.. _ifs_fesom_model_control: Control Simulation ################## The IFS-FESOM control simulation covers 10 years (1990-1999) and serves to assess whether the coupled system starts close to radiative equilibrium and to characterise any model drift present from the initialization. Both the atmosphere and ocean use a horizontal grid resolution of 5 km (NG5 mesh for FESOM). +----------+------------------------------------+--------------+-------------+-------------------------------------------------+-----------------------------------+ | Model | Spatial Resolution | Time | Realization | Experiment ID | DestinE Data Lake (bridge site) | +==========+====================================+==============+=============+=================================================+===================================+ | IFS-FESOM| 5km atmosphere; 5km ocean | 1990-1999 | 1 | climatedt-gen2-ifs-fesom-baseline-cont-5km-r1 | LUMI data bridge | +----------+------------------------------------+--------------+-------------+-------------------------------------------------+-----------------------------------+ Forcing data ------------ In this control simulation all external radiative forcings, including greenhouse gases, ozone, and aerosol concentrations, are kept constant at year 1990 levels. .. Assessment of the Control Simulation .. ------------------------------------- .. Gregory Plot .. ^^^^^^^^^^^^ .. The Gregory plot (:numref:`ifs-fesom_cont_gregory`) summarizes the co-evolution of global mean surface air temperature (TAS) and net top-of-atmosphere (TOA) radiation over the 10-year control period. The simulation starts in year 0 near the ERA5 1990 reference value of 14.24 °C with a positive TOA imbalance of approximately +0.4 W m\ :sup:`-2`, which is below the CERES observed mean of 0.88 W m\ :sup:`-2`, indicating that the system is initially absorbing more energy than it emits but less than observed. Over the following years, TAS drifts upward by roughly 0.15 K, reaching approximately 14.38 °C by year 9, while the TOA flux decreases steadily and crosses zero around year 6, ultimately reaching approximately -0.2 W m\ :sup:`-2` by the end of the simulation. The negative Gregory slope of -2.78 W m\ :sup:`-2` K\ :sup:`-1` indicates a strong net radiative feedback that acts to restore the energy balance as the surface warms. The transition from positive to negative TOA flux, combined with the ongoing surface warming, suggests that the coupled system has not yet reached equilibrium and is adjusting from its initialization state. Large interannual scatter in the TOA flux reflects the natural internal variability of the climate system over a short 10-year window. .. .. figure:: ../../../../evaluation/mn5/figures/Tco2559_Control_Gregory_IFS-FESOM.png .. :name: ifs-fesom_cont_gregory .. Gregory plot for the IFS-FESOM control simulation. Each dot represents one simulation year, colored by year number (0–9). The x-axis shows globally averaged annual surface air temperature and the y-axis the net TOA radiative flux. The green dashed line marks the ERA5 1990 reference temperature (14.24 °C), the orange dashed line indicates the CERES observed mean TOA flux (0.88 W m\ :sup:`-2`), and the black line shows the Gregory regression with a slope of -2.78 W m\ :sup:`-2` K\ :sup:`-1`. .. Ocean Temperature Drift .. ^^^^^^^^^^^^^^^^^^^^^^^ .. The Hovmoeller diagram of global mean ocean temperature anomalies (:numref:`ifs-fesom_cont_temp_ocean`) reveals a vertically differentiated drift structure over the 10-year control period. The uppermost layers (0–200 m) exhibit a pronounced seasonal cycle superimposed on a progressive cooling trend, with cold anomalies intensifying from near zero at initialization to approximately -0.4 to -0.6 °C by 1999. This upper-ocean cooling is consistent with the negative TOA flux developing in the second half of the simulation (see :numref:`ifs-fesom_cont_gregory`), suggesting that excess heat loss at the surface drives a net cooling of the mixed layer. At intermediate depths (roughly 500–1500 m), a persistent warm anomaly of approximately +0.1 to +0.2 °C develops, likely reflecting a redistribution of heat from the surface to the subsurface through deepening of the thermocline or enhanced vertical mixing during the adjustment phase. The deep ocean (below 2000 m) remains largely stable with near-zero anomalies, indicating that the model drift has not yet propagated to abyssal depths within this 10-year window. .. .. figure:: ../../../../evaluation/ifs_fesom_eval/figures/control_temperature_ocean.png .. :name: ifs-fesom_cont_temp_ocean .. Time-depth Hovmoeller diagram of global mean ocean temperature anomalies (°C) relative to the initial state in the IFS-FESOM control simulation (1990–1999). .. Ocean Salinity Drift .. ^^^^^^^^^^^^^^^^^^^^ .. The Hovmoeller diagram of global mean ocean salinity anomalies (:numref:`ifs-fesom_cont_sal_ocean`) shows a complementary vertical structure to the temperature drift. The near-surface layer displays a strong seasonal cycle, while the upper 100–200 m progressively develops positive salinity anomalies reaching approximately +0.10 to +0.15 PSU by 1997–1999, indicating a salinification of the upper ocean. Below this, at intermediate depths (roughly 200–700 m), a freshening signal intensifies over the decade, penetrating deeper with time. This pattern of upper-ocean salinification and subsurface freshening is consistent with an adjustment of the haline stratification during model spin-up, potentially driven by excessive surface evaporation or insufficient freshwater input in the early years of the coupled simulation. As with temperature, the deep ocean (below 1500 m) remains virtually unaffected, confirming that the drift is confined to the upper and intermediate water masses within the 10-year integration period. .. .. figure:: ../../../../evaluation/ifs_fesom_eval/figures/control_salinity_ocean.png .. :name: ifs-fesom_cont_sal_ocean .. Time-depth Hovmoeller diagram of global mean ocean salinity anomalies (PSU) relative to the initial state in the IFS-FESOM control simulation (1990–1999).