Development of a sequential tool, LMDZ-NEMO-med-V1, to conduct global-to-regional past climate simulation for the Mediterranean basin: an Early Holocene case study

Abstract

Abstract. Recently, major progress has been made in the simulation of the ocean dynamics of the Mediterranean using atmospheric and oceanic models with high spatial resolution. High resolution is essential to accurately capture the synoptic variability required to initiate intermediate- and deep-water formation, the engine of the Mediterranean thermohaline circulation (MTC). In paleoclimate studies, one major problem with the simulation of regional climate changes is that boundary conditions are not available from observations or data reconstruction to drive high-resolution regional models. One consistent way to advance paleoclimate modelling is to use a comprehensive global-to-regional approach. However, this approach needs long-term integration to reach equilibrium (hundreds of years), implying enormous computational resources. To tackle this issue, a sequential architecture of a global–regional modelling platform has been developed for the first time and is described in detail in this paper. First of all, the platform is validated for the historical period. It is then used to investigate the climate and in particular, the oceanic circulation, during the Early Holocene. This period was characterised by a large reorganisation of the MTC that strongly affected oxygen supply to the intermediate and deep waters, which ultimately led to an anoxic crisis (called sapropel). Beyond the case study shown here, this platform may be applied to a large number of paleoclimate contexts from the Quaternary to the Pliocene, as long as regional tectonics remain mostly unchanged. For example, the climate responses of the Mediterranean basin during the last interglacial period (LIG), the Last Glacial Maximum (LGM) and the Late Pliocene all present interesting scientific challenges which may be addressed using this numerical platform.

Modelling the water isotopes distribution in the Mediterranean Sea using a high-resolution oceanic model (NEMO-MED12-watiso-v1.0): Evaluation of model results against in-situ observations

“…Compared to other large ocean basins, the Mediterranean Sea can be considered ideal to improve our understanding of the processes that influence and drive oxygen isotope variations, and to further develop the existing modelling approach, because (i) the water residence time is relatively short (∼ 100 years; Millot and Taupier-Letage (2005)); (ii) all major forcing mechanisms are present, including air-sea interaction, buoyancy fluxes and wind forcing, with a well-studied salinity and water isotope structure (e.g., Pierre, 1999); (iii) a well marked δ 18 O sw of the surface waters of the eastern Mediterranean basin (value up to 2.2 ‰, Gat et al, 1996) has the potential as an oceanographic tracer to trace the process of deep water formation and the thermohaline circulation variability; (iv) a high spatial resolution regional model (NEMO-MED12) is available, which is essential for the simulation of realistic ocean dynamics, and which can then be used for past climate simulation with the adapted coupled regional model (Vadsaria et al, 2020). Over the last decades, considerable progress has been achieved in our understanding of the processes and mechanisms governing the distribution of water isotopes in the Mediterranean Sea, through high quality sampling and measurements (e.g., Gat et al, 1996;Pierre, 1999;LeGrande and Schmidt, 2006).…”