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

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.

Sea Surface Temperature Variability and Marine Heatwaves in the Black Sea

“…This could be related to the highest SST variability in 1994, as shown in the EOF analysis (Figure 4c,d). In addition, [2,26] found a warm peak in 1994 occurring in the second half of the year, confirming our results. Except for 1994, the temporal basin's average MHW duration (Figure 6c) shows a general increase from roughly 6 days to >10 days in the last decade (2010-2020), with a maximum of ~24 days in 2020.…”

“…This event may be linked to the high variability observed in the EOF analysis (Figure 4c,d). This showed that the second half of 1994 had the highest variability over the entire study period, confirming prior research findings [2,26].…”

“…The Black Sea is a semi-enclosed basin that receives salty Mediterranean water through the Marmara Sea via the Bosphorus channel (Figure 1). The surface layer of the Black Sea is particularly vulnerable to changing environmental and climatic conditions [24][25][26]. The Black Sea is the largest inland basin in the world, with an area of approximately 4.2 × 10 5 km 2 , a volume of 5.3 × 10 5 km 3 , and a maximum depth of 2200 m [22,26].…”

“…The surface layer of the Black Sea is particularly vulnerable to changing environmental and climatic conditions [24][25][26]. The Black Sea is the largest inland basin in the world, with an area of approximately 4.2 × 10 5 km 2 , a volume of 5.3 × 10 5 km 3 , and a maximum depth of 2200 m [22,26]. The northwestern region of the Black Sea is classified as an estuary, with a positive net freshwater balance (i.e., precipitation >> evaporation) [27,28].…”

Monitoring and Forecasting the Ocean State and Biogeochemical Processes in the Black Sea: Recent Developments in the Copernicus Marine Service

“…Finally, the geostrophic currents estimated from the time-averaged sea level (1993-2019) reveal the Rim current, which surrounds the entire Black Sea and forms a large-scale cyclonic gyre (Figure 7). Major details are provided in [47].…”