ZONALLY SYMMETRICAL EKMAN-CISK MECHANISM AND 30—60-DAY LOW FREQUENCY OSCILLATION NEAR THE EQUATOR

Based on the baroclinic semi-geostrophic model,the effects of zonally symmetrical Ekman- CISK mechanism on the characteristics of 30—60-day low frequency oscillation(LFO)near the equator are investigated.It is found that the theoretical results are in good agreement with the observational features of LFO.Besides,the planetary-scale LFO with the period of 30—60 d could be triggered by the Ekman-CISK mechanism,and the growth rate of perturbation with wave number 1 is in order of O (10^(-6)s^(-1)).The zonal propagation of LFO and the corresponding longitudinal-height structure of physical quantities are also discussed in detail.

PROPAGATION OF 30—60 DAY LOW FREQUENCY OSCILLATIONS AND THEIR INFLUENCE UPON THE SUBTROPICAL WESTERLIES JET STREAM DURING NORTHERN HEMISPHERE WINTER

Based on daily ECMWF gridpoint data of two winters during 1981—1983 including an ENSO year,propagation of low frequency oscillations(LFO)during Northern Hemisphere winters and their influences upon 30—60 day oscillations of the subtropical jet stream are studied with the sta- tistical methods as complex empirical orthogonal function(CEOF)and so on.Results show that in the winter of a normal year(1981—1982),30—60 day oscillations in the subtropical zone are mainly in the northern and southern flanks of exit region of jet stream.In the ENSO year(1982— 1983),they are mainly in the vicinity of entrance and exit regions of jet stream.Intraseasonal changes of subtropical jet stream manifested themselves as latitudinal fluctuation or longitudinal progression or regression of about 40 day period.There are marked differences between propagat- ing passages of low frequency modes responsible for changes of subtropical jet stream in the normal year(1981—1982)and in the ENSO year(1982—1983).Changes of oscillation amplitude show obvious phases.In general,the one in late winter is stronger than that in early winter,strongest one occurs in February.

南海地区大气30～60天低频振荡及其对南海夏季风的可能影响

本文利用1979~2008年美国国家环境预报中心（NCEP）和美国国家大气研究中心（NCAR）第二套再分析资料、美国国家海洋和大气管理局（NOAA）的向外长波辐射（OLR）资料及1979~2007年全球降水资料（CMAP）,分析了南海地区热带大气的低频振荡及其对南海夏季风的可能影响。结果表明,夏半年南海地区存在显著的30~60天的周期振荡。当30~60天低频振荡处于活跃位相时,南海及其周围地区的低层大气为低频西南风,南海和菲律宾北部为低频气旋流场且为正的位涡度,对应着增强的南海夏季风槽和南海夏季风;当30~60天低频振荡处于不活跃位相时,情形正好相反。进一步的研究揭示出,夏半年30~60天低频振荡变化的空间型与夏半年平均场的年际变化的空间分布非常相似,并且南海及其附近地区的30~60天低频振荡活动的年际变化对夏半年平均场的年际变化有显著的贡献。强、弱南海夏季风年30~60天低频振荡活动的比较也说明,强的南海夏季风年30~60天低频振荡活跃状态发生的概率大于不活跃状态发生的概率,而弱的南海夏季风年则是不活跃状态发生的概率大于活跃状态发生的概率。因此,南海地区30~60天低频振荡对南海夏季风很可能有重要影响,当30~60天低频振荡的活跃状态处于主导时,南海夏季风往往会偏强;反之,如果不活跃状态处于主导时,南海夏季风往往会偏弱。

ANALYSIS ON THE SOURCE AND SINK OF KINETIC ENERGY OF ATMOSPHERIC 30—60 DAY PERIOD OSCILLATION AND THE PROBABLE CAUSES

Based on ECMWF daily grid point data in summer (May—August),1981,the distribution features of the source and sink of kinetic energy of atmosphere 30—60 day oscillation,including its horizontal distribution characteristics and its vertical structure characteristics,are investigated systematically with diagnostic analysis methods over a latitude belt between 80°N and 60°S.Also,the probable reasons for the existence of the source and sink of low frequency kinetic energy (LFKE) are discussed preliminarily.Results show that the horizontal distribution of the sources and sinks of kinetic energy of atmospheric 30—60 day oscillation is extremely different.The significant sources and sinks of LFKE mainly exist in the oceans and the coastal regions of continents or islands in the mid-high latitudes.It is also found that,in the vertical direction,the sources and sinks of kinetic energy of 30—60 day oscillation display barotropic structure in the mid-high latitudes of both hemispheres,but dispaly baroclinic structure in the equtorial region,and in the horizontal direction, the sources and sinks mainly display zonal wave-like distribution.The source and sink of LFKE are determinded by ageostrophic wind effect,frictional effect,interaction between sub-grid-scale systems,nonlinear interaction,and the flux-divergence of LFKE transported by transient wind. There are some regional reasons for the generation of sources and sinks which are not completely identical in different areas.

Nonlinear Three-Wave Interaction among Barotropic Rossby Waves in a Large-scale Forced Barotropic Flow

In this paper. the coupling equations describing nonlinear three-wave interaction amongRossby waves including the forcing of an external vorticity source are obtained. Under certainconditions, the coupling equations with a constant amplitude forcing, the stability analysis indicates that when the amplitude of the external forcing increases to a certain extent, a pitchforkbifurcation occurs. Also. it is shown fi-o m numerical results that the bifurcation can lead to chaoticbehavior of' strange' attractor. For the obtained three-variable equation, when the amplitude ofmodulated external forcing gradually increases, a Period-doubling bifurcation is found to lead tochaotic behavior. Thus, in a nonlinear three-wave coupling model in the large-scale forcedbarotropic atmospheric flow, chaotic behavior can be observed. This chaotic behavior can explainin part 30-60-day low-flequency oscillations observed in mid-high latitudes.