Impacts of horizontal resolution on simulated climate statistics in ECHAM4

The sensitivity of a general circulation model to changes in resolution is studied using the Max Planck Institute for Meteorology (MPI) 19-level model, ECHAM4. Simulations extending over a period between 10 and 15 years, with observed sea surface temperatures as lower boundary conditions from 1979 onward, have been performed using four different horizontal resolutions, T21, T30, T42 and T106. The atmospheric time-mean state and the intraseasonal variability are compared to the European Centre for Medium Range Weather Forecasts (ECMWF) reanalyses and a few other observational datasets. The T30, T42 and T106 simulations are similar in many respects, whereas the T21 simula- tion is qualitatively different. Several effects related to model resolution could be identi- fied, such as increasing tropical upper tropospheric warming with increasing resolution. This is due to more vigorous tropical convection, larger ice water content and, hence, increasing cirrus cloud greenhouse effect. Associated with this increasing warming at higher resolution is a poleward expansion of the zonally averaged circulation regime. On the other hand, the zonally asymmetric component of the circulation, i.e., the stationary waves, improve with higher resolution. Also, higher resolution has a positive impact on regional precipitation patterns which are affected by orography such as the summer mon- soon precipitation over India. Intraseasonal variability has been analyzed only for the higher resolution models, T42 and T106. Compared to the ECMWF reanalyses, both models are able to simulate the intrasea- sonal geopotential height variability, eddy fluxes of heat and momentum, and eddy kinetic energy with reasonable accuracy. This applies to transient eddies in both the bandpass and lowpass regime and to the stationary eddies as well. Some biases can be identified which are more or less independent of resolution. These include the mislocation of the Azores high and the overestimation of its intensity, a cold bias in the polar upper troposphere and lower stratosphere and the poleward and upward displacement of the maxima of geopotential height variability, momentum fluxes and eddy kinetic energy. An important finding is that the operational ECMWF analyses, which have been widely used for model validation, considerably overestimate the lowpass variability, as compared to the reanalyses, due to frequent changes of the forecast model and data assimilation scheme. This implies that the results from our investigations are not directly comparable to previous investigations that used operational analyses for validation

The evolution of the global aerosol system in a transient climate simulation from 1860 to 2100

The evolution of the global aerosol system from 1860 to 2100 is investigated through a transient atmosphere-ocean General Circulation Model climate simulation with interactively coupled atmospheric aerosol and oceanic biogeochemistry modules. The microphysical aerosol module HAM incorporates the major global aerosol cycles with prognostic treatment of their composition, size distribution, and mixing state. Based on an SRES A1B emission scenario, the global mean sulfate burden is projected to peak in 2020 while black carbon and particulate organic matter show a lagged peak around 2070. From present day to future conditions the anthropogenic aerosol burden shifts generally from the northern high-latitudes to the developing low-latitude source regions with impacts on regional climate. Atmospheric residence- and aging-times show significant alterations under varying climatic and pollution conditions. Concurrently, the aerosol mixing state changes with an increasing aerosol mass fraction residing in the internally mixed accumulation mode. The associated increase in black carbon causes a more than threefold increase of its co-single scattering albedo from 1860 to 2100. Mid-visible aerosol optical depth increases from pre-industrial times, predominantly from the aerosol fine fraction, peaks at 0.26 around the sulfate peak in 2020 and maintains a high level thereafter, due to the continuing increase in carbonaceous aerosols. The global mean anthropogenic top of the atmosphere clear-sky short-wave direct aerosol radiative perturbation intensifies to −1.1 W m^−2 around 2020 and weakens after 2050 to −0.6 W m^−2, owing to an increase in atmospheric absorption. The demonstrated modifications in the aerosol residence- and aging-times, the microphysical state, and radiative properties challenge simplistic approaches to estimate the aerosol radiative effects from emission projections

ENSO impact on midlatitude circulation patterns in future climate change projections

The remote influence of the leading mode of interannual variability in the Tropics, the El Niño/Southern Oscillation (ENSO), on the northern hemispheric midlatitude circulation in future climate is investigated. For this, IPCC SRES scenarios of the latest version of the coupled climate model ECHAM5/MPI-OM are used. In ensembles of future climate change projections it is found, that a changing state of ENSO with increased variability has a pronounced influence on the dominant midlatitude circulation pattern, namely the Pacific North America (PNA) pattern and the North Atlantic Oscillation (NAO). More explicitly, in the 21st and 22nd century, a positive (negative) phase of ENSO is more likely followed by a positive (negative) PNA index and negative (positive) NAO index than it is observed in the 20th century. Correlation coefficients between the winter mean Niño3.4 index and the NAO index increase substantially from the 20th centur

A Normal-Mode Approach to Jovian Atmospheric Dynamics

We propose a nonlinear, quasi-geostrophic, baroclinic model of Jovian atmospheric dynamics, in which vertical variations of velocity are represented by a truncated sum over a complete set of orthogonal functions obtained by a separation of variables of the linearized quasi-geostrophic potential vorticity equation. A set of equations for the time variation of the mode amplitudes in the nonlinear case is then derived. We show that for a planet with a neutrally stable, fluid interior instead of a solid lower boundary, the baroclinic mode represents motions in the interior, and is not affected by the baroclinic modes. One consequence of this is that a normal-mode model with one baroclinic mode is dynamically equivalent to a one layer model with solid lower topography. We also show that for motions in Jupiter's cloudy lower troposphere, the stratosphere behaves nearly as a rigid lid, so that the normal-mode model is applicable to Jupiter. We test the accuracy of the normal-mode model for Jupiter using two simple problem forced, vertically propagating Rossby waves, using two and three baroclinic modes and baroclinic instability, using two baroclinic modes. We find that the normal-road model provide qualitatively correct results, even with only a very limited number of vertical degrees of freedom

Forcing of the quasi-biennial oscillation from a broad spectrum of atmospheric waves

[1] The circulation of the stratosphere, and its influence on the trace constituent distribution, is an important component of the climate system, which must be included in simulations of global climate change. However, the ability to simulate a dominant stratospheric phenomenon, the Quasi-Biennial Oscillation (QBO) in equatorial zonal wind, is an outstanding challenge in climate modeling. Although confined to the tropics, the QBO affects the circulation and the interannual variability of the entire stratosphere, parts of the mesosphere and possibly also of the troposphere. Here we show that the QBO is successfully simulated in a general circulation model (GCM) of the newest generation. Key factors are a sufficient spatial resolution, a realistic simulation of tropical convection, and the consideration of the effects of gravity waves. From this simulation it is inferred that a broad spectrum of atmospheric waves is necessary to generate the QBO in the model

The Antarctic circumpolar wave in a coupled ocean–atmosphere GCM

A phenomenon called the Antarctic Circumpolar Wave (ACW), suggested earlier from fragmentary observational evidence, has been simulated realistically in an extended integration of a Max Planck Institute coupled general circulation model. The ACW both in the observations and in the model constitutes a mode of the coupled ocean–atmosphere–sea-ice system that inhabits the high latitudes of the Southern Hemisphere. It is characterized by anomalies of such climate variables as sea surface temperature, sea level pressure, meridional wind, and sea ice that exhibit intricate and evolving spatial phase relations to each other. The simulated ACW signal in the ocean propagates eastward over most of the high-latitude Southern Ocean, mainly advected along in the Antarctic Circumpolar Current. On average, it completes a circuit entirely around the Southern Ocean but is strongly dissipated in the South Atlantic and in the southern Indian Ocean, just marginally maintaining statistical significance in these areas until it reaches the South Pacific where it is reenergized. In extreme cases, the complete circumpolar propagation is more clear, requiring about 12–16 yr to complete the circuit. This, coupled with the dominant zonal wavenumber 3 pattern of the ACW, results in the local reappearance of energy peaks about every 4–5 yr. The oceanic component of the mode is forced by the atmosphere via fluxes of heat. The overlying atmosphere establishes patterns of sea level pressure that mainly consist of a standing wave and are associated with the Pacific–South American (PSA) oscillation described in earlier works. The PSA, like its counterpart in the North Pacific, appears to be a natural mode of the high southern latitudes. There is some ENSO-related signal in the ACW forced by anomalous latent heat release associated with precipitation anomalies in the central and western tropical Pacific. However, ENSO-related forcing explains at most 30% of the ACW variance and, generally, much less. It is hypothesized that the ACW as an entity represents the net result of moving oceanic climate anomalies interacting with a spatially fixed atmospheric forcing pattern. As the SST moves into and out of phase with the resonant background pattern it is selectively amplified or dissipated, an idea supported by several independent analyses. A simplified ocean heat budget model seems to also support this idea

Cloud microphysics and aerosol indirect effects in the global climate model ECHAM5-HAM

The double-moment cloud microphysics scheme from ECHAM4 has been coupled to the size-resolved aerosol scheme ECHAM5-HAM. ECHAM5-HAM predicts the aerosol mass and number concentrations and the aerosol mixing state. This results in a much better agreement with observed vertical profiles of the black carbon and aerosol mass mixing ratios than with the previous version ECHAM4, where only the different aerosol mass mixing ratios were predicted. Also, the simulated liquid, ice and total water content and the cloud droplet and ice crystal number concentrations as a function of temperature in stratiform mixed-phase clouds between 0 and –35°C agree much better with aircraft observations in the ECHAM5 simulations. ECHAM5 performs better because more realistic aerosol concentrations are available for cloud droplet nucleation and because the Bergeron-Findeisen process is parameterized as being more efficient. The total anthropogenic aerosol effect includes the direct, semi-direct and indirect effects and is defined as the difference in the top-of-the-atmosphere net radiation between present-day and pre-industrial times. It amounts to –1.8 W m^−2 in ECHAM5, when a relative humidity dependent cloud cover scheme and present-day aerosol emissions representative for the year 2000 are used. It is larger when either a statistical cloud cover scheme or a different aerosol emission inventory are employed

Eavaluation of the hydrological cycle in the ECHAM5 model

This study investigates the impact of model resolution on the hydrological cycle in a suite of model simulations using a new version of the Max Planck Institute for Meteorology atmospheric general circulation model (AGCM). Special attention is paid to the evaluation of precipitation on the regional scale by comparing model simulations with observational data in a number of catchments representing the major river systems on the earth in different climate zones. It is found that an increased vertical resolution, from 19 to 31 atmospheric layers, has a beneficial effect on simulated precipitation with respect to both the annual mean and the annual cycle. On the other hand, the influence of increased horizontal resolution, from T63 to T106, is comparatively small. Most of the improvements at higher vertical resolution, on the scale of a catchment, are due to large-scale moisture transport, whereas the impact of local water recycling through evapotranspiration is somewhat smaller. At high horizontal and vertical resolution (T106L31) the model captures most features of the observed hydrological cycle over land, and also the local and remote precipitation response to El Niño–Southern Oscillation (ENSO) events. Major deficiencies are the overestimation of precipitation over the oceans, especially at higher vertical resolution, along steep mountain slopes and during the Asian summer monsoon season, whereas a dry bias exists over Australia. In addition, the model fails to reproduce the observed precipitation response to ENSO variability in the Indian Ocean and Africa. This might be related to missing coupled air–sea feedbacks in an AGCM forced with observed sea surface temperatures