Sebastian Grayek

Assessment of the Black Sea observing system. A focus on 2005-2012 Argo campaigns

An observing system in the Black Sea combining remote sensing data such as sea level anomalies from altimetry, sea surface temperature from satellite radiometer and data from Argo floats has been analyzed with the aim to quantify the contribution of different information sources when reconstructing the ocean state. The main research questions are: (1) do Argo float measurements substantially impact the quality of estimates, (2) what is the dependence of this quality upon the data and sampling used, and (3) are there specific Black Sea issues? Numerical model output and statistical analysis were used for this purpose. It has been demonstrated that the statistical method performs in a consistent way reproducing known geophysical patterns. Maximum footprints of sea level, salinity and temperature were illustrated, most of them clearly connected with specific thermohaline conditions and the general circulation. Reduced analysis capabilities were identified as associated with a low level of dynamical coupling between the shelf and the open ocean, mesoscale dynamics and representation of diapycnic processes in the models. The accuracy of Argo pressure measurements appeared very important to resolve the extremely sharp stratification in the upper layers. The present-day number of Argo floats operating in the Black Sea of about 10, seems optimal for operational purposes.

On the impact of salinity observations on state estimates in Ems Estuary

The hydrodynamics of Ems Estuary are dominated by tides and their interaction with buoyancy forcing. Such an environment is challenging for any effort to bring together observations and model results. In this study, we investigate how salinity measurements in the Ems Estuary affect the reconstruction of the salinity field. Similar to the traditional observing system experiments, the impact of specific observational arrays is simulated in the framework of statistical experiments. The experimental algorithm mainly relies on the model covariance matrix. Each experiment results in an estimate of the reconstruction error. The analysed observation configurations involve single and multiple, as well as stationary and non-stationary observing arrays. Generally, the reconstruction of the ocean state improves with increasing the density of observations. It appears that certain locations are more favourable for reconstruction than others. In fact, the regions separating the main dynamical realms resist strongest to the reconstruction effort. Extending the covariance matrix by the temporal cross-covariances between the model grid points enables to evaluate the impact of observations taken from a moving platform. This approach further improves the outcome of the experiments, resulting in reconstruction errors near zero with the exception of the tidal river. The cross-covariance information is able to tackle even the irregular dynamics arising on the border between the different physical regimes.

Understanding the Dynamics of the Oxic‐Anoxic Interface in the Black Sea

The Black Sea, the largest semienclosed anoxic basin on Earth, can be considered as an excellent natural laboratory for oxic and anoxic biogeochemical processes. The suboxic zone, a thin interface between oxic and anoxic waters, still remains poorly understood because it has been undersampled. This has led to alternative concepts regarding the underlying processes that create it. Existing hypotheses suggest that the interface originates either by isopycnal intrusions that introduce oxygen or the dynamics of manganese redox cycling that are associated with the sinking of particles or chemosynthetic bacteria. Here we reexamine these concepts using high-resolution oxygen, sulfide, nitrate, and particle concentration profiles obtained with sensors deployed on profiling floats. Our results show an extremely stable structure in density space over the entire basin with the exception of areas near the Bosporus plume and in the southern areas dominated by coastal anticyclones. The absence of large-scale horizontal intrusive signatures in the open-sea supports a hypothesis prioritizing the role of biogeochemical processes.

Ocean Forecasting: From Regional to Coastal Scales

In the past years, the Helmholtz Zentrum Geesthacht put in place the Coastal Observing System for the North and Arctic Seas (COSYNA) in the frame of which different aspects of forecasting the marine environment have been developed. This paper describes these developments, which are based on recent advances in coastal ocean forecasting in the field of 10 numerical modelling, data assimilation and observational array design. The region of interest is the North and Baltic Sea; most of the coastal examples discussed in the paper are for the German Bight. Several pre-operational applications are presented exemplifying the outcome of using the best available science in coastal ocean predictions. They help to identify new challenges; most of them are associated with resolving the non-linear behavior of coastal ocean, which for the studied region, is manifested by the tidal distortion and generation of shallow-water tides. Led by the motivation to maximize the 15 benefit from observations, the authors focus on the integration of observations and modelling by using advanced statistical methods. The coastal and regional ocean forecasting systems do not run in isolation, but are linked, either weakly by just using forcing data, or interactively by using two-way nesting or unstructured-grid models. Therefore the problem of downscaling and upscaling, which currently attracts much attention, is also addressed. One example shown is the coupling of the coarse-resolution regional models by using a two-way nesting method with fine resolution in the area of connecting 20 straits. The major part of the paper presents illustrations from assimilation of remote sensing, in situ and HF radar data, prediction of wind waves and storm surges, as well as possible applications to search and rescue operations, and modelling support for assessing the environmental impact of wind parks. Concepts for seamless approaches to link coastal and regional forecasting systems are also presented and the two examples given illustrate (1) an application of unstructured-grid model for the Ems Estuary, and (2) the potential influence of the information from coastal observatories or coastal forecasting 25 systems on the regional models. Ocean Sci. Discuss., doi:10.5194/os-2016-25, 2016 Manuscript under review for journal Ocean Sci. Published: 9 May 2016 c