Causes of change in Northern Hemisphere winter meridional winds and regional hydroclimate

A critical aspect of human-induced climate change is how it will affect precipitation around the world. Broadly speaking, warming increases atmospheric moisture holding capacity, intensifies moisture transports and makes sub-tropical dry regions drier and tropical and mid-to-high-latitude wet regions wetter. Extra-tropical precipitation patterns vary strongly with longitude, however, owing to the control exerted by the storm tracks and quasi-stationary highs and lows or stationary waves. Regional precipitation change will, therefore, also depend on how these aspects of the circulation respond. Current climate models robustly predict a change in the Northern Hemisphere (NH) winter stationary wave field that brings wetting southerlies to the west coast of North America, and drying northerlies to interior southwest North America and the eastern Mediterranean. Here we show that this change in the meridional wind field is caused by strengthened zonal mean westerlies in the sub-tropical upper troposphere, which alters the character of intermediate-scale stationary waves. Thus, a robust and easily understood model response to global warming is the prime cause of these regional wind changes. However, the majority of models probably overestimate the magnitude of this response because of biases in their climatological representation of the relevant waves, suggesting that winter season wetting of the North American west coast will be notably less than projected by the multi-model mean

Predicting climate change using response theory: global averages and spatial patterns

The provision of accurate methods for predicting the climate response to anthropogenic and natural forcings is a key contemporary scientific challenge. Using a simplified and efficient open-source general circulation model of the atmosphere featuring O(105105) degrees of freedom, we show how it is possible to approach such a problem using nonequilibrium statistical mechanics. Response theory allows one to practically compute the time-dependent measure supported on the pullback attractor of the climate system, whose dynamics is non-autonomous as a result of time-dependent forcings. We propose a simple yet efficient method for predicting—at any lead time and in an ensemble sense—the change in climate properties resulting from increase in the concentration of CO22 using test perturbation model runs. We assess strengths and limitations of the response theory in predicting the changes in the globally averaged values of surface temperature and of the yearly total precipitation, as well as in their spatial patterns. The quality of the predictions obtained for the surface temperature fields is rather good, while in the case of precipitation a good skill is observed only for the global average. We also show how it is possible to define accurately concepts like the inertia of the climate system or to predict when climate change is detectable given a scenario of forcing. Our analysis can be extended for dealing with more complex portfolios of forcings and can be adapted to treat, in principle, any climate observable. Our conclusion is that climate change is indeed a problem that can be effectively seen through a statistical mechanical lens, and that there is great potential for optimizing the current coordinated modelling exercises run for the preparation of the subsequent reports of the Intergovernmental Panel for Climate Change

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

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

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

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

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

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

Klimaatprojecties voor Europa: modelvergelijkingen en een analyse van de voorspelbaarheid

Abstract niet beschikbaarThe predictability of the European climate is investigated on three aspects. First, for 3 GCM models (i.e. ECHAM4-OPYC3, Hadcm3, CCCma) is determined how an enhanced greenhousegas-concentration influences the temperature, precipitation and pressure. This influence is determined for different spatial scales, i.e. global, regional (Europe) and local (De Bilt). The changes are expressed in a quality number, which compares the greenhouse signal with the capability of the models to describe the mean and the variability of the current climate. The second aspect considered is the way the probability density function (PDF) of the temperature changes due to an enhanced greenhousegas-concentration. The KNMI-model ECBilt shows for the end of this century not only a shifted, but also a narrower PDF. Specifically investigated is the effect on cold winter temperatures. For an event occurring once in 10 years, the shift is 1.2 WC, whereas the narrower PDF causes an extra 1.5 WC, being 2.7 WC in total. The third aspect is the surge level at the Dutch coast. Not only the effect of the greenhouse climate is investigated, but also is determined if the observed record can be used for estimating the surge level belonging to the (socially relevant) return period of 10.000 year. The research shows that the available records of 100 year deficit considerably. For the greenhouse climate, ECBilt indicates the excitation of so-called 'super-storms': storms of a extreme rareness, with an other intensity-frequency relation than the extreme storms we know from the current climate. For return periods longer than several centuries, the statistics of the extremes for wind and surge in the ECBilt greenhouse experiment are determined by these super-storms.SG-NO