Thomas Reichler

How Well Do Coupled Models Simulate Today's Climate?

Information about climate and how it responds to increased greenhouse gas concentrations depends heavily on insight gained from numerical simulations by coupled climate models. The confidence placed in quantitative estimates of the rate and magnitude of future climate change is therefore strongly related to the quality of these models. In this study, we test the realism of several generations of coupled climate models, including those used for the 1995, 2001, and 2007 reports of the Intergovernmental Panel on Climate Change (IPCC). By validating against observations of present climate, we show that the coupled models have been steadily improving over time and that the best models are converging toward a level of accuracy that is similar to observation-based analyses of the atmosphere.

On the Effective Number of Climate Models

AbstractProjections of future climate change are increasingly based on the output of many different models. Typically, the mean over all model simulations is considered as the optimal prediction, with the underlying assumption that different models provide statistically independent information evenly distributed around the true state. However, there is reason to believe that this is not the best assumption. Coupled models are of comparable complexity and are constructed in similar ways. Some models share parts of the same code and some models are even developed at the same center. Therefore, the limitations of these models tend to be fairly similar, contributing to the well-known problem of common model biases and possibly to an unrealistically small spread in the outcomes of model predictions.This study attempts to quantify the extent of this problem by asking how many models there effectively are and how to best determine this number. Quantifying the effective number of models is achieved by evaluating ...

Analysis and Reduction of Systematic Errors through a Seamless Approach to Modeling Weather and Climate

Abstract The reduction of systematic errors is a continuing challenge for model development. Feedbacks and compensating errors in climate models often make finding the source of a systematic error difficult. In this paper, it is shown how model development can benefit from the use of the same model across a range of temporal and spatial scales. Two particular systematic errors are examined: tropical circulation and precipitation distribution, and summer land surface temperature and moisture biases over Northern Hemisphere continental regions. Each of these errors affects the model performance on time scales ranging from a few days to several decades. In both cases, the characteristics of the long-time-scale errors are found to develop during the first few days of simulation, before any large-scale feedbacks have taken place. The ability to compare the model diagnostics from the first few days of a forecast, initialized from a realistic atmospheric state, directly with observations has allowed physical def...

The coupled stratosphere-troposphere response to impulsive forcing from the troposphere

A simple atmospheric general circulation model (GCM) is used to investigate the transient response of the stratosphere–troposphere system to externally imposed pulses of lower-tropospheric planetary wave activity. The atmospheric GCM is a dry, hydrostatic, global primitive-equations model, whose circulation includes an active polar vortex and a tropospheric jet maintained by baroclinic eddies. Planetary wave activity pulses are generated by a perturbation of the solid lower boundary that grow and decay over a period of 10 days. The planetary wave pulses propagate upward and break in the stratosphere. Subsequently, a zonal-mean circulation anomaly propagates downward, often into the troposphere, at lags of 30–100 days. The evolution of the response is found to be dependent on the state of the stratosphere– troposphere system at the time the pulse is generated. In particular, on the basis of a large ensemble of these simulations, it is found that the length of time the signal takes to propagate downward from the stratosphere is controlled by initial anomalies in the zonal-mean circulation and in the zonal-mean wave drag. Criteria based on these anomaly patterns can be used, therefore, to predict the long-term surface response of the stratosphere–troposphere system to a planetary wave pulse up to 90 days after the pulse is generated. In an independent test, it is verified that the initial states that most strongly satisfy these criteria respond in the expected way to the lower-tropospheric wave activity pulse.

Breaking down the tropospheric circulation response by forcing

This study describes simulated changes in the general circulation during the twentieth and twenty-first centuries due to a number of individual direct radiative forcings and warming sea surface temperatures, by examining very long time-slice simulations created with an enhanced version of the Geophysical Fluid Dynamics Laboratories Atmospheric Model AM 2.1. We examine the effects of changing stratospheric ozone, greenhouse gas concentrations, and sea surface temperatures individually and in combination over both hemispheres. Data reveal robust poleward shifts in zonal mean circulation features in present-day simulations compared to a pre-industrial control, and in future simulations compared to present-day. We document the seasonality and significance of these shifts, and find that the combined response is well approximated by the sum of the individual responses. Our results suggest that warming sea surface temperatures are the main driver of circulation change over both hemispheres, and we project that the southern hemisphere jet will continue to shift poleward, albeit more slowly during the summer due to expected ozone recovery in the stratosphere.

Long-Range Predictability in the Tropics. Part II: 30–60-Day Variability

Abstract It is suggested that the slow evolution of the tropical Madden–Julian oscillation (MJO) has the potential to improve the predictability of tropical and extratropical circulation systems at lead times beyond 2 weeks. In practice, however, the MJO phenomenon is extremely difficult to predict because of the lack of good observations, problems with ocean forecasts, and well-known model deficiencies. In this study, the potential skill in forecasting tropical intraseasonal variability is investigated by eliminating all those errors. This is accomplished by conducting five ensemble predictability experiments with a complex general circulation model and by verifying them under the perfect model assumption. The experiments are forced with different combinations of initial and boundary conditions to explore their sensitivity to uncertainties in those conditions. When “perfect” initial and boundary conditions are provided, the model produces a realistic climatology and variability as compared to reanalysis,...

Multidecadal Drought Cycles in the Great Basin Recorded by the Great Salt Lake: Modulation from a Transition-Phase Teleconnection

This study investigates the meteorological conditions associated with multidecadal drought cycles as revealed by lake level fluctuation of the Great Salt Lake (GSL). The analysis combined instrumental, proxy, and simulation datasets, including the Twentieth Century Reanalysis version 2, the North American Drought Atlas, and a 2000-yr control simulation of the GFDL Coupled Model, version 2.1 (CM2.1). Statistical evidence from the spectral coherence analysis points to a phase shift amounting to 6‐9 yr between the wet‐dry cyclesin theGreatBasinandthewarm–coolphasesoftheinterdecadalPacificoscillation(IPO).Diagnosesof the sea surface temperature and atmospheric circulation anomaliesattribute such a phase shift to a distinctive teleconnectionwavetrainthatdevelopsduringthetransitionpoints betweentheIPO’swarmandcoolphases. This teleconnection wave train forms recurrent circulation anomalies centered over the southeastern Gulf of Alaska; this directs moisture flux across the Great Basin and subsequently drives wet‐dry conditions over the Great Basin and the GSL watershed. The IPO life cycle therefore modulates local droughts‐pluvials in a quarter-phase manner.

Changes in the Atmospheric Circulation as Indicator of Climate Change

Publisher Summary The strength, direction, and steadiness of the prevailing winds are crucial for climate. Winds associated with the atmospheric circulation lead to transports of heat and moisture from remote areas and thereby modify the local characteristics of climate in important ways. Specific names, such as extratropical Westerlies, tropical Trades, and equatorial Doldrums remind us of the significance of winds for the climate of a region and for the human societies living in it. This chapter discusses changes in the structure of the atmospheric circulation and its associated winds that have taken place during recent decades. These changes are best described as poleward displacements of major wind and pressure systems throughout the global three-dimensional atmosphere. The associated trends are important indicators of climate change and are likely to have profound influences on ecosystems and societies. It is focused on two important examples: first, tropical circulation change related to a poleward expansion of the Hadley cell (HC) and second, extratropical circulation change, as manifested by a poleward shift of the zone of high westerly winds in the midlatitudes, also known as an enhanced positive phase of the annular modes (AMs). Although both changes are associated with similar poleward displacements, it still remains to be seen whether the two phenomena are directly connected. The most intriguing challenges regarding the atmospheric circulation and climate change are to understand what the nature of this change is, what the consequences for surface climate are, and what the underlying causes and mechanisms are.

Long‐term evolution of the cold point tropical tropopause: Simulation results and attribution analysis

The height, pressure, and temperature of the cold point tropical tropopause areexamined in three 140 year simulations of a coupled chemistry climate model.Tropopause height increases approximately steadily in the simulations at a mean rate of63 ± 3 m/decade (2s confidence interval). The pressure trend changes near the year2000 from 1.03 ± 0.30 hPa/decade in the past to 0.55 ± 0.06 hPa/decade for thefuture. The trend in tropopause temperature changes even more markedly from 0.13 ±0.07 K/decade in the past to +0.254 ± 0.014 K/decade in the future. The tropopause datawere fit using regression by terms representing total column ozone, tropical mean seasurface temperatures, and tropical mass upwelling. Tropopause height and pressureclosely follow the upwelling term, whereas tropopause temperature is primarily relatedto sea surface temperature and ozone. The change in tropopause temperature trend near theyear 2000 is related to the change in the sign of the ozone trend with the sea surfacetemperature having an increased role after 2040. A conceptual model is used to estimatetropopause changes. The results confirm the regression analysis in showing theimportance of upper tropospheric warming (connected with sea surface temperature)and stratospheric cooling (connected with CO

Use of radio occultation for long‐term tropopause studies: Uncertainties, biases, and instabilities

[1] Research suggests that changes in tropopause structure can both indicate and impact changes in the global climate system. The Global Positioning System radio occultation (RO) technique shows tremendous potential for monitoring the global tropopause because of its precision, temporal consistency, and global measurement density. This study examines the capability of RO to monitor the global tropopause by addressing three specific objectives: (1) quantify sources of uncertainty in individual RO tropopause measurements, (2) examine mean bias and long-term stability of RO tropopause parameters with respect to those obtained from radiosondes, and (3) distinguish between differences due to processing and RO instrument differences by comparing tropopause parameters from different RO products. In this study, we make use of data from four different RO missions, including the recent Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). RO tropopause uncertainty is shown to be largely due to the use of a highly nonlinear tropopause definition (1.6 K or 510 m), although uncertainties in the RO derived temperature profiles themselves (0.25 K or 75 m) are still significant. Global mean temperature and height biases between RO instruments and radiosondes are within 0.5 K and 75 m. One long-term RO data set examined in this study appeared to contain spurious temperature trends, but these have since been corrected. Tropopause measurements from different RO instruments are generally within 41 m and 0.1 K for the globe. Dissimilarly processed temperature data, however, can differ by as much as 2 K in the mean. These results confirm the precision of RO data, but also demonstrate the importance of consistent processing for long-term tropopause temperature studies. Tropopause height data do not appear to be significantly affected by the differences in processing examined in this study.