PIRATA: A Sustained Observing System for Tropical Atlantic Climate Research and Forecasting

Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) is a multinational program initiated in 1997 in the tropical Atlantic to improve our understanding and ability to predict ocean-atmosphere variability. PIRATA consists of a network of moored buoys providing meteorological and oceanographic data transmitted in real time to address fundamental scientific questions as well as societal needs. The network is maintained through dedicated yearly cruises, which allow for extensive complementary shipboard measurements and provide platforms for deployment of other components of the Tropical Atlantic Observing System. This paper describes network enhancements, scientific accomplishments and successes obtained from the last 10 years of observations, and additional results enabled by cooperation with other national and international programs. Capacity building activities and the role of PIRATA in a future Tropical Atlantic Observing System that is presently being optimized are also described

Combinations of single-top-quark production cross-section measurements and vertical bar f(LV)V(tb)vertical bar determinations at root s=7 and 8 TeV with the ATLAS and CMS experiments

This paper presents the combinations of single-top-quark production cross-section measurements by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at = 7 and 8 TeV corresponding to integrated luminosities of 1.17 to 5.1 fb(-1) at = 7 TeV and 12.2 to 20.3 fb(-1) at = 8 TeV. These combinations are performed per centre-of-mass energy and for each production mode: t-channel, tW, and s-channel. The combined t-channel cross-sections are 67.5 +/- 5.7 pb and 87.7 +/- 5.8 pb at = 7 and 8 TeV respectively. The combined tW cross-sections are 16.3 +/- 4.1 pb and 23.1 +/- 3.6 pb at = 7 and 8 TeV respectively. For the s-channel cross-section, the combination yields 4.9 +/- 1.4 pb at = 8 TeV. The square of the magnitude of the CKM matrix element V-tb multiplied by a form factor f(LV) is determined for each production mode and centre-of-mass energy, using the ratio of the measured cross-section to its theoretical prediction. It is assumed that the top-quark-related CKM matrix elements obey the relation |V-td|, |V-ts| << |V-tb|. All the |f(LV)V(tb)|(2) determinations, extracted from individual ratios at = 7 and 8 TeV, are combined, resulting in |f(LV)V(tb)| = 1.02 +/- 0.04 (meas.) +/- 0.02 (theo.). All combined measurements are consistent with their corresponding Standard Model predictions.Peer reviewe

Ocean Barotropic Vorticity Balances: Theory and Application to Numerical Models

International audienceAbstract The barotropic vorticity (BV) balance is fundamental when interpreting the ocean gyre circulation. Here we propose an intercomparison of vorticity equations for the depth‐integrated flow applied to ocean models. We review four distinct variants of the BV balances, each giving access to diagnostic equations for the depth‐integrated ocean circulation, either meridional, across geostrophic contours or its divergence. We then formulate those balances in the Vorticity Balances in NEMO (VoBiN) diagnostic package aimed at the NEMO ocean platform and more generally C‐grid ocean models. We show that spatial discretization of the equations of motion have profound implications for those vorticity balances. Finally, we diagnose the main balances of a global ocean climate simulation. In all vorticity balances, topographic torques arise from interactions of the flow with slanting topography. We identify significant spurious topographic torques related to the model's C‐grid discretizations, and we suggest ways to address them. In the depth‐integrated and BV balances, bottom vortex stretching and bottom pressure torque drive the flow interaction with topography, respectively. Contrary to Sverdrup theory, the wind stress curl, although dominant in the interior Subtropics, becomes a minor player anywhere significant bottom velocities prevail. The geostrophic contour vorticity balance highlights the limits of barotropic models of the ocean circulation through the so‐called JEBAR term. Finally, the transport divergence vorticity balance stresses the limitations of Ekman plus geostrophic dynamics for the mass balance closure in ocean models. This framework should encourage ocean modellers to diagnose more routinely momentum and vorticity equations

Dynamics of the Atlantic Marine Intertropical Convergence Zone

International audienceA generalized ω-equation is used to identify the contributions from different processes that force upward motions in the Atlantic Marine ITCZ (AMI) from a numerical mesoscale simulation of June 2010. This ω-equation separates the diabatic heating contributions, which lie at the core of the Weak Temperature Gradient (WTG) framework, from the dynamical terms. Three layers of atmosphere are found with different balance. In the Marine Atmospheric Boundary-Layer (MABL), the upward motions in the AMI are induced by the frontogenesis and buoyancy components, which are regulated by the ageostrophic adjustment due to the presence of thermal-wind imbalance. The balance of these three processes well captures the variability of the vertical velocity and the associated precipitation, meaning that boundary-layer processes play a central role in the AMI dynamics. In the layer [600-2,000 m], a zone of strong vertical wind-shear just above the MABL, the upward motions are induced by the ageostrophic adjustment and radiative components, which are counteracted by evaporation of convective precipitation. Above 2,000 m the ascending motions are driven by the deep convection heating, as expected by the WTG framework, and more surprisingly by the ageostrophic adjustment term within the Tropical Easterly Jet. Thanks to the use of the ω-equation, these results extend the current WTG framework to the boundary layer, where it is not expected to hold. In the free troposphere, the WTG framework only accounts for half of the AMI ascent, the other half being forced by the dynamical terms

Two-way one-dimensional high-resolution air-sea coupled modelling applied to Mediterranean heavy rain events

International audienceSouth-eastern France is prone to heavy rain events during the autumn. For these extreme precipitation events, the Mediterranean Sea fuels the atmospheric boundary layer in heat and moisture and sometimes contributes to flooding, owing to the large swell and waves produced in these situations. The aim of this study is to examine how the severe atmospheric conditions over the sea associated with these events alter the ocean mixed layer, and what feedback the ocean contributes to the precipitation events. To address these questions, a high-resolution air-sea coupled system is developed between the atmospheric MESO-NH model and a one-dimensional ocean model. It is applied for short range (24 h) and high-resolution (2-3 km) simulations of three representative torrential rainfall events: 12-13 November 1999 (Aude case), 8-9 September 2002 (Gard case) and 3 December 2003 (Hérault case). In those meteorological situations characterized by moderate to intense low-level winds, the Mediterranean Sea globally loses energy, to the benefit of the convective precipitating systems. The result is an overall decrease of the thermal content all along the simulation of the events. Significant cooling and deepening of the ocean mixed layer are found in the areas of intense low-level winds. A notable result of the study concerns the impact of the torrential rainfall on the ocean mixed layer. The most important disturbances of the ocean mixed layer are indeed found underneath the heavy precipitation. The salt content is decreased all along the ocean mixed layer depth, but more significantly in the first ten metres near the air-sea interface, with the formation of a salt barrier. By performing both two-way and one-way coupled simulations, it is found that the interactive coupling tends to moderate both the atmospheric and ocean responses compared with the one-way mode. The differences between the two-way and one-way coupled simulations are, however, found to be relatively small considering the atmospheric short-range forecast

Effects of the air–sea coupling time frequency on the ocean response during Mediterranean intense events

International audienceThe near-sea surface meteorological conditions associated with the Mediterranean heavy precipitation events constitute, on a short time scale, a strong forcing on the ocean mixed layer. This study addresses the question of the optimal time frequency of the atmospheric forcing to drive an ocean model in order to make it able to capture the fine scale ocean mixed layer response to severe meteorological conditions. The coupling time frequency should allow the ocean model to reproduce the formation of internal low-salty boundary layers due to sudden input of intense precipitation, as well as the cooling and deepening of the ocean mixed layer through large latent heat fluxes and stress under the intense low-level jet associated with these events. In this study, the one-dimensional ocean model is driven by 2.4-km atmospheric simulated fields on a case of Mediterranean heavy precipitation, varying the time resolution of the atmospheric forcing. The results show that using a finer temporal resolution than 1 h for the atmospheric forcing is not necessary, but a coarser temporal resolution (3 or 6 h) modifies the event course and intensity perceived by the ocean. Consequently, when using a too coarse temporal resolution forcing, typically 6 h, the ocean model fails to reproduce the ocean mixed layer fine scale response under the heavy rainfall pulses and the strong wind gusts

An Eddy-Diffusivity Mass-Flux Parameterization for Modeling Oceanic Convection

International audienceA new one-dimensional (1-D) parameterization of penetrative convection has been developed in order to have a better representation of the vertical mixing in ocean general circulation models. Our approach is inspired from atmospheric parameterizations of shallow convection which assumes that in the convective boundary layer, the subgrid-scale fluxes result from two different mixing scales: small eddies, which are represented by an Eddy-Diffusivity (ED) contribution, and large eddies associated with thermals, which are represented by a mass-flux contribution. In the present work, the local (small eddies) and nonlocal (large eddies) contributions are unified into an Eddy-Diffusivity-Mass-Flux (EDMF) parameterization which treats simultaneously the whole vertical mixing. EDMF is implemented in the community ocean model NEMO and tested in its 1-D column version. Deepening of dense water in analytic cases, successfully reproduced in LES simulations, is more realistic with EDMF than with standard diffusion parameterizations. Also the convective events observed in the western Mediterranean at the Lion station and in the North Pacific Ocean at the PAPA station are more realistic in terms of sequencing and amplitude with EDMF

Numerical investigation of an oceanic resonant regime induced by hurricane winds

International audienceThe oceanic mixed layer (OML) response to an idealized hurricane with different propagation speeds is investigated using a two-layer reduced gravity ocean model. First, the model performances are examined with respect to available observations relative to Hurricane Frances (2004). Then, 11 idealized simulations are performed with a Holland (Mon Weather Rev 108(8):1212-1218, 1980) symmetric wind profile as surface forcing with storm propagation speeds ranging from 2 to 12 m s−1. By varying this parameter, the phasing between atmospheric and oceanic scales is modified. Consequently, it leads to different momentum exchanges between the hurricane and the OML and to various oceanic responses. The present study determines how OML momentum and heat budgets depend on this parameter. The kinetic energy flux due to surface wind stress is found to strongly depend on the propagation speed and on the cross-track distance from the hurricane center. A resonant regime between surface winds and near-inertial currents is clearly identified. This regime maximizes locally the energy flux into the OML. For fast-moving hurricanes (\textgreater6 m s−1), the ratio of kinetic energy converted into turbulence depends only on the wind stress energy input. For slow-moving hurricanes (\textless6 m s−1), the upwelling induced by current divergence enhances this conversion by shallowing the OML depth. Regarding the thermodynamic response, two regimes are identified with respect to the propagation speed. For slow-moving hurricanes, the upwelling combined with a sharp temperature gradient at the OML base formed in the leading part of the storm maximizes the oceanic heat loss. For fast propagation speeds, the resonance mechanism sets up the cold wake on the right side of the hurricane track. These results suggest that the propagation speed is a parameter as important as the surface wind speed to accurately describe the oceanic response to a moving hurricane

Response of the atmospheric boundary layer to a mesoscale oceanic eddy in the northeast Atlantic

Fields of air-sea turbulent fluxes and bulk variables were derived from satellite sensor data from February to April 2001, over a region of the northeast Atlantic where a field experiment, Programme Oce´an Multidisciplinaire Meso Echelle (POMME), was conducted. The satellite products are in good agreement with in situ data in terms of heat fluxes, sea surface temperature, and wind speed. The central part of the experimental domain presented a cyclonic eddy in the ocean, which corresponded to a cold sea surface temperature (SST) anomaly. Winds were weaker within the eddy than outside of it, with lower latent and sensible heat loss. In order to analyze the relationship between the SST and wind anomalies, three numerical experiments were conducted with a regional atmospheric model. Three 3-month runs of the model were performed, using a realistic SST field, a smoothed SST field in which the cold SST was not present (reference run), and an SST field where the cold anomaly was increased by two degrees, successively. The fields simulated with the realistic SST were consistent with satellite sensor derived observations. In particular, the weak wind area over the cold SST anomaly was successfully rendered, whereas it was not present in the forcing fields. Taken individually, the three runs did not reveal the presence of secondary circulations. However, anomalous secondary circulations were clearly identified with respect to the reference run. The origin of the latter circulations was investigated with the Giordani and Planton generalization of the Sawyer-Eliassen equations. According to our results, differential heating induced by the cold SST anomaly mostly altered the vertical wind through the effect of friction and only marginally through pressure gradient forces. In the upper part of the boundary layer, the wind speed increased (decreased) over (downstream) the cold SST. We found that stability was the main factor that induced the simulated patterns of the friction term in the diagnostic equations. Therefore our results show that mesoscale wind patterns were significantly affected by SST gradients through the effect of stability, in a region of low oceanic eddy activity