Mediterranean winter rainfall in phase with African monsoons during the past 1.36 million years

Mediterranean climates are characterized by strong seasonal contrasts between dry summers and wet winters. Changes in winter rainfall are critical for regional socioeconomic development, but are difficult to simulate accurately1 and reconstruct on Quaternary timescales. This is partly because regional hydroclimate records that cover multiple glacial–interglacial cycles2,3 with different orbital geometries, global ice volume and atmospheric greenhouse gas concentrations are scarce. Moreover, the underlying mechanisms of change and their persistence remain unexplored. Here we show that, over the past 1.36 million years, wet winters in the northcentral Mediterranean tend to occur with high contrasts in local, seasonal insolation and a vigorous African summer monsoon. Our proxy time series from Lake Ohrid on the Balkan Peninsula, together with a 784,000-year transient climate model hindcast, suggest that increased sea surface temperatures amplify local cyclone development and refuel North Atlantic low-pressure systems that enter the Mediterranean during phases of low continental ice volume and high concentrations of atmospheric greenhouse gases. A comparison with modern reanalysis data shows that current drivers of the amount of rainfall in the Mediterranean share some similarities to those that drive the reconstructed increases in precipitation. Our data cover multiple insolation maxima and are therefore an important benchmark for testing climate model performance

Klimaatvariaties op een tijdschaal van een tiental jaren

Abstract niet beschikbaarA coupled atmosphere/ocean/sea-ice Model of Intermediate Complexity (ECBilt) was developed. With ECBilt we aim at deriving qualitative information on physical processes and feedbacks that may be crucial for explaining climate variability on time-scales of decades to millennia, as well as on the potential existence of instabilities in the climate system that may lead to rapid climate transitions. The model is very efficient, so that long integrations as well as "idealized" experiments, sensitivity experiments and ensemble climate integrations are possible.With the first generation ECBilt model (ECBilt1) we have investigated the interannual to decadal climate variability in the North Atlantic area and the Antarctic Circumpolar Wave (ACW), the possible effects of modulations in solar irradiance on the earth climate, the possiblity for the occurrence of rapid transitions in a 40 kY integration of ECBilt for present day orbital parameters and the predictability of climate on decadal time scales. In all of the above mentioned studies we have done experiments to test the robustness of the results by introducing variations in internal and/or external parameters. We have improved ECBilt1 in several aspects. In the atmospheric model the ratiation code was replaced. The new radiation scheme is a linearisation of the Morcrette scheme and includes the possibility of variations in trace gases like CO2. We are presentyl working on improving the boundary layer scheme. The ocean model was replaced by the MOM model of GFDL and by the CLIO model of the University of Louvain la Neuve. Both models are state of the art ocean models. MOM is less expensive than CLIO and will be used for millennial time-scale variability. CLIO will be used for studying the decadal time-scales. CLIO had a dynamic sea-ice model. With ECBilt2 coupled to CLIO we have performed greenhouse gas concentration scenario experiments and we have also studied the decadal variability in the Arctic which is associated with the dynamics of sea-ice. ECBilt2 will be integrated in the IMAGE model of RIVM for climate change scenario studies.SG-NO

Intrinsic limits to predictability of abrupt regional climate change in IPCCSRES scenarios

[1] We used an ensemble climate-model experiment to explore the timing and nature of an abrupt regional climate change within the 21st century. In response to global warming a North-Atlantic climate transition occurs, which affects climate over Greenland and northwestern Europe. For a high IPCC non-mitigation emission scenario the transition has a high probability to occur before 2100. In a lower IPCC scenario the probability is lower and the transition threshold is approached more gradually. We found that close to the threshold the evolution of the system becomes sensitive to small perturbations. Consequently, natural climate fluctuations limit the predictability of the timing of crossing the transition threshold, and thus of the abrupt climate change, most strongly for the lower IPCC scenario. No transition is projected for a mitigation scenario, in which CO2-equivalent concentrations are stabilized below the IPCC-scenario range

The influence of ocean convection patterns on high-latitude climate projections

The mean state and variability of deep convection in the ocean influence the North Atlantic climate. Using an ensemble experiment with a coupled atmosphere-ocean-sea ice model, it is shown that cooling and subdued warming areas can occur over the North Atlantic Ocean and adjacent landmasses under global warming. Different "present-day" convection patterns in the Greenland-Iceland-Norway (GIN) Sea result in different future surface-air temperature changes. At higher latitudes, the more effective positive sea ice feedback increases the likelihood of changes in convection causing a regional cooling that is larger than the warming brought about by the enhanced greenhouse effect. The modeled freshening of deep ocean layers in the North Atlantic in a time period preceding a reorganization of GIN Sea convection is consistent with recent observations. Low-frequency internal variability in the ocean model has relatively little impact on the response patterns

Decadal variability in high northern latitudes as simulated by an intermediate-complexity climate model

A 2500 year integration has been performed with a global coupled atmospheric-sea-ice-ocean model of intermediate complexity with the main objective of studying the climate variability in polar regions on decadal time-scales and longer. The atmospheric component is the ECBILT model, a spectral T21 three-level quasi-geostrophic model that includes a representation of horizontal and vertical beat transfers as well as of the hydrological cycle. ECBILT is coupled to the CLIO model, which consists of a primitive-equation free-surface ocean general circulation model and a dynamic-thermodynamic sea-ice model. Comparison of model results with observations shows that the ECBILT CLIO model is able to reproduce reasonably well the climate of the high northern latitudes. The dominant mode of coupled variability between the atmospheric circulation and sea-ice cover in the simulation consists of an annular mode for geopotential height at 800 hPa and of a dipole between the Barents and Labrador Seas for the sea-ice concentration which are similar to observed patterns of variability. In addition, the simulation displays strong decadal variability in the sea-ice volume, with a significant peak at about 18 years. Positive volume anomalies are caused by (1) a decrease in ice export through Fram Strait associated with more anticyclonic winds at high latitudes, (2) modifications in the freezing/melting rates in the Arctic due to lower air temperature and higher surface albedo, and (3) a weaker heat flux at the ice base in the Barents and Kara Seas caused by a lower inflow, of warm Atlantic water. Opposite anomalies occur during the volume-decrease phase of the oscillation

Large sea-ice volume anomalies simulated in a coupled climate model

The processes leading to the formation of a large,anomaly of sea-ice volume integrated over the Northern Hemisphere have been investigated in a coarse-resolution three-dimensional atmosphere-ocean-sea-ice model. This anomaly lasts for about 20 years and reaches 8.6 x 10(3) km(3), i.e. 6 standard deviations. It is associated with a maximum surface cooling of more than 5 degreesC in the annual mean close to Spitzbergen. The trigger of the large event in the model is a sequence of years presenting an atmospheric circulation characterised by negative geopotential height anomalies over Greenland and positive ones in the Barents and Norwegian seas. This atmospheric anomaly induces an increase in the ice volume as well as of the ice extent. Sea ice survives in the area located close to Spitzbergen where deep convection occurs during normal years. This is associated with a shut down of convection in that area and thus by a strong reduction of the upward heat flux from the ocean to the atmosphere that strongly amplifies the initial cooling. The long sequence of following years presenting the same type of anomaly in the atmosphere is not just a rare realisation of the intrinsic atmospheric variability. A positive feedback between the modification of surface conditions and the atmospheric circulation reinforces the initial atmospheric anomaly. Finally, the large event stops when convection increases again as a result of an increase in ocean surface density close to Spitzbergen. This density increase is due to both the advection of a positive salinity anomaly created in the Arctic because of brine rejection and to local ice formation southward of Spitzbergen during the cold event. Sensitivity experiments performed with the model have shown that the frequency of the events is very sensitive to the mean climate simulated by the model, the frequency being higher in a colder climate

A mechanism of decadal variability of the sea-ice volume in the Northern Hemisphere

A long-term simulation performed with a coarse-resolution, global, atmosphere-ocean-sea-ice model displays strong decadal variability of the sea-ice volume in the Northern Hemisphere with a significant peak at about 15-18 years. This model results from the coupling of ECBILT, a spectral T21, 3-level quasi-geostrophic atmospheric model, and CLIO, a sea-ice-ocean general circulation model. First, the mechanism underlying the variability of ice volume in the model was studied by performing correlation analyses between the simulated variables. In a second step, a series of additional sensitivity experiments was performed in order to illustrate the role of specific physical processes. This as allowed us to identify a feedback loop in the ice-ocean system, which proceeds as follows: an increase in At tic sea-ice volume induces an increase in the salinity there. This salinity anomaly is transported to the Greenland Sea where it promotes convective activity. This warms up the surface oceanic layer and the atmosphere in winter and induces a decrease of the ice volume, completing half a cycle. The changes in ice volume are driven by a geopotential height pattern characterised by centres of action of opposite signs over Greenland and the Barents-Kara-Central Arctic area. Thermodynamic feedback between the ice and the atmosphere appear also to be very important for the persistence of the oscillation. The dynamical response of the atmosphere to sea-ice and temperature anomalies at surface plays a smaller role

Simulated changes in vegetation distribution, land carbon storage, and atmospheric CO2 in response to a collapse of the North Atlantic thermohaline circulation

Measurements on glacial ice show that atmospheric CO2 varied by 20ppmv with large iceberg discharges into the North Atlantic (NA) and themost prominent Dansgaard/ Oeschger (D/O) climate fluctuations. CO2variations during less pronounced D/O events were smaller than a fewppm. The D/O fluctuations have been linked to changes in the NAThermohaline Circulation (THC). Here, we analyse how abrupt changes inthe NA THC affect the terrestrial carbon cycle by forcing theLund-Potsdam-Jena Dynamic Global Vegetation Model with climateperturbations from freshwater experiments with the ECBILT-CLIOgeneral circulation model. Changes in the marine carbon cycle are notaddressed. Modelled NA THC collapsed and recovered after about amillennium in response to prescribed freshwater forcing. The initialcooling of several Kelvin over Eurasia causes a reduction ofextant boreal and temperate forests and a decrease in carbon storage inhigh northern latitudes, whereas improved growing conditions andslower soil decomposition rates lead to enhanced storage inmid-latitudes. The magnitude and evolution of global terrestrialcarbon storage in response to abrupt THC changes depends sensitivelyon the initial climate conditions. Terrestrial storage varies between-67 and +50 PgC for arange of experiments that start at different times during the last21,000 years. Simulated peak-to-peak differences in atmospheric CO2and d13C are between {6 and 18 ppmv} and $0.18$ and $0.30$~\mypermil and compatible with the ice core CO2 record