The Effect of Boreal Summer Intraseasonal Oscillation on Mixed Layer and Upper Ocean Temperature over the South China Sea

Intraseasonal oscillation of the mixed layer and upper ocean temperature has been found to occur over the South China Sea(SCS)in the summer monsoon season based on the multiple reanalysis and observational data in this study.The method of composite analysis and an upper ocean temperature equation assisted the analysis of physical mechanisms.The results show that the mixed layer depth(MLD)in the SCS has a significant oscillation with a 30-60 d period over the SCS region,which is closely related to boreal summer intraseasonal oscillation(BSISO)activities.The MLD can increase(decrease)during the positive(negative)phase of the BSISO and usually lags behind by approximately one-eighth of the lifecycle(5 days)of the BSISO-related convection.The BSISO may cause periodic anomalies at the air-sea boundary,such as wind stress and heat flux,so it can play a dominant role in modulating the variation in MLD.There also are significant intraseasonal seawater temperature anomalies in both the surface and subsurface layers of the SCS.In addition,during the initial phase of the BSISO,the temperature anomaly signals of the thermocline are obviously opposite to the sea surface temperature(SST),especially in the southern SCS.According to the results from the analysis of the temperature equation,the vertical entrainment term caused by BSISO-related wind stress is stronger than the thermal forcing during the initial stage of convection,and it is more significant in the southern SCS.

Influence of three types of boreal summer intraseasonal oscillation on summer precipitation over the Yangtze River Valley

本文研究了三种来源于印度洋的北半球夏季季节内振荡(BSISO)对长江中下游地区降水的影响.结果表明,在所有BSISO类型中水汽辐合对降水异常的贡献最大.在经典型中,伴随着自副热带西太平洋向西北太平洋传播的正(负)对流异常的气旋(反气旋)使得降水在-2(-1至3)侯减少(增加).在向东扩展型中,由于在印度洋的正(负)对流异常激发的遥响应在西北太平洋引起反气旋(气旋),降水在-2至0(1至3)侯增加(减少).在向北偶极子型中,由于伴随着正(负)对流异常的气旋(反气旋)从中国南海移动到西北太平洋,降水在-2至0(1至3)侯减少(增加).

The Boreal Summer Intraseasonal Oscillation Simulated by Four Chinese AGCMs Participating in the CMIP5 Project

The performances of four Chinese AGCMs participating in the Coupled Model Intercomparison Project Phase 5 （CMIP5） in the simulation of the boreal summer intraseasonal oscillation （BSISO） are assessed. The authors focus on the major characteristics of BSISO： the intensity, significant period, and propagation. The results show that the four AGCMs can reproduce boreal summer intraseasonal signals of precipitation; however their limitations are also evident. Compared with the Climate Prediction Center Merged Analysis of Precipitation （CMAP） data, the models underestimate the strength of the intraseasonal oscillation （ISO） over the eastern equatorial Indian Ocean （IO） during the boreal summer （May to October）, but overestimate the intraseasonal variability over the western Pacific （WP）. In the model results, the westward propagation dominates, whereas the eastward propagation dominates in the CMAP data. The northward propagation in these models is tilted southwest-northeast, which is also different from the CMAP result. Thus, there is not a northeast-southwest tilted rain belt revolution off the equator during the BSISO＇s eastward journey in the models. The biases of the BSISO are consistent with the summer mean state, especially the vertical shear. Analysis also shows that there is a positive feedback between the intraseasonal precipitation and the summer mean precipitation. The positive feedback processes may amplify the models＇ biases in the BSISO simulation.

Factors Limiting the Forecast Skill of the Boreal Summer Intraseasonal Oscillation in a Subseasonal-to-Seasonal Model

In this study,we evaluate the forecast skill of the subseasonal-to-seasonal(S2S)prediction model of the Beijing Climate Center(BCC)for the boreal summer intraseasonal oscillation(BSISO).We also discuss the key factors that inhibit the BSISO forecast skill in this model.Based on the bivariate anomaly correlation coefficient(ACC)of the BSISO index,defined by the first two EOF modes of outgoing longwave radiation and 850-hPa zonal wind anomalies over the Asian monsoon region,we found that the hindcast skill degraded as the lead time increased.The ACC dropped to below 0.5for lead times of 11 days and longer when the predicted BSISO showed weakened strength and insignificant northward propagation.To identify what causes the weakened forecast skill of BSISO at the forecast lead time of 11 days,we diagnosed the main mechanisms responsible for the BSISO northward propagation.The same analysis was also carried out using the observations and the outputs of the four-day forecast lead that successfully predicted the observed northward-propagating BSISO.We found that the lack of northward propagation at the 11-day forecast lead was due to insufficient increases in low-level cyclonic vorticity,moistening and warm temperature anomalies to the north of the convection,which were induced by the interaction between background mean flows and BSISO-related anomalous fields.The BCC S2S model can predict the background monsoon circulations,such as the low-level southerly and the northerly and easterly vertical shears,but has limited capability in forecasting the distributions of circulation and moisture anomalies.

Effects of air-sea coupling on the eastward propagating boreal winter intraseasonal oscillation over the tropical Indian Ocean

本文通过分析比较SINTEX-F海气耦合模式两组试验(一组是热带海洋大气和海洋完全耦合,一组是除了印度洋外,其它海洋有海气耦合)模拟结果,研究冬季印度洋海气耦合对季节内振荡(MJO)向东传播的影响。当冬季印度洋有海气耦合时,海温异常的非对称分布会加强沿着5°S-10°S纬带上的向东传播的MJO。当暖的海温总是出现在对流的东侧时,其会导致边界层异常辐合,使得水汽增加,有利于对流向东传播。另外,冬季印度洋海温的年际变化可以调制海气耦合对东传MJO的影响效果,负(正)印度洋偶极子年和正(负)印度洋海盆年海气耦合对MJO起了增强(减弱)的作用。这主要是印度洋海温的年际变化可以导致背景风场的变化,通过风-海温-蒸发反馈机制,增加或减少水汽的纬向非对称性,进而增强或减弱MJO的向东传播。

Intraseasonal Oscillation of Tropospheric Ozone over the Indian Summer Monsoon Region

Boreal summer intraseasonal oscillation(BSISO) of lower tropospheric ozone is observed in the Indian summer monsoon(ISM) region on the basis of ERA-Interim reanalysis data and ozonesonde data from the World Ozone and Ultraviolet Radiation Data Centre. The 30–60-day intraseasonal variation of lower-tropospheric ozone shows a northwest–southeast pattern with northeastward propagation in the ISM region. The most significant ozone variations are observed in the Maritime Continent and western North Pacific. In the tropics, ozone anomalies extend from the surface to 300 hPa; however, in extratropical areas, it is mainly observed under 500 hPa. Precipitation caused by BSISO plays a dominant role in modulating the BSISO of lower-tropospheric ozone in the tropics, causing negative/positive ozone anomalies in phases 1–3/5–6. As the BSISO propagates northeastward to the western North Pacific, horizontal transport becomes relatively more important, increasing/reducing tropospheric ozone via anticyclonic/cyclonic anomalies over the western North Pacific in phases 3–4/7–8.As two extreme conditions of the ISM, most of its active/break events occur in BSISO phases 4–7/1–8 when suppressed/enhanced convection appears over the equatorial eastern Indian Ocean and enhanced/suppressed convection appears over India, the Bay of Bengal, and the South China Sea. As a result, the BSISO of tropospheric ozone shows significant positive/negative anomalies over the Maritime Continent, as well as negative/positive anomalies over India, the Bay of Bengal,and the South China Sea in active/break spells of the ISM. This BSISO of tropospheric ozone is more remarkable in break spells than in active spells of the ISM, due to the stronger amplitude of BSISO in the former.

Pre-summer Persistent Heavy Rain over Southern China and Its Relationship with Intra-seasonal Oscillation of Tropical Atmosphere

Based on daily precipitation data supplied by the Chinese meteorological administration,hourly reanalysis datasets provided by the ECMWF and daily outgoing long wave radiation supplied by the NOAA,the evolution regularity of continuous heavy precipitation over Southern China(SC)from April to June in 1979-2020 was systematically analyzed.The interaction between specific humidity and circulation field at the background-scale,the intra-seasonal-scale and the synoptic-scale,and its influence on persistent heavy precipitation over the SC during the April-June rainy season were quantitatively diagnosed and analyzed.The results are as follows.Persistent heavy rainfall events(PHREs)over the SC during the April-June rainy season occur frequently from mid-May to mid-and late-June,exhibiting significant intra-seasonal oscillation(10-30-day)features.Vertically integrated moisture flux convergence(VIMFC)can well represent the variation of the PHREs.A multiscale quantitative diagnosis of the VIMFC shows that the pre-summer PHREs over the SC are mainly affected by the background water vapor(greater than 30 days),intraseasonal circulation disturbance(10-30-day)and background circulation(greater than 30 days),and water vapor convergences are the main factor.The SC is under the control of a warm and humid background and a strong intraseasonal cyclonic circulation,with strong convergence and ascending movements and abundant water vapor conditions during the period of the PHREs.Meanwhile,the westward inter-seasonal oscillation of tropical atmosphere keeps the precipitation system over the SC for several consecutive days,eventually leading to the occurrence,development and persistence of heavy precipitation.

Extremely Active Tropical Cyclone Activities over the Western North Pacific and South China Sea in Summer 2018: Joint Effects of Decaying La Nina and Intraseasonal Oscillation

In summer 2018,a total of 18 tropical cyclones(TCs)formed in the western North Pacific(WNP)and South China Sea(SCS),among which 8 TCs landed in China,ranking respectively the second and the first highest since 1951.Most of these TCs travelled northwest to northward,bringing in heavy rainfall and strong winds in eastern China and Japan.The present study investigates the impacts of decaying La Nina and intraseasonal oscillation(ISO)on the extremely active TCs over the WNP and SCS in summer 2018 by use of correlation and composite analyses.It is found that the La Nina episode from October 2017 to March 2018 led to above-normal sea surface temperature(SST)over central–western Pacific,lower sea level pressure and 500-hPa geopotential height over WNP,and abnormally strong convective activities over the western Pacific in summer 2018.These preceding oceanic thermal conditions and their effects on circulation anomalies are favorable to TC genesis in summer.Detailed examination reveals that the monsoon trough was located further north and east,inducing more TCs in northern and eastern WNP;and the more eastward WNP subtropical high as well as the significant wave train with a"-+-+"height anomaly pattern over the midlatitude Eurasia–North Pacific region facilitated the northwest to northward TC tracks.Further analyses reveal that two successively active periods of Madden–Julian Oscillation(MJO)occurred in summer 2018 and the boreal summer intraseasonal oscillation(BSISO)was also active over WNP,propagating northward significantly,corresponding to the more northward TC tracks.The MJO was stagnant over the Maritime Continent to western Pacific,leading to notably enhanced convection in the lower troposphere and divergence in the upper troposphere,conducive to TC occurrences.In a word,the extremely active TC activities over the WNP and SCS in summer 2018 are closely linked with the decaying La Nina,and the MJO and BSISO;their joint effects result in increased TC occurrences and the TC tracks being shifted more northwest to northward than normal.

Boreal Summer Intraseasonal Oscillation in the Asian–Pacific Monsoon Region Simulated in CAMS-CSM

The boreal summer intraseasonal oscillation(BSISO) is simulated by the Climate System Model(CSM) developed at the Chinese Academy of Meteorological Sciences(CAMS), China Meteorological Administration. Firstly, the results indicate that this new model is able to reasonably simulate the annual cycle and seasonal mean of the precipitation, as well as the vertical shear of large-scale zonal wind in the tropics. The model also reproduces the eastward and northward propagating oscillation signals similar to those found in observations. The simulation of BSISO is generally in agreement with the observations in terms of variance center, periodicity, and propagation, with the exception that the magnitude of BSISO anomalous convections are underestimated during both its eastward propagation along the equator and its northward propagation over the Asian–Pacific summer monsoon region. Our preliminary evaluation of the simulated BSISO by CAMS-CSM suggests that this new model has the capability, to a certain extent, to capture the BSISO features, including its propagation zonally along the equator and meridionally over the Asian monsoon region.

Influence of the Boreal Summer Intraseasonal Oscillation on Extreme Temperature Events in the Northern Hemisphere

The impact of the boreal summer intraseasonal oscillation （BSISO） on extreme hot and cool events was investig-ated, by analyzing the observed and reanalysis data for the period from 1983 to 2012. It is found that the frequency of the extreme events in middle and high latitudes is significantly modulated by the BSISO convection in the tropics, with a 3-9-day lag. During phases 1 and 2 when the BSISO positive rainfall anomaly is primarily located over a northwest-southeast oriented belt extending from India to Maritime Continent and a negative rainfall anomaly ap- pears in western North Pacific, the frequency of extreme hot events is 40% more than the frequency of non-extreme hot events. Most noticeable increase appears in midlatitude North Pacific （north of 40°N） and higher-latitude polar region. Two physical mechanisms are primarily responsible for the change of the extreme frequency. First, an upper-tropo-spheric Rossby wave train （due to the wave energy propagation） is generated in response to a negative heating anom-aly over tropical western North Pacific in phases 1 and 2. This wave train consists of a strong high pressure anomaly center northeast of Japan, a weak low pressure anomaly center over Alaska, and a strong high pressure anomaly cen-ter over the western coast of United States. Easterly anomalies to the south of the two strong midlatitude high pres-sure centers weaken the climatological subtropical jet along 40°N, which is accompanied by anomalous subsidence and warming in North Pacific north of 40°N. Second, an enhanced monsoonal heating over South Asia and East Asia sets up a transverse monsoonal overturning circulation, with large-scale ascending （descending） anomalies over trop-ical Indian （Pacific） Ocean. Both the processes favor more frequent extreme hot events in higher-latitude Northern Hemisphere. An anomalous atmospheric general circulation model is used to confirm the tropical heating effect.

Comparison of the Structure and Evolution of Intraseasonal Oscillations Before and After Onset of the Asian Summer Monsoon

High-resolution satellite-derived data and NCEP-NCAR reanalysis data are used to investigate intraseasonal oscillations （ISO） over the tropical Indian Ocean.A composite evolution of the ISO life cycle is constructed,including the initiation,development,and propagation of rainfall anomalies over the tropical Indian Ocean.The characteristics of ISO over the tropical Indian Ocean are profoundly different before and after the onset of the Indian summer monsoon.Positive precipitation anomalies before monsoon onset appear one phase earlier than those after monsoon onset.Before monsoon onset,precipitation anomalies associated with ISO first initiate in the western tropical Indian Ocean and then propagate eastward along the equator.After monsoon onset,convective anomalies propagate northward over the Indian summer monsoon region after an initial eastward propagation over the equatorial Indian Ocean.Surface wind convergence and air-sea interaction play critical roles in initiating each new cycle of ISO convection.

热带海气相互作用对大气BSISO年际振荡的影响

利用卫星观测OLR资料以及海气耦合数值模拟试验结果,从每年波-频分析结果提取了各种传播模态的强度指数序列,分析了热带北半球夏季季节内振荡(BSISO)各种传播模态的年际变化谱特征,探讨了热带各海区海气相互作用对其影响。主要结果如下:赤道外西传波和印度洋北传波以准2 a为显著振荡周期,赤道东传波、南海北传波和西太平洋北传波则都包含准2 a和准5 a两种周期,南海北传波是5种指数中惟一以准5 a为最主要周期振荡的模态。热带印度洋、西太平洋、东太平洋各海区海气相互作用对各指数准2 a振荡、准5 a振荡既有加强作用,也有削弱作用。各海区比较而言,对赤道东传波准2 a和准5 a振荡、南海北传波准2 a和准5 a振荡起最大加强作用的是西太平洋海区海气相互作用;对赤道外西传波准2 a振荡、西太平洋北传波准2 a和准5 a振荡起最大加强作用的是印度洋海区海气相互作用。

海洋性大陆地形对夏季季节内振荡的影响:基于2020年9月个例的数值模拟分析

海洋性大陆(Maritime Continent,MC)是夏季大气季节内振荡(the Boreal Summer IntraSeasonal Oscillation,BSISO)传播的必经途径,而MC对于BSISO结构和传播产生的重要的影响机制很不清楚.针对此问题,利用高精度数值模式对一次BSISO事件展开数值模拟试验研究.选取2020年8-9月的一次BSISO事件,利用高精度数值模式WRF(Weather Research and Forecasting model)对本次BSISO过程进行了近一个月的数值模拟.发现WRF控制试验合理模拟出与再分析资料中相近的北传低层风场以及明显具有BSISO特征的西北-东南倾斜的雨带,并合理地捕捉了本次BSISO事件的传播特征和平均状态.为了研究MC地形对本次事件传播和强度的影响,在WRF模式中去除了MC地区的地形,开展了敏感性试验.在去除地形的敏感性试验中,BSISO低空风加强,传播更加平滑,整体降水幅度增加,而在岛屿上水汽大幅增加,降水量减少.在地形高度为零的情况下,纬向平流大大增强,从而增强了海上对流,促进了BSISO的加强和传播.此数值模拟试验研究揭示了MC地形对BSISO降水结构、传播和幅度的影响.

Mechanism of Regional Subseasonal Precipitation in the Strongest and Weakest East Asian Summer Monsoon Subseasonal Variation Years

Using the National Center for Environment Prediction Climate Forecast System Reanalysis coupled dataset during 1979–2010,we selected four subseasonal indexes from the 16 East Asian Summer Monsoon(EASM)indexes to characterize the subseasonal variability of the entire EASM system.The strongest(1996)and weakest(1998)years of the subseasonal variation were revealed based on these subseasonal EASM indexes.Furthermore,three rainfall concentration areas were defined in East Asia,and these areas were dissected by the atmospheric midlatitude jet stream axis and the position of the Western North Pacific Subtropical High(WNPSH).Then,the subseasonal effects of the WNPSH,the South Asian High(SAH),the Mongolian Cyclone(MC),and the Boreal Summer Intraseasonal Oscillation(BSISO)on each rainfall concentration area were studied in the strongest and weakest subseasonal variation years of the EASM.During the summer of 1998,the WNPSH and the SAH were stable in the more southern region,which not only blocked the northward progression of the BSISO but also caused the MC to advance southward.Therefore,the summer of 1998 was the weakest subseasonal variability of the EASM,but with significant subseasonal precipitation episodes in the northern and central rainfall areas.However,in 1996,the BSISO repeatedly spread northward in the south rainfall area because of the weak intensities and northern positions of the WNPSH and the SAH,which caused significant subseasonal precipitation episodes.In addition,MC was blocked to the north of approximately 42°N with a weak subseasonal rainfall.

Decadal changes of the intraseasonal oscillation during 1979–2016

The Intraseasonal Oscillation (ISO) is the cornerstone for 2–8-week subseasonal prediction. Understanding the decadal variation of the ISO is important for improving subseasonal prediction;however, there is still a gap in our knowledge of ISO dynamics. Here, we presented a method, an Empirical Orthogonal Function (EOF) analysis of 11-year-sliding ISO evolution, to objectively detect decadal variation of the ISO originated from the equatorial Indian Ocean (EIO) during 1979–2016. The results show that the properties of ISO have a notable decadal change since 1998 for both boreal summer and boreal winter seasons, mainly in its evolution rather than in its intensity at origin. During the pre-1998 epoch, the boreal summer intraseasonal oscillation (BSISO), was confined to the Indian Ocean;since 1998, however, it propagated northeastward across the Maritime Continent (MC) and the intraseasonal variability over the western North Pacific was significant enhanced. On the other hand, the boreal-winter ISO, usually known as Madden-Julian Oscillation (MJO) shows minor changes in MC ‘barrier effect’ between the two epochs, and continuously propagates eastward across the MC. The MJO only shows suppressed activity over the central equatorial Pacific in the post-1998 epoch. These decadal changes are related to the eastern Pacific cooling during the ‘global warming hiatus’ period rather than to the four-decade global warming. Results here provide a set of potential precursors for foreseeing ISO propagation under different mean states.

夏季热带印度洋季节内振荡的北向传播特征

利用高分辨率的卫星资料和再分析资料,详细地探究1985—2017年夏季(5—10月)热带印度洋季节内振荡(TISO)的北向传播过程和特征。结果表明,印度洋TISO的北向传播可以分为3类:稳定型,对流信号起源于赤道以南,印度洋西部,稳定地向北传播至印度半岛北部;衰减型,前期与稳定型相似,但向北传播至孟加拉湾附近后迅速衰减;增强型,前期在赤道附近信号较弱,大约10天后,对流信号从印度半岛南部开始显著增强,并发展至喜马拉雅山脉以南。TISO北向传播过程中伴随显著的东风切变异常、海表面温度异常和边界层水汽扰动,三者在不同类型的北向传播中起不同的作用。对流事件北侧的海温正异常会促进对流的北向传播,在稳定型和衰减型的赤道传播过程中都起到显著的作用,增强型传播过程中海温正异常在对流南侧更显著,会抑制对流的北向传播。东风垂直切变机制为稳定型和增强型的稳定北向传播提供持续的动力,在事件后期的影响更加显著。边界层水汽扰动的经向不对称性在稳定型事件前后、衰减型事件前期以及增强型事件发生时加剧大气的不稳定性,诱导对流系统向北移动。研究结果有助于提高东南亚夏季季节内降水预测的准确性。

热带夏季风场与对流场季节内振荡传播模比较

利用1979-2007年卫星观测日平均OLR资料以及NCEP/DOE第2套再分析资料中的风场资料,采用有限区域波一频分析、合成分析等方法,分析对比对流层高、低层风场与对流场所表征的热带北半球夏季季节内振荡(BSISO)各种传播模态谱分布气候特征及其年际异常。结果表明:各要素反映的BSISO各种模态的气候特征及其年际变化存在一定差异,总体而言对流层低层风(850hPa纬向风或经向风)与对流比较一致。850hPa经向风(纬向风)所反映的纬向(经向)传播BSISO谱分布气候特征与对流情况最相似。在ENSO发展年,850hPa经向风反映的赤道东传波加强趋势与对流较为一致;850hPa纬向风、经向风反映的北传波变化趋势都与对流相似。在ENSO衰减年,850hPa纬向风(经向风)反映的赤道东传波(赤道外西传波)减弱趋势与对流较为一致;对流以及850hPa经向风、200hPa纬向风和200hPa经向风4种要素都能体现南海及周边地区北传波明显减弱这一特征。对流和850hPa纬向风所反映的北传波与印度洋偶极子模态之间关系一致。

亚洲热带气候态夏季风涌传播特征及其对中国降水的影响

基于1979—2020年逐日的NOAA向外长波辐射资料、NCEP/NCAR再分析风场资料,以及全球CMAP再分析降水资料,探讨了气候态亚洲热带夏季风涌的传播过程及与我国夏季相应的降水联系。分析结果表明,主汛期亚洲热带气候态夏季风季节内振荡(CISO)活动是亚洲夏季风活动的主要特征,随时间北传的亚洲热带夏季风CISO称为亚洲热带夏季风涌,主要有南亚夏季风涌和南海夏季风涌。亚洲热带夏季风涌的传播可分为四个阶段。在亚洲热带夏季风涌的发展阶段,印度洋区域低频气旋与对流活跃,孟加拉湾和南海热带区域被低频东风控制,我国大部分地区无降水发生,降水中心位于两广地区。当进入亚洲热带夏季风涌活跃阶段,孟加拉湾和南海热带地区低频气旋和对流活跃,东亚低频“PJ”波列显著,我国降水中心北移到长江以南的附近区域。亚洲热带夏季风涌减弱阶段,孟加拉湾与南海低频气旋消亡,对流减弱,低频西风加强,日本南部附近为低频反气旋控制,我国长江中下游低频南风活跃,降水中心也北移到长江中下游地区,而华南地区已基本无降水,此阶段的大气低频环流场与亚洲热带夏季风涌发展阶段基本相反。进入亚洲热带夏季风涌间歇阶段时,孟加拉湾和南海热带地区低频反气旋活跃,对流不显著,日本南部附近的低频反气旋北移减弱,我国东部基本在低频南风的控制下,降水中心也逐步北移到华北-朝鲜半岛一带,此时的大气低频环流场与亚洲季风涌活跃阶段基本相反。

北半球夏季大气低频振荡演变特征及其与华北夏季降水的关系

本文采用1981~2010年夏季5~10月逐日的(10°S~50°N,40°E~160°E)范围内向外长波辐射OLR(Outgoing Longwave Radiation)资料和850 hPa层纬向风速资料(简称U850)作经验EOF(Empirical Orthogonal Function)分解,重新计算北半球夏季大气低频振荡BSISO(Boreal Summer Intraseasonal Oscillation)指数,并分析了其演变特征及其对华北夏季降水的影响规律。结果表明:(1)在北半球夏季印度洋—西北太平洋地区存在两种明显的低频信号,一种是BSISO1,空间分布呈西北—东南倾斜状,从热带印度洋向东北方向传播,振荡周期约为45 d;另一种是BSISO2,空间分布呈西南—东北倾斜状,从西北太平洋向西北方向传播,振荡周期约为20 d。(2)BSISO主要是通过影响大气环流和水汽输送来影响华北夏季降水过程。在500 hPa层,BSISO信号会造成华北地区东部副热带高压位置南北移动和强度发生变化来影响华北夏季降水;在850 hPa层,BSISO信号会通过伴随的气旋性或反气旋性异常环流影响向华北的水汽输送来影响华北夏季降水。(3)虽然热带大气季节内振荡MJO(Madden-Julian Oscillation)信号在全年都存在,但其变化在冬半年尤其冬季振幅最大,在夏季最小。BSISO信号变化在夏半年尤其夏季振幅最大。因此,利用热带大气低频信号开展延伸期降水过程预测,冬半年可以重点考虑MJO的影响,夏半年重点考虑BSISO的影响。

IOD对北半球夏季季节内振荡强度的可能影响

利用1979—1998年NCEP/NCAR再分析资料和GISST逐月海温资料,分析了印度洋偶极子(IOD)对季节内振荡强度的影响。结果表明:在非洲地区,IOD对季节内振荡强度存在显著影响,其对SLP季节内振荡强度的影响主要集中在非洲中部、西部和南部地区。在亚洲地区,IOD对季节内振荡强度也存在显著影响。在亚洲季风区的大部分地区,IOD与200 hPa纬向风(U)、高度场(H)和经向风(V)的季节内振荡强度都存在显著关联。IOD和ENSO对季节内振荡强度的作用差别显著,在有些地区甚至相反。进一步分析了1994年200 hPa上U、V的季节内振荡强度分布,并给出了1994年低频风场的传播特征。