Felix Bunzel

The effects of mixing on age of air

Mean age of air (AoA) measures the mean transit time of air parcels along the Brewer-Dobson circulation (BDC) starting from their entry into the stratosphere. AoA is determined both by transport along the residual circulation and by two-way mass exchange (mixing). The relative roles of residual circulation transport and two-way mixing for AoA, and for projected AoA changes are not well understood. Here effects of mixing on AoA are quantified by contrasting AoA with the transit time of hypothetical transport solely by the residual circulation. Based on climate model simulations, we find additional aging by mixing throughout most of the lower stratosphere, except in the extratropical lowermost stratosphere where mixing reduces AoA. We use a simple Lagrangian model to reconstruct the distribution of AoA in the GCM and to illustrate the effects of mixing at different locations in the stratosphere. Predicted future reduction in AoA associated with an intensified BDC is equally due to faster transport along the residual circulation as well as reduced aging by mixing. A tropical leaky pipe model is used to derive a mixing efficiency, measured by the ratio of the two-way mixing mass flux and the net (residual) mass flux across the subtropical boundary. The mixing efficiency remains close to constant in a future climate, suggesting that the strength of two-way mixing is tightly coupled to the strength of the residual circulation in the lower stratosphere. This implies that mixing generally amplifies changes in AoA due to uniform changes in the residual circulation.

The role of stratosphere-troposphere coupling in the occurrence of extreme winter cold spells over Northern Europe

Received 26 June 2012; accepted 26 August 2012; published 12 October 2012. [1] Extreme cold spells over Northern Europe during winter are examined in order to address the question to what degree and in which ways stratospheric dynamics may influence the state of the troposphere. The study is based on 500 years of a preindustrial control simulation with a comprehensive global climate model which well resolves the stratosphere, the MPI Earth System Model. Geopotential height anomalies leading to cold air outbreaks leave imprints throughout the atmosphere including the middle and lower stratosphere. A significant connection between tropospheric winter cold spells over Northern Europe and erosion of the stratospheric polar vortex is detected up to 30 hPa. In about 40 percent of the cases, the extreme cold spells are preceded by dynamical disturbances in the stratosphere. The strong warmings associated with the deceleration of the stratospheric jet cause the tropopause height to decrease over high latitudes. The compression of the tropospheric column below favors the development of high pressure anomalies and blocking signatures over polar regions. This in turn leads to the advection of cold air towards Northern Europe and the establishment of a negative annular mode pattern in the troposphere. Anomalies in the residual mean meridional circulation during the stratospheric weak vortex events contribute to the warming of the lower stratosphere, but are not key in the mechanism through which the stratosphere impacts the troposphere.

The Brewer-Dobson circulation in a changing climate : import of the model configuration

AbstractMost climate models simulate a strengthening of the Brewer–Dobson circulation (BDC) under a changing climate. However, the magnitude of the trend as well as the underlying mechanisms varies significantly among the models. In this work the impact of both vertical resolution and vertical extent of a model on the simulated BDC change is investigated by analyzing sensitivity simulations performed with the general circulation model ECHAM6 in three different model configurations for three different climate states. Tropical upwelling velocities and age of stratospheric air are used as measures for the strength of the BDC. Both consistently show a BDC strengthening from the preindustrial to the future climate state for all configurations of the model. However, the amplitude and origin of this change vary between the different setups. Analyses of the tropical upward mass flux indicate that in the model with a lid at 10 hPa the BDC strengthening at 70 hPa is primarily produced by resolved wave drag, while i...

Multi-model assessment of the impact of soil moisture initialization on mid-latitude summer predictability

Land surface initial conditions have been recognized as a potential source of predictability in sub-seasonal to seasonal forecast systems, at least for near-surface air temperature prediction over the mid-latitude continents. Yet, few studies have systematically explored such an influence over a sufficient hindcast period and in a multi-model framework to produce a robust quantitative assessment. Here, a dedicated set of twin experiments has been carried out with boreal summer retrospective forecasts over the 1992–2010 period performed by five different global coupled ocean–atmosphere models. The impact of a realistic versus climatological soil moisture initialization is assessed in two regions with high potential previously identified as hotspots of land–atmosphere coupling, namely the North American Great Plains and South-Eastern Europe. Over the latter region, temperature predictions show a significant improvement, especially over the Balkans. Forecast systems better simulate the warmest summers if they follow pronounced dry initial anomalies. It is hypothesized that models manage to capture a positive feedback between high temperature and low soil moisture content prone to dominate over other processes during the warmest summers in this region. Over the Great Plains, however, improving the soil moisture initialization does not lead to any robust gain of forecast quality for near-surface temperature. It is suggested that models biases prevent the forecast systems from making the most of the improved initial conditions.

Seasonal climate forecasts significantly affected by observational uncertainty of Arctic sea ice concentration

We investigate how observational uncertainty in satellite-retrieved sea ice concentrations affects seasonal climate predictions. To do so, we initialize hindcast simulations with the Max Planck Institute Earth System Model every 1 May and 1 November from 1981 to 2011 with two different sea ice concentration data sets, one based on the NASA Team and one on the Bootstrap algorithm. For hindcasts started in November, initial differences in Arctic sea ice area and surface temperature decrease rapidly throughout the freezing period. For hindcasts started in May, initial differences in sea ice area increase over time. By the end of the melting period, this causes significant differences in 2 meter air temperature of regionally more than 3∘C. Hindcast skill for surface temperatures over Europe and North America is higher with Bootstrap initialization during summer and with NASA Team initialization during winter. This implies that the observational uncertainty also affects forecasts of teleconnections that depend on northern hemispheric climate indices.

Improved Seasonal Prediction of European Summer Temperatures With New Five‐Layer Soil‐Hydrology Scheme

We evaluate the impact of a new five-layer soil-hydrology scheme on seasonal hindcast skill of 2 m temperatures over Europe obtained with the Max Planck Institute Earth System Model (MPI-ESM). Assimilation experiments from 1981 to 2010 and 10-member seasonal hindcasts initialized on 1 May each year are performed with MPI-ESM in two soil configurations, one using a bucket scheme and one a new five-layer soil-hydrology scheme. We find the seasonal hindcast skill for European summer temperatures to improve with the five-layer scheme compared to the bucket scheme and investigate possible causes for these improvements. First, improved indirect soil moisture assimilation allows for enhanced soil moisture-temperature feedbacks in the hindcasts. Additionally, this leads to improved prediction of anomalies in the 500 hPa geopotential height surface, reflecting more realistic atmospheric circulation patterns over Europe.

Improved teleconnection-based dynamical seasonal predictions of boreal winter

Climate and weather variability in the North Atlantic region is determined largely by the North Atlantic Oscillation (NAO). The potential for skillful seasonal forecasts of the winter NAO using an ensemble-based dynamical prediction system has only recently been demonstrated. Here we show that the winter predictability can be significantly improved by refining a dynamical ensemble through subsampling. We enhance prediction skill of surface temperature, precipitation, and sea level pressure over essential parts of the Northern Hemisphere by retaining only the ensemble members whose NAO state is close to a “first guess” NAO prediction based on a statistical analysis of the initial autumn state of the ocean, sea ice, land, and stratosphere. The correlation coefficient between the reforecasted and observation-based winter NAO is significantly increased from 0.49 to 0.83 over a reforecast period from 1982 to 2016, and from 0.42 to 0.86 for a forecast period from 2001 to 2017. Our novel approach represents a successful and robust alternative to further increasing the ensemble size, and potentially can be used in operational seasonal prediction systems. Plain Language Summary Predicting Northern Hemisphere winter conditions, which are controlled largely by fluctuations in the pressure filed over the North Atlantic (North Atlantic Oscillation, NAO), for the next season is a major challenge. Most state-of-the-art seasonal prediction systems show a correlation between observed and predicted NAOs of less than 0.30. Our novel approach uses dynamical links (teleconnections) between the autumn state of sea surface temperature in the North Atlantic, Arctic sea ice, snow in Eurasia, and stratosphere temperature over the Northern Hemisphere as predictors of the NAO in the subsequent winter to subsample a dynamical reforecast ensemble. We select only the ensemble members that consistently reproduce winter NAO states that evolve in accordance with the autumn state of these predictors. As a result the winter NAO prediction skill increases to a correlation value of 0.83. Considering these well established NAO teleconnections in our Earth system model leads to an improved prediction skill of European winter conditions, that is, surface temperature, precipitation, and sea level pressure. Our results advance seasonal prediction of European weather to a level that is usually limited to tropical regions and are relevant for a variety of societal sectors, such as global and national economies and energy and water resources.

A Higher‐resolution Version of the Max Planck Institute Earth System Model (MPI‐ESM1.2‐HR)

The MPI‐ESM1.2 is the latest version of the Max Planck Institute Earth System Model and is the baseline for the Coupled Model Intercomparison Project Phase 6 and current seasonal and decadal climate predictions. This paper evaluates a coupled higher‐resolution version (MPI‐ESM1.2‐HR) in comparison with its lower‐resolved version (MPI‐ESM1.2‐LR). We focus on basic oceanic and atmospheric mean states and selected modes of variability, the El Nino/Southern Oscillation and the North Atlantic Oscillation. The increase in atmospheric resolution in MPI‐ESM1.2‐HR reduces the biases of upper‐level zonal wind and atmospheric jet stream position in the northern extratropics. This results in a decrease of the storm track bias over the northern North Atlantic, for both winter and summer season. The blocking frequency over the European region is improved in summer, and North Atlantic Oscillation and related storm track variations improve in winter. Stable Atlantic meridional overturning circulations are found with magnitudes of ~16 Sv for MPI‐ESM1.2‐HR and ~20 Sv for MPI‐ESM1.2‐LR at 26°N. A strong sea surface temperature bias of ~5°C along with a too zonal North Atlantic current is present in both versions. The sea surface temperature bias in the eastern tropical Atlantic is reduced by ~1°C due to higher‐resolved orography in MPI‐ESM‐HR, and the region of the cold‐tongue bias is reduced in the tropical Pacific. MPI‐ESM1.2‐HR has a well‐balanced radiation budget and its climate sensitivity is explicitly tuned to 3 K. Although the obtained reductions in long‐standing biases are modest, the improvements in atmospheric dynamics make this model well suited for prediction and impact studies.

Numerical studies of stratosphere-troposphere dynamical coupling in a changing climate

In this thesis the behaviour of stratosphere-troposphere dynamical coupling under a changing climate is investigated. The response of the main features of stratospheric dynamics to a changing climate is evaluated, and the implications of stratospheric changes for the troposphere-surface system are assessed. A prominent feature of stratospheric dynamics is the stratospheric meridional overturning circulation, the Brewer-Dobson Circulation (BDC). The BDC is driven by the dissipation and breaking of different types of upward propagating atmospheric waves which originate from the troposphere. Recent modelling studies showed that the tropospheric warming and simultaneous stratospheric cooling, induced by increasing greenhouse gas (GHG) concentrations, modify the driving forces of the BDC in a changing climate by exerting an upward shift on the critical layers for wave dissipation. The vast majority of climate models consistently simulates a BDC strengthening for the last decades. Observational datasets, however, indicate no significant change or even a slight decrease in BDC strength over the last 30 years. Both significance and origin of this discrepancy between model and observational data are studied in this thesis. First the mechanisms that drive the BDC are investigated. Although most models produce a qualitatively similar response, an acceleration of the BDC in a changing climate, the mechanisms causing this acceleration are still not fully understood. In some models the simulated BDC strengthening is mainly due to the drag originating from resolved large-scale waves, while other models find the parameterised small-scale wave drag to yield the more prominent contribution to the positive trend in BDC strength. It is also unclear to what extent the simulated BDC response depends on the representation of the stratosphere in a model. In order to investigate the impact of both vertical resolution and vertical extent of a model on the simulated BDC trend, sensitivity simulations with the General Circulation Model ECHAM6 were performed for three different model configurations and for three different sets of boundary conditions, representing preindustrial, present-day, and future climate states. Tropical upwelling velocities and age of stratospheric air are used as a measure for the strength of the BDC. Both consistently show a BDC strengthening from the preindustrial to the future time slice for all configurations of the model. However, the amplitude and origin of this change vary among the different setups. Analyses of the tropical upward mass flux indicate that in the model with a lid at 10 hPa (low top) the BDC strengthening at 70 hPa is primarily produced by resolved wave drag, while in the model with a higher lid (0.01 hPa, high top) the parameterised wave drag yields the main contribution to the BDC increase. This implies that consistent changes in the BDC originate from different causes when the stratosphere is not sufficiently resolved in a model. Furthermore, the effect of enhancing the horizontal diffusion in the upper model layers to avoid resolved wave reflection at the model lid is quantified, and a possible link to the different behaviour of the low-top model with regard to the origin of the BDC change is identified. Trends in the strength of the BDC may also be confused with or masked by natural BDC variability. In order to further investigate the discrepancy between model and observational data in terms of the BDC strength, the natural BDC variability is assessed in this study from a multi-centennial preindustrial control simulation, per-