A regional atmosphere-ocean climate system model (CCLMv5.0clm7-NEMOv3.3-NEMOv3.6) over Europe including three marginal seas: on its stability and performance

Abstract

Abstract. The frequency of extreme events has changed, having a direct impact on human lives. Regional climate models help us to predict these regional climate changes. This work presents an atmosphere-ocean coupled regional climate system model (RCSM, with the atmospheric component COSMO-CLM and the ocean component NEMO) over the European domain, including three marginal seas: the Mediterranean, the North and the Baltic Seas. To test the model, we evaluate a simulation of more than one hundred years (1900–2009) with a spatial grid resolution of about 25 km. The simulation was nested into a coupled global simulation with the model MPI-ESM, whose ocean temperature and salinity was nudged to an MPI-ESM ocean-ice component forced with the 20th Century Reanalysis (20CR). The evaluation shows the robustness of the RCSM and discusses the added value by the coupled marginal seas over an atmosphere-only simulation. The coupled system runs stable for the complete 20th century and provides a better representation of extreme temperatures compared to the atmosphere-only model. The produced long-term dataset will allow an improved study of extreme events, helping us to better understand the processes leading to meteorological and climate extremes and their prediction.

Added Value of Atmosphere-Ocean Coupling in a Century-Long Regional Climate Simulation

“…The SST values in the 20CR reanalysis are taken from the UK Met Office HadISST1.1 (Hadley Centre sea ice and sea surface temperature) dataset [28], as the atmosphere-land model system, creating the reanalysis needed lower boundary conditions. Thus, the SST values used in our uncoupled experiment are in the end originated from the HadISST dataset, but as our previous study [19] shows, the SST values in the global and uncoupled system have a cold bias compared to the observations. Nevertheless, they are not independent from the observed data.…”

“…Modelled data is needed for this, since observational data is not always available for this time range, or it has only low resolution, as well as available only over land. The stability of the simulation during the twentieth century is evaluated in Reference [19], where the authors present that the climate system is stable, and it does not have any continuous drift. Nevertheless, it is not free from biases.…”

“…Despite this, a transient century-long simulation might show some improvements also over the continental region, since the system has a long time to adapt and progress with the more complex climate system dynamics. Reference [19] analyze the effect of coupling regarding extremes with the help of daily observations from Germany stations. They found, that in Germany the coupled system improves the values of the climate change indices related to extreme temperatures compared to the uncoupled version, while for related precipitation indices the uncoupled simulation is more skillful.…”

“…The coupled system gets its driving data at the boundaries from the same assimilation experiment of MPI-ESM-LR as the uncoupled simulation. To illustrate the computational side of running a coupled system, Reference [19] calculated for how long does the coupled and the uncoupled system run when using the same resources (same amount of nodes for the whole system), and they found that the coupled system needs ca. five times more time for the integration.…”

“…five times more time for the integration. For more details on the coupled system, we also refer to Reference [19], who investigated the stability of the same simulations.…”