ALBERT

Description: Orbital forcing does not only exert direct insolation effects, but also alters climate indirectly through feedback mechanisms that modify atmosphere and ocean dynamics and meridional heat and moisture transfers. We investigate the regional effects of these changes by detailed analysis of atmosphere and ocean circulation and heat transports in a coupled atmosphere-ocean-sea ice-biosphere general circulation model (ECHAM5/JSBACH/MPI-OM). We perform long term quasi equilibrium simulations under pre-industrial, mid-Holocene (6000 years before present – yBP), and Eemian (125 000 yBP) orbital boundary conditions. Compared to pre-industrial climate, Eemian and Holocene temperatures show generally warmer conditions at higher and cooler conditions at lower latitudes. Changes in sea-ice cover, ocean heat transports, and atmospheric circulation patterns lead to pronounced regional heterogeneity. Over Europe, the warming is most pronounced over the north-eastern part in accordance with recent reconstructions for the Holocene. We attribute this warming to enhanced ocean circulation in the Nordic Seas and enhanced ocean-atmosphere heat flux over the Barents Shelf in conduction with retreat of sea ice and intensified winter storm tracks over northern Europe.

Description: Changes in the Earth's orbit lead to changes in the seasonal and meridional distribution of insolation. We quantify the influence of orbitally induced changes on the seasonal temperature cycle in a transient simulation of the last 6000 years – from the mid-Holocene to today – using a coupled atmosphere-ocean general circulation model (ECHAM5/MPI-OM) including a land surface model (JSBACH). The seasonal temperature cycle responds directly to the insolation changes almost everywhere. In the Northern Hemisphere, its amplitude decreases according to an increase in winter insolation and a decrease in summer insolation. In the Southern Hemisphere, the opposite is true. Over the Arctic Ocean, decreasing summer insolation leads to an increase in sea-ice cover. The insulating effect of sea ice between the ocean and the atmosphere leads to decreasing heat flux and favors more "continental" conditions over the Arctic Ocean in winter, resulting in strongly decreasing temperatures. Consequently, there are two competing effects: the direct response to insolation changes and a sea-ice insulation effect. The sea-ice insulation effect is stronger, and thus an increase in the amplitude of the seasonal temperature cycle over the Arctic Ocean occurs. This increase is strongest over the Barents Shelf and influences the temperature response over northern Europe. We compare our modeled seasonal temperatures over Europe to paleo reconstructions. We find better agreements in winter temperatures than in summer temperatures and better agreements in northern Europe than in southern Europe, since the model does not reproduce the southern European Holocene summer cooling inferred from the paleo reconstructions. The temperature reconstructions for northern Europe support the notion of the influence of the sea-ice insulation effect on the evolution of the seasonal temperature cycle.

Description: Water availability is fundamental to societies and ecosystems, but our understanding of variations in hydroclimate (including extreme events, flooding, and decadal periods of drought) is limited because of a paucity of modern instrumental observations that are distributed unevenly across the globe and only span parts of the 20th and 21st centuries. Such data coverage is insufficient for characterizing hydroclimate and its associated dynamics because of its multidecadal-to-centennial variability and highly regionalized spatial signature. High-resolution (seasonal to decadal) hydroclimatic proxies that span all or parts of the Common Era (CE) and paleoclimate model simulations are therefore important tools for augmenting our understanding of hydroclimate variability. In particular, the comparison of the two sources of information is critical for addressing the uncertainties and limitations of both, while enriching each of their interpretations. We review the principal proxy data available for hydroclimatic reconstructions over the CE and highlight contemporary understanding of how these proxies are interpreted as hydroclimate indicators. We also review the available last-millennium simulations from fully-coupled climate models and discuss several outstanding challenges associated with simulating hydroclimate variability and change over the CE. A specific review of simulated hydroclimatic changes forced by volcanic events is provided, as well as a discussion of expected improvements in estimated forcings, models and their implementation in the future. Our review of hydroclimatic proxies and last-millennium model simulations is used as the basis for articulating a variety of considerations and best practices for how to perform proxy-model comparisons of CE hydroclimate. This discussion provides a framework for how best to evaluate hydroclimate variability and its associated dynamics using these comparisons, as well as how they can better inform interpretations of both proxy data and model simulations. We subsequently explore means of using proxy-model comparisons to better constrain and characterize future hydroclimate risks. This is explored specifically in the context of several examples that demonstrate how proxy-model comparisons can be used to quantitatively constrain future hydroclimatic risks as estimated from climate model projections.

Description: The pre-industrial millennium is among the periods selected by the Paleoclimate Model Intercomparison Project (PMIP) for experiments contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and the fourth phase of the PMIP (PMIP4). The past1000 transient simulations serve to investigate the response to (mainly) natural forcing under background conditions not too different from today, and to discriminate between forced and internally generated variability on interannual to centennial timescales. This paper describes the motivation and the experimental set-ups for the PMIP4-CMIP6 past1000 simulations, and discusses the forcing agents orbital, solar, volcanic, and land use/land cover changes, and variations in greenhouse gas concentrations. The past1000 simulations covering the pre-industrial millennium from 850 Common Era (CE) to 1849 CE have to be complemented by historical simulations (1850 to 2014 CE) following the CMIP6 protocol. The external forcings for the past1000 experiments have been adapted to provide a seamless transition across these time periods. Protocols for the past1000 simulations have been divided into three tiers. A default forcing data set has been defined for the Tier 1 (the CMIP6 past1000) experiment. However, the PMIP community has maintained the flexibility to conduct coordinated sensitivity experiments to explore uncertainty in forcing reconstructions as well as parameter uncertainty in dedicated Tier 2 simulations. Additional experiments (Tier 3) are defined to foster collaborative model experiments focusing on the early instrumental period and to extend the temporal range and the scope of the simulations. This paper outlines current and future research foci and common analyses for collaborative work between the PMIP and the observational communities (reconstructions, instrumental data).

Description: Analyses of a 500-yr control integration with the non-flux-adjusted coupled atmosphere–sea ice–ocean model ECHAM5/Max-Planck-Institute Ocean Model (MPI-OM) show pronounced multidecadal fluctuations of the Atlantic overturning circulation and the associated meridional heat transport. The period of the oscillations is about 70–80 yr. The low-frequency variability of the meridional overturning circulation (MOC) contributes substantially to sea surface temperature and sea ice fluctuations in the North Atlantic. The strength of the overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, and the time-varying control on the freshwater export from the Arctic to the convection sites modulates the overturning circulation. The variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport. The relatively high resolution in the deep-water formation region and the Arctic Ocean suggests that a better representation of convective and frontal processes not only leads to an improvement in the mean state but also introduces new mechanisms determining multidecadal variability in large-scale ocean circulation.

Description: It is investigated how changes in the North Atlantic meridional overturning circulation (MOC) might be reliably detected within a few decades, using the observations provided by the RAPID-MOC 26°N array. Previously, detectability of MOC changes had been investigated with a univariate MOC time series exhibiting strong internal variability, which would prohibit the detection of MOC changes within a few decades. Here, a modification of K. Hasselmann’s fingerprint technique is used: (simulated) observations are projected onto a time-independent spatial pattern of natural variability to derive a time-dependent detection variable. The fixed spatial pattern of natural variability is derived by regressing the zonal density gradient along 26°N against the strength of the MOC at 26°N within the coupled ECHAM5/Max Planck Institute Ocean Model’s (MPI-OM) control climate simulation. This pattern is confirmed against the observed anomalies found between the 1957 and the 2004 hydrographic occupations of the section. Onto this fixed spatial pattern of natural variability, both the existing hydrographic observations and simulated observations mimicking the RAPID-MOC 26°N array in three realizations of the Intergovernmental Panel on Climate Change (IPCC) scenario A1B are projected. For a random observation error of 0.01 kg m−3, and only using zonal density gradients between 1700- and 3100-m depth, statistically significant detection occurs with 95% reliability after about 30 yr, in the model and climate change scenario analyzed here. Compared to using a single MOC time series as the detection variable, continuous observations of zonal density gradients reduce the detection time by 50%. For the five hydrographic occupations of the 26°N transect, none of the analyzed depth ranges shows a significant trend between 1957 and 2004, implying that there was no MOC trend over the past 50 yr.

Description: The decadal–centennial variations of East Asian summer monsoon (EASM) and the associated rainfall change during the past millennium are simulated using the earth system model developed at the Max Planck Institute for Meteorology. The model was driven by up-to-date reconstructions of external forcing including the recent low-amplitude estimates of solar variations. Analysis of the simulations indicates that the EASM is generally strong during the Medieval Warm Period (MWP; A.D. 1000–1100) and weak during the Little Ice Age (LIA; A.D. 1600–1700). The monsoon rainband exhibits a meridional tripolar pattern during both epochs. Excessive (deficient) precipitation is found over northern China (35°–42°N, 100°–120°E) but deficient (excessive) precipitation is seen along the Yangtze River valley (27°–34°N, 100°–120°E) during the MWP (LIA). Both similarities and disparities of the rainfall pattern between the model results herein and the proxy data have been compared, and reconstructions from Chinese historical documents and some geological evidence support the results. The changes of the EASM circulation including the subtropical westerly jet stream in the upper troposphere and the western Pacific subtropical high (WPSH) in the middle and lower troposphere are consistent with the meridional shift of the monsoon rain belt during both epochs. The meridional monsoon circulation changes are accompanied with anomalous southerly (northerly) winds between 20° and 50°N during the MWP (LIA). The land–sea thermal contrast change caused by the effective radiative forcing leads to the MWP and LIA monsoon changes. The “warmer land–colder ocean” anomaly pattern during the MWP favors a stronger monsoon, while the “colder land–warmer ocean” anomaly pattern during the LIA favors a weaker monsoon.

Description: Responses of summer [June–August (JJA)] temperature and precipitation to large volcanic eruptions are analyzed using the millennial simulations of the earth system model developed at the Max Planck Institute for Meteorology. The model was driven by up-to-date reconstructions of external forcing, including natural forcing (solar and volcanic) and anthropogenic forcing (land-cover change and greenhouse gases). Cooling anomalies after large volcanic eruptions are seen on a nearly global scale. The cooling in the Northern Hemisphere (NH) is stronger than in the Southern Hemisphere (SH), and cooling is stronger over the continents than over the oceans. The precipitation decreases in the tropical and subtropical regions in the first summer after large volcanic eruptions. The cooling, with amplitudes of up to −0.6°C, is also seen over eastern China. East Asia is dominated by northerly wind anomalies, and the corresponding summer rainfall exhibits a coherent reduction over the entirety of eastern China. The tropospheric mean temperature anomalies indicate that there is coherent cooling over East Asia and the tropical ocean after large volcanic eruptions. The cooling over the middle-to-high latitudes of East Asia is stronger than over the tropical ocean. This temperature anomaly pattern suggests a reduced land–sea thermal contrast and favors a weaker East Asian summer monsoon (EASM) circulation. Analysis of the radiative fluxes at the top of the atmosphere (TOA) suggests that the reduction in shortwave radiation after large volcanic eruptions is nearly twice as large as the reduction in emitted longwave radiation, a net loss of radiative energy that cools the surface and lower troposphere.

Description: This study aims at improving the forecast skill of climate predictions through the use of ocean synthesis data for initial conditions of a coupled climate model. For this purpose, the coupled model of the Max Planck Institute (MPI) for Meteorology, which consists of the atmosphere model ECHAM5 and the MPI Ocean Model (MPI-OM), is initialized with oceanic synthesis fields available from the German contribution to Estimating the Circulation and Climate of the Ocean (GECCO) project. The use of an anomaly coupling scheme during the initialization avoids the main problems with drift in the climate predictions. Thus, the coupled model is continuously forced to follow the density anomalies of the GECCO synthesis over the period 1952–2001. Hindcast experiments are initialized from this experiment at constant intervals. The results show predictive skill through the initialization up to the decadal time scale, particularly over the North Atlantic. Viewed over the time scales analyzed here (annual, 5-yr, and 10-yr mean), greater skill for the North Atlantic sea surface temperature (SST) is obtained in the hindcast experiments than in either a damped persistence or trend forecast. The Atlantic meridional overturning circulation hindcast closely follows that of the GECCO oceanic synthesis. Hindcasts of global-mean temperature do not obtain greater skill than either damped persistence or a trend forecast, owing to the SST errors in the GECCO synthesis, outside the North Atlantic. An ensemble of forecast experiments is subsequently performed over the period 2002–11. North Atlantic SST from the forecast experiment agrees well with observations until the year 2007, and it is higher than if simulated without the oceanic initialization (averaged over the forecast period). The results confirm that both the initial and the boundary conditions must be accounted for in decadal climate predictions.