T2: Energy budget of the ocean surface mixed layer

Principal investigators: Dr. Jeff Carpenter (Helmholtz Center Geesthacht), Dr. Lars Umlauf (Leibniz Center for Baltic Research), Prof. Hans Burchard (Leibniz Center for Baltic Research)

Sea surface temperature on 25 September 2007 from an eddy-resolving GETM simulation of the Baltic Sea (left), and AVHRR remote sensing data (right). Shown in the left panel is only the high-resolution area of the central Baltic Sea as described in Holtermann et al. (2014). Note that the colour shading is similar but not identical in these panels. The right panel is based on data from the German Federal Maritime and Hydrographic Agency (BSH); courtesy of H. Siegel (IOW).

The energy pathways of the submesoscales, which exist in the range between the mesoscale (on the order of 100 km) and the largest turbulent eddies(order of 1 m), will be quantified and parameterised so they can be incorporated into global climate models. This will be done using a numerical approach consisting of two different model studies specialised in both turbulent flows and regional ocean processes, aswell as a dedicated field program using the Baltic Sea as a "natural laboratory" for the measurement of submesoscale energy pathways.

The surface mixing layer (SML) is the ocean side of the air-seainterface through which the fluxes of energy, momentum and tracers have to pass in a coupled atmosphere-ocean system. Pathways and transformations of energy, momentum and tracers in the SML arecomplex, highly variable, and not sufficiently understood. Even in high-resolution ocean models, energy and momentum budgets are energetically inconsistent because the additional energy reservoirs and transformation processes due to unresolved processes (e.g.,mesoscale/submesoscale motions, surface waves) are either ignored or not correctly taken into account. In coarse-resolution climate models, the situation is even worse. The goal of this subproject is thereforeto investigate energy transport and transformation processes in the SML that are relevant for the ocean-atmosphere coupling in climate models.

Our major efforts to understand the energy budget of the SML will be conducted through the use of idealised Large Eddy Simulations (LES), high-resolution ocean modelling, and coordinated field surveys including high-resolution turbulence observations. The results from the LES and field work will be used in a realistic model to understand the energy pathways associated with submesoscale motions in the SML, and to test the developed parameterisations.

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Focussing on the ocean surface mixed layer

Our scope is to investigate the sub-mesoscale structures and the surface mixed layer instabilities in order to develop new parameterisations of energy consistent pathways.

Evridiki Chrysagi, PhD student in T2

My name is Evridiki and I’m PhD candidate working with Prof. Dr. Hans Burchard, in subproject T2. Our research will be focused mainly on the ocean surface mixed layer which is a highly complex and energetic region. The upper ocean is characterized by a relative shallow mixing layer with weak stratification due to turbulent mixing. Our scope is to investigate the sub-mesoscale structures and the surface mixed layer instabilities in order to develop new parameterisations of energy consistent pathways, associated with these motions. For that we will use the General Estuarine Transport Model (GETM) which includes turbulence closure models provided by GOTM, diagnostic tools for the numerical mixing and dissipation but also adaptive vertical coordinates that can resolve the sub-mesoscale features. The configurations will include idealized high resolution simulations as well as hindcast simulations of the Central Baltic Sea. In order to validate the model, the results will be combined with field observations.

Surface salinity field and eddy formation in an idealized high resolution upwelling simulation. The model domain is a re-entrant channel with periodic boundary conditions forced by wind stress.