Ocean reanalyses integrate models and observations to provide a continuous and consistent reconstruction of the past physical and biogeochemical ocean state and variability. We present a reanalysis of the Mediterranean Sea biogeochemistry at a 1/24^o resolution developed within the Copernicus Marine Service framework. The reanalysis is based on the Biogeochemical Flux Model (BFM) coupled with a variational data assimilation scheme (3DVarBio) and forced by the NEMO–OceanVar Mediterranean reanalysis and the ERA5 atmospheric reanalysis. Covering the 1999–2021 period, the reanalysis assimilates ESA-CCI satellite chlorophyll data and integrates EMODnet data as initial conditions, in addition to considering World Ocean Atlas data at the Atlantic boundary, CO2 atmospheric observations, and yearly estimates of riverine nutrient inputs.

With the use of multiple observation sources (remote, in situ, and BGC-Argo), the quality of the biogeochemical reanalysis is qualitatively and quantitatively assessed at three validation levels. Results of the first validation-level indicate an overall pretty good reanalysis skill in simulating basin-wide values and variability in the biogeochemical variables, such as phytoplankton biomass, net primary production and CO2 air-sea flux. Then, chlorophyll, nutrients, oxygen, and carbonate system variables show also satisfactory uncertainty in reproducing in situ observations at the mesoscale and weekly temporal scale. The uncertainty increases for a few variables (i.e., oxygen and ammonium) in the mesopelagic layers. Finally, using specific and process-oriented skill metrics based on BGC-Argo data, the vertical dynamics of phytoplankton and nitrate are positively assessed.

As a consequence of the continuous increases in temperature, salinity and atmospheric CO2 in the Mediterranean Sea over the last 20 years, the reanalysis results indicate basin-wide biogeochemical signals of surface deoxygenation, increase in alkalinity and dissolved inorganic carbon concentrations, and decrease in pH at the surface. The new, high-resolution reanalysis, open and freely available from the Copernicus Marine Service, allows users from different communities to investigate the spatial and temporal variability in 12 biogeochemical variables and fluxes at different scales (from the mesoscale to the basin-wide scale and from daily to multiyear scales) and the interaction between physical and biogeochemical processes shaping Mediterranean marine ecosystem functioning.

How to cite: Cossarini, G., Salon, S., Feudalel, L., Bolzon, G., Coidessa, G., Solidoro, C., Di Biagio, V., Amadio, C., Lazzari, P., and Brosich, A.: High-Resolution Reanalysis of the Mediterranean Sea Biogeochemistry , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11207, https://doi.org/10.5194/egusphere-egu22-11207, 2022.

Extreme events can have great impacts on the functioning of marine ecosystems. In this work we describe a novel method that identifies and characterises extreme events in marine ecosystems at the basin scale, by accounting for the persistence of the events within a certain impacted area and over a specific time duration. Extreme events are firstly identified as peaks over a predefined threshold (i.e. the 99th percentile) computed from a local timeseries. Then, a series of extreme events that are connected over space and time is defined as an extreme event wave (EEW) and characterised by a set of indexes describing their initiation, extent, duration and strength.  
We applied the method to surface chlorophyll concentration, that is an essential ocean variable representative of the marine ecosystem state and evolution. Since high frequency and seamless data in time and space are mandatory to properly detect extreme events on the basin scale, we used daily data provided by a numerical model. In particular, we employed data of winter-spring surface chlorophyll concentration provided by the coupled hydrodynamic-biogeochemical model MITgcm-BFM at 1/12° horizontal resolution, validated in the Mediterranean Sea in the 1994-2012 period. Our method allowed us to identify and characterise surface chlorophyll EEWs occurring in the considered period and also to provide a bio-regionalisation of the Mediterranean Sea associated to different regimes of chlorophyll dynamics, by means of a fuzzy classification of EEW indexes.

How to cite: Di Biagio, V., Cossarini, G., Salon, S., and Solidoro, C.: Extreme event waves in marine ecosystems: method and application to the surface chlorophyll in Mediterranean Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9624, https://doi.org/10.5194/egusphere-egu22-9624, 2022.

We used monthly means by Copernicus Marine Services for the Black Sea basin to calculate a series of metrics related to wind-driven upwelling dynamics (Upwelling Index) and examine the relationship with nutrient and plankton environment. We then use these to objectively describe upwelling signals in terms of their frequency, intensity and duration during summer months over 26 years (1993 - 2019). We found that an increase or a decrease in the sea surface temperature is associated with a reduction (or increase) in upwelling events, a decrease/increase in the intensity of upwelling, and a decrease/increase in the cumulative upwelling intensity, with differences between Romanian Black sea areas. Nitrate supply by coastal upwelling has been estimated by combining sea surface temperature and salinity for the in-situ data for the North-Western Black Sea shallow waters. The seasonal vertical transport induced by wind forcing was assessed by daily wind data retrieved from the Copernicus Marine Service data was used. 

How to cite: Mihailov, M. E., Lazar, L., Chirosca, G., Popov, P., Pantea, E. D., Vatu, N., Andrescu, D., and Chirosca, A. V.: Climatological analysis on the North-Western Black Sea upwelling phenomena, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12873, https://doi.org/10.5194/egusphere-egu22-12873, 2022.

Mesoscale eddies are ubiquitous energetic features that alter the biogeochemical regimes of the oceans by blending large-scale gradients, isolating and transporting water masses over large distances, and by locally shallowing or deepening isopycnals [1]. While several studies highlighted mesoscale biogeochemical mechanisms in the open ocean, the difficulties affecting altimetry products in nearshore regions constitute a strong barrier to the observation-based characterization of nearshore biogeochemical eddy dynamics.

Because of their transitional nature, capturing observational snapshots of eddies with satisfactory coverage is challenging. To overcome this difficulty, composite analyses consist of gathering a large number of near-eddy data instances (from observation or model results) and analyzing their variations according to their relative position to nearby eddies. The method thus aims at characterizing average eddy-induced perturbations and provided the basis for many of the recent advances in the field [2].

The BGC-Argo program provides a powerful asset for eddy composite studies, resulting from 1) the large availability of data provided under the hood of common technical protocols, 2) the richness of characterized biogeochemical variables, and 3) the continuity of data acquisition which facilitates the characterization of local anomalies.

Here, we evaluate three Black Sea altimetry data sets (2011-2019) and compare their adequacy to characterize eddy-induced subsurface oxygen and salinity signatures by applying a common composite analysis framework exploiting in-situ data acquired by BGC-Argo profilers.

The locations, contours, and properties of eddies are obtained by applying the py-eddy-tracker procedure [3] to three altimetric sets, that differ in terms of along-track and gridding processing, and spatial resolution. For comparison, we consider equivalent CMEMS BS-MFC model products [4]. Oxygen and salinity subsurface anomalies are then obtained from BGC-Argo profiles and relocated in eddy-centric coordinates specifically for each altimetric product.

The most recent altimetric data set, issued from the ESA EO4SIBS project, provides eddy properties that are closer to model simulations, in particular for coastal anticyclones. More importantly, subsurface signatures reconstructed from BGC-Argo are more consistent when EO4SIBS is used to express eddy-centric coordinates.

We propose that the estimated error on the reconstructed mean anomaly may serve to qualify the accuracy of gridded altimetry products and that BGC-Argo data provide a strong asset in that regard.

Besides, we reveal intense subsurface oxygen anomalies whose structure supports the hypothesis that the mesoscale contribution to Black Sea oxygen dynamics extends beyond transport and involves net biogeochemical processes.

[1] D. J. McGillicuddy, (2016), Ann. Rev. Mar. Sci., 8, 125–159.

[2] P. Gaube, D. J. McGillicuddy, Jr, D. B. Chelton, M. J. Behrenfeld, P. G. Strutton, (2014), J. Geophys. Res. C: Oceans, 119, 8195–8220.

[3] E. Mason, A. Pascual, J. C. McWilliams, (2014), J. Atmos. Ocean. Technol., 31, 1181–1188.

[4] Ciliberti, S. A., et al. (2021), Journal of Marine Science and Engineering, 9(10), 1146.

How to cite: Capet, A., Taburet, G., Mason, E., Pujol, M.-I., D'Ortenzio, F., Grégoire, M., and Rio, M.-H.: Using Argo to characterize altimetric products: a study of eddy-induced subsurface oxygen anomalies in the Black Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13265, https://doi.org/10.5194/egusphere-egu22-13265, 2022.

Data from profiling floats in the Black Sea revealed complex temporal and spatial relationships between physical variables on the one hand and oxygen, chlorophyll (Chl-a) and the backscattering coefficient at 700 nm (bbp) on the other. It was demonstrated that the temperature-stratified upper layer and salinity-stratified layers below 50 m provide the ecological niche responsible for the major variability of the BGC system in the euphotic layer. There, around the depth of the minimum potential vorticity, the subsurface chlorophyll maximum presents a major BGC characteristics. Data analysis revealed some limits in understanding the details of BGC dynamics, as well as their dependence on physical drivers. One example is the fact that while bbp and Chl-a weakly follow the long-term changes in temperature and salinity, their responses to individual cooling events appear much stronger than what is observed in the physical fields. To fully account for different interdependences, a feedforward backpropagation neural network (NN) was used. The NN learnes from data recorded by profiling floats and predicts BGC states using physical measurements only. The skill was very high, particularly for oxygen, but it reduced when the NN was applied to older data because the interrelationships between the physical and BGC properties have changed recently. One indication of such change is the missing overshooting of either Chl-a or bbp penetration depth in winter reported in earlier studies. The BGC states reconstructed by the NN from physical data produced by a coupled physical-BGC operational model outperform the BGC output of the same coupled model. Therefore, the use of data from profiling floats, physical properties from numerical models and NN appeared a powerful tool to reconstruct the 4D dynamics of euphotic zone. Basin wide patterns and temporal variabilities of oxygen, bbp and Chl-a are also analyzed. Of particular interest is the reconstruction of short-living BGC features, particularly in the area of coastal anticyclones, which are difficult to observe basin-wide with available floats.

How to cite: Stanev, E., Wahle, K., and Staneva, J.: Dynamics of the euphotic zone in the Black Sea: The synergy of data from profiling floats, machine learning and numerical modeling , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7197, https://doi.org/10.5194/egusphere-egu22-7197, 2022.

The thermohaline and dynamic characteristics of the upper ocean can be affected by the way the sunlight is absorbed and scattered on the surface layer. Changes in light penetration can be investigated through the turbidity of the layer, which is determined by a synthesis of terrestrial inputs (atmospheric and riverine), and the biological activity in a region, of both natural and anthropogenic origin. To examine the effects that turbidity variability has on the surface-layer characteristics of the ocean, a twin modeling sensitivity experiment was performed, using as a case study the Aegean Sea, NE Mediterranean. The Aegean Sea is an oligotrophic region with most nutrient inputs located in the northern coasts of the basin, creating a north-to-south chlorophyll-a gradient, with the highest concentrations on the North and lowest on the South. The first experiment corresponds to a very clear ocean, and the second incorporates the turbidity field, varying according to chlorophyll-a concentration.
The experiments were implemented using the NEMO models' ocean component (v3.6) for the region (34.05-41.16 °N, 22.29-28.98 °E) and for the period 1997-2001, discretized on a 1/36° Arakawa-C grid, with 75 partial step vertical levels. Atmospheric inputs are ERA-5 reanalysis products of the ECMWF service, whereas inputs for initial and boundary values have been derived from the Copernicus database. First, a two-band light penetration scheme was applied, using a Jerlov Type-I extinction depth at 23.0 m, representing the constant-low turbidity field. An RGB scheme was applied for the second experiment, using the multiyear monthly mean of the chlorophyll-a concentration variable derived from the ESA-CCI service. The sea surface variables' response is examined for the final year of the experiment. The results indicate that the RGB-scheme experiment estimates elevated sea surface salinity and temperature values, with the most significant difference in salinity located in the northern part of the basin, where there is a strong influence of the inflow of Black Sea Water from the Dardanelles Straits. Elevated eddy kinetic energy is observed in the gyres formed in the Cretan Sea.

How to cite: Metheniti, V., Vervatis, V., Karageorgis, A. P., Kampanis, N., and Sofianos, S.: Response of the Aegean Sea surface characteristics to turbidity variations, using numerical simulations., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3773, https://doi.org/10.5194/egusphere-egu22-3773, 2022.

In a warming world, society is facing major climate-related challenges and impacts, such as marine heat waves (MHW) that have devastating effects on ecosystems, threaten economies and strengthen severe storms. MHWs are substantially increasing in intensity, duration and frequency worldwide and particularly in the Mediterranean Sea. This semi-enclosed and relatively small basin responds rapidly to global warming experiencing strong spatial variations that require specific consideration, in particular to better understand the drivers, mechanisms and consequences of such extreme events on the physical, biogeochemical and biological components of the oceans.

This study proposes a comprehensive characterization of MHWs in the Mediterranean at sub-regional scale from surface to sub-surface and from open to coastal waters, using remote sensing and multi-platform in situ observations. First, the long-term evolution of MHW characteristics (mean and maximum intensities, mean duration and frequency) is analysed at sub-regional scale using the satellite observations of sea surface temperature over the last four decades. Then, the propagation of sub-regional MHWs into the ocean interior and the associated modified stratification are examined through the use of vertical hydrographic profiles from profiling floats. Finally, the ocean response to extreme temperature events is also investigated in the coastal ocean complementing the satellite observations with mooring data in the near-shore waters of the Balearic Islands.

A smart platform has been implemented to monitor, visualize and share timely information on sub-regional MHWs, from event detection in real-time to long-term variations in response to climate change, to diverse stakeholders (e.g., scientific community, educators in marine science, environmental agencies and policy decision-makers). The “Sub-regional Mediterranean Marine Heat Waves” visualization tool will help to implement adaptive management, to establish adaptation strategy and to support the marine conservation and sustainable management of the oceans in a warming world.

How to cite: Juza, M., Fernandez-Mora, A., and Tintore, J.: Sub-regional marine heat waves in the Mediterranean Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9382, https://doi.org/10.5194/egusphere-egu22-9382, 2022.

On 7 April 2021, an exceptional bloom of the scyphomedusa Rhizostoma pulmo was observed in the Gulf of Trieste (Italy). Blooms of these species in the northern Adriatic Sea have been reported since the late 1800s, however, the density of jellyfish observed in 2021 reached dozens of specimens per cubic meter. In this work, we analyze the bloom from a multi-platform approach using observation and model data at different time scales. This study aims to contextualize the oceanographic/environmental conditions that may have contributed to the exceptional aggregation of the scyphomedusa Rhizostoma pulmo along the northernmost coast of the Adriatic Sea. Our study shows that 1) the bloom was probably enabled by anomalous warm sea conditions during winter 2020, allowing specimens that reproduced in 2020 to survive and reach considerable abundance and sizes by early 2021; 2) strong wind events, such as the Bora wind for the Gulf of Trieste, enhanced upwelling and mixing processes in the gulf thus bringing the jellyfish present in deeper waters to the surface and clustering them along the coast.

How to cite: Reyes Suarez, N. C., Ursella, L., Tirelli, V., Ličer, M., Querin, S., and Cardin, V.: How physical properties can unveil the biological processes in the water column: a multi-platform study of the extreme bloom of Rhizostoma pulmo jellyfish in the Gulf of Trieste,  April 2021 case study., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2337, https://doi.org/10.5194/egusphere-egu22-2337, 2022.