L4: Energy-Consistent Ocean-Atmosphere Coupling

Principal investigators: Dr. Nils Brüggemann (Universität Hamburg), Dr. Cathy Hohenegger (Max Planck Institute for Meteorology), Dr. Stephan Juricke (Jacobs University), Dr. Lars Umlauf (Leibniz Institute for Baltic Sea Research Warnemünde)

Figure 1

Recent studies highlight the importance of small-scale coherent structures associated with atmospheric convection and (sub-)mesoscale ocean dynamics on atmosphere-ocean feedbacks in key regions of the climate system. In state-of-the-art coupled climate models, such small-scale atmosphere-ocean feedbacks are either ignored or rely on parameterizations, which may lead to model biases and energetic inconsistencies. In this subproject, we investigate these small-scale coupling mechanisms using coupled simulations on a global scale, based on resolutions allowing us, for the first time, to directly simulate the underlying key processes. More specifically, we focus on the following topics:

(i) With the help of storm-resolving coupled simulations and data from a large-scale field experiment conducted in the tropical Atlantic, we investigate the interaction of shallow atmospheric convection and near-surface processes in the ocean. A special focus here is on the dynamics of thin (meter-scale) "diurnal warm layers" and "rain layers" at the ocean surface, their role in mediating air-sea fluxes, and their feedback with atmospheric convection (see figure 1).

Figure 2

(ii) We also study the effect of parameterized submesoscale dynamics like baroclinic instability and symmetric instability on atmosphere-ocean coupling (see figure 2). To this end, the role of submesoscale dynamics on atmosphere-ocean feedbacks in selected key regions of the climate system is investigated with the help of coupled global simulations at unprecedented resolution (see figure 3). These simulations are also used to develop and test parameterizations of these effects in more coarsely resolved global ocean-atmosphere models. Our final goal is to obtain an integral estimate of the role of submesoscale dynamics on the coupling between the atmosphere and the ocean.

(iii) Finally, we aim to clarify the effect of resolved mesoscale eddies on air-sea coupling (see figure 2), develop a new stochastic parameterization that represents such effects in surface fluxes of heat and momentum, and investigate the global and regional impact of the parameterization in coupled atmosphere-ocean simulations that cannot use high enough resolution to represent those processes explicitly.

Figure 3

An In-Depth Study of Diurnal Warm Layers: Quantification of Air-Sea Interactions

I am fascinated by such theoretical results but also by their various fields of application.

Mira Shevchenko, Postdoc, L4

In July 2021 I joined the TRR 181 as a postdoctoral researcher in the project L4, “Energy-Consistent Ocean-Atmosphere Coupling”. Within this project I am studying the phenomenon of diurnal warm layers (DWLs) in the ocean. It describes the warming of the sea surface in certain areas during daytime (by up to 2K, though in particular cases also higher fluctuations have been observed) compared to the surrounding ocean that usually keeps an almost constant surface temperature.

From the point of view of air-sea interactions the appearance of DWLs is of particular interest, since such differential heating can promote a sea breeze like convective movement and, as a result, serve as a cloud building mechanism. Moreover, as such warm spots appear due to solar radiation, one can also expect a feedback behaviour caused by an increase in the cloud cover. 

The presence of DWLs as well as their influence on the cloud amount is well documented in the literature, at least in the qualitative sense. Moreover, this phenomenon has been confirmed in idealised simulation studies. However, most modern global coupled simulations do not capture this mechanism, since it requires a high vertical resolution of the sea levels in order to correctly represent the heat transport (involving only about 20m in the vertical), but also a high horizontal resolution in the atmosphere that would permit to directly resolve convection. My work in the project consists in implementing a simulation that would incorporate both these features. This has been made possible thanks to recent model development advances for the ICON models at the Max Planck Institute for Meteorology. A subsequent analysis of the output will improve the understanding of the phenomenon itself, in particular permitting to quantify the feedback mechanisms, but it will also clarify how significant of an influence the correct representation of DWLs has on the global cloud amount, and, as a consequence, on the climate described by the simulation. Such results would, moreover, enable a parametrisation of this phenomenon such that it can be included in lower resolution models in order to improve their performance.

Within the project L4 I work under supervision of Cathy Hohenegger at the MPI for Meteorology and collaborate mainly with Nils Brüggemann, Lars Umlauf and Mira Schmitt who already implemented a set of thin layer ocean simulations and contributed significantly to my understanding of the mechanisms involved. 

Prior to joining the TRR 181 I spent several years doing research in Probability Theory. After obtaining my Master’s degree at the HU Berlin I went on to complete my PhD at the TU Dortmund with a research stay at the University of Lille. During my doctorate I studied stochastic (partial) differential equations driven by random processes or fields with long memory, i.e. such that the increment correlation decays only slowly over time. An example is the fractional Brownian motion. Using techniques from the Malliavin-Stein toolkit (providing a definition for multiple stochastic integrals with respect to Gaussian processes and many limiting results for those) I proved in several collaborations limit theorems for certain functionals of the solutions of such equations. From the practical point of view, this enabled me to derive results in mathematical statistics and provide estimators for different quantities in such equations as well as show their asymptotic properties.

After defending my dissertation I stayed at the TU Dortmund as a postdoctoral researcher. During this time I studied (in another collaboration) random fields on a sphere. Such objects are used in cosmology to describe cosmic microwave background, but they can also be applied to analyse other random spherical observations such as, for instance, temperature defects.

I am fascinated by such theoretical results but also by their various fields of application. I hope to be able to use some of the models that I studied in order to assess the impact of DWLs and/or to describe other phenomena in the atmosphere and ocean that would help advance the understanding and modelling of physical processes on different scales.

The Impact of Submesoscales on the Air-Sea Exchange

I will investigate on the potential impact of submesoscale dynamics on the sea surface temperature or the influence of wind on instabilities at ocean fronts.

Moritz Epke, PhD, L4

Hello everyone, my name is Moritz Epke and I am pleased to give you a small impression of my work at TRR. I am part of the subproject L4 „Energy consistent ocean atmosphere coupling”, which investigates small scale and balanced processes and their impact on feedback mechanism between atmosphere and ocean. Before I go into more detail, maybe a few words about my background. I moved to Hamburg to study theoretical mechanical engineering at the Hamburg University of Technology. My interest in the physics of fluids grew and grew through my studies and drove me to focus on this topic and related numerical solution approaches. In my thesis I developed and implemented a lattice Boltzmann scheme to efficiently simulate non-isothermal flows, which I benchmarked on standard testcases like Rayleigh-Bénard convection in a cavity and which I used to simulate the internal cooling of a turbine blade by a turbulent flow.

While most engineering applications have setups with scales from less than a centimeter as in a pipe flow, or up to a few hundred meters as in a large ship, the ocean and the atmosphere have scales that are orders of magnitude higher. Even if we make use of clever approximation techniques to simplify the governing equations in order to reduce the computational effort, we can only carry out coupled climate simulations with roughly tenkilometer (ocean) grid spacing on a modern supercomputer. In such a simulation an 80km ocean eddy would only be coarsely resolved. The computational surplus to resolve more scales in long-term simulations is simply too high. What cannot be resolved is usually parameterized or neglected. If parameterized, a model is developed which is based at best on a physical relationship between the relevant parameters. These parameterizations are then tested and optimized in idealized or regional setups. If now such parameterizations or insufficient parameterizations are used, the model is most likely subject to biases. These types of biases might have a strong impact on the energy consistency.

In the first phase of my PhD I am using an ICON submesoscale telescope simulation, which is based on an unstructured grid and allows us (for a short time period) to use an extremely fine spatial resolutions of up to 600m in the focus region. If we look again at an 80km ocean eddy, which is now well resolved, we can see small scale coherent structures that we associate with the submesoscale (see figure) and define to be smaller than the first baroclinic Rossby radius of deformation. It is an objective to understand and quantify the impact of submesoscale dynamics like baroclinic and symmetric instabilities on the downward heat and energy transfer and their role for ocean-atmosphere interactions. Here, I will investigate on the potential impact of submesoscale dynamics on the sea surface temperature or the influence of wind on instabilities at ocean fronts. Therewith, I aim to obtain a better understanding of submesoscale dynamics and their role in the coupled ocean-atmosphere system. This improved understanding might ultimately lead to improved parameterizations and therewith less biases in the coupled climate models.

  • Peng, J.-P., Dräger-Dietel, J., North, R. P., & Umlauf, L. (2021). Diurnal Variability of Frontal Dynamics, Instability, and Turbulence in a Submesoscale Upwelling Filament, J. Phys.Oceanogr.51(9), 2825-2843, doi: https://doi.org/10.1175/JPO-D-21-0033.1.

  • Li, Q., Bruggemann, J., Burchard, H., Klingbeil, K., Umlauf, L., & Bolding, K. (2021). Integrating CVMix into GOTM (v6.0): a consistent framework for testing, comparing, and applying ocean mixing schemes. Geosci. Model Dev., doi: https://doi.org/10.5194/gmd-14-4261-2021.

  • Carpenter, J. R. , Rodrigues, A., Schultze, L. K. P., Merckelbach, L. M., Suzuki, N., Baschek, B. & Umlauf, L. (2020). Shear Instability and Turbulence Within a Submesoscale Front Following a Storm. Geophys. Res. Lett., doi: https://doi.org/10.1029/2020GL090365.

  • Rackow, T., & Juricke, S (2019). Flow‐dependent stochastic coupling for climate models with high ocean‐to‐atmosphere resolution ratio. Q. J. Roy. Meteor. Soc., 1-17, https://doi.org/10.1002/qj.3674.