A High Resolution Reanalysis for the Mediterranean Sea

Abstract

In order to be able to forecast the weather and estimate future climate changes in the ocean, it is crucial to understand the past and the mechanisms responsible for the ocean variability. This is particularly true in a complex area such as the Mediterranean Sea with diverse dynamics like deep convection and overturning circulation. To this end, effective tools are ocean reanalyses or reconstructions of the past ocean state. Here we present a new physical reanalysis of the Mediterranean Sea at high resolution, developed in the Copernicus Marine Environment Monitoring Service (CMEMS) framework. The hydrodynamic model is based on the Nucleus for European Modelling of the Ocean (NEMO) combined with a variational data assimilation scheme (OceanVar). The model has a horizontal resolution of 1/24° and 141 unevenly distributed vertical z* levels. It provides daily and monthly temperature, salinity, current, sea level and mixed layer depth as well as hourly fields for surface velocities and sea level. ECMWF ERA-5 atmospheric fields force the model and daily boundary conditions in the Atlantic are taken from a global reanalysis. The reanalysis covers the 33 years from 1987 to 2019. Initialized from SeaDataNet climatology in January 1985, it reaches a nominal state after a 2-years spin-up. In-situ data from CTD, ARGO floats and XBT are assimilated into the model in combination with satellite altimetry observations. This reanalysis has been validated and assessed through comparison to in-situ and satellite observations as well as literature climatologies. The results show an overall improvement of the comparison with observations and a better representation of the main dynamics of the region compared to a previous, lower resolution (1/16°), reanalysis. Temperature and salinity RMSD are decreased by respectively 14 and 18%. The salinity biases at depth of the previous version are corrected. Climate signals show continuous increase of the temperature and salinity, confirming estimates from observations and other reanalysis. The new reanalysis will allow the study of physical processes at multi-scales, from the large scale to the transient small mesoscale structures and the selection of climate indicators for the basin.

High-Resolution Reanalysis of the Mediterranean Sea Biogeochemistry (1999–2019)

“…While the surface layer is very well simulated (Figures 6B,E), the reanalysis tends to overestimate the oxygen observations, with a positive bias of approximately 20 mmol/m 3 below 300-400 m (Supplementary Figure 1). Small errors in oxygen at the surface (e.g., RMSD values between 5 and 13 mmol/m 3 ; Figure 6E) confirm that the high-temporal resolution temperature dynamics at the mesoscale (Escudier et al, 2021), which drive saturation and biological production, are generally suitably reproduced. In the deep layers, the reanalysis does not fully capture the oxygen dynamics due to the observed bias, and the uncertainty increases in the mesopelagic zone of the water column, specifically in the western subbasins, where it overcomes 30 mmol/m 3 .…”

“…Recent MedBFM upgrades, including the assimilation of BGC-Argo float data and biooptical components, have been described in Cossarini et al (2019) and Terzić et al (2019Terzić et al ( , 2021, respectively. MedBFM is coupled off-line with the CMEMS Mediterranean Sea physical reanalysis system (Escudier et al, 2021), which has provided the necessary fields at a daily frequency for 21-year integration (currents, temperature, salinity, diffusivity, wind, and solar radiation) to force tracer transport, dependency of biochemical kinetics on the temperature, and air-sea interactions.…”

“…Although the boundary conditions data [i.e., Atlantic boundaries retrieved from the World Ocean Atlas and GLobal Ocean Data Analysis Project (GLODAP), and nutrient inputs obtained from Perseus] are climatological, the atmospheric (ERA5 data) and ocean dynamics forcing data (Escudier et al, 2021) are daily data, and the assimilated biogeochemical data (surface chlorophyll; Teruzzi et al, 2018) are weekly data, thus driving the high-frequency dynamics of the 21-year long reanalysis model simulation.…”

“…Chlorophyll simulation achieves a good skill (Figure 8) and captures important spatial and temporal signals, which have already been reported in the literature (Lavigne et al, 2015;Barbieux et al, 2019), the enhanced seasonal cycle in both the surface (i.e., winter/spring blooms, especially in the western regions) and subsurface (50-150 m, with DCM onset from April to October) layers and significant spatial variability both in terms of the zonal gradient and vertical displacement of the subsurface chlorophyll maximum (Figure 9). At the surface, the oxygen temporal variability reaches as high as approximately 50-60 mmol/m 3 following the seasonal saturation cycle, with important spatial differences among the subbasins attributed to the spatial gradient of the surface temperature (Escudier et al, 2021). In the subsurface layer, biological activity drives the variability associated with a maximum at a depth from approximately 50-100 m (i.e., just above the DCM), and important spatial differences are associated with the different levels of productivity in the various subbasins, with the western subbasins being more productive and shallower than are the eastern subbasins.…”

“…In the present study, we describe a novel version of the CMEMS biogeochemical reanalysis product covering the 1999-2019 period, with a spatial resolution as high as 1/24 • horizontally and 125 vertical levels, based on the most advanced version of the Mediterranean Sea Biogeochemical Flux Model system (MedBFM) operationally employed in CMEMS for short-term prediction purposes (Salon et al, 2019). Additionally, the biogeochemical reanalysis includes the latest update of the assimilation scheme (Teruzzi et al, 2018) of ocean color data (Colella et al, 2021) and off-line coupling with the latest release of a physical reanalysis (Escudier et al, 2021). The reanalysis results are available as daily and monthly 3D fields of 12 biogeochemical variables and fluxes through the CMEMS service.…”