Spin‐up of UK Earth System Model 1 (UKESM1) for CMIP6

For simulations intended to study the influence of anthropogenic forcing on climate, temporal stability of the Earth's natural heat, freshwater and biogeochemical budgets is critical. Achieving such coupled model equilibration is scientifically and computationally challenging. We describe the protocol used to spin‐up the UK Earth system model (UKESM1) with respect to pre‐industrial forcing for use in the 6th Coupled Model Intercomparison Project (CMIP6). Due to the high computational cost of UKESM1's atmospheric model, especially when running with interactive full chemistry and aerosols, spin‐up primarily used parallel configurations using only ocean/land components. For the ocean, the resulting spin‐up permitted the carbon and heat contents of the ocean's full volume to approach equilibrium over ~5000 years. On land, a spin‐up of ~1000 years brought UKESM1's dynamic vegetation and soil carbon reservoirs towards near‐equilibrium. The end‐states of these parallel ocean‐ and land‐only phases then initialised a multi‐centennial period of spin‐up with the full Earth system model, prior to this simulation continuing as the UKESM1 CMIP6 pre‐industrial control (piControl). The realism of the fully‐coupled spin‐up was assessed for a range of ocean and land properties, as was the degree of equilibration for key variables. Lessons drawn include the importance of consistent interface physics across ocean‐ and land‐only models and the coupled (parent) model, the extreme simulation duration required to approach equilibration targets, and the occurrence of significant regional land carbon drifts despite global‐scale equilibration. Overall, the UKESM1 spin‐up underscores the expense involved and argues in favour of future development of more efficient spin‐up techniques

Characteristics and conditions of production of transient luminous events observed over a maritime storm

International audienceOn the night of 15/16 November 2007, cameras in southern France detected 30 transient luminous events (TLEs) over a storm located in the Corsican region (France). Among these TLEs, 19 were sprites, 6 were halos, and 5 were elves. For 26 of them, a positive “parent” cloud‐to‐ground lightning (P+CG) flash was identified. The peak current of the P+CG flashes for the sprites had an average value of 63 kA and had a maximum value of 125 kA. The flashes for the halos and the elves had average values of 272 and 351 kA, respectively, and they had maximum values of 312 and 384 kA, respectively. No TLEs were detected after negative CG flashes with very large peak currents. Among the 26 P+CG flashes, 23 were located in a stratiform region with reflectivity values lower than 45 dBZ. The CG flashes in this region were classified into two groups according to the time interval separating them from the following flash: one group with values less than 2 s and one with values greater than 2 s. About 79% of all CGs were produced in a sequence of at least two flashes less than 2 s apart. For 65.5% of the sequences, the first flash was positive with an average peak current of 73 kA, while the later +CG flashes in a sequence had much lower peak currents. Several triangulated sprites were found to be shifted from their P+CG flashes by about 10 to 50 km and preferentially downstream. The observations suggest that the P+CG flashes can initiate both sprites and other CG flashes in a storm

Simulated anthropogenic CO₂ storage and acidification of the Mediterranean Sea

International audienceConstraints on the Mediterranean Sea's storage of anthropogenic CO 2 are limited, coming only from data-based approaches that disagree by more than a factor of two. Here we simulate this marginal sea's anthropogenic carbon storage by applying a perturbation approach in a high-resolution regional model. Our model simulates that, between 1800 and 2001, basin-wide CO 2 storage by the Mediterranean Sea has increased by 1.0 Pg C, a lower limit based on the model's weak deep-water ventilation, as revealed by evaluation with CFC-12. Furthermore, by testing a data-based approach (transit time distribution) in our model, comparing simulated anthropogenic CO 2 to values computed from simulated CFC-12 and physical variables, we conclude that the associated basin-wide storage of 1.7 Pg, published previously, must be an upper bound. Out of the total simulated storage of 1.0 Pg C, 75 % comes from the air-sea flux into the Mediterranean Sea and 25 % comes from net transport from the Atlantic across the Strait of Gibraltar. Sensitivity tests indicate that the Mediterranean Sea's higher total alkalinity, relative to the global-ocean mean, enhances the Mediterranean's total inventory of anthropogenic carbon by 10 %. Yet the corresponding average anthropogenic change in surface pH does not differ significantly from the globalocean average, despite higher total alkalinity. In Mediterranean deep waters, the pH change is estimated to be between −0.005 and −0.06 pH units

Characteristics and conditions of production of TLEs observed over a maritime storm

The coordinated European campaign of TLE observation in 2007 involved several cameras in southern France and northern Spain and lasted several months of summer, fall, and winter. Thanks to good conditions of observation at the camera locations and to several periods of long-duration storm activity, a large proportion of the TLEs (~90%) were observed from October to January over the Mediterranean Sea. Different types of TLEs (sprites, halos, elves) were produced by these maritime storms.Preprin

Characteristics and conditions of production of TLEs observed over a maritime storm

The coordinated European campaign of TLE observation in 2007 involved several cameras in southern France and northern Spain and lasted several months of summer, fall, and winter. Thanks to good conditions of observation at the camera locations and to several periods of long-duration storm activity, a large proportion of the TLEs (~90%) were observed from October to January over the Mediterranean Sea. Different types of TLEs (sprites, halos, elves) were produced by these maritime storms

Characteristics and conditions of production of TLEs observed over a maritime storm

The coordinated European campaign of TLE observation in 2007 involved several cameras in southern France and northern Spain and lasted several months of summer, fall, and winter. Thanks to good conditions of observation at the camera locations and to several periods of long-duration storm activity, a large proportion of the TLEs (~90%) were observed from October to January over the Mediterranean Sea. Different types of TLEs (sprites, halos, elves) were produced by these maritime storms

New insights into the organic carbon export in the Mediterranean Sea from 3-D modeling

International audienceThe Mediterranean Sea is one of the most oligotrophic regions of the oceans, and nutrients have been shown to limit both phytoplankton and bacterial activities, resulting in a potential major role of dissolved organic carbon (DOC) export in the biological pump. Strong DOC accumulation in surface waters is already well documented, though measurements of DOC stocks and export flux are still sparse and associated with major uncertainties. This study provides the first basin-scale overview and analysis of organic carbon stocks and export fluxes in the MediterraneanSea through a modeling approach based on a coupled model combining a mechanistic biogeochemical model (Eco3M-MED) and a high-resolution (eddy-resolving) hydrodynamic simulation (NEMO-MED12). The model is shown to reproduce the main spatial and seasonal biogeochemical characteristics of the Mediterranean Sea. Model estimations of carbon export are also of the same order of magnitude as estimations from in situ observations, and their respective spatial patterns are mutually consistent. Strong differences between the western and eastern basins are evidenced by the model for organic carbon export. Though less oligotrophic than the eastern basin, the western basin only supports 39 %of organic carbon (particulate and dissolved) export. Another major result is that except for the Alboran Sea, the DOC contribution to organic carbon export is higher than that of particulate organic carbon (POC) throughout the MediterraneanSea, especially in the eastern basin. This paper also investigates the seasonality of DOC and POC exports as well asthe differences in the processes involved in DOC and POC exports in light of intracellular quotas. Finally, according tothe model, strong phosphate limitation of both bacteria and phytoplankton growth is one of the main drivers of DOC accumulation and therefore of export

Characteristics and conditions of production of transient luminous events observed over a maritime storm

On the night of 15/16 November 2007, cameras in southern France detected 30 transient luminous events (TLEs) over a storm located in the Corsican region (France). Among these TLEs, 19 were sprites, 6 were halos, and 5 were elves. For 26 of them, a positive “parent” cloud-to-ground lightning (P+CG) flash was identified. The peak current of the P+CG flashes for the sprites had an average value of 63 kA and had a maximum value of 125 kA. The flashes for the halos and the elves had average values of 272 and 351 kA, respectively, and they had maximum values of 312 and 384 kA, respectively. No TLEs were detected after negative CG flashes with very large peak currents. Among the 26 P+CG flashes, 23 were located in a stratiform region with reflectivity values lower than 45 dBZ. The CG flashes in this region were classified into two groups according to the time interval separating them from the following flash: one group with values less than 2 s and one with values greater than 2 s. About 79% of all CGs were produced in a sequence of at least two flashes less than 2 s apart. For 65.5% of the sequences, the first flash was positive with an average peak current of 73 kA, while the later +CG flashes in a sequence had much lower peak currents. Several triangulated sprites were found to be shifted from their P+CG flashes by about 10 to 50 km and preferentially downstream. The observations suggest that the P+CG flashes can initiate both sprites and other CG flashes in a storm.Peer Reviewe

A fully coupled Mediterranean regional climate system model: design and evaluation of the ocean component for the 1980–2012 period

A fully coupled regional climate system model (CNRM-RCSM4) dedicated to the Mediterranean region is described and evaluated using a multidecadal hindcast simulation (1980–2012) driven by global atmosphere and ocean reanalysis. CNRM-RCSM4 includes the regional representation of the atmosphere (ALADIN-Climate model), land surface (ISBA model), rivers (TRIP model) and the ocean (NEMOMED8 model), with a daily coupling by the OASIS coupler. This model aims to reproduce the regional climate system with as few constraints as possible: there is no surface salinity, temperature relaxation, or flux correction; the Black Sea budget is parameterised and river runoffs (except for the Nile) are fully coupled. The atmospheric component of CNRM-RCSM4 is evaluated in a companion paper; here, we focus on the air–sea fluxes, river discharges, surface ocean characteristics, deep water formation phenomena and the Mediterranean thermohaline circulation. Long-term stability, mean seasonal cycle, interannual variability and decadal trends are evaluated using basin-scale climatologies and in-situ measurements when available. We demonstrate that the simulation shows overall good behaviour in agreement with state-of-the-art Mediterranean RCSMs. An overestimation of the shortwave radiation and latent heat loss as well as a cold Sea Surface Temperature (SST) bias and a slight trend in the bottom layers are the primary current deficiencies. Further, CNRM-RCSM4 shows high skill in reproducing the interannual to decadal variability for air–sea fluxes, river runoffs, sea surface temperature and salinity as well as open-sea deep convection, including a realistic simulation of the Eastern Mediterranean Transient. We conclude that CNRM-RCSM4 is a mature modelling tool allowing the climate variability of the Mediterranean regional climate system to be studied and understood. It is used in hindcast and scenario modes in the HyMeX and Med-CORDEX programs