Impacts of SST and SST Anomalies on Low-Frequency Oscillation in the Tropical Atmosphere

Considering the multiscale character of LFO effects of SST on LFO in the tropical atmosphere （low-frequency oscillation） in the tropical atmosphere, the are discussed by using an absolute ageostrophic, baroclinic model. Here, SST effects include sea surface heating and forcing of SST anomalies （SSTAs）. Studies of the influences of sea surface heating on LFO frequency and stability show that sea surface heating can slow the speed of waves and lower their frequency when SST is comparatively low; while higher SST leads to unstable waves and less periods of LFO. Since the impact of a SSTA on ultra-long waves is more evident than that on kilometer-scale waves, long-wave approximation is used when we continue to study the effect of SSTAs. Results indicate that SSTAs can lead to a longer period of LFO, and make waves unstable. In other words, positive （negative） SSTAs can make waves decay （grow）.     

大气中的季节内振荡

本文依据最新的分析研究结果,系统地讨论了大气中的季节内(30—60天)振荡。内容包括大气季节内振荡的全球分布特征;热带大气30—60天振荡的基本特征;中高纬度大气30—60天振荡;热带和中高纬度大气30—60天振荡的相互影响;以及大气季节内振荡的动力学机制。 本文不仅对热带大气30—60天振荡提出了一些新的研究结果,特别是关于中高纬度大气30—60天振荡的讨论,以及关于热带和中高纬度大气30—60天振荡主要通过低频波列相互联系和相互影响的研究,将使我们对大气季节内振荡有比较全面的认识。     

赤道以外热带大气中30—50天振荡的一个动力学研究

本文的动力学分析和模式大气的计算结果表明,在积云对流加热的反馈作用下,热带大气可以产生一种CISK-Rossby型波动。这种波与经典的热带Rossby波不同,它既可以向西也能向东移动,而且在热带大气通常的加热情况下其相速度同热带30—50天振荡的活动相一致。同时,CISK-Rossby型波动具有频散特性,其能量频散可以说明30—50天大气振荡的二维Rossby波列特征。因此可以认为,CISK-Rossby型波动是赤道以外热带大气30—50天振荡的主要激发和驱动机制。     

Intraseasonal Oscillation in the Tropical Indian Ocean

1. Introduction The intraseasonal oscillation (ISO or Madden- Julian Oscillation, MJO) in the tropical atmosphere has been studied extensively, including its existence, structure, evolution and propagation (Madden and Ju- lian, 1971; Murakami, et al., 198…     

The Dynamical Influence of Land－Sea Contrast and Sea Surface Temperature on Intraseasonal Oscillation in Tropical Atmosphere

An equatorial β-plane model which includes realistic non-uniform land-sea contrast and the underlying surface temperature distribution is used to simulate the 30-60 day oscillation (LFO) processes in tropical atmosphere, with emphasis on its longitude-dependent evolution and convective seesaw between Indian and the western Pacific oceans.The model simulated the twice-amplification of the disturbances over Indian and the western Pacific oceans while they are travelling eastward. It reproduced the dipole structure caused by the out-of-phase oscillation of the active centres in these two areas and the periodical transition between the phases of LFO. It is suggested that the convective seesaw is the result of interaction of the internal dynamics of tropical atmosphere with the zonally non-uniform thermal forcing from underlying surface. The convective activities are suppressed over Indonesia mari-time continents whilst they are favoured over the Indian Ocean and western Pacific warm waters, so there formed two active oscillation centres. The feedback of convection with large-scale flow slows down the propagation of disturb-ances when they are intensifying over these two areas, therefore they manifest a kind of quasi-stationary component to favor the ‘dipole’ structure. Whereas the disturbances weaken and speed up over the eastern Pacific cold water re-gion due to the interaction of sensible heating and evaporation with perturbational wind. Therefore the two major centers just show out-of-phase oscillation during onecycle around the latitudinal beltBy introducing the SST anomalies in El Ni?o and La Ni?a years into the surface temperature, we also show that they have significant influence on LFO processes. In an anomalously warm year, the LFO disturbances dissipate more slowly over the central-eastern Pacific region and can travel farther eastward; whilst in an anomalously cold year, the opposite is true.     

CISK-related Rossby Waves in the Tropical Atmosphere

In terms of a baroclinic quasi-geostrophic wave-filtering technique in connection with a dimensionless paramet-er, η(z), of condensation-released latent heat that indicates the CISK mechanism, a model is established for des-cribing tropical atmosphere CISK-Rossby waves alongside its analytical solution. Theoretical study shows that there exists pronounced difference between Rossby waves, CISK-involving and classic, and the former can be used to in-terpret some aspects of the low-frequency oscillation in the tropical atmosphere     

Bimodal representation of the tropical intraseasonal oscillation

The tropical intraseasonal oscillation (ISO) shows distinct variability centers and propagation patterns between boreal winter and summer. To accurately represent the state of the ISO at any particular time of a year, a bimodal ISO index was developed. It consists of Madden-Julian Oscillation (MJO) mode with predominant eastward propagation along the equator and Boreal Summer ISO (BSISO) mode with prominent northward propagation and large variability in off-equatorial monsoon trough regions. The spatial–temporal patterns of the MJO and BSISO modes are identified with the extended empirical orthogonal function analysis of 31?years (1979–2009) OLR data for the December–February and June–August period, respectively. The dominant mode of the ISO at any given time can be judged by the proportions of the OLR anomalies projected onto the two modes. The bimodal ISO index provides objective and quantitative measures on the annual and interannual variations of the predominant ISO modes. It is shown that from December to April the MJO mode dominates while from June to October the BSISO mode dominates. May and November are transitional months when the predominant mode changes from one to the other. It is also shown that the fractional variance reconstructed based on the bimodal index is significantly higher than the counterpart reconstructed based on the Wheeler and Hendon’s index. The bimodal ISO index provides a reliable real time monitoring skill, too. The method and results provide critical information in assessing models’ performance to reproduce the ISO and developing further research on predictability of the ISO and are also useful for a variety of scientific and practical purposes.     

Lidar Measurements of Aerosols in the Tropical Atmosphere

Measurements of atmospheric aerosols and trace gases using the Laser radar (lidar) techniques, have been in pro-gress since 1985 at the Indian Institute of Tropical Meteorology, Pune (18o32’N, 73o51’E, 559 m AMSL), India. These observations carried out during nighttime in the lower atmosphere (up to 5.5 km AGL), employing an Argon ion / Helium-Neon lidar provided information on the nature, size, concentration and other characteristics of the constituents present in the tropical atmosphere. The time-height variations in aerosol concentration and associated layer structure exhibit marked differences between the post-sunset and pre-sunrise periods besides their seasonal va-riation with maximum concentration during pre-monsoon / winter and minimum concentration during monsoon months. These observations also revealed the influence of the terrain of the experimental site and some selected me-teorological parameters on the aerosol vertical distributions. The special observations of aerosol vertical profiles ob-tained in the nighttime atmospheric boundary layer during October 1986 through September 1989 showed that the most probable occurrence of mixing depth lies between 450 and 550 m, and the multiple stably stratified aerosol lay-ers present above the mixing depth with maximum frequency of occurrence at around 750 m. This information on nighttime mixing depth / stable layer derived from lidar aerosol observations showed good agreement with the height of the ground-based shear layer / elevated layer observed by the simultaneously operated sodar at the lidar site.     

热带大气运动的特征

本文用小参数展开方法,探讨了热带大气中的特征运动及其基本属性.理论分析表明: 热带地区的超长波系统有中纬度长波的类似性质.其快过程是地转适应过程,有重力惯性波的活动;而慢过程在零级近似下是地转风,流场具有水平无辐散性质,在一级近似下是类似Rossby波的慢波,运动满足位涡度守恒的条件. 热带大气中的中间尺度运动,时间尺度远大于平流时间时为旋转风,是一种定常涡旋;时间尺度近于平流时间时,运动是水平无辐散的,且保持相对涡度守恒;若时间尺度远小于平流时间,则表现为重力波的活动. 热带大气中水平尺度近于10~6米     

Principal oscillation pattern analysis of the tropical 30- to 60-day oscillation

In Part I (Storch and Xu 1990) the principal oscillation pattern (POP) analysis of 200 mb equatorial velocity potential leads to the definition of a bivariate (POP-) index of the tropical 30- to 60-day oscillation. Using the POP prediction scheme this index is predictable for a few days in advance. In Part 11, the prediction of the equatorial velocity potential field, made by the POP method and made by two GCMs, is investigated. The POP index forecast can incorporate skillful forecasts of the equatorial velocity potential () field. Its ensemble correlation skill score passes the 0.50 level at 7 days, whereas persistence passes after 3 days. If there is a strong 30- to 60-day oscillation signal in the initial state, useful forecasts of more than 20 days are sometimes possible; if the initial signal is weak, the POP forecast fails. Also, the forecast skill of two GCMs is considered. The NCAR T31 GCM appears to be quite skillful in predicting the equatorial -field, and in particular the 30- to 60-day oscillation. Its skill, however, is less than that of the POP scheme. The CNRM T42 GCM seems not to be able to predict the regular development associated with the tropical 30- to 60-day oscillation. The power of the POP index in explaining the equatorial x-field is a measure of the strength and dominance of the 30- to 60-day oscillation. This measure at day 0 is an a priori indicator of the NCAR T31 GCM's skill in predicting the equatorial velocity potential field.The National Center for Atmospheric Research is sponsored by the National Science Foundation     

大气准双周振荡的研究进展

大气准双周振荡(Quasi-Biweekly Oscillation,QBWO)主要是指时间尺度为10~20 d的振荡,它是大气中重要的低频系统之一,是在研究季风天气及其相联系的季风系统时被发现的。QBWO对大范围的长期天气变化及异常有着重要影响,且具有全球性和多季性。本文较系统总结了近年来QBWO的研究成果,主要对其结构、活动特征,以及它对天气气候的影响及其激发维持机制等进行了综述。在此基础上,对今后的一些研究重点进行了展望。     

Principal oscillation pattern analysis of the 30- to 60-day oscillation in the tropical troposphere

The Principal Oscillation Pattern technique is used to derive an index of the 30- to 60-day oscillation in the tropical troposphere. In the 200-mb equatorial velocity potential field, one dominant pair of POPS is found. Its properties compare very well with the properties of the oscillation identified in previous studies. In particular, a good correlation between the time evolution of the POP coefficients and area-averaged outgoing long-wave radiation (ORL) is found. The POPS are derived from a 2-year subinterval of the whole 5-year data set. This leaves independent data for subsequent verification. The patterns and their characteristic numbers are almost unchanged if the whole data set is analysed. Also, the analysis is insensitive to changes of the analysis area: if the analysis is limited to 90°-longitude equatorial sectors, the signal is also identified and its patterns are consistent with the patterns derived from the full data set. Interestingly, the signal is best defined in the eastern hemisphere. The POPS may be used to derive associated correlation patterns of other quantities in winter and summer separately. The path of the oscillation has a marked annual cycle: in northern winter it migrates from the Indian Ocean across northern Australia into the region of the South Pacific Convergence Zone (SPCZ) and in northern summer it moves from the Indian Ocean across South Asia along the intertropical Convergence Zone (ITCZ) to South America. The POP coefficient may be seen as a bivariate index of the state (phase and strength) of the 30- to 60-day oscillation. Since the POP technique incorporates a prediction equation for the phase of the POP coefficients, the POP model allows for the prediction of the complex amplitude of the oscillation. In a sequence of forecast experiments, of which about two-thirds used independent data, the POP forecasts were found to be useful in about half of all cases for lead times of several days. The correlation and RMS skills were calculated for the POP forecast and for persistence. The POP forecast appears to be considerably better with respect to both measures. The correlation skill scores 60% after 7 days. The POP forecast is most skillful in northern winter and if strong signals are present with minima of velocity potential in the eastern hemisphere.     

中国热带大气季节内振荡研究进展

热带大气季节内振荡(包括MJO)是大气环流的重要系统,它的活动及异常既对其他系统有一定的作用,也对长期天气和短期气候有明显影响。因此,热带大气季节内振荡一直是大气科学的前沿研究课题之一。文中对近5—10年中国学者的有关研究工作及其进展做了简要回顾和综合,主要包括:(1)热带大气季节内振荡特别是MJO的动力学机制;(2)热带大气季节内振荡以及MJO的数值模拟问题,特别是大气非绝热加热廓线对模式模拟MJO的重要作用;(3)热带大气季节内振荡和MJO,特别是在赤道西太平洋地区,与ENSO的相互作用关系;(4)热带大气季节内振荡(包括MJO)及其流场形势对西太平洋台风活动的重要影响,即MJO对西北太平洋台风生成数的调制作用,以及热带大气季节内低频气旋性(LFC)和反气旋性(LFAC)流场对西太平洋台风路径的影响;(5)热带大气季节内振荡(包括MJO)的活动及异常对东亚和南亚夏季风建立、活动异常的影响,以及它们与中国降水异常的密切关系。     

热带大气中的地形强迫运动

本文研究了在基本气流无切变和有切变的情况下地形对低伟大气大尺度定常运动的影响。从赤道β平面的线性化正压原始方程出发,求得一个解析形式的地形影响函数。对它的分析表明:大气的受迫运动不仅取决于越过山脉的基本气流的方向,而且还与山脉所处的纬度以及山脉的尺度有关。当适度的西风越过山脉时,会在背风侧形成地形槽。在线性模式中,基本气流的东西风切变对地形强迫波的经向伸展范围起限制作用。利用本文的模式,对东南亚近赤道地区的气压场和流场进行模拟,其结果与观测事实定性一致。最后,本文还讨论了越赤道气流通道的形成机制。      

正压大气中的地形性振荡

本文讨论了正压大气中的一种地形性振荡,这种振荡是由地形对行星涡度的周期性强迫而产生的共振作用所引起的。它具有两个明显的特征,即能流方向与波尺度无关及振荡的周期为中长期天气尺度。     

热带大气动力学研究

热带大气控制着约地球面积的一半,热带大气的运动、热带地区的天气气候情况对人类有重要的影响;地球大气运动的基本能源是太阳辐射,而热带大气获得太阳能最多,加之热带大气有充足的水份,从运动能量的观点,热带大气又是能源之所在。因此,热带大气运动的研究是气象学的重要课题,越来越引起人们的重视。但是,目前人们对热带大气运动规律的了解还相当不够,热带地区天气预报(包括数值预报)的准确率还很低;而热     

热带太平洋地区海气系统的耦合振荡

本文讨论了东太平洋赤道海温和太平洋月平均云量距平的关系,结果表明:(1)东太平洋赤道海温距平和中太平洋赤道云量距平有很好的正相关,而与东、西太平洋赤道云量距平有很好负相关。所以太平洋赤道上空应该存在二个距平的东西向环流。(2)云和海温存在周期为34—38个月的耦合振荡,我们提出了云-辐射-海温机制来作解释。(3)在东太平洋海温暖水月的前12个月到后6个月期间,东太平洋赤道云量是负距平的(即偏少),这表明此期间可能是大气在影响海洋,亦即Walker环流影响海温变化,而不是海温影响了Walker 环流。海温和Walker环流是相互作用的,这种相互作用组成了大气和海洋之间的一种耦合振荡。(4)东太平洋海温、中太平洋云和北半球中、西太平洋信风亦有很好的相关。     

Activities of low-frequency waves in the tropical atmosphere and enso

ENSO, particularly the occurrence of ENSO is still an important research object in climatic variation. Using the ECMWF data, the relationship between ENSO and the activities of low-frequency waves in the tropical atmosphere is analyzed in this paper. It is shown that the occurrence of ENSO is closely related to the intraseasonal oscillation and the quasi-stationary waves (the period > 90 days) in the tropical atmosphere. Associated with the occurrence of El Nino event, the kinetic energy of low-frequency waves has obvious variation: the kinetic energy of atmospheric intraseasonal (30–60 days) oscillation (ISO) decreases abruptly and the kinetic energy of quasi-stationary waves increases abruptly. Moreover, the ISO and quasi-stationary waves propagate eastward clearly corresponding to El Nino; but they clearly propagate westward in La Nina cases.     

El Nino/Southern oscillation signals in the global tropical ocean

The monthly mean sea surface temperature data of 6 areas are used to study the El Nino/Southern Oscillation signals in the global tropical ocean. These areas are in the 5oN-5oS latitude zone at 1) eastern Pacific (110o-l40oW), 2) western Atlantic (30o-50oW), 3) eastern Atlantic (10oW-10oE), 4) western Indian Ocean (30o-50oE), 5) central Indian Ocean (70o-90oE) and 6) far western Pacific (120o-140oE), and the data cover the 120-month period of December 1968 to November 1978.A power spectrum analysts shows that the characteristic time of the El Nino/Southern Oscillation (about 3-4 years) appears not only in the eastern Pacific but also in other areas of the tropics except for the western Pa-cific, where the spectrum is of white noise. The amplitude of oscillation in the eastern Pacific is about 4 times larger than the others, making the El Nino/Southern Oscillation signal the strongest in this area. According to a cross-spectrum analysis, there is no time lag between the variation in the central Indian Ocean and that in the eastern Pacific. These two areas oscillate simultaneously and comprise the main feature of the El Nino/ Southern Oscillation. Other tropical areas are related with time lags, as shown by correlation and coherence calculations.It should be noted that the sea surface temperature in the eastern Pacific oscillates in phase with that in the Indian Ocean, while the pressure oscillations in these two areas are out of phase with each other, according to the Southern Oscillation definition. It is suggested that the Southern Oscillation cannot be explained simply by the sea surface temperature anomalies.Variations in the far western equatorial Pacific do not have the time scale of the El Nino/Southern Oscilla-tion, perhaps because it is a buffer zone between the monsoon system and the trade wind system.     

The tropical very low-frequency oscillation on interannual scale

This is a review on the studies of tropical very low-frequency oscillation (VLFO) on interannual scale, mainly on the recent researches undertaken by Chinese scientists which are not well known outside of the country.This paper summarizes the basic features of VLFO in the tropics, the characteristic time and spacial structure of oscillation, especially the new concept of Low Latitude Oscillation consisted of two components: the well-known Southern Oscillation (SO) and the so-called Northern Oscillation (NO). A large number of evidences have been provided to illustrate the relationship between VLFO in tropics and the climate variation in China, such as the long-term variation of north Pacific high, the frequency of typhoon and the cyclone over the East China Sea, the summer monsoon rainfall in Yangtze valley basin and the cold summer disaster in Northeast China, and so on. Finally throw some lights on the nature of VLFO on imerannual scale.