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Mikhail Dobrynin, Jens Murawski, Johanna Baehr, and Tatiana Ilyina

Abstract

Surface waves in the ocean respond to variability and changes of climate. Observations and modeling studies indicate trends in wave height over the past decades. Nevertheless, it is currently impossible to discern whether these trends are the result of climate variability or change. The output of an Earth system model (EC-EARTH) produced within phase 5 of the Coupled Model Intercomparison Project (CMIP5) is used here to force a global Wave Model (WAM) in order to study the response of waves to different climate regimes. A control simulation was run to determine the natural (unforced) model variability. A simplified fingerprint approach was used to calculate positive and negative limits of natural variability for wind speed and significant wave height, which were then compared to different (forced) climate regimes over the historical period (1850–2010) and in the future climate change scenario RCP8.5 (2010–2100). Detectable climate change signals were found in the current decade (2010–20) in the North Atlantic, equatorial Pacific, and Southern Ocean. Until the year 2060, climate change signals are detectable in 60% of the global ocean area. The authors show that climate change acts to generate detectable trends in wind speed and significant wave height that exceed the positive and the negative ranges of natural variability in different regions of the ocean. Moreover, in more than 3% of the ocean area, the climate change signal is reversible such that trends exceeded both positive and negative limits of natural variability at different points in time. These changes are attributed to local (due to local wind) and remote (due to swell) factors.

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Andrey Pleskachevsky, Mikhail Dobrynin, Alexander V. Babanin, Heinz Günther, and Emil Stanev

Abstract

This paper studies the impact of the surface waves on the turbulent mixing. The satellite observations of suspended particulate matter (SPM) at the ocean surface as an indicator of turbulent quantities of the flow are used. In a water column, SPM builds a vertical profile depending on settling velocities of the particles and on vertical mixing processes; thus, SPM is a perfect marker to study the turbulent quantities of the flow. Satellite observations in the North Sea show that surface SPM concentrations, in locations of its deposition, grow rapidly and build plume-shaped, long (many kilometers) uninterrupted and consistent structures during a storm. Also, satellites reveal that SPM rapidly sinks to the seabed after the storm peak has passed and wave height decreases (i.e., in the absence of strong turbulence).

The nonbreaking wave-induced turbulence has been discussed, parameterized, and implemented into an equation of evolution of turbulent kinetic energy (TKE) in the frame of mean-flow concept, which can be used in existing circulation models. The ratio between dissipated and total wave energy is used to describe the influence of wave damping on the mean flow. The numerical tests reproduce experiments in a wave tank very well and are supported by observations of SPM in the North Sea. Their results show that the motion of an individual nonbreaking wave includes turbulent fluctuations if the critical Reynolds number for wave motion is exceeded, independent of the presence of currents due to wind or tides. These fluctuations can produce high diffusivity and strongly influence mixing in the upper water layer of the ocean.

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Gil Lemos, Alvaro Semedo, Mikhail Dobrynin, Melisa Menendez, and Pedro M. A. Miranda

Abstract

A quantile-based bias-correction method is applied to a seven-member dynamic ensemble of global wave climate simulations with the aim of reducing the significant wave height H S, mean wave period T m, and mean wave direction (MWD) biases, in comparison with the ERA5 reanalysis. The corresponding projected changes toward the end of the twenty-first century are assessed. Seven CMIP5 EC-EARTH runs (single forcing) were used to force seven wave model (WAM) realizations (single model), following the RCP8.5 scenario (single scenario). The biases for the 1979–2005 reference period (present climate) are corrected using the empirical Gumbel quantile mapping and empirical quantile mapping methods. The same bias-correction parameters are applied to the H S, T m (and wave energy flux P w), and MWD future climate projections for the 2081–2100 period. The bias-corrected projected changes show increases in the annual mean H S (14%), T m (6.5%), and P w (30%) in the Southern Hemisphere and decreases in the Northern Hemisphere (mainly in the North Atlantic Ocean) that are more pronounced during local winter. For the upper quantiles, the bias-corrected projected changes are more striking during local summer, up to 120%, for P w. After bias correction, the magnitude of the H S, T m, and P w original projected changes has generally increased. These results, albeit consistent with recent studies, show the relevance of a quantile-based bias-correction method in the estimation of the future projected changes in swave climate that is able to deal with the misrepresentation of extreme phenomena, especially along the tropical and subtropical latitudes.

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