Abstract. SURFEX is a new externalized land and ocean surface platform that describes the surface fluxes and the evolution of four types of surfaces: nature, town, inland water and ocean. It is mostly based on pre-existing, well-validated scientific models that are continuously improved. The motivation for the building of SURFEX is to use strictly identical scientific models in a high range of applications in order to mutualise the research and development efforts. SURFEX can be run in offline mode (0-D or 2-D runs) or in coupled mode (from mesoscale models to numerical weather prediction and climate models). An assimilation mode is included for numerical weather prediction and monitoring. In addition to momentum, heat and water fluxes, SURFEX is able to simulate fluxes of carbon dioxide, chemical species, continental aerosols, sea salt and snow particles. The main principles of the organisation of the surface are described first. Then, a survey is made of the scientific module (including the coupling strategy). Finally, the main applications of the code are summarised. The validation work undertaken shows that replacing the pre-existing surface models by SURFEX in these applications is usually associated with improved skill, as the numerous scientific developments contained in this community code are used to good advantage.

[1] The formation of the Atlantic cold tongue (ACT) is the dominant seasonal sea surface temperature signal in the eastern equatorial Atlantic (EEA). A comprehensive analysis of variability in its spatial extent, temperature, and onset is presented. Then, the physical mechanisms which initiate ACT onset, as well as the feedbacks from the ACT to the maritime boundary layer, and how the ACT influences the onset of the West African monsoon (WAM) are discussed. We argue that in the EEA, the air-sea coupling between the ACT and WAM occurs in two phases. From March to mid-June, the ACT results from the intensification of the southeastern trades associated with the St. Helena anticyclone. Steering of surface winds by the basin shape of the EEA imparts optimal wind stress for generating the maximum upwelling south of the equator. During the second phase (mid-June-August), wind speeds north of the equator increase as a result of the northward progression of the intensifying trades and as a result of significant surface heat flux gradients produced by the differential cooling between the ACT and the tropical waters circulating in the Gulf of Guinea (GG). It is anticipated that the atmospheric divergence induced at low levels north of the equator reduces convection over the GG and that increased northward winds shift convection over land. Correlations between the ACT and the WAM onset dates over the last 26 years measure as much as 0.8. This suggests that the ACT plays a key role in the WAM onset.

[1] Torrential rains often occur in the western Mediterranean region during the fall season when the Mediterranean Sea is still warm. The Mediterranean Sea acts in moistening and warming the low level of the atmosphere. Then, the southerly to easterly flow that prevails before and during torrential rainfall events, transports the conditionally unstable air toward the coasts where the convection can develop often triggered by a flow interaction with orography. This study examines the sensitivity to the sea surface temperature (SST) of very short range (18-24 hours) high-resolution (2.4 km) forecasts of heavy precipitation events. Three torrential rainfall events were selected as representative of extreme rainfall events that occurred over southern France: two cases of quasistationary mesoscale convective systems and one other case characterized by a slow moving frontal perturbation. For each case, a number of runs is performed with the MESO-NH research model using several SST fields differing in their resolution or/and their average value over the Mediterranean basin. Results show that an increase (a decrease) of SST by several degrees, on average, intensifies (weakens) the convection. The convection can even be stopped with strong decrease of SST. Impacts on the stationary behavior of the systems have also been pointed out. A more detailed SST field influences the mesoscale patterns of the sea surface heat fluxes but have almost no significant effect on the convection and the low-level jets forecast. Eventually, the SST has been allowed to evolve during the runs through the action of the air-sea interface fluxes, resulting in local effects such as significant cooling of the SST beneath the low-level jet but almost no impact on the very short range forecasts of heavy precipitation.Citation: Lebeaupin, C., V. Ducrocq, and H. Giordani (2006), Sensitivity of torrential rain events to the sea surface temperature based on high-resolution numerical forecasts,

The tropical Atlantic is home to multiple coupled climate variations covering a wide range of timescales and impacting societally relevant phenomena such as continental rainfall, Atlantic hurricane activity, oceanic biological productivity, and atmospheric circulation in the equatorial Pacific. The tropical Atlantic also connects the southern

During winter 2012–2013, open‐ocean deep convection which is a major driver for the thermohaline circulation and ventilation of the ocean, occurred in the Gulf of Lions (Northwestern Mediterranean Sea) and has been thoroughly documented thanks in particular to the deployment of several gliders, Argo profiling floats, several dedicated ship cruises, and a mooring array during a period of about a year. Thanks to these intense observational efforts, we show that deep convection reached the bottom in winter early in February 2013 in a area of maximum 28 ± 3 109normalm2. We present new quantitative results with estimates of heat and salt content at the subbasin scale at different time scales (on the seasonal scale to a 10 days basis) through optimal interpolation techniques, and robust estimates of the deep water formation rate of 2.0 ±0.2 Sv. We provide an overview of the spatiotemporal coverage that has been reached throughout the seasons this year and we highlight some results based on data analysis and numerical modeling that are presented in this special issue. They concern key circulation features for the deep convection and the subsequent bloom such as Submesoscale Coherent Vortices (SCVs), the plumes, and symmetric instability at the edge of the deep convection area.

SURFEX is a new externalized land and ocean surface platform that describes the surface fluxes and the evolution of four types of surface: nature, town, inland water and ocean. It can be run either coupled or in offline mode. It is mostly based on pre-existing, well validated scientific models. It can be used in offline mode (from point scale to global runs) or fully coupled with an atmospheric model. SURFEX is able to simulate fluxes of carbon dioxide, chemical species, continental aerosols, sea salt and snow particles. It also includes a data assimilation module. The main principles of the organization of the surface are described first. Then, a survey is made of the scientific module (including the coupling strategy). Finally the main applications of the code are summarized. The current applications are extremely diverse, ranging from surface monitoring and hydrology to numerical weather prediction and global climate simulations. The validation work undertaken shows that replacing the pre-existing surface models by SURFEX in these applications is usually associated with improved skill, as the numerous scientific developments contained in this community code are used to good advantage

International audienceA comparison of June 2005 and June 2006 sea surface temperatures in the eastern equatorial Atlantic exhibits large variability in the properties of the equatorial cold tongue, with far colder temperatures in 2005 than in 2006. This difference is found to result mainly from a time shift in the development of the cold tongue between the two years. Easterlies were observed to be stronger in the western tropical Atlantic in April–May 2005 than in April–May 2006, and these winds favorably preconditioned oceanic subsurface conditions in the eastern Atlantic. However, it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid-May 2005 over a broad region extending from 20°W to the African coast and from 6°S to the equator. This particular event underscores the ability of local intraseasonal wind stress variability in the Gulf of Guinea to initiate the cold tongue season and thus to dramatically impact the SST in the eastern equatorial Atlantic. Such intraseasonal wind intensifications are of potential importance for year-to-year variability in the onset of the African monsoon

Estimating vertical velocity in the oceanic upper layers is a key issue for understanding ocean dynamics and the transport of biogeochemical elements. This paper aims to identify the physical sources of vertical velocity associated with sub-mesoscale dynamics (fronts, eddies) and mixed-layer depth (MLD) structures, using (a) an ocean adaptation of the generalized Q-vector form of the ω-equation deduced from a primitive equation system which takes into account the turbulent buoyancy and momentum fluxes and (b) an application of this diagnostic method for an ocean simulation of the Programme Océan Multidisciplinaire Méso Echelle (POMME) field experiment in the North-Eastern Atlantic. The approach indicates that wsources can play a significant role in the ocean dynamics and strongly depend on the dynamical structure (anticyclonic eddy, front, MLD, etc.). Our results stress the important contribution of the ageostrophic forcing, even under quasigeostrophic conditions. The turbulent w-forcing was split into two components associated with the spatial variability of (a) the buoyancy and momentum (Ekman pumping) surface fluxes and (b) the MLD. Process (b) represents the trapping of the buoyancy and momentum surface energy into the MLD structure and is identified as an atmosphere/ oceanic mixed-layer coupling. The momentum-trapping process is 10 to 100 times stronger than the Ekman pumping and is at least 1,000 times stronger than the buoyancy w-sources. When this decomposition is applied to a filamentary mixed-layer structure simulated during the POMME experiment, we find that the associated vertical velocity is created by trapping the surface wind-stress energy into this structure and not by Ekman pumping.