首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Our research focuses on the analysis of extreme high maximum air temperature events (EXHTEs) in the Czech Republic in the period 1961–2010, their climatological characteristics, and on the identification of synoptic-scale circulation conditions conductive to them. EXHTEs are detected using the Weather Extremity Index (WEI) combining return periods of daily maximum air temperature, duration of events, and the extent of the affected area. We selected 37 EXHTEs as non-overlapping periods with the highest WEI. Some long EXHTEs were divided into several shorter synoptically homogeneous episodes. Using the two-level divisive clustering of 700 hPa air temperature and wind field anomalies, we obtained four main variants of synoptic-scale circulation conditions. The most frequent variant associated with extreme episodes is characterized by a westerly flow connected with a high pressure ridge extending northeastward from North Africa over Central Europe or with an anticyclone centered over the Central Mediterranean. The most extreme episodes occurred during the variant characterized by an easterly flow between a high pressure area to the northeast and a low pressure area to the southeast.  相似文献   

2.
A self-contained derivation of the IPESD models [Majda, A.J., Klein, R., 2003. Systematic multi-scale models for the tropics. J. Atmos. Sci. 60, 393–408] governing synoptic and planetary scale tropical flows is provided. This derivation demonstrates the analytic tractability of the model and the effect of zonally and meridionally tilted synoptic scale heating on the forcing of planetary scale flows through upscale momentum and temperature fluxes. Exploiting the analytic tractability of the models, different aspects of the planetary scale forcing are traced to meridional and vertical tilts in the synoptic scale heating profile. Variants of the archetypal IPESD models for the Madden–Julian oscillation (MJO) presented in Majda and Biello [Majda, A.J., Biello, J.A., 2004. A multi-scale model for tropical intraseasonal oscillations. Proc. Natl. Acad. Sci. 101, 4736–4741; Biello, J.A., Majda, A.J., 2005. A new multi-scale model for the Madden–Julian oscillation. J. Atmos. Sci. 62, 1694–1721] are studied. In addition to vertically tilted synoptic scale heating, the models discussed herein incorporate upscale zonal momentum flux due to meridional flux convergence arising from meridionally tilted heating. The effect of a boundary layer momentum drag at the base of the free troposphere is also systematically incorporated into the IPESD models. Both meridional tilts and lower boundary layer drag are shown to meridionally confine the MJO westerly wind burst and drive a planetary scale barotropic flow. Meridionally tilted heating can also greatly strengthen the wind burst at the base of the troposphere and modify its vertical profile. The competing effects of meridionally tilted, and off-equatorial heating can also significantly weaken the MJO winds. Appendices are provided which discuss generalizations and a solution algorithm for the IPESD models.  相似文献   

3.
To better understand the relationship between anticyclones in Siberia and cold-air activities and temperature changes in East Asia, this study proposes a 2D anticyclone identification method based on a deep-learning model, Mask R-CNN, which can reliably detect the changes in the morphological characteristics of anticyclones. Using the new method, the authors identified the southeastward-extending Siberian cold high (SEESCH), which greatly affects wintertime temperatures in China. This type of cold high is one of the main synoptic systems (45.7%) emerging from Siberia in winter. Cold air carried by SEESCH has a significant negative correlation with the temperature changes in the downstream area, and 52% of SEESCHs are accompanied by cold-air accumulation in North and East China, which has a significant impact on regional cooling. These results provide clues for studying the interconnection between SEESCHs and extreme cold events.摘要为了更好地研究西伯利亚地区反气旋与冷空气活动,东亚地区气温变化之间的关联, 本文提出一种基于Mask R-CNN的反气旋识别方法, 能够较为准确地刻画反气旋形态特征变化. 使用该方法能够识别对中国冬季气温具有较大影响的东南延伸型西伯利亚冷高压(SEESCH), 这种冷高压是冬季出现在西伯利亚地区的主要天气系统之一(45.7%). SEESCH携带的冷空气与下游地区温度变化呈显著负相关, 52%的SEESCH伴随着华北华东地区冷空气聚集, 对区域降温有显著影响. 这些结果为研究 SEESCH 与极端寒冷事件之间的联系提供线索.  相似文献   

4.
This paper investigates the main sources and features of the Mediterranean synoptic cyclones affecting the basin, using the cyclone tracks. The cyclones’ tracks are identified using sea level pressure (SLP) from the NCEP/NCAR reanalysis data for the period 1956–2013. The identified cyclones are classified into two categories: basin affected and basin non-affected. Most of the basin-affected (non-affected) cyclones are internal (external), i.e., generated inside (outside) the Mediterranean basin. This study reveals four (five) main sources of internal (external) cyclones. These four (five) main sources generated about 63.76% (57.25%) of the internal (external) cyclones. Seasonal analysis shows that most of the basin-affected internal (external) cyclones were generated in the winter (spring) season. The lowest number of cyclones were found in the summer. Moreover, the synoptic study of the atmospheric systems accompanied the highest- and lowest-generated years demonstrates that the deepening of the north Europe cyclones and the relative positions of Azores- and Siberian-high systems represent the important factors that influence the number of internal cyclones. Essential factors influencing the external cyclones are the strength of the maximum upper wind, Azores high, Siberian high, and orientations of their ridges.  相似文献   

5.
Synoptic patterns identified by an automated procedure employing principal- component analysis and a two-stage cluster analysis, and backward trajectory analysis clustered by the HYSPLIT4.9 model were used to examine air quality patterns over¨ Uru¨mqi, China, one of the most heavily polluted cities in the world. Six synoptic patterns representing different atmospheric circulation patterns and air-mass characteristics were classified during the winter heating periods from 2001 to 2008, and seven trajectory clusters representing different paths of air masses arriving at ürümqi were calculated during the winter heating periods from 2005 to 2008. Then air quality was evaluated using these two approaches, and significant variations were found across both synoptic patterns and trajectory clusters. The heaviest air-pollution episodes occurred when ürümqi was either in an extremely cold, strong anticyclone or at the front of a migrating cyclone. Both conditions were characterized by with light winds, cold, wet surface air, and relatively dry upper air. ürümqi was predominately influenced by air masses from the southwest and from local areas. Air pollution index (API) levels were highest for air masses originating from the southwest with a longer path or for the local area, because of transport from semi-desert/desert regions by strong winds and because of local heavy pollution emissions, respectively. The interactions between these two analytical approaches showed that poor diffusion conditions, together with local circulation, enhanced air pollution, besides, regional air-mass transport caused by strong winds contributed to serious air quality under relatively good diffusion conditions.  相似文献   

6.
The potential of Petrosyants-Gushchina integrated circulation index in respect to the problem of synoptic analysis in low latitudes is considered. The compositional models of synoptic situations, which led to the formation of extreme conditions in weather and climate over the Pacific Ocean, are constructed using the computed indices according to the data of NCEP/NCAR reanalysis for 1950–2002 separately for El Niño and La Niña events. It is demonstrated that the barometric circulation pattern of synoptic processes in tropics, being reproduced using indices, indicates the main peculiarities of atmospheric circulation in low latitudes and can be the quantitative indicator of the whole circulation system of El Niño-Southern Oscillation over the Pacific Ocean.  相似文献   

7.
Demonstrated is an influence of synoptic processes on the seasonal dynamics of the Vietnamese Current in the South China Sea varying its direction under the influence of the monsoon. The spring season of 1999 is used as an example of the transition from the winter to summer. In winter, under the influence of the northeastern monsoon, the current is directed from north to south and in the summer, at the southwestern monsoon, in the opposite direction. In the spring, two zones of different water modification are formed: an impact is observed of both the leaving winter monsoon and the coming summer monsoon. Considered is an atmospheric process of low-gradient field type, when the pressure field is characterized by the low pressure gradient over the whole South China Sea. It is revealed that the Vietnamese Current moves in the summer regime (from south to north) in the northern and southern parts and keeps the winter regime (from north to south) in the central part.  相似文献   

8.
A thorough analysis of a proxy El Nino/Southern Oscillation (ENSO) record indicates that a bifurcation occurred in the ENSO system sometime around 5,000 years b.p. As a result of this bifurcation the attractor became higher dimensional and a new mechanism of instability was introduced. As a consequence of these changes the system switched from a dynamics where the normal condition (La Nina) was dominant to a dynamics characterized by more frequent and stronger El Nino events.  相似文献   

9.
The atmospheric conditions that lead to strong offshore surface winds in Southern California, commonly referred to as Santa Ana winds, are investigated using the North American Regional Reanalysis and a 12-year, 6-km resolution regional climate simulation of Southern California. We first construct an index to characterize Santa Ana events based on offshore wind strength. This index is then used to identify the average synoptic conditions associated with Santa Ana events—a high pressure anomaly over the Great Basin. This pressure anomaly causes offshore geostrophic winds roughly perpendicular to the region’s mountain ranges, which in turn cause surface flow as the offshore momentum is transferred to the surface. We find, however, that there are large variations in the synoptic conditions during Santa Ana conditions, and that there are many days with strong offshore flow and weak synoptic forcing. This is due to local thermodynamic forcing that also causes strong offshore surface flow: a large temperature gradient between the cold desert surface and the warm ocean air at the same altitude creates an offshore pressure gradient at that altitude, in turn causing katabatic-like offshore flow in a thin layer near the surface. We quantify the contribution of “synoptic” and “local thermodynamic” mechanisms using a bivariate linear regression model, and find that, unless synoptic conditions force strongly onshore winds, the local thermodynamic forcing is the primary control on Santa Ana variability.  相似文献   

10.
The zonal momentum generation in forced stationary waves may exceed the requirement for momentum balance after long, if the waves do not change their patterns. This suggests that the changes in stationary wave patterns would be required by maintenance of momentum balance over the external forcings. It will be found that the low frequency anomalies like blocking regimes may produce reversed zonal momentum variations, if they happen in the observed centre areas. The zonal momentum balance in the stationary waves may be maintained effectively by alternation between the normal and blocking circulation regimes. Thus, from the point of long-term zonal momentum balance, we may explain the geographical distributions of the blocking centres and the seasonal variations in blocking areas and frequencies.  相似文献   

11.
Recent Progress in the Impact of the Tibetan Plateau on Climate in China   总被引：14，自引：0，他引：14  
Studies of the impacts of the Tibetan Plateau （TP） on climate in China in the last four years are reviewed. It is reported that temperature and precipitation over the TP have increased during recent decades. From satellite data analysis, it is demonstrated that most of the precipitation over the TP is from deep convection clouds. Moreover, the huge TP mechanical forcing and extraordinary elevated thermal forcing impose remarkable impacts upon local circulation and global climate. In winter and spring, stream flow is deflected by a large obstacle and appears as an asymmetric dipole, making East Asia much colder than mid Asia in winter and forming persistent rainfall in late winter and early spring over South China. In late spring, TP heating contributes to the establishment and intensification of the South Asian high and the abrupt seasonal transition of the surrounding circulations. In summer, TP heating in conjunction with the TP air pump cause the deviating stream field to resemble a cyclonic spiral, converging towards and rising over the TP. Therefore, the prominent Asian monsoon climate over East Asia and the dry climate over mid Asia in summer are forced by both TP local forcing and Eurasian continental forcing.
Due to the longer memory of snow and soil moisture, the TP thermal status both in summer and in late winter and spring can influence the variation of Eastern Asian summer rainfall. A combined index using both snow cover over the TP and the ENSO index in winter shows a better seasonal forecast.
On the other hand, strong sensible heating over the Tibetan Plateau in spring contributes significantly to anchor the earliest Asian monsoon being over the eastern Bay of Bengal （BOB） and the western Indochina peninsula. Qualitative prediction of the BOB monsoon onset was attempted by using the sign of meridional temperature gradient in March in the upper troposphere, or at 400 hPa over the TP. It is also demonstrated by a numerical experiment and theoretical study that the heating over the TP lea  相似文献   

12.
  相似文献   

13.
  相似文献   

14.
Hall  Richard J.  Hanna  Edward  Chen  Linling 《Climate Dynamics》2021,56(1-2):457-473

We investigate winter Arctic Amplification (AA) on synoptic timescales and at regional scales using a daily version of the Arctic Amplification Index (AAI) and examine causes on a synoptic scale. The persistence, frequency and intensity of high AAI events show significant increases over the Arctic. Similarly, low AAI events are decreasing in frequency, persistence and intensity. In both cases, there are regional variations in these trends, in terms of significance and timing. Significant trends in increasing persistence, frequency and intensity of high AAI events in winter are concentrated in the period 2000–2009, with few significant trends before and after this. There are some decreases in sea-ice concentration in response to synoptic-scale AA events and these AA events can contribute to the decadal trends in AA found in other studies. A sectoral analysis of the Arctic indicates that in the Beaufort–Chukchi and East Siberian–Laptev Seas, synoptic scale high AAI events can be driven by tropical teleconnections while in other Arctic sectors, it is the intrusion of moisture-transporting synoptic cyclones into the Arctic that is most important in synoptic-scale AA. The presence of Rossby wave breaking during high AAI events is indicative of forcing from lower latitudes, modulated by variations in the jet stream. An important conclusion is that the increased persistence, frequency and intensity of synoptic-scale high AAI events make significant contributions to the interannual trend in AA.

  相似文献   

15.
The evolution of El Ni?o-Southern Oscillation (ENSO) variability can be characterized by various ocean–atmosphere feedbacks, for example, the influence of ENSO related sea surface temperature (SST) variability on the low-level wind and surface heat fluxes in the equatorial tropical Pacific, which in turn affects the evolution of the SST. An analysis of these feedbacks requires physically consistent observational data sets. Availability of various reanalysis data sets produced during the last 15?years provides such an opportunity. A consolidated estimate of ocean surface fluxes based on multiple reanalyses also helps understand biases in ENSO predictions and simulations from climate models. In this paper, the intensity and the spatial structure of ocean–atmosphere feedback terms (precipitation, surface wind stress, and ocean surface heat flux) associated with ENSO are evaluated for six different reanalysis products. The analysis provides an estimate for the feedback terms that could be used for model validation studies. The analysis includes the robustness of the estimate across different reanalyses. Results show that one of the “coupled” reanalysis among the six investigated is closer to the ensemble mean of the results, suggesting that the coupled data assimilation may have the potential to better capture the overall atmosphere–ocean feedback processes associated with ENSO than the uncoupled ones.  相似文献   

16.
A depth map (close to that of the thermocline as defined by 20℃) of climatically maximum seatemperature anomaly was created at the subsurface of the tropical Pacific and Indian Ocean, based on which the evolving sea-temperature anomaly at this depth map from 1960 to 2000 was statistically analyzed. It is noted that the evolving sea temperature anomaly at this depth map can be better analyzed than the evolving sea surface one. For example, during the ENSO event in the tropical Pacific, the seatemperature anomaly signals travel counter-clockwise within the range of 10°S-10°N, and while moving, the signals change in intensity or even type. If Dipole is used in the tropical Indian Ocean for analyzing the depth map of maximum sea-temperature anomaly, the sea-temperature anomalies of the eastern and western Indian Oceans would be negatively correlated in statistical sense (Dipole in real physical sense), which is unlike the sea surface temperature anomaly based analysis which demonstrates that the inter-annual positive and negative changes only occur on the gradients of the western and eastern temperature anomalies. Further analysis shows that the development of ENSO and Dipole has a time lag features statistically, with the sea-temperature anomaly in the eastern equatorial Pacific changing earlier (by three months or so). And the linkage between these two changes is a pair of coupled evolving Walker circulations that move reversely in the equatorial Pacific and Indian Oceans.  相似文献   

17.
The intermediate water upwelling zone is discovered in November and December in the Peter the Great Bay (the Sea of Japan) on the basis of the data of the repeated hydrological section along 132° E. Taking account of the two-layer density field structure obtained from the observational data and applying the corresponding upwelling model [20], its main characteristics are computed. The upwelling zone width amounts to 6.2 km, the stratification destruction time is 69 h, and the vertical speed component is 4.3 × 10−2 cm/s. The upwelling zone is clearly pronounced both at the temperature and salinity fields and at the biogenic element field.  相似文献   

18.
The harmonic analyses of monthly mean total ozone in the atmosphere over the Northern Hemisphere for 26 years (1960-1985) are made by using the Fourier expansion. The analysed results show that there is obviously a quasi-biennial oscillation (QBO) in the interannual variations of the amplitudes of total ozone. Generally, the amplitudes of wavenumber 1 and 2 during the westerly of the equatorial QBO are larger than those during the easterly. In the early winter, the amplitude of wavenumber 1 during the easterly phase is larger, and in the late winter, it is larger during the westerly phase. These are in good agreement with the observational distributions.  相似文献   

19.
A field campaign was conducted to collect high quality vertical wind speed and temperature profiles with simultaneous turbulent momentum and sensible heat fluxes over the flat Gobi surface located at 39°09′N, 100°06′E at an elevation of 1,458 m during the Pilot Intensive Observation Period in 1990. Careful evaluation of the observations in near-neutral conditions supports a value of the von Karman constant close to 0.39, which is in good agreement with the results obtained from many other field experiments conducted in low elevation regions. In near-neutral stratification the turbulent Prandtl number is found to be 1 but with lower confidence due to scarcity and scatter of the data points. For an expanded stability range, exponents of −1/4 and −1/2 are respectively best fitted to the functional relations for the non-dimensional wind and temperature profile functions in unstable stratification but linear relations still hold for stable stratification in this high elevation region.  相似文献   

20.
We analyze the ability of an oceanic monitoring array to detect potential changes in the North Atlantic meridional overturning circulation (MOC). The observing array is ‘deployed’ into a numerical model (ECHAM5/MPI-OM), and simulates the measurements of density and wind stress at 26°N in the Atlantic. The simulated array mimics the continuous monitoring system deployed in the framework of the UK Rapid Climate Change program. We analyze a set of three realizations of a climate change scenario (IPCC A1B), in which – within the considered time-horizon of 200 years – the MOC weakens, but does not collapse. For the detection analysis, we assume that the natural variability of the MOC is known from an independent source, the control run. Our detection approach accounts for the effects of observation errors, infrequent observations, autocorrelated internal variability, and uncertainty in the initial conditions. Continuous observation with the simulated array for approximately 60 years yields a statistically significant (p < 0.05) detection with 95 percent reliability assuming a random observation error of 1 Sv (1 Sv = 106 m3 s?1). Observing continuously with an observation error of 3 Sv yields a detection time of about 90 years (with 95 percent reliability). Repeated hydrographic transects every 5 years/ 20 years result in a detection time of about 90 years/120 years, with 95 percent reliability and an assumed observation error of 3 Sv. An observation error of 3 Sv (one standard deviation) is a plausible estimate of the observation error associated with the RAPID UK 26°N array.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号