ALBERT

Description: 〈h3〉Abstract〈/h3〉 〈p〉Using the sub-seasonal to seasonal forecast model of Beijing Climate Center, several key physical parameters are perturbed by the Latin hypercube sampling method to find a better configuration for representation of Madden–Julian oscillation (MJO) in the free-run simulation. We find that although model simulation is especially sensitive to some parameters, there are overall no significant linear relationships between model skill and any one of the parameters, and the optimum performance can be obtained by combined perturbations of multiple parameters. By optimization, MJO’s spectrum, intensity, spatial structure and propagation, as well as the mean state and variance, are all improved to some extent, suggesting the correspondence and interrelation of model’s performances in simulating different characteristics of MJO. Further, several sets of initialized hindcasts using the optimized parameters are conducted, and their results are compared with the hindcasts using only improved initial conditions. We show that with an optimized model, the forecast of MJO beyond 3-week lead time is not improved, and the maximum useful skill is only slightly increased, implying that a decrease of model error does not always translate into an increase of forecast skill at all lead time. However, the skill is obviously enhanced during lead times of 2–3 weeks for forecasts in most seasons and initial phases except for a few cases. Particularly, the deficiency in forecasting MJO’s propagation from the Indian Ocean to the Pacific is relieved, further highlighting the positive contribution of reducing model error compared to previous work that only reduced initial condition error. In this study, we also show benefits of multi-scheme ensemble strategy in describing uncertainties of model error and initial condition error and thus improving MJO forecast.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉The accurate characterization of seasonal and inter-annual site-level wind energy variability is essential during wind project development. Understanding the features and probability of low-wind years is of particular interest to developers and financers. However, a dearth of long-term, hub-height wind observations makes these characterizations challenging, and thus techniques to improve these characterizations are of great value. To improve resource characterization, we explicitly link wind resource variability (at hub-height, and at specific sites) to regional and synoptic scale wind regimes. Our approach involves statistical clustering of high-resolution modeled wind data, and is applied to California for a period covering 1980–2015. With this approach, we investigate the unique meteorological patterns driving low and high wind years at five separate wind project sites. We also find wind regime changes over the 36-year period consistent with global warming: wind regimes associated with anomalously hot summer days increased at half a day per year and stagnant conditions increased at one-third days per year. Despite these changes, the average annual resource potential remained constant at all project sites. Additionally, we identify correlations between climate modes and wind regime frequency, a linkage valuable for resource characterization and forecasting. Our general approach can be applied in any location and may benefit many aspects of wind energy resource evaluation and forecasting.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉A new set of CORDEX simulations over South America, together with their coarser-resolution driving Global Climate Models (GCMs) are used to investigate added value of Regional Climate Models (RCMs) in reproducing mean climate conditions over the continent. There are two types of simulations with different lateral boundary conditions: five hindcast simulations use re-analysis as boundary conditions, and five other historical simulations use GCMs outputs. Multi-model ensemble means and individual simulations are evaluated against two or three observation-based gridded datasets for 2-m surface air temperature and total precipitation. The analysis is performed for summer and winter, over a common period from 1990 to 2004. Results indicate that added value of RCMs is dependent on driving fields, surface properties of the area, season and variable considered. A robust added value for RCMs driven by ERA-Interim is obtained in reproducing the summer climatology of surface air temperature over tropical and subtropical latitudes. Mixed results can be seen, however, for summer precipitation climatology in both hindcast and historical experiments. For winter, there is no noticeable improvement by the RCMs for the large-scale precipitation and surface air temperature climatology. To further understand the added value of RCMs, models deviations from observation are decomposed according to different terms that reflect the observational uncertainty, the representativeness error, the interpolation error, and the actual performance of the model. Regions where these errors are not negligible, such as in complex terrain regions, among others, can be identified. There is a clear need for complementary assessment to understand better the real value added by RCMs.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉The model fidelity in simulating the Northern Hemisphere storm track interannual variability and the connections of this variability to the low frequency atmospheric variations and oceanic variations are examined based on the atmospheric European Centre for Medium-Range Weather Forecasts (ECMWF) model and coupled NCAR Community Climate System Model (CCSM) systems at different horizontal resolutions. The atmospheric general circulation model (AGCM) runs are forced by observed sea surface temperatures (SST) with varying atmospheric resolutions, while the coupled general circulation model (CGCM) runs have a fixed atmospheric resolution but varying oceanic resolutions. The phases, between the North Pacific (NP) and North Atlantic (NA) sectors, of the simulated hemisphere-scale Empirical Orthogonal Function (EOF) modes of the storm track fluctuations change with the model resolution, suggesting the storm track variability in NP and NA basins are largely independent. The models can qualitatively reproduce the basin-scale EOFs of both NP and NA storm track variability. These EOFs are not sensitive to either atmospheric or oceanic model horizontal resolutions, but their magnitudes from the CGCM runs are substantially underestimated. The storm track variations over NP basin are hybrid of internal atmospheric variations and external forcing from the underlying conditions, but the fluctuations over the NA basin are merely atmospheric internal variability. The NP storm track variability from SST forcing accounts for 4.4% of the total variance in observations, while it only has less than 2% of the total in all AGCM simulations. The external forcing to the storm track variations is more realistically reproduced in the higher atmospheric resolution runs. The air–sea coupling makes the SST feedbacks to the atmospheric internal variability, absent in the atmospheric ECMWF model hindcasts, emerge in the coupled CCSM simulations.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Urban land use in East China has undergone considerable change since the 1990s. How such change affects both in situ and remote climate conditions is investigated through numerical modelling experiments with the Community Atmosphere Model Version 5.1. The results show that urbanization causes an increase in surface temperature due to reduced surface albedo but a decrease in specific humidity due to locally reduced surface evaporation. The change in specific humidity overwhelms the surface temperature change effect, leading to locally reduced precipitation. It is noted that urbanization causes changes in climate conditions not only locally but also remotely. Anomalous low-level divergence associated with the reduced precipitation in situ prevents the northward progression of the East Asian summer monsoon. As a result, the major monsoon rain band is strengthened and confined over South China and the tropical Asian monsoon zone along 12°–25°N. The increase of rainfall in the tropical zone, on one hand, induces the local overturning cell, leading to anomalous subsidence over mid-latitude Asia and the equatorial zone, and, on the other hand, perturbs the Subtropical Jet, generating a Rossby wave train disseminating along the Jet. Both of these processes cause anomalous dry and hot conditions over mid-latitude Asia.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Uncertainties in future changes of temperature and precipitation over the homogenous monsoon regions of India are investigated using the CMIP5 and CESM-LE datasets. The uncertainty is partitioned into epistemic (model) and aleatoric (internal variability) components for each season using the RCP8.5 scenario. The uncertainty in temperature change is dominated by epistemic uncertainty that increases over time. The uncertainty in precipitation change shows a more complex picture. Aleatoric uncertainty can remain quite large and comparable to epistemic uncertainty till the latter part of the twenty-first Century especially during the JJA and SON seasons. Much of the rainfall uncertainty is in the more arid Northwest region with the West Central region (part of the core monsoon area) exhibiting lower uncertainties. Considerable increase in rainfall is seen during the SON season indicating an extended monsoon season. During the DJF season aleatoric uncertainty is much larger than epistemic uncertainty over much of the century and shows considerable decadal scale variability. Using the 40-member CESM-LE ensemble to analyze the influence of ensemble size on aleatoric uncertainty we find that low ensemble sizes can lead to an underestimate of the uncertainty.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉In this study, we investigate the role of the Asian summer monsoon (ASM) anticyclone in the distribution of ozone over the southern India and tropical Indian Ocean. We present the horizontal and vertical structure of ozone in the upper troposphere and lower stratosphere (UTLS) region. The analysis shows that the region within the ASM anticyclone has low ozone, and high tropopause altitude, as compared to the region outside the anticyclone during boreal summer. The southern edge of the ASM anticyclone, i.e. the southern India and tropical Indian Ocean show a remarkably high ozone concentration in the UTLS region during summer. Analysis of daily fields shows that ozone concentration in the upper troposphere over the southern India and tropical Indian Ocean increases with the strength of the tropical easterly jet, which is an outcome of ASM circulation. Different mechanisms responsible for the ozone enhancement in the UTLS region over the tropical Indian region have been discussed in this paper. The in situ ozonesonde observations from six Indian stations also support the space-based Aura-MLS observations, concluding that ASM anticyclone effectively transports ozone from the mid-latitude stratosphere to deep tropics. Shear generated turbulence and mixing in the vicinity of easterly jet also likely to play a minor role in the local ozone distribution.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Global climate is a multi-scale system whose subsystems interact complexly. Notably, the Tropical-Andean region has a strong rainfall variability because of the confluence of many global climate processes altered by morphological features. An approach for a synthetical climate description is the use of global indicators and their regional teleconnections. However, typically this is carried out using filters and correlations, which results in seasonal and inter-annual teleconnections information, which are difficult to integrate into a modeling framework. A new methodology, based on rainfall signal extraction using dynamic-harmonic-regressions (DHR) and stochastic-multiple-linear-regressions (SMLR) between rainfall components and global signals for searching intra-annual and inter-annual teleconnections, is proposed. DHR gives non-stationary inter-annual trends and intra-annual quasi-periodic oscillations for monthly rainfall measurements. Time-variable amplitudes of quasi-periodical oscillations are crucial for finding intra-annual teleconnections using SMLR, while trends are better suited for the case of inter-annual ones. The methodology is tested over a Tropical-Andean region in southern Ecuador. The following results were obtained: (1) trans-Niño-Index (TNI) and Tropical-South-Atlantic signals are strongly connected to inter-annual and intra-annual time-scales. (2) However, TNI progressively weakens its relation with intra-annual components; meanwhile, El-Niño-Southern-Oscillation 3 gains ground for such time-scales. (3) Finally, an inter-annual connection with the North-Atlantic-Oscillation (NAO) is revealed. These results are consistent with previous literature, although the TNI and NAO connections are interesting findings, taking into account the differences in the connected scales. These results show the methodology’s capability of unraveling global teleconnections in different space and time scales using attributes embedded in an integral mathematical framework, which could be interesting for other purposes—such as the analysis of climate mechanisms or climate modeling.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Decadal changes in the teleconnection between the central tropical Pacific and the Southern Hemisphere extratropics are studied using the NCEP–NCAR reanalysis data. Concurrent and lagged relationships show that teleconnection strength in austral spring was weak (strong) before (after) 1996/1997. This decadal change coincides in time with the climate regime shift in the Pacific in the 1990s known from many studies. We show that, after the regime shift, the concurrent and delayed teleconnection with the Southern Hemisphere extratropics is insignificant in September and abruptly increases in October. Penetration of the stratospheric anomaly into the troposphere in October can indicate interacting tropospheric and stratospheric pathways of the teleconnection to strongly enhance the central tropical Pacific impact since the late 1990s. The results give evidence that the Southern Annular Mode seems to be connecting element between the two pathways in the recent decades. The common tendencies in the eastward shift of the tropical anomalies and zonal wave 1 phase in the Antarctic stratosphere in austral spring have been demonstrated. The difference between the central Pacific and eastern Pacific teleconnections is consistent with that known from previous studies and new tendencies in their decadal changes and delayed effects have been revealed. It has been found that the central Pacific contributions to the Pacific decadal oscillation and to the Northern Hemisphere stratosphere have also increased significantly after the 1990s. This characterizes the central tropical Pacific as one of the key regions impacting climate and teleconnection not only in the Southern Hemisphere, but also in the Northern Hemisphere. Our findings are consistent with and further develop the recent studies of the stratosphere–troposphere coupling in austral spring, and emphasize significant contribution of the delayed tropical signals to the climate variability in austral spring in both hemispheres.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉A simple theoretical model is constructed to understand the cause of a peculiar cooling trend in North Pacific under the background of the greenhouse gases induced global warming during the past 50 years. It is found that the North Pacific cooling is caused by the increase of surface upward latent heat flux due to the atmosphere and the decrease of surface downward shortwave radiative flux. The former is attributed to enhanced low-level westerlies, while the latter is caused by the increase of stratus cloud over North Pacific. An atmosphere general circulation model is utilized to investigate the cause of the wind and low-level cloud changes. It is found that the strengthened westerly in North Pacific is the result of an atmospheric teleconnection pattern forced by the SSTAs warming in the tropical Pacific. The SSTAs warming in other tropical basins, along with the local cooling in North Pacific, tends to reduce the tropical Pacific SSTAs forcing effect. In addition, the increased local low-level cloud response to the tropical Pacific SSTAs forcing is also responsible for the cooling trend in North Pacific. The increased local stratus cloud may enhance the cooling through a positive feedback among the SST, atmospheric static stability and stratus cloud.〈/p〉