Klaus Arpe

Eavaluation of the hydrological cycle in the ECHAM5 model

Abstract 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 a...

Oceanic control of decadal North Atlantic sea level pressure variability in winter

The predictability of winter‐time North Atlantic sea level pressure (SLP) variability has been investigated by means of an ensemble of integrations with an atmospheric general circulation model (AGCM) forced by observed sea surface temperatures (SSTs) for the period 1951–1994. The results imply that the SLP variations on timescales of several years to decades may be predictable, provided the SST anomalies themselves used to drive the AGCM can be predicted. The model, however, suffers from systematic errors, and the simulated centers of action are shifted relative to those observed.

Connection between Caspian sea level variability and ENSO

The problem of the world greatest lake, the Caspian Sea, level changes attracts the increased attention due to its environmental consequences and unique natural characteristics. Despite the huge number of studies aimed to explain the reasons of the sea level variations the underlying mechanism has not yet been clarified. The important question is to what extent the CSL variability is linked to changes in the global climate system and to what extent it can be explained by internal natural variations in the Caspian regional hydrological system. In this study an evidence of a link between the El Nino/Southern Oscillation phenomenon and changes of the Caspian Sea level is presented. This link was also found to be dominating in numerical experiments with the ECHAM4 atmospheric general circulation model on the 20th century climate.

Impact of global warming on the Asian winter monsoon in a coupled GCM

The Asian winter monsoon (AWM) response to the global warming was investigated through a long-term integration of the transient greenhouse warming with the ECHAM4/OPYC3 CGCM. The physics of the response was studied through analyses of the impact of the global warming on the variations of the ocean and land contrast near the ground in the Asian and western Pacific region and the east Asian trough and jet stream in the middle and upper troposphere. Forcing of transient eddy activity on the zonal circulation over the Asian and western Pacific region was also analyzed. It is found that in the global warming scenario the winter northeasterlies along the Pacific coast of the Eurasian continent weaken systematically and significantly, and intensity of the AWM reduces evidently, but the AWM variances on the interannual and interdecadal scales are not affected much by the global warming. It is suggested that the global warming makes the climate over the most part of Asia to be milder with enhanced moisture in winter. In the global warming scenario the contrasts of the sea level pressure and the near-surface temperature between the Asian continent and the Pacific Ocean become significantly smaller, northward and eastward shifts and weakening of the east Asian trough and jet stream in the middle and upper troposphere are found. As a consequence, the cold air in the AWM originating from the east Asian trough and high latitudes is less powerful. In addition, feedback of the transient activity also makes a considerable contribution to the higher-latitude shift of the jet stream over the North Pacific in the global warming scenario.

Climate predictability experiments with a general circulation model

The atmospheric response to the evolution of the global sea surface temperatures from 1979 to 1992 is studied using the Max-Planck-Institut 19 level atmospheric general circulation model, ECHAM3 at T 42 resolution. Five separate 14-year integrations are performed and results are presented for each individual realization and for the ensemble-averaged response. The results are compared to a 30-year control integration using a climate monthly mean state of the sea surface temperatures and to analysis data. It is found that the ECHAM3 model, by and large, does reproduce the observed response pattern to El Nino and La Niña. During the El Nino events, the subtropical jet streams in both hemispheres are intensified and displaced equatorward, and there is a tendency towards weak upper easterlies over the equator. The Southern Oscillation is a very stable feature of the integrations and is accurately reproduced in all experiments. The inter-annual variability at middle- and high-latitudes, on the other hand, is strongly dominated by chaotic dynamics, and the tropical SST forcing only modulates the atmospheric circulation. The potential predictability of the model is investigated for six different regions. Signal to noise ratio is large in most parts of the tropical belt, of medium strength in the western hemisphere and generally small over the European area. The ENSO signal is most pronounced during the boreal spring. A particularly strong signal in the precipitation field in the extratropics during spring can be found over the southern United States. Western Canada is normally warmer during the warm ENSO phase, while northern Europe is warmer than normal during the ENSO cold phase. The reason is advection of warm air due to a more intense Pacific low than normal during the warm ENSO phase and a more intense Icelandic low than normal during the cold ENSO phase, respectively.

A Comparison of Tropical Pacific Surface Wind Analyses

Abstract Surface wind analyses from three data assimilation systems are compared with independent wind observations from six buoys located in the Pacific within 8 deg of the equator. The period of comparison is 6 months (February to July 1987), with daily sampling. The agreement between the assimilation systems and the independent buoy data is disappointing. The longterm mean differences between the buoy and the assimilated zonal and meridional winds are as large as 3.1 m s−1, which is comparable to the size of the means themselves. The zonal and meridional daily wind correlations range between 0.66 and 0.17. The wind field agreement was actually better among the different systems than between any system and the buoys. However, the agreement among the analysis products was usually better for the zonal winds than for the meridional winds. For the time period and locations presented, the comparisons with the independent data show that no assimilation system is clearly superior to any of the others.

Variability of the Indian Monsoon in the ECHAM3 Model: Sensitivity to Sea Surface Temperature, Soil Moisture, and the Stratospheric Quasi-Biennial Oscillation

Abstract The variability of the monsoon is investigated using a set of 90-day forecasts [MONEG (Tropical Ocean Global Atmosphere Monsoon Numerical Experimentation Group) experiments] and a set of AMIP-type (Atmospheric Model Intercomparison Project) long-term simulations of the atmospheric circulation with the ECHAM3 model. The large-scale aspects of the summer monsoon circulation as represented by differences of dynamical quantities between the two extreme years 1987 and 1988 were reproduced well by the model in both kinds of experiments forced with observed sea surface temperature (SST). At the regional scale the difference of precipitation over India during summer 1987 and 1988 was well reproduced by the model in the 90-day forecasts using interannually varying SSTs; however, similarly good results were achieved in forecasts using climatological SSTs. The long-term simulations forced with interannually varying SST at the lower boundary of the atmosphere over a period of 14 years, on the other hand, onl...

Validation of the hydrological cycle of ERA 40

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Potential Predictability and AMIP Implications of Midlatitude Climate Variability in Two General Circulation Models

Abstract Ensembles of extended Atmospheric Model Intercomparison Project (AMIP) runs from the general circulation models of the National Centers for Environmental Prediction (formerly the National Meteorological Center) and the Max-Planck Institute (Hamburg, Germany) are used to estimate the potential predictability (PP) of an index of the Pacific–North America (PNA) mode of climate change. The PP of this pattern in “perfect” prediction experiments is 20%–25% of the index’s variance. The models, particularly that from MPI, capture virtually all of this variance in their hindcasts of the winter PNA for the period 1970–93. The high levels of internally generated model noise in the PNA simulations reconfirm the need for an ensemble averaging approach to climate prediction. This means that the forecasts ought to be expressed in a probabilistic manner. It is shown that the models’ skills are higher by about 50% during strong SST events in the tropical Pacific, so the probabilistic forecasts need to be conditio...

Simulation of the hydrological cycle over Europe : Model validation and impacts of increasing greenhouse gases

Different methods of estimating precipitation area means, based on observations, are compared with each other to investigate their usefulness for model validation. For the applications relevant to this study the ECMWF reanalyses provide a good and comprehensive data set for validation. The uncertainties of precipitation analyses, based on observed precipitation or from numerical weather forecasting schemes, are generally in the range of 20% but regionally much larger. The MPI atmospheric general circulation model is able to reproduce long term means of the main features of the hydrological cycle within the range of uncertainty of observational data, even for relatively small areas such as the Rhine river basin. Simulations with the MPI coupled general circulation model, assuming a further increase of anthropogenic greenhouse gases, show clear trends in temperature and precipitation for the next century which would have significant implications for human activity, e.g. a further increase of the sea level of the Caspian Sea and less water in the Rhine and the Danube. We have gained confidence in these results because trends in the temperature and precipitation in the coupled model simulations up to the present are partly confirmed by an atmospheric model simulation forced with observed SSTs and by observational data. We gained further confidence because the simulations with the same coupled model but using constant greenhouse gases do not show such trends. However, doubts arise from the fact that these trends are strong where the systematic errors of the model are large.