Madden-Julian variability in NCAR CAM2.0 and CCSM2.0

The Madden-Julian Oscillation (MJO) dominates tropical variability on time scales of 30–70 days. During the boreal winter/spring it is manifested as an eastward propagating disturbance, with a strong convective signature over the eastern hemisphere. Here, 20–100 day bandpass filtered outgoing longwave radiation (OLR) for the months of November–March from the National Center for Atmospheric Research Community Atmospheric Model Version 2.0 (NCAR CAM2.0) and the Community Coupled System Model Version 2.0 (CCSM2.0) models is projected onto the observed patterns of MJO convection. This provides for the analysis of the models within a standard framework. Additionally, only analyzing years when the lead/lag relationship of the simulated principal components lie in the observed phase-space better isolates the simulated MJO signal. CCSM2.0 yields a better representation of the MJO than CAM2.0 due to the presence of air-sea interaction. Even so, the amplitude and spatial extent of the intraseasonal convection are underestimated relative to observed OLR, with a pronounced underestimate of the near-equatorial convection. Due to the development of a split inter-tropical convergence zone in the western Pacific, which is independent of the MJO, the models are precluded from representing the low-level moisture convergence that is central to the eastward propagation of the MJO. Once the systematic model error is remedied the underlying capability of the models to simulate the MJO will be possible.     

Aerosol direct forcing of the summer Indian monsoon as simulated by the NCAR CAM3

In this study, the effects of aerosols on the simulation of the Indian monsoon by the NCAR Community Atmosphere Model CAM3 are measured and investigated. Monthly mean 3D mass concentrations of soil dust, black and organic carbons, sulfate, and sea salt, as output from the GOCART model, are interpolated to mid-month values and to the horizontal and vertical grids of CAM3. With these mid-month aerosol concentrations, CAM3 is run for a period of approximately 16 months, allowing for one complete episode of the Indian monsoon. Responses to the aerosols are measured by comparing the mean of an ensemble of aerosol-induced monsoon simulations to the mean of an ensemble of CAM3 simulations in which aerosols are omitted, following the method of Lau et al. (2006) in their experiment with the NASA finite volume general circulation model. Additionally, an ensemble of simulations of CAM3 using climatological mid-month aerosol concentrations from the MATCH model is composed for comparison. Results of this experiment indicate that the inclusion of aerosols results in drops in surface temperature and increases in precipitation over central India during the pre-monsoon months of March, April, and May. The presence of aerosols induces tropospheric shortwave heating over central India, which destabilizes the atmosphere for enhanced convection and precipitation. Reduced shortwave heating and enhanced evaporation at the surface during April and May results in reduced terrestrial emission to cool the lower troposphere, relative to simulations with no aerosols. This effect weakens the near-surface cyclonic circulation and, consequently, has a negative feedback on precipitation during the active monsoon months of June and July.     

Climate variability over the Greater Horn of Africa based on NCAR AGCM ensemble

Summary The variability and extreme wet anomalies in the Greater Horn of Africa (GHA) climate are investigated based on a multi-year National Center for Atmospheric Research (NCAR) AGCM ensemble data. While the GCM ensemble average reproduces realistic inter-annual variability of rainfall pattern over the GHA sub-region compared to observations, there is a distinct northward shift in the simulated regions of rainfall maxima throughout the season. However, in agreement with observations and many previous studies, the inter-annual variability derived from leading mode of EOF analysis is dominated by ENSO-related fluctuations. On the other hand, the spatial pattern corresponding to the second mode (EOF2) exhibits a unique dipole rainfall anomaly pattern (wet/dry conditions) over the northern/southern halves of our domain during all the three months of the short rains season. When the 3–10 year periodicity is filtered out from the 40-year EOF2 time series of the ensemble mean data, three distinct quasi-decadal regimes in the rainfall anomalies is exhibited for both monthly and seasonal mean data. It is also evident from our results that a combination of anomalous surface and mid-tropospheric flow from northwestern and eastern Atlantic Ocean and easterly flow from the Indian Ocean played a significant role in setting up the non-ENSO related 1961 floods. Coversely, during the ENSO-related 1997 floods, the mid-troposheric flow was characterized by anomalous westerly flow originating from the Congo rainforest that converged with the flow from Indian Ocean along the East Africa coast and over eastern/northeastern Kenya. The anomalous moisture flux convergence/divergence in both the ensemble and NCEP reanalysis is also consistent with the mid-trospheric flow anomalies that are associated with the two wet events.     

大气环流模式CAM中土壤冻融过程改进对东亚气候模拟的影响

本文在改进了大气环流模式NCAR CAM3.1中的土壤冻融过程参数化的基础上,模拟研究了改进的冻土过程对东亚气候模拟的影响.模拟结果分析表明,改进冻融过程参数化后,冬季欧亚大陆上大部分地区大气对地表的加热偏强,而夏季地表对大气的加热偏强,尤其是青藏高原对大气的加热作用显著增强.东亚气候对冻土过程参数化方案非常敏感,冬、...     

Direct Climatic Effect of Dust Aerosol in the NCAR Community Atmosphere Model Version 3 （CAM3）

Direct climate responses to dust shortwave and longwave radiative forcing (RF) are studied using the NCAR Community Atmosphere Model Version 3 (CAM3). The simulated RF at the top of the atmosphere (TOA) is-0.45 W m-2 in the solar spectrum and +0.09 W m-2 in the thermal spectrum on a global average. The magnitude of surface RF is larger than the TOA forcing, with global mean shortwave forcing of-1.76 W m-2 and longwave forcing of +0.31 W m-2 . As a result, dust aerosol causes the absorption of 1.1 W m-2 in t...     

Climate sensitivity of the NCAR Community Climate Model (CCM2) to horizontal resolution

The dependence on horizontal resolution of the climate simulated by the National Center for Atmospheric Research Community Climate Model (CCM2) is explored. Simulations employing R15, T21, T31, T42, T63, and T106 horizontal spectral truncations are compared. Parameters associated with the diagnostic cloud scheme are modified for each resolution to provide similar global average cloud radiative forcing at each resolution. Overall, as with earlier studies, there are large differences between the low resolution R15 and T21 simulations and the medium resolution T42 simulation. Many climate statistics show a monotonic signal with increasing resolution, with the largest variation occurring from low to medium resolution. Although the monotonic signal is often from the low resolution simulations toward atmospheric analyses, in some cases it continues beyond the analyses at the highest resolution. Where convergence occurs, it is not always to the atmospheric analyses, and the highest resolution simulations are not the best by all measures. Although many climate statistics converge, the processes that maintain the climate do not, especially when considered on a regional basis. The implication is that the finer scales are required to capture the nonlinear processes that force the medium scales. Overall, it appears that, at a minimum, T42 resolution is required, but higher resolution would be better. Applications at T42 should take into consideration how model errors indicated by these resolution signals might affect any findings.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

CCM3大气环流模式月-季尺度预报初步试验

以1991和1994年NCEP再分析格点资料作初始场, 用NCAR气候模式CCM3进行了48次月、季预报, 针对500 hPa高度和中国降水资料对其预测能力进行了检验.结果表明, CCM3对月尺度的高度场的预报有一定的能力.对中国160站的降水距平进行月尺度和季节预报, 其准确程度可以和目前经验预报的水平相当.比较使用实际海温和用外推法预测的海温两种下边界条件所作的预报结果发现, 两者的效果差异不大.     

Asian monsoon simulations by Community Climate Models CAM4 and CCSM4

This study examines the ability of Community Atmosphere Model (CAM) and Community Climate System Model (CCSM) to simulate the Asian summer monsoon, focusing particularly on inter-model comparison and the role of air–sea interaction. Two different versions of CAM, namely CAM4 and CAM5, are used for uncoupled simulations whereas coupled simulations are performed with CCSM4 model. Ensemble uncoupled simulations are performed for a 30 year time period whereas the coupled model is integrated for 100 years. Emphasis is placed on the simulation of monsoon precipitation by analyzing the interannual variability of the atmosphere-only simulations and sea surface temperature bias in the coupled simulation. It is found that both CAM4 and CAM5 adequately simulated monsoon precipitation, and considerably reduced systematic errors that occurred in predecessors of CAM4, although both tend to overestimate monsoon precipitation when compared with observations. The onset and cessation of the precipitation annual cycle, along with the mean climatology, are reasonably well captured in their simulations. In terms of monsoon interannual variability and its teleconnection with SST over the Pacific and Indian Ocean, both CAM4 and CAM5 showed modest skill. CAM5, with revised model physics, has significantly improved the simulation of the monsoon mean climatology and showed better skill than CAM4. Using idealized experiments with CAM5, it is seen that the adoption of new boundary layer schemes in CAM5 contributes the most to reduce the monsoon overestimation bias in its simulation. In the CCSM4 coupled simulations, several aspects of the monsoon simulation are improved by the inclusion of air–sea interaction, including the cross-variability of simulated precipitation and SST. A significant improvement is seen in the spatial distribution of monsoon mean climatology where a too-heavy monsoon precipitation, which occurred in CAM4, is rectified. A detailed investigation of this significant precipitation reduction showed that the large systematic cold SST errors in the Northern Indian Ocean reduces monsoon precipitation and delays onset by weakening local evaporation. Sensitivity experiments with CAM4 further confirmed these results by simulating a weak monsoon in the presence of cold biases in the Northern Indian Ocean. It is found that although the air–sea coupling rectifies the major weaknesses of the monsoon simulation, the SST bias in coupled simulations induces significant differences in monsoon precipitation. The overall simulation characteristics demonstrate that although the new model versions CAM4, CAM5 and CCSM4, are significantly improved, they still have major weaknesses in simulating Asian monsoon precipitation.     

Climate response to introduction of the ESA CCI land cover data to the NCAR CESM

The European Space Agency Climate Change Initiative Land Cover data (ESA CCI-LC, from 1992 to 2015) is introduced to the National Center for Atmospheric Research Community Earth System Model version 1.2.1 (NCAR CESM1.2.1). In comparison with the original land surface data in the Community Land Model version 4 (ORG), the new data features notable land use and land cover change (LULCC) with increased forests over northeastern Asia and Alaska by decreasing shrublands and grasslands. Overestimated bare land cover over the Tibetan Plateau (TP) and the Rocky Mountains in the ORG are corrected with the replacements by grasslands and shrublands respectively in the new data. The model simulation results show that with the introduction of the ESA CCI-LC, the simulated surface albedo, surface net radiation flux, sensible and latent heat fluxes are relatively improved over the regions where significant LULCC exists, such as northeastern Asia, Alaska, the TP, and Australia. Surface air temperature, precipitation, and atmospheric circulation are improved in boreal winter but degraded in summer. The winter warming over northeastern Asia results from increased longwave downwelling flux and adiabatic heating while the notable winter cooling over Alaska is attributed to strong cold advection followed by reduced longwave downwelling flux. LULCC alters precipitation by influencing water vapor content. In winter, LULCC affects atmospheric circulation via modulating baroclinicity while in summer, it influences land-sea thermal contrast, thus affecting the intensity of East Asian summer monsoon. LULCC also alters the simulated dust burden.

     

Climate variability and irregular migration to the European Union

The so-called ‘European Migrant Crisis’ has been blamed on armed conflict and economic misery, particularly in the Middle East and Sub-Saharan Africa. Some have suggested that this process has been exacerbated by climate change and weather events. In this paper, we evaluate these claims, focusing on the role of droughts in influencing irregular migration flows to the European Union. Drawing on temporally disaggregated data on the detection of unauthorized migrants at EU external borders, we examine how weather shocks affect irregular migration. We show that weather events may indeed influence migration. Yet, in contradiction to the findings from recent research, we find no evidence that a drought in a sending country increases unauthorized migration to the EU. If anything, and while not entirely conclusive, the incidence of drought seems rather to exert a negative, albeit moderate, impact on the size of migration flows, in particular for countries dependent on agriculture. Conversely, higher levels of rainfall increase migration. We interpret this as evidence that international migration is cost-prohibitive, and that adverse weather shocks reinforce existing financial barriers to migration.     

Sensitivity of glaciation to initial snow cover,CO2, snow albedo,and oceanic roughness in the NCAR CCM

We present results from numerical experiments made with a GCM, the NCAR CCM1, that were designed to estimate the annual balance between snow-fall accumulation and ablation for geographically important land regions for a variety of conditions. We also attempt to assess the reliability of these results by investigating model sensitivity to changes in prescribed physical parameters. Experiments were run with an initial imposition of 1 m of (midwinter) snowcover over all northern hemisphere land points. Over Alaska, western Canada, Siberia, and the Tibetan Plateau the model tended to retain this snow cover through the summer and in some cases increase its depth as well. We define these regions as glaciation sensitive and note some correspondence between them and source regions for the Pleistocene ice sheets. An experiment with greatly reduced CO₂ (100 ppm) showed a tendency towards spontaneous glaciation, i.e., the model remained snow-covered throughout the summer over the same geographic regions noted above. With 200 ppm CO₂ (roughly equal to values at the last glacial maximum), snow cover over these regions did not quite survive the summer on a consistent basis. Combining 200 ppm CO₂ and 1 m of initial northern hemisphere snow cover yielded glaciation-sensitive conditions, agreeing remarkably well with locations undergoing glaciation during the Pleistocene. To assess the reliability of these results, we have determined minimal model uncertainty by varying two of the empirical coefficients in the model within physically plausible ranges. In one case surface roughness of all ocean gridpoints was reduced by an order of magnitude, leading to local 10% reductions in precipitation (snowfall), a change hard to distinguish from inherent model variability. In the other case, the fraction of a land grid square assumed to be occupied by snow cover for albedo purposes was varied from one-half to unity. Large changes occurred in the degree of summer melting, and in some cases the sign of the net balance changed as fractional snow cover was changed. We conclude that the model may be able to reveal regions sensitive to glaciation, but that it cannot yield a reliable quantitative computation of the magnitude of the net snow accumulation that can be implicitly or explicitly integrated through time.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dilmenil     

Multidecadal to decadal variability in the equatorial Indian Ocean subsurface temperature and the forcing mechanisms

Climate Dynamics - The Tropical Indian Ocean (TIO) is seen to exhibit robust warming after the 1950s. Most of the previous studies on the Indian Ocean (IO) surface and subsurface temperature...     

Changes of interannual NAO variability in response to greenhouse gases forcing

Observations show that there was change in interannual North Atlantic Oscillation (NAO) variability in the mid-1970s. This change was characterized by an eastward shift of the NAO action centres, a poleward shift of zonal wind anomalies and a downstream extension of climate anomalies associated with the NAO. The NAO interannual variability for the period after the mid-1970s has an annular mode structure that penetrates deeply into the stratosphere, indicating a strengthened relationship between the NAO and the Arctic Oscillation (AO) and strengthened stratosphere-troposphere coupling. In this study we have investigated possible causes of these changes in the NAO by carrying out experiments with an atmospheric GCM. The model is forced either by doubling CO₂, or increasing sea surface temperatures (SST), or both. In the case of SST forcing the SST anomaly is derived from a coupled model simulation forced by increasing CO₂. Results indicate that SST and CO₂ change both force a poleward and eastward shift in the pattern of interannual NAO variability and the associated poleward shift of zonal wind anomalies, similar to the observations. The effect of SST change can be understood in terms of mean changes in the troposphere. The direct effect of CO₂ change, in contrast, can not be understood in terms of mean changes in the troposphere. However, there is a significant response in the stratosphere, characterized by a strengthened climatological polar vortex with strongly enhanced interannual variability. In this case, the NAO interannual variability has a strong link with the variability over the North Pacific, as in the annular AO pattern, and is also strongly related to the stratospheric vortex, indicating strengthened stratosphere-troposphere coupling. The similarity of changes in many characteristics of NAO interannual variability between the model response to doubling CO₂ and those in observations in the mid-1970s implies that the increase of greenhouse gas concentration in the atmosphere, and the resulting changes in the stratosphere, might have played an important role in the multidecadal change of interannual NAO variability and its associated climate anomalies during the late twentieth century. The weak change in mean westerlies in the troposphere in response to CO₂ change implies that enhanced and eastward extended mid-latitude westerlies in the troposphere might not be a necessary condition for the poleward and eastward shift of the NAO action centres in the mid-1970s.     

Sensitivity of ecosystem models to the spatial resolution of the NCAR Community Climate Model CCM2

 This study evaluates the sensitivity of ecosystem models to changes in the horizontal resolution of version 2 of the National Centre for Atmospheric Research Community Climate Model (CCM2). A previous study has shown that the distributions of natural ecosystems predicted by vegetation models using coarse resolution present-day climate simulations are poorly simulated. It is usually assumed that increasing the spatial resolution of general circulation models (GCMs) will improve the simulation of climate, and hence will increase our level of confidence in the use of GCM output for impacts studies. The principal goals of this study is to investigate this hypothesis and to identify which biomes are more affected by the changes in spatial resolution of the forcing climate. The ecosystem models used are the BIOME-1 model and a version of the Holdridge scheme. The climate simulations come from a set of experiments in which CCM2 was run with increasing horizontal resolutions. The biome distributions predicted using CCM2 climates are compared against biome distributions predicted using observed climate datasets. Results show that increasing the resolution of CCM2 produces a significant improvement of the global-scale vegetation prediction, indicating that a higher level of confidence can be vested in the global-scale prediction of natural ecosystems using medium and high resolution GCMs. However, not all biomes are equally affected by the increased spatial resolution, and although certain biome distributions are improved (e.g. hot desert, tropical seasonal forest), others remain globally poorly predicted even at high resolution (e.g. grasses and xerophytic woods). In addition, these results show that some climatic biases are enhanced with increasing resolution (e.g. in mountain ranges), resulting in the inadequate prediction of biomes. Received: 4 March 1997 / Accepted: 10 December 1997     

Tropical Indian Ocean subsurface temperature variability and the forcing mechanisms

The first two leading modes of interannual variability of sea surface temperature in the Tropical Indian Ocean (TIO) are governed by El Niño Southern Oscillation and Indian Ocean Dipole (IOD) respectively. TIO subsurface however does not co-vary with the surface. The patterns of the first mode of TIO subsurface temperature variability and their vertical structure are found to closely resemble the patterns of IOD and El Niño co-occurrence years. These co-occurrence years are characterized by a north–south subsurface dipole rather than a conventional IOD forced east–west dipole. This subsurface dipole is forced by wind stress curl anomalies, driven mainly by meridional shear in the zonal wind anomalies. A new subsurface dipole index (SDI) has been defined in this study to quantify the intensity of the north–south dipole mode. The SDI peaks during December to February (DJF), a season after the dipole mode index peaks. It is found that this subsurface north–south dipole is a manifestation of the internal mode of variability of the Indian Ocean forced by IOD but modulated by Pacific forcing. The seasonal evolution of thermocline, subsurface temperature and the corresponding leading modes of variability further support this hypothesis. Positive wind stress curl anomalies in the south and negative wind stress curl anomalies in the north of 5°S force (or intensify) downwelling and upwelling waves respectively during DJF. These waves induce strong subsurface warming in the south and cooling in the north (especially during DJF) and assist the formation and/or maintenance of the north–south subsurface dipole. A thick barrier layer forms in the southern TIO, supporting the long persistence of anomalous subsurface warming. To the best of our knowledge the existence of such north–south subsurface dipole in TIO is being reported for the first time.     

Extratropical cyclone variability in the Northern Hemisphere winter from the NCEP/NCAR reanalysis data

 The winter climatology of Northern Hemisphere cyclone activity was derived from 6-hourly NCEP/NCAR reanalysis data for the period from 1958 to 1999, using software which provides improved accuracy in cyclone identification in comparison to numerical tracking schemes. Cyclone characteristics over the Kuroshio and Gulfstream are very different to those over continental North America and the Arctic. Analysis of Northern Hemisphere cyclones shows secular and decadal-scale changes in cyclone frequency, intensity, lifetime and deepening rates. The western Pacific and Atlantic are characterized by an increase in cyclone intensity and deepening during the 42-year period, although the eastern Pacific and continental North America demonstrate opposite tendencies in most cyclone characteristics. There is an increase of the number of cyclones in the Arctic and in the western Pacific and a downward tendency over the Gulf Stream and subpolar Pacific. Decadal scale variability in cyclone activity over the Atlantic and Pacific exhibits south-north dipole-like patterns. Atlantic and Pacific cyclone activity associated with the NAO and PNA is analyzed. Atlantic cyclone frequency demonstrates a high correlation with NAO and reflects the NAO shift in the mid 1970s, associated with considerable changes in European storm tracks. The PNA is largely linked to the eastern Pacific cyclone frequencies, and controls cyclone activity over the Gulf region and the North American coast during the last two decades. Assessment of the accuracy of the results and comparison with those derived using numerical algorithms, shows that biases inherent in numerical procedures are not negligible. Received: 7 July 2000 / Accepted: 30 November 2000     

Climatic variability of the sub-surface sea temperatures in the Aegean-Black Sea system and relation to meteorological forcing

Non-smoothed yearly temperature records with minimal statistical uncertainties are constructed for winter and summer of the period 1950–2000 in two areas in the Aegean Sea, for the sub-surface layer of 80–120?m, and two areas in the Black Sea, for the sub-surface layer of sigma-theta isopycnals between 14.5 and 15.4. The specific areas are selected mostly because of the dense hydrographic-data coverage they have during the period 1950–2000. Two trend regimes appear in both Seas: a period of decreasing sea temperatures from the early/mid 1960s to the early/mid 1990s and an apparent warming afterwards. Trends in sea temperatures correlate with trends in the North Atlantic Oscillation (NAO) and partly the East Atlantic West Russian (EAWR) indexes, but the signs of NAO and/or EAWR cannot sufficiently justify the winter-to-winter temperature changes in the entire study area. In examining the wind flows in the sea-level-pressure maps for characteristic winters in which local peaks in the sea-temperature records occur, we identify particular sea-level-pressure structures that are not accounted for by the typical North-Atlantic or East Atlantic-West Russia positive or negative dipoles. In addition, there are winters when the Siberian High induces local maxima in sea-temperatures in the study area. A spectral-coherence analysis of the unfiltered winter sea-temperature and the corresponding teleconnection NAO/EAWR records, shows that common spectral and coherence peaks exist at ~5–6, ~9–10 and ~15–17?years.