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.     

An investigation of QBO signals in the east Asian and Indian monsoon in GCM experiments

Observations have shown that the monsoon is a highly variable phenomenon of the tropical troposphere, which exhibits significant variance in the temporal range of two to three years. The reason for this specific interannual variability has not yet been identified unequivocally. Observational analyses have also shown that EI Niño indices or western Pacific SSTs exhibit some power in the two to three year period range and therefore it was suggested that an ocean-atmosphere interaction could excite and support such a cycle. Similar mechanisms include land-surface-atmosphere interaction as a possible driving mechanism. A rather different explanation could be provided by a forcing mechanism based on the quasi-biennial oscillation of the zonal wind in the lower equatorial stratosphere (QBO). The QBO is a phenomenon driven by equatorial waves with periods of some days which are excited in the troposphere. Provided that the monsoon circulation reacts to the modulation of tropopause conditions as forced by the QBO, this could explain monsoon variability in the quasi-biennial window. The possibility of a QBO-driven monsoon variability is investigated in this study in a number of general circulation model experiments where the QBO is assimilated to externally controlled phase states. These experiments show that the boreal summer monsoon is significantly influenced by the QBO. A QBO westerly phase implies less precipitation in the western Pacific, but more in India, in agreement with observations. The austral summer monsoon is exposed to similar but weaker mechanisms and the precipitation does not change significantly.     

中国日降雨量变化特征分析

将旋转主成分分析和复变量莫莱特小波分析应用于14a中国境内实测与ECMWF再分析日降雨量资料,研究其年际、季际与季节内变化的时空分布特征和再分析日降雨量资料的有效性。结果表明:从实测资料中分析的降水特征,都可以由再分析资料再现出来。除周内振荡大有出入外,其余频率的振荡二者大致相符或基本吻合。因此,周内振荡还不能被再分析数据较好地刻划,但是与旱涝灾害和季风爆发联系密切的周际以上振荡,在再分析日降雨量中基本上能被正确地表现出来。     

Evaluation of daily precipitation in China from ECMWF and NCEP reanalyses

ＩＮＴＲＯＤＵＣＴＩＯＮＷｅａｔｈｅｒｆｏｒｅｃａｓｔｉｎｇａｓａｍａｊｏｒｓｕｂｊｅｃｔｉｎａｔｍｏｓｐｈｅｒｉｃｓｃｉｅｎｃｅｈａｓｄｅｖｅｌｏｐｅｄｓｉｎｃｅｔｈｅ 1 950’ｓｉｎｔｏａｍｏｄｅｒｎｓｃｉｅｎｃｅ .Ｎｕｍｅｒｉｃａｌｗｅａｔｈｅｒｆｏｒｅｃａｓｔｍｏｄｅｌｓａｒｅｅｘｔｅｎｓｉｖｅｌｙａｎｄｆｒｅｑｕｅｎｔｌｙｕｓｅｄｔｏｃｈｅｃｋｔｈｅｔｈｅｏｒｉｅｓ,ｒｕｌｅｏｕｔｔｈｅｏｌｄｉｎｃｏｒｒｅｃｔｏｎｅｓａｎｄｐｒｅｓｅｎｔｎｅｗｉｄｅａｓ,ａｎｄｓｕｇｇｅｓｔｍｅｔｈｏｄｓ…     

Prediction of the Caspian Sea level using ECMWF seasonal forecasts and reanalysis

The hydrological budget of the Caspian Sea (CS) is investigated using the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERAi) and seasonal forecast (FCST) data with the aim of predicting the Caspian Sea Level (CSL) some months ahead. Precipitation and evaporation are used. After precipitation events over the Volga River, the discharge (Volga River discharge (VRD)) follows with delays, which are parameterized. The components of the water budget from ERAi and FCSTs are integrated to obtain time series of the CSL. Observations of the CSL and the VRD are used for comparison and tuning. The quality of ERAi data is sufficiently good to calculate the time variability of the CSL with a satisfactory accuracy. Already the storage of water within the Volga Basin allows forecasts of the CSL a few months ahead, and using the FCSTs of precipitation improves the CSL forecasts. The evaporation in the seasonal forecasts is deficient due to unrealistic sea surface temperatures over the CS. Impacts of different water budget terms on the CSL variability are shown by a variety of validation tools. The importance of precipitation anomalies over the catchment of the Volga River is confirmed, but also impacts from the two southern rivers (Sefidrud and Kura River) and the evaporation over the CS become obvious for some periods. When pushing the FCSTs beyond the limits of the seasonal FCSTs to 1 year, considerable forecast skill can still be found. Validating only FCSTs by the present approach, which show the same trend as one based on a statistical method, significantly enhances the skill scores.     

The hydrological cycle in the ECMWF short range forecasts

Precipitation and latent heat flux forecasts by the European Centre for Medium Range Weather Forecasts (ECMWF) model have been compared with other estimates of these quantities. In the Northern Hemisphere extra-tropics the latent heat flux over oceans and the precipitation over continents in the short range forecasts are probably good estimates of the truth. The day-to-day as well as the interannual variability in these latitudes seem to be realistic.In the Southern Hemisphere extra-tropics there is a strong spin-up in the precipitation forecasts probably with too low precipitation amounts in the short range forecasts. It is speculated that inconsistent use of satellite data leads to a weakening of large-scale rising motions between 40 and 60°S. Also the latent heat flux in these latitudes is probably too low due to a too moist 1000 mb humidity analysis.Over subtropical deserts the precipitation amounts in the forecasts agree with climatological estimates. Contrary to climatological estimates this precipitation is not evaporated but runs off.In the tropics, especially over mountainous areas, the short range forecasts (average for the first 24 h) with the present model tend to overpredict precipitation amounts, but still with reasonable distributions. Averages between days 1 and 2 probably give a good estimate of the truth except over the eastern Pacific where there is an overestimation, also in the medium range forecasts. Strong underestimation of latent heat fluxes over tropical oceans in the short range forecasts have been considerably reduced with a recent model change. There are still areas, e.g. the Southern Hemisphere subtropical Pacific, with too low evaporation due to too moist 1000 mb analyses probably in connection with an inconsistent use of satellite observations.The interannual variability of monthly mean evaporation and precipitation in the short range forecasts reflects partly atmospheric anomalies, but especially in the tropics, and also larger amplitude variations due to changes in the analysis/forecasting scheme.     

Changes in the structural and magnetic properties of Ni–substituted hematite prepared from metal oxinates

Samples prepared by the novel method based on the thermal decomposition at 700 °C in air of mixed Fe and Ni oxinates were characterized by thermogravimetric analysis, X-ray diffraction, magnetization measurements, and variable-temperature Mössbauer spectroscopy. It is found that the combustion treatment produces Ni-hematite and trevorite, the fraction of the latter increasing with the increment of Ni in the metal oxinates. Results indicate that the substitution of Ni²⁺ for Fe³⁺ in the hematite structure, which was found to be less than 5.3 mol%, is accompanied by the presence of oxygen and structural vacancy sites. Both the metal replacement and the presence of defects cause the a and c cell hematite parameters to decrease. The Néel temperature and the difference between the saturation fields for the antiferromagnetic AF and the weakly ferromagnetic WF phases are also found to decrease with nickel content. These effects are due to the fact that the magnetic behavior of hematite depends on the presence of Ni, vacancy sites, and lattice distortion. The large decrease in the Néel temperature with Ni doping suggests that structural vacancies are also present On the contrary, Ni incorporation does not appreciably affect the Morin temperature and the temperature range in which both AF and WF phases coexist. It is suggested that these effects are probably due to the differing effects that Ni and defects can produce on the magnetic dipolar and the single ion anisotropies.     

A more accurate scheme for calculating Earth’s skin temperature

The theoretical framework of the vertical discretization of a ground column for calculating Earth’s skin temperature is presented. The suggested discretization is derived from the evenly heat-content discretization with the optimal effective thickness for layer-temperature simulation. For the same level number, the suggested discretization is more accurate in skin temperature as well as surface ground heat flux simulations than those used in some state-of-the-art models. A proposed scheme (“op(3,2,0)”) can reduce the normalized root–mean–square error (or RMSE/STD ratio) of the calculated surface ground heat flux of a cropland site significantly to 2% (or 0.9 W m^?2), from 11% (or 5 W m^?2) by a 5-layer scheme used in ECMWF, from 19% (or 8 W m^?2) by a 5-layer scheme used in ECHAM, and from 74% (or 32 W m^?2) by a single-layer scheme used in the UCLA GCM. Better accuracy can be achieved by including more layers to the vertical discretization. Similar improvements are expected for other locations with different land types since the numerical error is inherited into the models for all the land types. The proposed scheme can be easily implemented into state-of-the-art climate models for the temperature simulation of snow, ice and soil.     

Vegetation context and climatic limits of the Early Pleistocene hominin dispersal in Europe

The vegetation and the climatic context in which the first hominins entered and dispersed in Europe during the Early Pleistocene are reconstructed, using literature review and a new climatic simulation. Both in situ fauna and in situ pollen at the twelve early hominin sites under consideration indicate the occurrence of open landscapes: grasslands or forested steppes. The presence of ancient hominins (Homo of the erectus group) in Europe is only possible at the transition from glacial to interglacial periods, the full glacial being too cold for them and the transition interglacial to glacial too forested. Glacial–interglacial cycles forced by obliquity showed paralleled vegetation successions, which repeated c. 42 times during the course of the Early Pleistocene (2.58–0.78 Ma), providing 42 narrow windows of opportunity for hominins to disperse into Europe.The climatic conditions of this Early Pleistocene vegetation at glacial-interglacial transitions are compared with a climatic simulation for 9 ka ago without ice sheet, as this time period is so far the best analogue available. The climate at the beginning of the present interglacial displayed a stronger seasonality than now. Forest cover would not have been hampered though, clearly indicating that other factors linked to refugial location and soils leave this period relatively free of forests. Similar situations with an offset between climate and vegetation at the beginning of interglacials repeated themselves throughout the Quaternary and benefitted the early hominins when colonising Europe.The duration of this open phase of vegetation at the glacial–interglacial transition was long enough to allow colonisation from the Levant to the Atlantic.The twelve sites fall within rather narrow ranges of summer precipitation and temperature of the coldest month, suggesting the hominins had only a very low tolerance to climate variability.     

The intraseasonal oscillation in ECHAM4 Part I: coupled to a comprehensive ocean model

This study discusses the representation of the intraseasonal oscillation (ISO) in three simulations with the ECHAM4 atmosphere general circulation model (GCM). First, the model is forced by AMIP sea surface temperatures (SST), then coupled to the OPYC3 global ocean GCM and third forced by OPYC3 SSTs to clarify possible air-sea interactions and connections of the ISO and the ENSO cycle. The simulations are compared to ECMWF reanalysis data and NOAA outgoing longwave radiation (OLR) observations. Although previous studies have shown that the ECHAM4 GCM simulates an ISO-like oscillation, the main deficits are an overly fast eastward propagation and an eastward displacement of the main ISO activity, which is shown with a composite analysis of daily data between 1984 to 1988 for the reanalysis and the AMIP simulation, 25 years of the coupled integration, and a five year subset of the coupled SST output used for the OPYC3 forced atmosphere GCM experiment. These deficits are common to many atmospheric GCMs. The composites are obtained by principal oscillation pattern (POP). The POPs are also used to investigate the propagation speed and the interannual variability of the main ISO activity. The present coupled model version reveals no clear improvements in the ISO simulation compared to the uncoupled version forced with OPYC3 SSTs, although it is shown that the modeled ISO influences the simulated high-frequency SST variability in the coupled GCM. Within the current analysis, ECHAM4 forced by AMIP SSTs provides the most reasonable ISO simulation. However, it is shown that the maximum amplitudes of the annual cycle of the ISO variability in all analyzed model versions are reached too late in the year (spring and summer) compared to the observations (winter and spring). Additionally, the ENSO cycle influences the interannual variability of the ISO, which is revealed by 20 years of daily reanalysis data and 100 years of the coupled integration. The ENSO cycle is simulated by the coupled model, although there is a roughly 1 K cold bias in the East Pacific in the coupled model. This leads to a diminished influence of the ENSO cycle on the spatial variability of the modeled ISO activity compared to observations. This points out the strong sensitivity of the SST on the ISO activity. Small biases in the SST appear to cause large deterioration in the modeled ISO.