Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave

The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly. We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes in the preceding two winters combined with the re-emergence of cold ocean water masses. Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme European heat waves since the 1980s indicating that it is a common factor in the development of these events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position of the Jet Stream that favours the development of high surface temperatures over Central Europe during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major European summer temperature extremes in order to provide better advance warning measures of these high societal impact events

Reconstructing extreme AMOC events through nudging of the ocean surface: a perfect model approach

While the Atlantic Meridional Overturning Circulation (AMOC) is thought to be a crucial component of the North Atlantic climate, past changes in its strength are challenging to quantify, and only limited information is available. In this study, we use a perfect model approach with the IPSL-CM5A-LR model to assess the performance of several surface nudging techniques in reconstructing the variability of the AMOC. Special attention is given to the reproducibility of an extreme positive AMOC peak from a preindustrial control simulation. Nudging includes standard relaxation techniques towards the sea surface temperature and salinity anomalies of this target control simulation, and/or the prescription of the wind-stress fields. Surface nudging approaches using standard fixed restoring terms succeed in reproducing most of the target AMOC variability, including the timing of the extreme event, but systematically underestimate its amplitude. A detailed analysis of the AMOC variability mechanisms reveals that the underestimation of the extreme AMOC maximum comes from a deficit in the formation of the dense water masses in the main convection region, located south of Iceland in the model. This issue is largely corrected after introducing a novel surface nudging approach, which uses a varying restoring coefficient that is proportional to the simulated mixed layer depth, which, in essence, keeps the restoring time scale constant. This new technique substantially improves water mass transformation in the regions of convection, and in particular, the formation of the densest waters, which are key for the representation of the AMOC extreme. It is therefore a promising strategy that may help to better constrain the AMOC variability and other ocean features in the models. As this restoring technique only uses surface data, for which better and longer observations are available, it opens up opportunities for improved reconstructions of the AMOC over the last few decades

Re-emergence of North Atlantic subsurface ocean temperature anomalies in a seasonal forecast system

A high-resolution coupled ocean atmosphere model is used to study the effects of seasonal re-emergence of North Atlantic subsurface ocean temperature anomalies on northern hemisphere winter climate. A 50-member control ensemble is integrated from 1 September 2007 to 28 February 2008 and compared with a parallel ensemble with perturbed ocean initial conditions. The perturbation consists of a density-compensated subsurface Atlantic temperature anomaly corresponding to the observed subsurface temperature anomaly for September 2010. The experiment is repeated for two atmosphere horizontal resolutions (~ 60 km and ~ 25 km) in order to determine whether the sensitivity of the atmosphere to re-emerging temperature anomalies is dependent on resolution. A wide range of re-emergence behavior is found within the perturbed ensembles. While the observations seem to indicate that most of the re-emergence is occurring in November, most members of the ensemble show re-emergence occurring later in the winter. However, when re-emergence does occur it is preceded by an atmospheric pressure pattern that induces a strong flow of cold, dry air over the mid-latitude Atlantic, and enhances oceanic latent heat loss. In response to re-emergence (negative SST anomalies), there is reduced latent heat loss, less atmospheric convection, a reduction in eddy kinetic energy and positive low-level pressure anomalies downstream. Within the framework of a seasonal forecast system the results highlight the atmospheric conditions required for re-emergence to take place and the physical processes that may lead to a significant effect on the winter atmospheric circulation

Advances in reconstructing the AMOC using sea surface observations of salinity

The Atlantic meridional overturning circulation (AMOC) is one of the main drivers of climate variability at decadal and longer time scales. As there are no direct multi-decadal observations of this key circulation, the reconstruction of past AMOC variations is essential. This work presents a step forward in reconstructing the AMOC using climate models and time-varying surface nudging of salinity and temperature data, for which independent multi-decadal observed series are available. A number of nudging protocols are explored in a perfect model framework to best reproduce the AMOC variability accommodating to the characteristics of SST and SSS available products. As reference SST products with sufficient space and time coverage are available, we here choose to focus on the limitations associated to SSS products with the goal of providing protocols using independent salinity products. We consider a global gridded dataset and, additionally, a coarser SSS dataset restricted to the Atlantic and with a quite low spatial resolution (order of 10 degrees vs. 2 for the model grid). We show how, using the latter, we can improve the efficiency of the nudging on the AMOC reconstruction by adding a high-resolution annual cycle to the coarse resolution SSS product as well as a spatial downscaling to account for SSS gradient. The final protocol retained for the coarse SSS data is able to reconstruct a 100-year long AMOC period (average of 10.18 Sv and a standard deviation of 1.39 Sv), with a correlation of 0.76 to the target and a RMSE of 0.99 Sv. These values can be respectively compared to 0.85 and 0.75 Sv when using the global salinity surface observations. This work provides a first step towards understanding the limitations and prospects of historical AMOC reconstructions using different sea surface salinity datasets for the surface nudging

A correlative and quantitative imaging approach enabling characterization of primary cell-cell communication: Case of human CD4+ T cell-macrophage immunological synapses

Cell-to-cell communication engages signaling and spatiotemporal reorganization events driven by highly context-dependent and dynamic intercellular interactions, which are difficult to capture within heterogeneous primary cell cultures. Here, we present a straightforward correlative imaging approach utilizing commonly available instrumentation to sample large numbers of cell-cell interaction events, allowing qualitative and quantitative characterization of rare functioning cell-conjugates based on calcium signals. We applied this approach to examine a previously uncharacterized immunological synapse, investigating autologous human blood CD4+ T cells and monocyte-derived macrophages (MDMs) forming functional conjugates in vitro. Populations of signaling conjugates were visualized, tracked and analyzed by combining live imaging, calcium recording and multivariate statistical analysis. Correlative immunofluorescence was added to quantify endogenous molecular recruitments at the cell-cell junction. By analyzing a large number of rare conjugates, we were able to define calcium signatures associated with different states of CD4+ T cell-MDM interactions. Quantitative image analysis of immunostained conjugates detected the propensity of endogenous T cell surface markers and intracellular organelles to polarize towards cell-cell junctions with high and sustained calcium signaling profiles, hence defining immunological synapses. Overall, we developed a broadly applicable approach enabling detailed single cell- and population-based investigations of rare cell-cell communication events with primary cells

Essai d'optimisation d'un reseau agrometeorologique.

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Expression of thrombopoietin by umbilical vein endothelial cells.

International audienceCurrent data suggest that the primary source of thrombopoietin (TPO) is the liver. Extra-hepatic sites for TPO production have been demonstrated essentially through the study of the expression of TPO mRNA. In this work, we report that TPO is expressed at low levels by endothelial cells (EC) derived from the umbilical vein (HUVEC). Both TPO mRNA and the protein are expressed and the protein is functional as assessed by biological assay. Expression of TPO by HUVEC may be useful to study the regulation of the production of this cytokine and to understand the apparent specific interactions between mature megakaryocytes and EC in the bone marrow

Expression of thrombopoietin by umbilical vein endothelial cells.

International audienceCurrent data suggest that the primary source of thrombopoietin (TPO) is the liver. Extra-hepatic sites for TPO production have been demonstrated essentially through the study of the expression of TPO mRNA. In this work, we report that TPO is expressed at low levels by endothelial cells (EC) derived from the umbilical vein (HUVEC). Both TPO mRNA and the protein are expressed and the protein is functional as assessed by biological assay. Expression of TPO by HUVEC may be useful to study the regulation of the production of this cytokine and to understand the apparent specific interactions between mature megakaryocytes and EC in the bone marrow