Heat and salt redistribution within the Mediterranean Sea in the Med-CORDEX model ensemble

Abstract

Characterizing and understanding the basic functioning of the Mediterranean Sea in terms of heat and salt redistribution within the basin is a crucial issue to predict its evolution. Here we quantify and analyze the heat and salt transfers using a simple box model consisting of four layers in the vertical for each of the two (western and eastern) basins. Namely, we box-average 14 regional simulations of the Med-CORDEX ensemble plus a regional and a global reanalysis, computing for each of them the heat and salt exchanges between layers. First, we analyze in detail the mechanisms behind heat and salt redistribution at different time scales from the outputs of a single simulation (NEMOMED8). We show that in the western basin the transfer between layer 1 (0–150 m) and layer 2 (150–600 m) is upwards for most models both for heat and salt, while in the eastern basin both transfers are downwards. A feature common to both basins is that the transports are smaller in summer than in winter due to the enhanced stratification, which dampen the mixing between layers. From the comparison of the 16 simulations we observe that the spread between models is much larger than the ensemble average for the salt transfer and for the heat transfer between layer 1 and layer 2. At lower layers (below 600 m) there is a set of models showing a good agreement between them, while others are not correlated with any other. The mechanisms behind the ensemble spread are not straightforward. First, to have a coarse resolution prevents the model to correctly represent the heat and salt redistribution in the basin. Second, those models with a very different initial stratification also show a very different redistribution, especially at intermediate and deep layers. Finally, the assimilation of data seems to perturb the heat and salt redistribution. Besides this, the differences among regional models that share similar spatial resolution and initial conditions are induced by more subtle mechanisms which depend on the variable and process analyzed. In order to reduce the uncertainties in the Mediterranean regional climate projections further modelling studies and better observational datasets are needed to constrain the main sources of discrepancies among models. In the absence of those, an ensemble modelling approach as the one followed in the Med-CORDEX initiative seems to be the best solution to evaluate model uncertainties into the future climate projections. © 2016 Springer-Verlag Berlin Heidelber

Improving sea level simulation in Mediterranean regional climate models

“…(2)), and thus only the actual expansion is represented, the interannual variability is similar but a stong positive trend appears (4.48 mm year −1 ). This is due to the warming trend present in LMDZ-MED and already identified by Llasses et al (2016). This trend was driven by the warming of the deepest layers in the Levantine basin and attributed to a too short spin-up, thus being unrealistic.…”

Evolution of Mediterranean Sea water properties under climate change scenarios in the Med-CORDEX ensemble

“…In summary, in the surface and upper layers, the discrepancies between all the simulations and the reanalyses are in the same range of those obtained in previous works. They are mainly attributed to uncertainties in the surface heat and freshwater fluxes (Nabat et al 2014;Harzallah et al 2018;Llasses et al 2018;Darmaraki et al 2019). In the intermediate and deep layers, the difficulties of the ocean circulation models to reproduce the salinity and temperature trends have also been reported.…”

“…In the intermediate and deep layers, the difficulties of the ocean circulation models to reproduce the salinity and temperature trends have also been reported. Llasses et al (2018) analyzed 14 hindcast simulations from state-of-the-art ocean models and established that present-day regional models can provide reasonable estimates in the surface and upper layers but below 150 m are less reliable. In our case, the trends obtained for AWIs, LMDs, and CNRMs for the temperature and also for ENEA-CNRM-HIST for the salinity are in the range of variability of the state-ofthe-art models for the Mediterranean.…”

Unexpected Covariant Behavior of the Aegean and Ionian Seas in the Period 1987–2008 by Means of a Nondimensional Sea Surface Height Index

“…This study aims at analyzing the dynamics of the Adriatic‐Ionian‐Aegean Sea system using a multimodel approach. The advantages of using a multimodel approach in the Mediterranean region have been shown in the analysis of heat and salt budget (Harzallah et al, ; Llasses et al, ), but also in analysis of atmospheric circulation over the area (Flaounas et al, ). These studies have shown that a multimodel approach is able to describe the main features and mechanisms driving the atmosphere‐ocean circulation over the Mediterranean region (Ruti et al, ).…”

“…MedMIT12 (Harzallah et al, ; Llasses et al, ) is the MITgcm (Table , Adcroft et al, ) adapted to the Mediterranean region and implemented at a horizontal resolution of 1/12° with 75 vertical levels. The model covers the whole Mediterranean Sea plus a buffer zone including a small portion of the near Atlantic Ocean.…”