Improving sea level simulation in Mediterranean regional climate models

For now, the question about future sea level change in the Mediterranean remains a challenge. Previous climate modelling attempts to estimate future sea level change in the Mediterranean did not meet a consensus. The low resolution of CMIP-type models prevents an accurate representation of important small scales processes acting over the Mediterranean region. For this reason among others, the use of high resolution regional ocean modelling has been recommended in literature to address the question of ongoing and future Mediterranean sea level change in response to climate change or greenhouse gases emissions. Also, it has been shown that east Atlantic sea level variability is the dominant driver of the Mediterranean variability at interannual and interdecadal scales. However, up to now, long-term regional simulations of the Mediterranean Sea do not integrate the full sea level information from the Atlantic, which is a substantial shortcoming when analysing Mediterranean sea level response. In the present study we analyse different approaches followed by state-of-the-art regional climate models to simulate Mediterranean sea level variability. Additionally we present a new simulation which incorporates improved information of Atlantic sea level forcing at the lateral boundary. We evaluate the skills of the different simulations in the frame of long-term hindcast simulations spanning from 1980 to 2012 analysing sea level variability from seasonal to multidecadal scales. Results from the new simulation show a substantial improvement in the modelled Mediterranean sea level signal. This confirms that Mediterranean mean sea level is strongly influenced by the Atlantic conditions, and thus suggests that the quality of the information in the lateral boundary conditions (LBCs) is crucial for the good modelling of Mediterranean sea level. We also found that the regional differences inside the basin, that are induced by circulation changes, are model-dependent and thus not affected by the LBCs. Finally, we argue that a correct configuration of LBCs in the Atlantic should be used for future Mediterranean simulations, which cover hindcast period, but also for scenarios

The SURFEXv7.2 land and ocean surface platform for coupled or offline simulation of Earth surface variables and fluxes

CC Attribution 3.0 License.Final revised paper also available at http://www.geosci-model-dev.net/6/929/2013/gmd-6-929-2013.pdfInternational audienceSURFEX 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

Modelling of the Mediterranean circulation using atmospheric fields from the WRF model at different space-time resolutions

International audienceIn the framework of the MORCE-MED project, a two-way ocean-atmosphere coupling is developed between the Weather Research and Forecasting (WRF) atmospheric model and the NEMO ocean model over the Mediterranean basin. The future ocean-atmosphere coupled system is part of the future regional numerical platform including also the modelling of the continental superficial layers, atmospheric chemistry and marine biogeochemistry. Forced by global reanalyses or by the global climatic numerical system outputs, the whole regional coupled model aims to study the impacts of the climate change over the Mediterranean basin. Before applying the full two-way interactive coupling between the ocean and atmospheric regional models for long-term simulations over the Mediterranean, the forcing mode is considered through a sensitivity study. The downscaling of the NCEP reanalyses over the full Mediterranean basin with a 20-km resolution has been done with the WRF model between August 1998 and July 1999. The daily atmospheric fields obtained are then used to drive the NEMO model (with a 6-8 km resolution) over the Mediterranean Sea in a perpetual mode during a spinup of 8 years. Then, three experiments are done for a period of 4 years. The first experiment (or control experiment) is the continuity of the spinup. In the second experiment, a higher temporal resolution is used and the frequency of the forcing is 3 hours, which allows a good representation of the diurnal cycle and of the extreme air-sea exchanges that occur with a short duration during severe meteorological events. In the third experiment, a finer spatial resolution of the forcing is applied over the Gulf of Lions area in order to approach the ocean model resolution and to well represent the channelling of the Mistral and Tramontane. The benefit of increasing the space-time resolutions of the atmospheric forcing is estimated by a comparison of the ocean model performances to represent the general Mediterranean circulation as the characteristics of the mixed layer, of the deep convection and of the upwellings between the control experiment and the two sensitivity experiments, and by a comparison of our experiments to observations and climatologies

Numerical mesoscale air-sea coupling over the Gulf of Lions during two Tramontane/Mistral events

International audienceThe near-sea surface meteorological conditions associated with strong wind events constitute a strong forcing on the ocean mixed layer. The Gulf of Lions is one of the most windy region of the Mediterranean basin, with frequent Mistral and Tramontane events. These northerly and north-westerly low-level flows, generally induced by a cyclogenesis in the Ligurian basin, are channelled and accelerated in the Rhône and Aude valleys, respectively. They transport cold continental air over sea and induce strong momentum and heat exchanges at the air-sea interface. The local continental shelf circulation with sometimes transient coastal upwellings is also sensitive to these intense meteorological events. This study addresses the question of the sea surface scheme used in mesoscale atmospheric numerical modelling to represent the ocean mixed layer response under these severe wind events. Several slab ocean models have been used coupled with the Weather Research and Forecasting (WRF) model at 21 and 7-km resolution and applied on two Mistral/Tramontane cases (23-26 March 1998 and 5-9 November 1999): (i) a slab model based on the transport divergence equation where the mixed layer evolution is only driven by the wind stress; (ii) a slab model where the temperature is the only prognostic variable and evolves according to the net surface heat budget and (iii) the complete slab scheme from Price [1981]. The coupled simulations were also compared to two basic simulations, one using a constant sea surface temperature (SST) field during all of the model integration and another using a 6 hourly-update SST reanalysis. In this study, we mainly focused on the slab models performances. We identified the processes involved in the ocean mixed layer response under Mistral and Tramontane situations at mesoscale, i. e. local and fast cooling and deepening, and finally we investigated the feedbacks of an interactive ocean mixed layer on the atmospheric simulation

Numerical mesoscale air-sea coupling over the Gulf of Lions during two Tramontane/Mistral events

International audienceThe near-sea surface meteorological conditions associated with strong wind events constitute a strong forcing on the ocean mixed layer. The Gulf of Lions is one of the most windy region of the Mediterranean basin, with frequent Mistral and Tramontane events. These northerly and north-westerly low-level flows, generally induced by a cyclogenesis in the Ligurian basin, are channelled and accelerated in the Rhône and Aude valleys, respectively. They transport cold continental air over sea and induce strong momentum and heat exchanges at the air-sea interface. The local continental shelf circulation with sometimes transient coastal upwellings is also sensitive to these intense meteorological events. This study addresses the question of the sea surface scheme used in mesoscale atmospheric numerical modelling to represent the ocean mixed layer response under these severe wind events. Several slab ocean models have been used coupled with the Weather Research and Forecasting (WRF) model at 21 and 7-km resolution and applied on two Mistral/Tramontane cases (23-26 March 1998 and 5-9 November 1999): (i) a slab model based on the transport divergence equation where the mixed layer evolution is only driven by the wind stress; (ii) a slab model where the temperature is the only prognostic variable and evolves according to the net surface heat budget and (iii) the complete slab scheme from Price [1981]. The coupled simulations were also compared to two basic simulations, one using a constant sea surface temperature (SST) field during all of the model integration and another using a 6 hourly-update SST reanalysis. In this study, we mainly focused on the slab models performances. We identified the processes involved in the ocean mixed layer response under Mistral and Tramontane situations at mesoscale, i. e. local and fast cooling and deepening, and finally we investigated the feedbacks of an interactive ocean mixed layer on the atmospheric simulation

Numerical mesoscale air-sea coupling over the Gulf of Lions during two Tramontane/Mistral events

International audienceThe near-sea surface meteorological conditions associated with strong wind events constitute a strong forcing on the ocean mixed layer. The Gulf of Lions is one of the most windy region of the Mediterranean basin, with frequent Mistral and Tramontane events. These northerly and north-westerly low-level flows, generally induced by a cyclogenesis in the Ligurian basin, are channelled and accelerated in the Rhône and Aude valleys, respectively. They transport cold continental air over sea and induce strong momentum and heat exchanges at the air-sea interface. The local continental shelf circulation with sometimes transient coastal upwellings is also sensitive to these intense meteorological events. This study addresses the question of the sea surface scheme used in mesoscale atmospheric numerical modelling to represent the ocean mixed layer response under these severe wind events. Several slab ocean models have been used coupled with the Weather Research and Forecasting (WRF) model at 21 and 7-km resolution and applied on two Mistral/Tramontane cases (23-26 March 1998 and 5-9 November 1999): (i) a slab model based on the transport divergence equation where the mixed layer evolution is only driven by the wind stress; (ii) a slab model where the temperature is the only prognostic variable and evolves according to the net surface heat budget and (iii) the complete slab scheme from Price [1981]. The coupled simulations were also compared to two basic simulations, one using a constant sea surface temperature (SST) field during all of the model integration and another using a 6 hourly-update SST reanalysis. In this study, we mainly focused on the slab models performances. We identified the processes involved in the ocean mixed layer response under Mistral and Tramontane situations at mesoscale, i. e. local and fast cooling and deepening, and finally we investigated the feedbacks of an interactive ocean mixed layer on the atmospheric simulation

Ocean response in numerical mesoscale modelling during high-wind events over the Gulf of Lions

International audienceThe near-sea surface meteorological conditions associated with strong wind events constitute a strong forcing on the ocean mixed layer. The Gulf of Lions is one of the most windy region of the Mediterranean basin, with frequent Mistral and Tramontane events. These northerly and north-westerly low-level flows, generally induced by a cyclogenesis in the Ligurian basin, transport cold continental air over sea and induce strong momentum and heat exchanges at the air-sea interface. The local continental shelf circulation with sometimes transient coastal upwellings is also sensitive to these intense meteorological events. A preliminary study addresses the question of the sea surface scheme used in mesoscale atmospheric numerical modelling to represent the ocean mixed layer response under these severe wind events. Several slab ocean models have been used coupled with the Weather Research and Forecasting (WRF) model at 21 and 7-km resolution and applied on two Mistral/Tramontane cases. We mainly focused on the slab models performances to represent the ocean mixed layer response under Mistral and Tramontane situations at mesoscale, i. e. local and fast cooling and deepening, and finally we investigated the feedbacks of an interactive ocean mixed layer on the atmospheric simulation. In a second experimental set, the downscaling of the NCEP reanalyses over the full Mediterranean basin has been done with the WRF model between August 1998 and July 1999. The atmospheric fields obtained are then used to drive the regional NEMO-MED12 ocean model with a 1/12° resolution in a perpetual mode. The benefit of increasing the space and time resolutions of the atmospheric forcing (20 to nearly 7 km; daily to 3-hourly) is estimated by a comparison of the ocean model performances to represent the general Mediterranean circulation as the characteristics of the mixed layer, of the deep convection and of the upwellings between the sensitivity experiments, and by a comparison of our experiments to observations and climatologies. A special focus on the local 3D circulation in the Gulf of Lions under high-wind events in these simulations will be presented during the conference