Different Impact of Central Pacific and Eastern Pacific El Nio on the Duration of Sudden Stratospheric Warming     

Yuanpu LI  Wenshou TIAN 《大气科学进展》2017,34(6):771-782

The NCEP–NCAR reanalysis dataset and the Had ISST dataset(1959–2014) are used to analyze the impact of two types of El Nio events, i.e., eastern Pacific El Nio(EP-El Nio) and central Pacific El Nio(CP-El Nio) events, on the duration of major and minor sudden stratospheric warmings(SSWs) in Northern Hemisphere winter(November to February). Although the frequency of major and minor SSWs during different types of El Nio shows no distinct differences, the duration of both major and minor SSWs during CP-El Nio is shorter than that during EP-El Nio. The spatial distribution of geopotential height anomalies preceding major SSWs resembles the western Pacific(WP) teleconnection pattern, while the spatial distribution of geopotential height anomalies preceding minor SSWs bears similarity to the Pacific–North America(PNA)teleconnection pattern. An enhancement of the strength of both wavenumber 1 and wavenumber 2 is found before major SSWs. Before minor SSWs, wavenumber 1 is also strengthened, but wavenumber 2 is weakened. The analysis also reveals that EP-El Nio tends to induce positive phases of PNA and WP teleconnections, while CP-El Nio induces negative-phase WP teleconnection. As the positive phases of the PNA and WP teleconnections are related to the strengthening of wavenumber 1, EP-El Nio causes an enhancement of wavenumber 1 in the high-latitude upper troposphere and an enhancement of the upward wave flux in the high-latitude stratosphere, accompanied by a negative anomaly in Eliassen–Palm flux divergence in the subpolar stratosphere, which accounts for the longer SSW duration during EP-El Nio than during CP-El Nio.  相似文献   

Interdecadal Variability of the Afro-Asian Summer Monsoon System     

Yi LI  Yihui DING  Weijing LI 《大气科学进展》2017,34(7):833-846

The Afro-Asian summer monsoon is a zonally planetary-scale system, with a large-scale rainbelt covering Africa, South Asia and East Asia on interdecadal timescales both in the past century(1901–2014) and during the last three decades(1979–2014). A recent abrupt change of precipitation occurred in the late 1990 s. Since then, the entire rainbelt of the Afro-Asia monsoon system has advanced northwards in a coordinated way. Consistent increases in precipitation over the Huanghe–Huaihe River valley and the Sahel are associated with the teleconnection pattern excited by the warm phase of the Atlantic Multidecadal Oscillation(AMO). A teleconnection wave train, with alternating cyclones/anticyclones, is detected in the upper troposphere. Along the teleconnection path, the configuration of circulation anomalies in North Africa is characterized by coupling of the upper-level anticyclone(divergence) with low-level thermal low pressure(convergence), facilitating the initiation and development of ascending motions in the Sahel. Similarly, in East Asia, a coupled circulation pattern also excites ascending motion in the Huanghe–Huaihe River valley. The synchronous increase in precipitation over the Sahel and Huanghe–Huaihe River valley can be attributed to the co-occurrences and in-phase changes of ascending motion. On the other hand, the warm phase of the AMO results in significant warming in the upper troposphere in North Africa and the northern part of East Asia. Such warming contributes to intensification of the tropical easterly jet through increasing the meridional pressure gradient both at the entrance region(East Asia) and the exit region(Africa). Accordingly, precipitation over the Sahel and Huanghe–Huaihe River valley intensifies, owing to ageostrophic secondary cells. The results of this study provide evidence for a consistent and holistic interdecadal change in the Afro-Asian summer monsoon.  相似文献   

Parallel Comparison of the 1982/83,1997/98 and 2015/16 Super El Nios and Their Effects on the Extratropical Stratosphere     

Jian RAO  Rongcai REN 《大气科学进展》2017,34(9):1121-1133

This study uses multiple sea surface temperature(SST) datasets to perform a parallel comparison of three super El Ni os and their effects on the stratosphere. The results show that, different from ordinary El Ni os, warm SST anomalies appear earliest in the western tropical Pacific and precede the super El Ni o peak by more than 18 months. In the previous winter,relative to the mature phase of El Ni o, as a precursor, North Pacific Oscillation-like circulation anomalies are observed. A Pacific–North America(PNA) teleconnection appears in the extratropical troposphere during the mature phase, in spite of the subtle differences between the intensities, as well as the zonal position, of the PNA lobes. Related to the negative rainfall response over the tropical Indian Ocean, the PNA teleconnection in the winter of 1997/98 is the strongest among the three super El Ni os. The northern winter stratosphere shows large anomalies in the polar cap temperature and the circumpolar westerly, if the interferences from other factors are linearly filtered from the circulation data. Associated with the positive PNA response in a super El Ni o winter, positive polar cap temperature anomalies and circumpolar easterly anomalies,though different in timing, are also observed in the mature winters of the three super El Ni os. The stratospheric polar vortex in the next winter relative to the 1982/83 and 1997/98 events is also anomalously weaker and warmer, and the stratospheric circulation conditions remain to be seen in the coming winter following the mature phase of the 2015/16 event.  相似文献   

A model of the upper branch of the meridional overturning of the southern ocean     

John Marshall  Timour Radko 《Progress in Oceanography》2006,70(2-4):331

A zonal-average model of the upper branch of the meridional overturning circulation of the southern ocean is constructed and used to discuss the processes – wind, buoyancy, eddy forcing and boundary conditions – that control its strength and sense of circulation. The geometry of the thermocline ‘wedge’, set by the mapping between the vertical spacing of buoyancy surfaces (the stratification) on the equatorial flank of the Antarctic Circumpolar Current and their outcrop at the sea surface, is seen to play a central role by setting the interior large-scale potential vorticity distribution. It is shown that the action of eddies mixing this potential vorticity field induces a residual flow in the meridional plane much as is observed, with upwelling of fluid around Antarctica, northward surface flow and subduction to form intermediate water. Along with this overturning circulation there is a concomitant air-sea buoyancy flux directed in to the ocean.  相似文献   

The life cycle of the intense IOP‐14 storm during CASPII. Part I: Analysis and simulations     

Zonghui Huo  Da‐Lin Zhang  John Gyakum 《大气与海洋》2013,51(1):51-80

Abstract

In this study, a 5‐day life‐cycle of the IOP‐14 storm during CASP II is examined using conventional observations and numerical simulations with a mesoscale version of the Canadian Regional Finite‐Element (RFE) model. Observational analysis reveals that the IOP‐14 storm forms from a lee trough, occurring along a strong baroclinic zone with an intense frontogenetic deformation, that interacts with an upper‐level travelling short‐wave trough across the Canadian Rockies. Then the storm experiences a slow, but nearly steady, growth while traversing the North American continent. It deepens explosively as it moves into the Atlantic Ocean. It appears that i) the enhanced large‐scale baroclinicity due to land‐sea temperature contrasts, ii) the tremendous latent heat release due to the transport of high‐θ_e air from the marine boundary layer, Hi) the decrease of surface drag and iv) the favourable westward tilt of the low with an amplifying trough all contribute to the explosive deepening of the storm.

Two consecutive simulations covering a total of 102 h during the storm development are carried out with a grid size of 50 km. The RFE model reproduces very well the formation of the surface low on the lee side of the Rockies, the track and deepening rates, the explosive development and decay of the storm, and various mesoscale phenomena (e.g., a “bent‐back” warm front, a “T‐bone” thermal pattern, a cold frontal “fracture”, an upper‐level “eye” and warm‐core structures), as verified by conventional observations, satellite imagery, flight‐level and dropsonde data from a research aircraft. It is found from potential vorticity (PV) analysis that the storm reaches its peak intensity as the upper‐level dry PV anomaly, the low‐level moist PV anomaly and surface thermal warmth are vertically superposed. PV inversions reveal that these anomalies contribute about 60%, 30% and 10%, respectively, to the 900‐hPa negative height perturbation. It is shown that the warm‐core structure near the cyclone centre is produced by advection of warmer air ahead of the cold front, rather than by adiabatic warming associated with subsidence.  相似文献   

Impacts of large-scale teleconnections on climate variability over Southwest Asia     

《Dynamics of Atmospheres and Oceans》2019

The atmospheric low frequency variability at a regional or global scale is represented by teleconnection. Using monthly dataset of the Climatic Research Unit (CRU) for the period 1971–2016, the impacts of four large-scale teleconnection patterns on the climate variability over Southwest Asia are investigated. The large-scale features include the El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) and the East Atlantic (EA) teleconnection patterns, as well as western tropical Indian Ocean (WTIO) sea surface temperature anomaly index. Results indicate that ENSO and EA are the first leading modes that explain variation of Southwest Asian precipitation, with positive (negative) anomalies during El Niño (La Niña) and the negative (positive) phase of EA. Variation of Southwest Asian near-surface temperature is most strongly related to WTIO index, with above-average (below-average) temperature during the positive (negative) phase of WTIO index, although the negative (positive) phase of NAO also favours the above-average (below-average) temperature. On the other hand, temperature (precipitation) over Southwest Asia shows the least response to ENSO (WTIO). ENSO and EA individually explain 13 percent annual variance of precipitation, while WTIO index explains 36 percent annual variance of near-surface temperature over Southwest Asia. Analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis Interim (ERA-Interim) data indicated establishments of negative (positive) geopotential height anomalies in the middle troposphere over Southwest Asia during El Niño (La Niña) or the negative (positive) phase of NAO, EA and WTIO. The response of precipitation variability over Southwest Asia to NAO is opposite to that expected from the geopotential height anomalies, but the correlation between precipitation and NAO is not statistically significant. Due to predictability of large-scale teleconnections, results of this study are encouraging for improvement of the state-of-the-art seasonal prediction of the climate over Southwest Asia.  相似文献   

Meridional Displacement of the East Asian Upper-tropospheric Westerly Jet and Its Relationship with the East Asian Summer Rainfall in CMIP5 Simulations     

Yuhan YAN  Chaofan LI  Riyu LU 《大气科学进展》2019,36(11):1203-1216

As the first leading mode of upper-tropospheric circulation in observations, the meridional displacement of the East Asian westerly jet (EAJ) varies closely with the East Asian rainfall in summer. In this study, the interannual variation of the EAJ meridional displacement and its relationship with the East Asian summer rainfall are evaluated, using the historical simulations of CMIP5 (phase 5 of the Coupled Model Intercomparison Project). The models can generally reproduce the meridional displacement of the EAJ, which is mainly manifested as the first principal mode in most of the simulations. For the relationship between the meridional displacement of the EAJ and East Asian rainfall, almost all the models depict a weaker correlation than observations and exhibit considerably large spread across the models. It is found that the discrepancy in the interannual relationship is closely related to the simulation of the climate mean state, including the climatological location of the westerly jet in Eurasia and rainfall bias in South Asia and the western North Pacific. In addition, a close relationship between the simulation discrepancy and intensity of EAJ variability is also found: the models with a stronger intensity of the EAJ meridional displacement tend to reproduce a closer interannual relationship, and vice versa.  相似文献   

2018年7月北半球极端天气气候事件及环流特征分析          下载免费PDF全文

王倩  翟盘茂  余荣 《大气科学学报》2019,42(1):28-35

2018年7月北半球天气气候显著异常,极端事件高发。欧洲、北非、东亚以及北美的大部分地区均遭受严重的高温热浪侵袭;印度、东南亚、中国西南部以及日本西部等地出现极端降水;西太平洋台风活动异常活跃,移动路径偏北。初步诊断表明,北半球中高纬度,由低层到高层稳定维持的异常高压系统是导致北半球中高纬度大部分地区高温热浪持续发生的直接原因。其中异常偏强、偏北的副热带高压,以及增强、东伸的南亚高压与东亚地区持续高温和极端降水事件直接相关;低层菲律宾周围异常活跃的对流活动和强盛的西南水汽输送共同导致南亚、东南亚地区极端降水发生。热带太平洋大部分地区偏暖的海温条件和菲律宾附近异常气旋性环流则与异常活跃的台风活动有关。更需要关注的是,北半球尤其是东亚地区大气环流的异常主要受海洋表面热力状况以及其他区域大气环流遥相关的影响。  相似文献   

Analysis and Numerical Simulations of the Teleconnection Between Indian Summer Monsoon and Precipitation in North China   总被引：4，自引：0，他引：4       下载免费PDF全文

LIU Yunyun  DING Yihui 《Acta Meteorologica Sinica》2008,22(4):489-501

In the early 1980s, Chinese meteorologists discovered the positive correlation in summer rainfall between India and North China and the correlation was later confirmed by some researches in and outside China. Based on a variety of meteorological data from 1951 to 2005 and numerical simulations, the present study investigates such a correlation between Indian summer monsoon （ISM） and precipitation in North China. Furthermore, we discuss the intrinsic relations of the positive （Northwest India）-negative （the Tibetan Plateau）-positive （North China） precipitation anomaly teleconnection pattern from two aspects of thermal and dynamical factors, which not only confirms the precipitation teleconnection previously discovered again, but also reveals the influence mechanism of the ISM on the rainfall in North China. The results show that： （1） When the ISM is strong （weak）, the precipitation in North China tends to be more （less） than normal; however, when the rainfall in North China is more （less） than normal, the probability of the strengthening （weakening） of the ISM is relatively lower. This implies that the ISM anomaly has more impact on the rainfall in North China. （2） The Indian low usually dominantly impacts the intensity of the ISM. When the Indian low deepens, the low troughs in mid-high latitudes are frequently strengthened, and the ridge of the western Pacific subtropical high （WPSH） extends westward. The southwesterly water vapor transport originated from low-latitudes and the southeasterly water vapor transport along the southwestern flank of the WPSH converge in North China, which is favorable for more rainfall there than normal, and vice versa. （3） The simulations from the regional climate model developed by National Climate Center （ReGCM_NCC） capture the salient feature of the precipitation teleconnection between India and North China. The simulated anomalous atmospheric existence of such a teleconnection from another circulations are close to observatio  相似文献   

A Study of the Teleconnections in the Asian-Pacific Monsoon Region   总被引：2，自引：0，他引：2       下载免费PDF全文

DING Yihui  LIU Yunyun 《Acta Meteorologica Sinica》2008,22(4):404-418

The interactions among the Asian-Pacific monsoon subsystems have significant impacts on the climatic regimes in the monsoon region and even the whole world. Based on the domestic and foreign related research, an analysis is made of four different teleconnection modes found in the Asian-Pacific monsoon region, which reveal clearly the interactions among the Indian summer monsoon （ISM）, the East Asian summer monsoon （EASM）, and the western North Pacific summer monsoon （WNPSM）. The results show that： （1） In the period of the Asian monsoon onset, the date of ISM onset is two weeks earlier than the beginning of the Meiyu over the Yangtze River Basin, and a teleconnection mode is set up from the southwestern India via the Bay of Bengal （BOB） to the Yangtze River Basin and southern Japan, i.e., the ＂southern＂ teleconnection of the Asian summer monsoon. （2） In the Asian monsoon culmination period, the precipitation of the Yangtze River Basin is influenced significantly by the WNPSM through their teleconnection relationship, and is negatively related to the WNPSM rainfall, that is, when the WNPSM is weaker than normal, the precipitation of the Yangtze River Basin is more than normal. （3） In contrast to the rainfall over the Yangtze River Basin, the precipitation of northern China （from the 4th pentad of July to the 3rd pentad of August） is positively related to the WNPSM. When the WNPSM is stronger than normal, the position of the western Pacific subtropical high （WPSH） becomes farther northeast than normal, the anomalous northeastward water vapor transport along the southwestern flank of WPSH is converged over northern China, providing adequate moisture for more rainfalls than normal there. （4） The summer rainfall in northern China has also a positive correlation with the ISM. During the peak period of ISM, a teleconnection pattern is formed from Northwest India via the Tibetan Plateau to northern China, i.e., the ＂northern＂ teleconnection of the Asian summer monsoon. The  相似文献