 and 1996.

We report on a new series of ground-based CCD observations at visual wavelengths, covering a period of 1255 days between May 1992 and November 1995, of the longest-lived asymmetric feature known in Saturn's atmosphere: the north polar spot (NPS). This completes our previous analysis of this feature during the period 1990–1991 (A. Sanchez-Lavega, J. Lecacheux, F. Colas, and P. Lagues, 1993,Science260,329–332). Longitude measurements of the NPS indicate an averaged longitudinal drift of −0.030 deg/day for the whole period 1990–1995 corresponding to a zonal velocity of 0.11 msec−1. These data, when combined with previous and new measurements of the NPS position on Voyager 1 and 2 images obtained in 1980 and 1981, indicate a long-term drift in longitude of the NPS with a constant angular acceleration of 1.1 × 10−5deg/(day)2. High-resolution Voyager 2 violet, blue, green, and orange images were used to measure the size and reflectivity of the NPS. Its structure is characterized by a bright elliptical core surrounded by a dark ring and a large uniform area. The contrast between all these features changes appreciably from violet to orange: the spot is dark in violet but bright in orange relative to its surroundings. The spot is embedded within a region seeded by a “field of bright clouds” with characteristic size 1000 km reminiscent of a cellular convection pattern. The NPS's east–west apparent size is shorter at violet–blue (about 7000 km as limited by a dark ring at these wavelengths) than at green–orange (about 11,000 km corresponding to the large uniform area). Green processed images show apparent spiral patterns within the NPS consistent with anticyclonic vorticity. The results of ground-based photometry of the north polar region (NPR) and the NPS in the red methane absorption bands and their adjacent continuum are consistent with a radiative transfer model of the cloud vertical structure consisting of a clear gas layer, a haze layer, and a semi-infinite cloud. In the context of this model the NPS cloud tops are slightly higher than neighboring clouds reaching a pressure level of 45 mbar. Calculations of the seasonal insolation at the north pole, together with a simple linear radiative response of the atmosphere to this heating at different altitudes, suggest temperature changes at the level of the NPS cloud tops which should influence the NPS dynamics. Because of the long lifetime of the NPS, and because its motions did not vary appreciably during the long observing period, we suggest that the main properties and dynamics of the NPS are insensitive to the external solar forcing.