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We present a model study that investigates to what extent it is possible to introduce ENSO variability to an Earth system Model of Intermediate Complexity (EMIC). The Zebiak-Cane ENSO model is dynamically coupled to the EMIC CLIMBER-3 alpha, which by itself exhibits no interannual or multidecadal variability. ENSO variability is introduced to CLIMBER-3 alpha by adding ENSO-related sea surface temperature anomalies to the upper layers of the model ocean. For the other coupling direction, changes in the mean CLIMBER-3 alpha climate on decadal time scales are used to change the background state of the ENSO model, achieving a two-way coupling. We compare typical ENSO-related patterns of a fully coupled pre-industrial model run to reanalysis data and point out the possibilities and limitations of this model configuration. Although introduced ENSO-related SST anomalies and other related variables like the Southern Oscillation Index are well reproduced by the EMIC in the forcing domain, teleconnections to other regions are damped, especially in meridional direction. The reason for this limitation is the atmospheric model, which does not sufficiently resolve the necessary transport mechanisms. Despite this limitation the presented coupling method may still be a useful tool in combination with higher resolution atmospheric models as being in development for the successor model CLIMBER-3 and possibly other EMICs.
Stratification-dependent mixing is employed in a coupled climate model of intermediate complexity with a 3- dimensional ocean component. Oceanic vertical diffusivity is calculated as kappa similar to N-alpha, where N is the local buoyancy frequency. The sensitivity of the Atlantic meridional overturning circulation (AMOC) to freshwater forcing is tested for exponents 0 <= a <= 2 by first slowly increasing, then decreasing the freshwater forcing over the North Atlantic, keeping the model close to equilibrium. The surface fresh anomaly imposed between 20 degrees N and 50 degrees N in the Atlantic reaches the deep ocean by vertical diffusion, and by AMOC advection via the northern convection sites. The fresh anomaly leads to enhanced stratification and thereby reduces vertical mixing stronger for higher values of a. Consequently, the freshwater anomaly reaches the northern deep water formation regions less diluted, and reduces the AMOC more strongly compared to lower values of a. Our findings indicate that modeled changes in the AMOC depend critically on the details of the mixing parameterization employed in the model.
We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland-Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland-Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing. Special attention is paid to changes in the subpolar gyre circulation. We find it sensitive to the overflow transport and the density of these water masses through baroclinic adjustments. The analysis of the governing equations confirms the presence of positive feedbacks inherent to the subpolar gyre and allows us to isolate the influence of the overflows on its dynamics.
The 8.2 ka event : abrupt transition of the subpolar gyre toward a modern North Atlantic circulation
(2010)
Climate model simulations of the 8.2 ka event show an abrupt strengthening of the Atlantic subpolar gyre that allows us to connect two major but apparently contradictory climate events of the early Holocene: the freshwater outburst from proglacial lakes and the onset of Labrador Sea water formation. The 8.2 ka event is the largest climatic signal of our present interglacial with a widespread cooling in the North Atlantic region about 8200 years before present. It coincides with a meltwater outburst from North American proglacial lakes that is believed to have weakened the Atlantic meridional overturning circulation and northward heat transport, followed by a recovery of the deep ocean circulation and rising temperatures after a few centuries. Marine proxy data, however, date the onset of deep water formation in Labrador Sea to the same time. The subsequent strengthening of the slope current system created a regional signal recorded as an abrupt and persistent surface temperature decrease. Although similarities in timing are compelling, a mechanism to reconcile these apparently contradictory events was missing. Our simulations show that an abrupt and persistent strengthening of the Atlantic subpolar gyre provides a plausible explanation. The intense freshwater pulse triggered a transition of the gyre circulation into a different mode of operation, stabilized by internal feedbacks and persistent after the cessation of the perturbation. As a direct consequence, deep water formation around its center intensifies. This corresponds to the modern flow regime and stabilizes the meridional overturning circulation, possibly contributing to the Holocene's climatic stability.
We present climatic consequences of the Representative Concentration Pathways (RCPs) using the coupled climate model CLIMBER-3 alpha, which contains a statistical-dynamical atmosphere and a three-dimensional ocean model. We compare those with emulations of 19 state-of-the-art atmosphere-ocean general circulation models (AOGCM) using MAGICC6. The RCPs are designed as standard scenarios for the forthcoming IPCC Fifth Assessment Report to span the full range of future greenhouse gas (GHG) concentrations pathways currently discussed. The lowest of the RCP scenarios, RCP3-PD, is projected in CLIMBER-3 alpha to imply a maximal warming by the middle of the 21st century slightly above 1.5 degrees C and a slow decline of temperatures thereafter, approaching today's level by 2500. We identify two mechanisms that slow down global cooling after GHG concentrations peak: The known inertia induced by mixing-related oceanic heat uptake; and a change in oceanic convection that enhances ocean heat loss in high latitudes, reducing the surface cooling rate by almost 50%. Steric sea level rise under the RCP3-PD scenario continues for 200 years after the peak in surface air temperatures, stabilizing around 2250 at 30 cm. This contrasts with around 1.3 m of steric sea level rise by 2250, and 2 m by 2500, under the highest scenario, RCP8.5. Maximum oceanic warming at intermediate depth (300-800 m) is found to exceed that of the sea surface by the second half of the 21st century under RCP3-PD. This intermediate-depth warming persists for centuries even after surface temperatures have returned to present-day values, with potential consequences for marine ecosystems, oceanic methane hydrates, and ice-shelf stability. Due to an enhanced land-ocean temperature contrast, all scenarios yield an intensification of monsoon rainfall under global warming.
The dynamics of the North Atlantic subpolar gyre (SPG) are assessed under present and glacial boundary conditions by investigating the SPG sensitivity to surface wind-stress changes in a coupled climate model. To this end, the gyre transport is decomposed in Ekman, thermohaline, and bottom transports. Surface wind-stress variations are found to play an important indirect role in SPG dynamics through their effect on water-mass densities. Our results suggest the existence of two dynamically distinct regimes of the SPG, depending on the absence or presence of deep water formation (DWF) in the Nordic Seas and a vigorous Greenland-Scotland ridge (GSR) overflow. In the first regime, the GSR overflow is weak and the SPG strength increases with wind-stress as a result of enhanced outcropping of isopycnals in the centre of the SPG. As soon as a vigorous GSR overflow is established, its associated positive density anomalies on the southern GSR slope reduce the SPG strength. This has implications for past glacial abrupt climate changes, insofar as these can be explained through latitudinal shifts in North Atlantic DWF sites and strengthening of the North Atlantic current. Regardless of the ultimate trigger, an abrupt shift of DWF into the Nordic Seas could result both in a drastic reduction of the SPG strength and a sudden reversal in its sensitivity to wind-stress variations. Our results could provide insight into changes in the horizontal ocean circulation during abrupt glacial climate changes, which have been largely neglected up to now in model studies.
We present the Potsdam Parallel Ice Sheet Model (PISM-PIK), developed at the Potsdam Institute for Climate Impact Research to be used for simulations of large-scale ice sheet-shelf systems. It is derived from the Parallel Ice Sheet Model (Bueler and Brown, 2009). Velocities are calculated by superposition of two shallow stress balance approximations within the entire ice covered region: the shallow ice approximation (SIA) is dominant in grounded regions and accounts for shear deformation parallel to the geoid. The plug-flow type shallow shelf approximation (SSA) dominates the velocity field in ice shelf regions and serves as a basal sliding velocity in grounded regions. Ice streams can be identified diagnostically as regions with a significant contribution of membrane stresses to the local momentum balance. All lateral boundaries in PISM-PIK are free to evolve, including the grounding line and ice fronts. Ice shelf margins in particular are modeled using Neumann boundary conditions for the SSA equations, reflecting a hydrostatic stress imbalance along the vertical calving face. The ice front position is modeled using a subgrid-scale representation of calving front motion (Albrecht et al., 2011) and a physically-motivated calving law based on horizontal spreading rates. The model is tested in experiments from the Marine Ice Sheet Model Intercomparison Project (MISMIP). A dynamic equilibrium simulation of Antarctica under present-day conditions is presented in Martin et al. (2011).
We present a dynamic equilibrium simulation of the ice sheet-shelf system on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK). The simulation is initialized with present-day conditions for bed topography and ice thickness and then run to steady state with constant present-day surface mass balance. Surface temperature and sub-shelf basal melt distribution are parameterized. Grounding lines and calving fronts are free to evolve, and their modeled equilibrium state is compared to observational data. A physically-motivated calving law based on horizontal spreading rates allows for realistic calving fronts for various types of shelves. Steady-state dynamics including surface velocity and ice flux are analyzed for whole Antarctica and the Ronne-Filchner and Ross ice shelf areas in particular. The results show that the different flow regimes in sheet and shelves, and the transition zone between them, are captured reasonably well, supporting the approach of superposition of SIA and SSA for the representation of fast motion of grounded ice. This approach also leads to a natural emergence of sliding-dominated flow in stream-like features in this new 3-D marine ice sheet model.
Thawing of permafrost and the associated release of carbon constitutes a positive feedback in the climate system, elevating the effect of anthropogenic GHG emissions on global-mean temperatures. Multiple factors have hindered the quantification of this feedback, which was not included in climate carbon-cycle models which participated in recent model intercomparisons (such as the Coupled Carbon Cycle Climate Model Intercomparison Project - (CMIP)-M-4). There are considerable uncertainties in the rate and extent of permafrost thaw, the hydrological and vegetation response to permafrost thaw, the decomposition timescales of freshly thawed organic material, the proportion of soil carbon that might be emitted as carbon dioxide via aerobic decomposition or as methane via anaerobic decomposition, and in the magnitude of the high latitude amplification of global warming that will drive permafrost degradation. Additionally, there are extensive and poorly characterized regional heterogeneities in soil properties, carbon content, and hydrology. Here, we couple a new permafrost module to a reduced complexity carbon-cycle climate model, which allows us to perform a large ensemble of simulations. The ensemble is designed to span the uncertainties listed above and thereby the results provide an estimate of the potential strength of the feedback from newly thawed permafrost carbon. For the high CO2 concentration scenario (RCP8.5), 33-114 GtC (giga tons of Carbon) are released by 2100 (68% uncertainty range). This leads to an additional warming of 0.04-0.23 degrees C. Though projected 21st century permafrost carbon emissions are relatively modest, ongoing permafrost thaw and slow but steady soil carbon decomposition means that, by 2300, about half of the potentially vulnerable permafrost carbon stock in the upper 3 m of soil layer (600-1000 GtC) could be released as CO2, with an extra 1-4% being released as methane. Our results also suggest that mitigation action in line with the lower scenario RCP3-PD could contain Arctic temperature increase sufficiently that thawing of the permafrost area is limited to 9-23% and the permafrost-carbon induced temperature increase does not exceed 0.04-0.16 degrees C by 2300.
Indian monsoon rainfall is vital for a large share of the world's population. Both reliably projecting India's future precipitation and unraveling abrupt cessations of monsoon rainfall found in paleorecords require improved understanding of its stability properties. While details of monsoon circulations and the associated rainfall are complex, full-season failure is dominated by large-scale positive feedbacks within the region. Here we find that in a comprehensive climate model, monsoon failure is possible but very rare under pre-industrial conditions, while under future warming it becomes much more frequent. We identify the fundamental intraseasonal feedbacks that are responsible for monsoon failure in the climate model, relate these to observational data, and build a statistically predictive model for such failure. This model provides a simple dynamical explanation for future changes in the frequency distribution of seasonal mean all-Indian rainfall. Forced only by global mean temperature and the strength of the Pacific Walker circulation in spring, it reproduces the trend as well as the multidecadal variability in the mean and skewness of the distribution, as found in the climate model. The approach offers an alternative perspective on large-scale monsoon variability as the result of internal instabilities modulated by pre-seasonal ambient climate conditions.