Description: The relative importance of the formation of different North Atlantic Deep Water masses on the meridional overturning is examined with a non-eddy-resolving version of the CME model. In contrast to a frequently held belief, convective deep-water formation south of the North Atlantic sill does not significantly force the large-scale overturning if an adequate overflow across the sill can be represented by the model. The sensitivity of the meridional transport to the surface thermohaline forcing is increased under alternate climatic conditions such as increased surface cooling or reduced overflow compared to the present-day situation. The results indicate that climate models may be too sensitive to decadal timescale variability of the surface forcing in subpolar regions.

Description: An ocean circulation model for process studies of the Subpolar North Atlantic is developed based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model (MOM) code. The basic model configuration is identical with that of the high-resolution model (with a grid size of 1/3° × 2/5°) of the World Ocean Circulation Experiment (WOCE) Community Modeling Effort (CME), except that the domain of integration is confined to the area from 43° to 65°N. Open boundary conditions are used for the inflows and outflows across the northern and southern boundaries. A comparison with the CME model covering the whole North Atlantic (from 15°S to 65°N) shows that the regional model, with inflow conditions at 43°N from a CME solution, is able to reproduce the CME results for the subpolar area. Thus the potential of a regional model lies in its use as an efficient tool for numerical experiments aiming at an identification of the key physical processes that determine the circulation and water mass transformations in the subpolar gyre. This study deals primarily with the representation and role of the overflow waters that enter the domain at the northern boundary. Sensitivity experiments show the effect of closed versus open boundaries, of different hydrographic conditions at inflow points, and of the representation of the narrow Faeroe Bank Channel. The representation of overflow processes in the Denmark Strait is the main controlling mechanism for the net transport of the deep boundary current along the Greenland continental slope and further downstream. Changes in the Faeroe Bank Channel throughflow conditions have a comparatively smaller effect on the deep transport in the western basin but strongly affect the water mass characteristics in the eastern North Atlantic. The deep water transport at Cape Farewell and further downstream is enhanced compared to the combined Denmark Strait and Iceland-Scotland overflows. This enhancement can be attributed to a barotropic recirculation in the Irminger Basin which is very sensitive to the outflow conditions in the Denmark Strait. The representation of both overflow regions determine the upper layer circulation in the Irminger and Iceland Basins, in particular the path of the North Atlantic Current.

Description: A series of numerical experiments with models of the Atlantic Ocean is analyzed with respect to the uptake of CFC‐11 and its export from the subpolar gyre with the North Atlantic Deep Water. We discuss the influence of parameterizations for air‐sea gas exchange and subgrid‐scale processes on the rate of CFC‐11 that enters the North Atlantic Ocean and its dependence on horizontal grid spacing in models from medium (4/3°) to eddy‐permitting (1/3°) horizontal resolution. Model results are compared with observational estimates of tracer inventories in order to evaluate to what degree the simulations capture realistic CFC distributions. While higher resolution is needed to model details of the CFC distribution, for example, in the Deep Western Boundary Current, the medium resolution models are able to simulate quantitatively satisfying CFC inventories in different water masses. Nevertheless, the inventories derived from the medium‐resolution experiments show a critical dependence on details of the parameterization of the mixing effect of mesoscale eddies and on the representation of bottom boundary layer processes. The numerical representation of eddy activity turns out to be of crucial importance in order to obtain modeled CFC inventories in agreement with observed values, which can be achieved either by carefully choosing the mixing parameterization or by applying higher horizontal resolution. The ratio of CFC‐11 being exported southward from the subpolar North Atlantic to the total CFC‐11 inventory in NADW does not vary significantly over the suite of model experiments.

Description: The ocean takes up a large fraction of the pertubation C02 that enters the atmosphere by human activity. A realistic representation of this uptake in numerical models is essential for future climate studies. Uptake of C02 or other atmospheric trace gases is strongly influenced by oceanic physical variability at spatial scales between 20 and 100 km. Our main goal is to study the effect of this mesoscale variability on the cumulative uptake of anthropogenic C02 and chlorofluorocarbons using an existing model of the ocean circulation in the Atlantic that resolves a significant part of that variability explicitly because of its grid spacing of about 20 km. Results are compared with simulated trace gas distribution obtained from a model with coarser resolution.

Description: A new, non-flux-corrected, global climate model is introduced, the Kiel Climate Model (KCM), which will be used to study internal climate variability from interannual to millennial time scales and climate predictability of the first and second kind. The version described here is a coarse-resolution version that will be employed in extended-range integrations of several millennia. KCM's performance in the tropical Pacific with respect to mean state, annual cycle, and El Nino-Southern Oscillation (ENSO) is described. Additionally, the tropical Pacific response to global warming is studied.Overall, climate drift in a multicentury control integration is small. However, KCM exhibits an equatorial cold bias at the surface of the order 1 degrees C, while strong warm biases of several degrees are simulated in the eastern tropical Pacific on both sides off the equator, with maxima near the coasts. The annual and semiannual cycles are realistically simulated in the eastern and western equatorial Pacific, respectively. ENSO performance compares favorably to observations with respect to both amplitude and period. An ensemble of eight greenhouse warming simulations was performed, in which the CO2 concentration was increased by 1% yr(-1) until doubling was reached, and stabilized thereafter. Warming of equatorial Pacific sea surface temperature (SST) is, to first order, zonally symmetric and leads to a sharpening of the thermocline. ENSO variability increases because of global warming: during the 30-yr period after CO2 doubling, the ensemble mean standard deviation of Nino-3 SST anomalies is increased by 26% relative to the control, and power in the ENSO band is almost doubled. The increased variability is due to both a strengthened (22%) thermocline feedback and an enhanced (52%) atmospheric sensitivity to SST; both are associated with changes in the basic state. Although variability increases in the mean, there is a large spread among ensemble members and hence a finite probability that in the "model world" no change in ENSO would be observed.

Description: A new release of the Max Planck Institute for Meteorology Earth System Model version 1.2 (MPI-ESM1.2) is presented. The development focused on correcting errors in and improving the physical processes representation, as well as improving the computational performance, versatility, and overall user friendliness. In addition to new radiation and aerosol parameterizations of the atmosphere, several relatively large, but partly compensating, coding errors in the model's cloud, convection, and turbulence parameterizations were corrected. The representation of land processes was refined by introducing a multilayer soil hydrology scheme, extending the land biogeochemistry to include the nitrogen cycle, replacing the soil and litter decomposition model and improving the representation of wildfires. The ocean biogeochemistry now represents cyanobacteria prognostically in order to capture the response of nitrogen fixation to changing climate conditions and further includes improved detritus settling and numerous other refinements. As something new, in addition to limiting drift and minimizing certain biases, the instrumental record warming was explicitly taken into account during the tuning process. To this end, a very high climate sensitivity of around 7 K caused by low-level clouds in the tropics as found in an intermediate model version was addressed, as it was not deemed possible to match observed warming otherwise. As a result, the model has a climate sensitivity to a doubling of CO2 over preindustrial conditions of 2.77 K, maintaining the previously identified highly nonlinear global mean response to increasing CO2 forcing, which nonetheless can be represented by a simple two-layer model.