Climate Signals in the Black Sea From a Multidecadal Eddy-Resolving Reanalysis

Leonardo Lima,Diana Azevedo,Andrea Cipollone,Romain Escudier,Elisaveta Peneva,Sergio Cretì,Francesco Palermo,Rita Lecci,Ali Aydoğdu,Eric Jansen,Mehmet Ilicak,Salvatore Causio,Emanuela Clementi,Simona Masina,Stefania Angela Ciliberti,Giovanni Coppini,Laura Stefanizzi

doi:10.3389/fmars.2021.710973

Abstract

Ocean reanalyses are becoming increasingly important to reconstruct and provide an overview of the ocean state from the past to the present-day. In this article, we present a Black Sea reanalysis covering the whole satellite altimetry era. In the scope of the Copernicus Marine Environment Monitoring Service, the Black Sea reanalysis system is produced using an advanced variational data assimilation method to combine the best available observations with a state-of-the-art ocean general circulation model. The hydrodynamical model is based on Nucleus for European Modeling of the Ocean, implemented for the Black Sea domain with a horizontal resolution of 1/27°× 1/36°, and 31 unevenly distributed vertical levels. The model is forced by the ECMWF ERA5 atmospheric reanalysis and climatological precipitation, whereas the sea surface temperature is relaxed to daily objective analysis fields. The model is online coupled to OceanVar, a 3D-Var ocean data assimilation scheme, to assimilate sea level anomaly along-track observations and in situ vertical profiles of temperature and salinity. Temperature fields present a continuous warming in the layer between 25 and 150 m, where the Black Sea Cold Intermediate Layer resides. This is an important signal of the Black Sea response to climate change. Sea surface temperature shows a basin-wide positive bias and the root mean square difference can reach 0.75°C along the Turkish coast in summer. The overall surface dynamic topography is well reproduced as well as the reanalysis can represent the main Black Sea circulation such as the Rim Current and the quasi-permanent anticyclonic Sevastopol and Batumi eddies. The system produces very accurate estimates of temperature, salinity and sea level which makes it suitable for understanding the Black Sea physical state in the last decades. Nevertheless, in order to improve the quality of the Black Sea reanalysis, new developments in ocean modeling and data assimilation are still important, and sustaining the Black Sea ocean observing system is crucial.

Highlights

The Black Sea is the largest land-locked basin in the world with an area of 4.2 × 105 km2, a volume of 5.3 × 105 km3 and a maximum depth of 2200 m (Özsoy and Ünlüata, 1997)
1/36o × 1/27o 31 Z levels 1993–2018 1988–1992 with T/S data assimilation ERA5 (1 h; 0.25 degree) 2 days 4-day centered at the analysis time CNR-CMEMS SST L4 REP SeaDataNet and CMEMS NRT in situ TAC CMEMS sea level along-track REP product for the European seas Model-based Time-averaged sea surface height (SSH) (1993–2012) from an integration with the assimilation of only in situ T and S 4 cm Monthly 2-D fields Oke and Sakov (2008) Ingleby and Huddleston (2007) Static 2-D field Incremental analysis update (IAU) Correlation length-scale of 20 km Below 300 m heat content anomalies for the Black Sea
The Black Sea reanalysis (BS-REA) system shows very satisfactory skills compared to the model simulation, which highlights the importance of using data assimilation to improve the model representation

Summary

Introduction

The Black Sea is the largest land-locked basin in the world with an area of 4.2 × 105 km, a volume of 5.3 × 105 km and a maximum depth of 2200 m (Özsoy and Ünlüata, 1997). The resulting salinity of about 18 psu in the upper layer forms a strong stratification all over the basin where a saltier water of Mediterranean origin, crossing the Marmara Sea and the Bosphorus Strait, becomes the major source of ventilation for the anoxic lower layer (Ünlülata et al, 1990; Stanev and Beckers, 1999; Stanev et al, 2001). Another main characteristic of the Black Sea is the Cold Intermediate Layer (CIL) formed at the depth of the winter convection (Özsoy and Ünlüata, 1997). The upper layer circulation of the Black Sea is dominated by the Rim Current, a quasi-permanent cyclonic jet following the bottom topography which interacts with several anti-cyclonic eddies (e.g., Batumi and Sevastopol) along its pathway in the basin (Oguz et al, 1993; Korotaev et al, 2003)

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Results

Conclusion