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2014 Vol. 31, No. 2

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Mechanisms of Atlantic Meridional Overturning Circulation (AMOC) Variability in a Coupled Ocean-Atmosphere GCM
Boyin HUANG, ZHU Jiang, YANG Haijun
2014, 31(2): 241-251. doi: 10.1007/s00376-013-3021-3
The mechanisms involved in the variability of Atlantic Meridional Overturning Circulation (AMOC) are studied using a 2000-yr control simulation of the coupled Fast Ocean-Atmosphere Model (FOAM). This study identifies a coupled mode between SST and surface heat flux in the North Atlantic at the decadal timescale, as well as a forcing mode of surface heat flux at the interannual timescale. The coupled mode is regulated by AMOC through meridional heat transport. The increase in surface heating in the North Atlantic weakens the AMOC approximately 10 yr later, and the weakened AMOC in turn decreases SST and sea surface salinity. The decreased SST results in an increase in surface heating in the North Atlantic, thus forming a positive feedback loop. Meanwhile, the weakened AMOC weakens northward heat transport and therefore lowers subsurface temperature approximately 19 yr later, which prevents the AMOC from weakening. In the forcing mode, the surface heat flux leads AMOC by approximately 4 yr.
Can Adaptive Observations Improve Tropical Cyclone Intensity Forecasts?
QIN Xiaohao, MU Mu
2014, 31(2): 252-262. doi: 10.1007/s00376-013-3008-0
In order to investigate whether adaptive observations can improve tropical cyclone (TC) intensity forecasts, observing system simulation experiments (OSSEs) were conducted for 20 TC cases originating in the western North Pacific during the 2010 season according to the conditional nonlinear optimal perturbation (CNOP) sensitivity, using the fifth version of the PSU/NCAR mesoscale model (MM5) and its 3DVAR assimilation system. A new intensity index was defined as the sum of the number of grid points within an allocated square centered at the corresponding forecast TC central position, that satisfy constraints associated with the Sea Level Pressure (SLP), near-surface horizontal wind speed, and accumulated convective precipitation. The higher the index value is, the more intense the TC is. The impacts of the CNOP sensitivity on the intensity forecast were then estimated. The OSSE results showed that for 15 of the 20 cases there were improvements, with reductions of forecast errors in the range of 0.12%-8.59%, which were much less than in track forecasts. The indication, therefore, is that the CNOP sensitivity has a generally positive effect on TC intensity forecasts, but only to a certain degree. We conclude that factors such as the use of a coupled model, or better initialization of the TC vortex, are more important for an accurate TC intensity forecast.
Analysis of Sampling Error Uncertainties and Trends in Maximum and Minimum Temperatures in China
HUA Wei, Samuel S. P. SHEN, WANG Huijun
2014, 31(2): 263-272. doi: 10.1007/s00376-013-2316-8
In this paper we report an analysis of sampling error uncertainties in mean maximum and minimum temperatures (Tmax and Tmin) carried out on monthly, seasonal and annual scales, including an examination of homogenized and original data collected at 731 meteorological stations across China for the period 1951-2004. Uncertainties of the gridded data and national average, linear trends and their uncertainties, as well as the homogenization effect on uncertainties are assessed. It is shown that the sampling error variances of homogenized Tmax and Tmin, which are larger in winter than in summer, have a marked northwest-southeast gradient distribution, while the sampling error variances of the original data are found to be larger and irregular. Tmax and Tmin increase in all months of the year in the study period 1951-2004, with the largest warming and uncertainties being 0.400C (10 yr)-1 0.269C (10 yr)-1 and 0.578C (10 yr)-1 0.211C (10 yr)-1 in February, and the least being 0.022C (10 yr)-1 0.085C (10 yr)-1 and 0.104C (10 yr)-1 0.070C (10 yr)-1 in August. Homogenization can remove large uncertainties in the original records resulting from various non-natural changes in China.
Simulated Spatial Distribution and Seasonal Variation of Atmospheric Methane over China: Contributions from Key Sources
ZHANG Dingyuan, LIAO Hong, WANG Yuesi
2014, 31(2): 283-292. doi: 10.1007/s00376-013-3018-y
We used the global atmospheric chemical transport model, GEOS-Chem, to simulate the spatial distribution and seasonal variation of surface-layer methane (CH4) in 2004, and quantify the impacts of individual domestic sources and foreign transport on CH4 concentrations over China. Simulated surface-layer CH4 concentrations over China exhibit maximum concentrations in summer and minimum concentrations in spring. The annual mean CH4 concentrations range from 1800 ppb over western China to 2300 ppb over the more populated eastern China. Foreign emissions were found to have large impacts on CH4 concentrations over China, contributing to about 85% of the CH4 concentrations over western China and about 80% of those over eastern China. The tagged simulation results showed that coal mining, livestock, and waste are the dominant domestic contributors to CH4 concentrations over China, accounting for 36%, 18%, and 16%, respectively, of the annual and national mean increase in CH4 concentration from all domestic emissions. Emissions from rice cultivation were found to make the largest contributions to CH4 concentrations over China in the summer, which is the key factor that leads to the maximum seasonal mean CH4 concentrations in summer.
Improvement of 6-15 Day Precipitation Forecasts Using a Time-Lagged Ensemble Method
JIE Weihua, WU Tongwen, WANG Jun, LI Weijing, LIU Xiangwen
2014, 31(2): 293-304. doi: 10.1007/s00376-013-3037-8
A time-lagged ensemble method is used to improve 6-15 day precipitation forecasts from the Beijing Climate Center Atmospheric General Circulation Model, version 2.0.1. The approach averages the deterministic predictions of precipitation from the most recent model run and from earlier runs, all at the same forecast valid time. This lagged average forecast (LAF) method assigns equal weight to each ensemble member and produces a forecast by taking the ensemble mean. Our analyses of the Equitable Threat Score, the Hanssen and Kuipers Score, and the frequency bias indicate that the LAF using five members at time-lagged intervals of 6 h improves 6-15 day forecasts of precipitation frequency above 1 mm d-1 and 5 mm d-1 in many regions of China, and is more effective than the LAF method with selection of the time-lagged interval of 12 or 24 h between ensemble members. In particular, significant improvements are seen over regions where the frequencies of rainfall days are higher than about 40%-50% in the summer season; these regions include northeastern and central to southern China, and the southeastern Tibetan Plateau.
Idealized Numerical Simulations of Tropical Cyclone Formation Associated with Monsoon Gyres
LIANG Jia, WU Liguang, ZONG Huijun
2014, 31(2): 305-315. doi: 10.1007/s00376-013-2282-1
Monsoon gyres have been identified as one of the important large-scale circulation patterns associated with tropical cyclone (TC) formation in the western North Pacific. A recent observational analysis indicated that most TCs form near the center of monsoon gyres or at the northeast end of the enhanced low-level southwesterly flows on the southeast-east periphery of monsoon gyres. In the present reported study, idealized numerical experiments were conducted to examine the tropical cyclogenesis associated with Rossby wave energy dispersion with an initial idealized monsoon gyre. The numerical simulations showed that the development of the low-level enhanced southwesterly flows on the southeast-east periphery of monsoon gyres can be induced by Rossby wave energy dispersion. Mesoscale convective systems emerged from the northeast end of the enhanced southwesterly flows with mid-level maximum relative vorticity. The simulated TC formed in the northeast of the monsoon gyre and moved westward towards the center of the monsoon gyre. The numerical experiment with a relatively smaller sized initial monsoon gyre showed the TC forming near the center of the initial monsoon gyre. The results of the present study suggest that Rossby wave energy dispersion can play an important role in TC formation in the presence of monsoon gyres.
An Observational Analysis of the Oceanic and Atmospheric Structure of Global-Scale Multi-decadal Variability
Peng LIU, Chung-Hsiung SUI
2014, 31(2): 316-330. doi: 10.1007/s00376-013-2305-y
The aim of the present study was to identify multi-decadal variability (MDV) relative to the current centennial global warming trend in available observation data. The centennial global warming trend was first identified in the global mean surface temperature (STgm) data. The MDV was identified based on three sets of climate variables, including sea surface temperature (SST), ocean temperature from the surface to 700 m, and the NCEP and ERA40 reanalysis datasets, respectively. All variables were detrended and low-pass filtered. Through three independent EOF analyses of the filtered variables, all results consistently showed two dominant modes, with their respective temporal variability resembling the Pacific Decadal Oscillation/Inter-decadal Pacific Oscillation (PDO/IPO) and the Atlantic Multi-decadal Oscillation (AMO). The spatial structure of the PDO-like oscillation is characterized by an ENSO-like structure and hemispheric symmetric features. The structure associated with the AMO-like oscillation exhibits hemispheric asymmetric features with anomalous warm air over Eurasia and warm SST in the Atlantic and Pacific basin north of 10S, and cold SST over the southern oceans. The Pacific and Atlantic MDV in upper-ocean temperature suggest that they are mutually linked. We also found that the PDO-like and AMO-like oscillations are almost equally important in global-scale MDV by EOF analyses. In the period 1975-2005, the evolution of the two oscillations has given rise to strong temperature trends and has contributed almost half of the STgm warming. Hereon, in the next decade, the two oscillations are expected to slow down the global warming trends.
A Test Pattern Identification Algorithm and Its Application to CINRAD/SA(B) Data
JIANG Yuan, LIU Liping
2014, 31(2): 331-343. doi: 10.1007/s00376-013-2315-9
A variety of faulty radar echoes may cause serious problems with radar data applications, especially radar data assimilation and quantitative precipitation estimates. In this study, ''test pattern'' caused by test signal or radar hardware failures in CINRAD (China New Generation Weather Radar) SA and SB radar operational observations are investigated. In order to distinguish the test pattern from other types of radar echoes, such as precipitation, clear air and other non-meteorological echoes, five feature parameters including the effective reflectivity data percentage (RZ), velocity RF (range folding) data percentage (RRF), missing velocity data percentage (RM), averaged along-azimuth reflectivity fluctuation (RNr,Z) and averaged along-beam reflectivity fluctuation (RNa,Z) are proposed. Based on the fuzzy logic method, a test pattern identification algorithm is developed, and the statistical results from all the different kinds of radar echoes indicate the performance of the algorithm. Analysis of two typical cases with heavy precipitation echoes located inside the test pattern are performed. The statistical results show that the test pattern identification algorithm performs well, since the test pattern is recognized in most cases. Besides, the algorithm can effectively remove the test pattern signal and retain strong precipitation echoes in heavy rainfall events.
Interference of the East Asian Winter Monsoon in the Impact of ENSO on the East Asian Summer Monsoon in Decaying Phases
FENG Juan*, CHEN Wen
2014, 31(2): 344-354. doi: 10.1007/s00376-013-3118-8
The variability of the East Asian winter monsoon (EAWM) can be divided into an ENSO-related part (EAWMEN) and an ENSO-unrelated part (EAWMres). The influence of EAWM res on the ENSO-East Asian summer monsoon (EASM) relationship in the decaying stages of ENSO is investigated in the present study. To achieve this, ENSO is divided into four groups based on the EAWMres: (1) weak EAWM res-El Nio (WEAWMres-EN); (2) strong EAWMres-El Nio (SEAWMres-EN); (3) weak EAWMres-La Nia (WEAWMres-LN); (4) strong EAWM res-La Nia (SEAWM res-LN). Composite results demonstrate that the EAWMres may enhance the atmospheric responses over East Asia to ENSO for WEAWMres-EN and SEAWMres-LN. The corresponding low-level anticyclonic (cyclonic) anomalies over the western North Pacific (WNP) associated with El Nio (La Nia) tend to be strong. Importantly, this feature may persist into the following summer, causing abundant rainfall in northern China for WEAWMres-EN cases and in southwestern China for SEAWMres-LN cases. In contrast, for the SEAWMres-EN and WEAWMres-LN groups, the EAWMres tends to weaken the atmospheric circulation anomalies associated with El Nio or La Nia. In these cases, the anomalous WNP anticyclone or cyclone tend to be reduced and confined to lower latitudes, which results in deficient summer rainfall in northern China for SEAWMres-EN and in southwestern China for WEAWMres-LN. Further study suggests that anomalous EAWM res may have an effect on the extra-tropical sea surface temperature anomaly, which persists into the ensuing summer and may interfere with the influences of ENSO.
Tropospheric Wet Refractivity Tomography Based on the BeiDou Satellite System
WANG Xiaoying, WANG Xianliang, DAI Ziqiang, KE Fuyang, CAO Yunchang, WANG Feifei, SONG Lianchun
2014, 31(2): 355-362. doi: 10.1007/s00376-013-2311-0
This paper presents a novel approach for assessing the precision of the wet refractivity field using BDS (BeiDou navigation satellite system) simulations only, GPS, and BDS+GPS for the Shenzhen and Hongkong GNSS network. The simulations are carried out by adding artificial noise to a real observation dataset. Instead of using the and parameters computed from slant wet delay, as in previous studies, we employ the Bias and RMS parameters, computed from the tomography results of total voxels, in order to obtain a more direct and comprehensive evaluation of the precision of the refractivity field determination. The results show that: (2) the precision of tropospheric wet refractivity estimated using BDS alone (only 9 satellites used) is basically comparable to that of GPS; (3) BDS+GPS (as of current operation) may not be able to significantly improve the data's spatial density for the application of refractivity tomography; and (4) any slight increase in the precision of refractivity tomography, particularly in the lower atmosphere, bears great significance for any applications dependent on the Chinese operational meteorological service.
Impact of Anthropogenic Heat Release on Regional Climate in Three Vast Urban Agglomerations in China
FENG Jinming, WANG Jun, YAN Zhongwei
2014, 31(2): 363-373. doi: 10.1007/s00376-013-3041-z
We simulated the impact of anthropogenic heat release (AHR) on the regional climate in three vast city agglomerations in China using the Weather Research and Forecasting model with nested high-resolution modeling. Based on energy consumption and high-quality land use data, we designed two scenarios to represent no-AHR and current-AHR conditions. By comparing the results of the two numerical experiments, changes of surface air temperature and precipitation due to AHR were quantified and analyzed. We concluded that AHR increases the temperature in these urbanized areas by about 0.5C-1C, and this increase is more pronounced in winter than in other seasons. The inclusion of AHR enhances the convergence of water vapor over urbanized areas. Together with the warming of the lower troposphere and the enhancement of ascending motions caused by AHR, the average convective available potential energy in urbanized areas is increased. Rainfall amounts in summer over urbanized areas are likely to increase and regional precipitation patterns to be altered to some extent.
Changes in Climate Regionalization Indices in China during 1961-2010
LIU Jing, ZHAI Panmao
2014, 31(2): 374-384. doi: 10.1007/s00376-013-3017-z
The regionalization of climate in China is based on a three-level classification in terms of lasting days for accumulated temperature (AT), aridity index, and July mean temperature. Based on daily meteorological observational data from 756 stations, trends and interdecadal variation in indices for classifying temperature zones, moisture regions and climatic sub-regions in the period 1961-2010 are discussed. Results reveal that the nationwide AT 10C (AT10) and its lasting days are basically increasing, while aridity in northern Xinjiang is decreasing. The increasing trend of July mean temperature in North China is found to be notably larger than in South China. In terms of their national averages, a marked step increase of AT10 and its lasting period, as well as July mean temperature occurred around 1997, while the aridity index presents no such clear change. By comparing regionalization areas for 1998-2010 with those for 1961-97, it is found that the semi-humid, semi-dry and dry regions in the sub-temperate zone, as well as the humid region in the middle subtropical zone, have experienced substantial shrinkage in terms of area. In contrast, the areas of semi-dry and dry regions in the warm temperate zone, as well as the humid region in the south subtropical zone, present drastically increasing trends. Owing to the influence of such step changes that took place in 1997, that particular point in time should be given close attention in future studies regarding the regionalization of climate in China.
The Limiting Effect of Deep Soil Water on Evapotranspiration of a Subtropical Coniferous Plantation Subjected to Seasonal Drought
TANG Yakun, WEN Xuefa, SUN Xiaomin, ZHANG Xinyu, WANG Huimin
2014, 31(2): 385-395. doi: 10.1007/s00376-013-2321-y
Seasonal drought is a common occurrence in humid climates. The year 2003 was the driest year during the period 1985-2011 in southeastern China. The objective of this study was to elucidate the impact of the exceptional drought in 2003, compared with eddy flux measurements during 2004-11, on the dynamics of evapotranspiration (ET) and related factors, as well as their underlying mechanisms, in a subtropical coniferous plantation in southeastern China. It was found that daily ET decreased from 5.34 to 1.84 mm during the intensive drought period and recovered to 4.80 mm during the subsquent recovering drought period. Path analysis indicated that ET was mainly determined by canopy conductance and deep soil water content (50 cm) during the intensive drought and recovering drought periods, respectively. The canopy conductance offset the positive effect of air vapor pressure deficit on ET when suffering drought stress, while the canopy conductance enhanced the positive effect of air temperature on ET during the late growing season. Because the fine roots of this plantation are mainly distributed in shallow soil, and the soil water in the upper 40 cm did not satisfy the demand for ET, stomatal closure and defoliation were evident as physiological responses to drought stress.
The Characteristics of Atmospheric Ice Nuclei Measured at Different Altitudes in the Huangshan Mountains in Southeast China
JIANG Hui, YIN Yan, YANG Lei, YANG Shaozhong, SU Hang, CHEN Kui
2014, 31(2): 396-406. doi: 10.1007/s00376-013-3048-5
The concentration of ice nuclei (IN) and the relationship with aerosol particles were measured and analyzed using three 5-L mixing cloud chambers and a static diffusion cloud chamber at three altitudes in the Huangshan Mountains in Southeast China from May to September 2011. The results showed that the mean total number concentration of IN on the highest peak of the Huangshan Mountains at an activation temperature (Ta) of20C was 16.6 L-1. When the supersaturation with respect to water (Sw) and with respect to ice (Si) were set to 5%, the average number concentrations of IN measured at an activation temperature of20C by the static diffusion cloud chamber were 0.89 and 0.105 L-1, respectively. A comparison of the concentrations of IN at three different altitudes showed that the concentration of IN at the foot of the mountains was higher than at the peak. A further calculation of the correlation between IN and the concentrations of aerosol particles of different size ranges showed that the IN concentration was well correlated with the concentration of aerosol particles in the size range of 1.2-20 m. It was also found that the IN concentration varied with meteorological conditions, such as wind speed, with higher IN concentrations often observed on days with strong wind. An analysis of the backward trajectories of air masses showed that low IN concentrations were often related to air masses travelling along southwest pathways, while higher IN concentrations were usually related to those transported along northeast pathways.
Evaluation of Snow Depth and Snow Cover Fraction Simulated by Two Versions of the Flexible Global Ocean-Atmosphere-Land System Model
XIA Kun, WANG Bin, LI Lijuan, SHEN Si, HUANG Wenyu, XU Shiming, DONG Li, LIU Li
2014, 31(2): 407-420. doi: 10.1007/s00376-013-3026-y
Based on historical runs, one of the core experiments of the fifth phase of the Coupled Model Intercomparison Project (CMIP5), the snow depth (SD) and snow cover fraction (SCF) simulated by two versions of the Flexible Global Ocean-Atmosphere-Land System (FGOALS) model, Grid-point Version 2 (g2) and Spectral Version 2 (s2), were validated against observational data. The results revealed that the spatial pattern of SD and SCF over the Northern Hemisphere (NH) are simulated well by both models, except over the Tibetan Plateau, with the average spatial correlation coefficient over all months being around 0.7 and 0.8 for SD and SCF, respectively. Although the onset of snow accumulation is captured well by the two models in terms of the annual cycle of SD and SCF, g2 overestimates SD/SCF over most mid- and high-latitude areas of the NH. Analysis showed that g2 produces lower temperatures than s2 because it considers the indirect effects of aerosols in its atmospheric component, which is the primary driver for the SD/SCF difference between the two models. In addition, both models simulate the significant decreasing trend of SCF well over (30-70N) in winter during the period 1971-94. However, as g2 has a weak response to an increase in the concentration of CO2 and lower climate sensitivity, it presents weaker interannual variation compared to s2.
Impacts of the Lowest Model Level Height on Tropical Cyclone Intensity and Structure
MA Zhanhong, FEI Jianfang, HUANG Xiaogang, CHENG Xiaoping
2014, 31(2): 421-434. doi: 10.1007/s00376-013-3044-9
Variable thicknesses in the lowest half- model level (LML) are often used in atmospheric models to compute surface diagnostic fields such as surface latent and sensible heat fluxes. The effects of the LML on simulated tropical cyclone (TC) evolution were investigated in this study using the Weather Research and Forecasting (WRF) model. The results demonstrated notable influences of the LML on TC evolution when the LML was placed below 12 m. The TC intensification rate decreased progressively with a lowering of the LML, but its ultimate intensity change was relatively small. The maximum 10-m winds showed different behavior to minimum sea level pressure and azimuthally-averaged tangential winds, and thus the wind-pressure relationship was changed accordingly by varying the LML. The TC circulation was more contracted in association with a higher LML. Surface latent heat fluxes were enhanced greatly by elevating the LML, wherein the wind speed at the LML played a dominant role. The changes in the wind speed at the LML were dependent not only on their profile differences, but also the different heights they were taken from. Due to the enhanced surface heat fluxes, more intense latent heat release occurred in the eyewall, which boosted the storm's intensification. A higher LML tended to produce a stronger storm, and therefore the surface friction was reinforced, which in turn induced stronger boundary layer inflow together with increased diabatic heating.
An Approach for Improving Short-Term Prediction of Summer Rainfall over North China by Decomposing Interannual and Decadal Variability
HAN Leqiong, LI Shuanglin, LIU Na
2014, 31(2): 435-448. doi: 10.1007/s00376-013-3016-0
A statistical downscaling approach was developed to improve seasonal-to-interannual prediction of summer rainfall over North China by considering the effect of decadal variability based on observational datasets and dynamical model outputs. Both predictands and predictors were first decomposed into interannual and decadal components. Two predictive equations were then built separately for the two distinct timescales by using multivariate linear regressions based on independent sample validation. For the interannual timescale, 850-hPa meridional wind and 500-hPa geopotential heights from multiple dynamical models' hindcasts and SSTs from observational datasets were used to construct predictors. For the decadal timescale, two well-known basin-scale SST decadal oscillation (the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation) indices were used as predictors. Then, the downscaled predictands were combined to represent the predicted/hindcasted total rainfall. The prediction was compared with the models' raw hindcasts and those from a similar approach but without timescale decomposition. In comparison to hindcasts from individual models or their multi-model ensemble mean, the skill of the present scheme was found to be significantly higher, with anomaly correlation coefficients increasing from nearly neutral to over 0.4 and with RMSE decreasing by up to 0.6 mm d-1. The improvements were also seen in the station-based temporal correlation of the predictions with observed rainfall, with the coefficients ranging from -0.1 to 0.87, obviously higher than the models' raw hindcasted rainfall results. Thus, the present approach exhibits a great advantage and may be appropriate for use in operational predictions.
An Analysis of Historical and Future Temperature Fluctuations over China Based on CMIP5 Simulations
LIU Yonghe, FENG Jinming, MA Zhuguo
2014, 31(2): 457-467. doi: 10.1007/s00376-013-3093-0
The trends and fluctuations of observed and CMIP5-simulated yearly mean surface air temperature over China were analyzed. In general, the historical simulations replicate the observed increase of temperature, but the multi-model ensemble (MME) mean does not accurately reproduce the drastic interannual fluctuations. The correlation coefficient of the MME mean with the observations over all runs and all models was 0.77, which was larger than the largest value (0.65) from any single model ensemble. The results showed that winter temperatures are increasing at a higher rate than summer temperatures, and that winter temperatures exhibit stronger interannual variations. It was also found that the models underestimate the differences between winter and summer rates. The ensemble empirical mode decomposition technique was used to obtain six intrinsic mode functions (IMFs) for the modeled temperature and observations. The periods of the first two IMFs of the MME mean were 3.2 and 7.2, which represented the cycle of 2-7-yr oscillations. The periods of the third and fourth IMFs were 14.7 and 35.2, which reflected a multi-decadal oscillation of climate change. The corresponding periods of the first four IMFs were 2.69, 7.24, 16.15 and 52.5 in the observed data. The models overestimate the period of low frequency oscillation of temperature, but underestimate the period of high frequency variation. The warming rates from different representative concentration pathways (RCPs) were calculated, and the results showed that the temperature will increase by approximately 0.9C, 2.4C, 3.2C and 6.1C in the next century under the RCP2.6, RCP4.5, RCP6.0 and RCP8.5 scenarios, respectively.
The Relationship between the El Nio/La Nia Cycle and the Transition Chains of Four Atmospheric Oscillations. Part I: The Four Oscillations
PENG Jingbei, CHEN Lieting, ZHANG Qingyun
2014, 31(2): 468-479. doi: 10.1007/s00376-013-2275-0
The first leading modes of the interannual variations in low-level circulation over the North and South Pacific are the Northern Oscillation (NO) and Southern Oscillation (SO), which are oscillations in sea level pressure anomalies (SLPAs) between the eastern and western Pacific Ocean. The second leading modes are the North Pacific Oscillation (NPO) and the Antarctic Oscillation (AAO), which reflect oscillations between the subtropics and the high and middle latitudes. The transition chains of these four oscillations were investigated using the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data. The general pattern of the transition chain between the NO and NPO was from the negative phase of the NO (NO-) to the positive phase of the NPO (NPO+), then from NO+ to NPO- to NO-. The whole transition chain took about 4-6 years. The general pattern and period of the transition between the SO and AAO were similar to those between the NO and NPO. In addition, the transition chains between the NO and NPO, and the SO and AAO, were almost simultaneous. The transition chains of the four oscillations were found to be closely connected, with the eastward propagations of SLPAs occurring along both sides of the Equator.
Trends in the Different Grades of Precipitation over South China during 1960-2010 and the Possible Link with Anthropogenic Aerosols
FU Chuanbo, DAN Li
2014, 31(2): 480-491. doi: 10.1007/s00376-013-2102-7
Using observed daily precipitation data to classify five levels of rainy days by strength in South China (SC), with an emphasis on the Pearl River Delta (PRD) region, the spatiotemporal variation of different grades of precipitation during the period 1960-2010 was analyzed and the possible link with anthropogenic aerosols examined. Statistical analysis showed that drizzle and small precipitation has significantly decreased, whereas medium to heavy precipitation has increased slightly over the past 50 years (although not statistically significant). Further data analysis suggested that the decline in drizzle and small precipitation probably has a strong link to increased concentrations of anthropogenic aerosols produced by large-scale human activities related to the rapid socioeconomic development of the PRD region. These aerosols may also have led to the obvious decreasing trend in horizontal visibility and sunshine duration in SC, which is statistically significant according to the t-test.
Quantifying the Response Strength of the Southern Stratospheric Polar Vortex to Indian Ocean Warming in Austral Summer
LI Shuanglin, CHEN Xiaoting
2014, 31(2): 492-503. doi: 10.1007/s00376-013-2322-x
A previous multiple-AGCM study suggested that Indian Ocean Warming (IOW) tends to warm and weaken the southern polar vortex. Such an impact is robust because of a qualitative consistency among the five AGCMs used. However, a significant difference exists in the modeled strengths, particularly in the stratosphere, with those in three of the AGCMs (CCM3, CAM3, and GFS) being four to five times as strong as those in the two other models (GFDL AM2, ECHAM5). As to which case reflects reality is an important issue not only for quantifying the role of tropical ocean warming in the recent modest recovery of the ozone hole over the Antarctic, but also for projecting its future trend. This issue is addressed in the present study through comparing the models' climatological mean states and intrinsic variability, particularly those influencing tropospheric signals to propagate upward and reach the stratosphere. The results suggest that differences in intrinsic variability of model atmospheres provide implications for the difference. Based on a comparison with observations, it is speculated that the impact in the real world may be closer to the modest one simulated by GFDL AM2 and ECHAM5, rather than the strong one simulated by the three other models (CCM3, CAM3 and GFS). In particular, IOW during the past 50 years may have dynamically induced a 1.0C warming in the polar lower stratosphere (~100 hPa), which canceled a fraction of radiative cooling due to ozone depletion.
Large-scale Circulation Anomalies Associated with Interannual Variation in Monthly Rainfall over South China from May to August
SU Qin, LU Riyu, LI Chaofan
2014, 31(2): 273-282. doi: 10.1007/s00376-013-3051-x
Interannual variation in summer rainfall over South China (SC) was investigated on the monthly timescale. It was found that monthly rainfall from May to August exhibits different features of variation, and the amounts are basically independent of each other. There is a significant negative correlation, however, between May and July SC rainfall, which is partially related to the developing phases of ENSO events. It was also found that stronger (weaker) lower-tropospheric winds over SC and the upstream parts are responsible for more (less) SC rainfall in every month from May to August. Despite this monthly consistent enhancement of horizontal winds, the wind anomalies exhibit distinct differences between May-June and July-August, due to the remarkable change in climatological winds between these two periods. More SC rainfall is associated with a lower-tropospheric anticyclonic anomaly over the SCS and the Philippine Sea in May and June, but with a cyclonic anomaly centered over SC in July and August.
New Evidence for Improving Omega Estimation by Explicitly Considering Horizontal Divergence
YUAN Zhuojian, QI Jindian, GAO Shouting, FENG Yerong, XU Pengcheng, WU Naigeng
2014, 31(2): 449-456. doi: 10.1007/s00376-013-3003-5
It is well known that the quasi-geostrophic (QG) omega equation with only two contributors respectively associated with vorticity advection (VA) and temperature advection is derived for midlatitude synoptic-scale systems only. Based on reliable reanalysis data, new evidence revealed by cyclonic and anticyclonic cases indicates that forecasters might sometimes experience problems by paying too much attention to the 500-hPa VA when estimating vertical motions not only in subtropical systems but also in systems meeting all the assumptions of the QG omega equation. Our investigations also showed that explicitly considering the vertical profiles of horizontal divergence could allow for better interpretation of vertical motions and weather in these real cases, suggesting that this equation might not be sufficient due to the presence of only two horizontal-divergence-related (HDR) mechanisms and the absence of other HDR mechanisms, e.g., frictional force, mountain barriers, diabatic/adiabatic processes, and acceleration/deceleration of air flows.