Modulation of Western North Pacific Tropical Cyclone Genesis by Intraseasonal Oscillation of the ITCZ: A Statistical Analysis

The present study investigates modulation of western North Pacific(WNP) tropical cyclone(TC) genesis in relation to different phases of the intraseasonal oscillation(ISO) of ITCZ convection during May to October in the period 1979-2008.The phases of the ITCZ ISO were determined based on 30-80-day filtered OLR anomalies averaged over the region(5-20 N,120-150 E).The number of TCs during the active phases was nearly three times more than during the inactive phases.The active(inactive) phases of ISO were characterized by low-level cyclonic(anticyclonic) circulation anomalies,higher(lower) midlevel relative humidity anomalies,and larger(smaller) vertical gradient anomalies of relative vorticity associated with enhanced(weakened) ITCZ convection anomalies.During the active phases,TCs tended to form in the center of the ITCZ region.Barotropic conversion from the low-level mean flow is suggested to be the major energy source for TC formation.The energy conversion mainly depended on the zonal and meridional gradients of the zonal flow during the active phases.However,barotropic conversion weakened greatly during the inactive phases.The relationship between the meridional gradient of absolute vorticity and low-level zonal flow indicates that the sign of the absolute vorticity gradient tends to be reversed during the two phases,whereas the same sign between zonal flow and the absolute vorticity gradient is more easily satisfied in the active phases.Thus,the barotropic instability of low-level zonal flow might be an important mechanism for TC formation over the WNP during the active phases of ISO.     

一个大羊环流模式中赤道西太平洋暖池海面温度对西风爆发的响应

用逐日的欧洲中期数值预报中心再分析(ERA)风应力,和由Haney公式结合ERA海表资料与预报海温计算出的热通量强迫一个全球大洋环流模式.并用逐日的模拟结果与TOGA-COARE(Tropical Ocean--Global Atmosphere--Coupled Ocean-AtmosphereResponse Experiment)浮标观测资料对比,分析模拟结果中暖池海区上层海洋热量平衡对西风爆发(WWB)的响应.在第一次WWB过程中,模拟与观测的主要差异在WWB期间,而造成差异的原因主要是模式中由下沉运动引起的增温和由强的纬向温度梯度引起的暖平流.初步认为下沉增温可能是差分格式本身和模式分辨率不足造成的.从热量平衡的结果看,第二次WWB事件的模拟比第一次更成功,两次差异可能与两次WWB事件的季节背景不同有关.     

亚洲季风区ITCZ对华北雨季异常的可能影响

分析了冬、春季亚洲季风区ITCZ活动对华北夏季降水的影响。用OLR资料定义了ITCZ综合指数，分析表明该指数能比较清晰、客观地综合反映出亚洲季风区深对流活动的强度和位置。用该指数分析了华北多雨年和少雨年前期热带大范围对流活动的变化特征。印度和东亚季风区的ITCZ活动在多雨和少雨年显示出明显不同。华北夏季降水与2-4月ITCZ综合指数的时滞相关性最明显。这种时滞相关可能与球面Rossby波的频散相联系，并可以为华北夏季降水的长期预报提供新的参考依据。低频振荡分析表明，多雨年南北两个半球的低频振荡强度较强，范围较大，并不断由南向北传播，这种传播很可能与副热带高压的北跳有关。而少雨年低频振荡在南半球或热带地区呈准静止状态。     

西北太平洋TC高频源地与GMS-SST暖水区及ITCZ的匹配关系

利用GMS高精度遥感海表温度(SST)反演资料、外逸长波辐射(OLR)反演资料和西北太平洋热带气旋(TC)源地及频数数据,分析了GMS-SST、赤道辐合带(ITCZ)、TC源地及频数的时空分布特征。研究发现:TC源地、发生频数以及强度的时空分布和变化均具有趋暖性。GMS-SST大于等于28℃的阈值条件为西北太平洋TC发生和维持的必要条件。并且存在10°N高频收缩轴,其与由GMS-SST大于等于28℃所定义的西北太平洋暖水区,GMS-OLR小于等于240 W.m-2定义的ITCZ的时空分布及变化有很好的相关性,即存在TC高频源地—GMS-SST暖水区—ITCZ三者间的匹配相关。     

北太平洋副热带逆流的气候特征

分析了从1992年10月到1996年10月的TOPEX/POSEIDON卫星高度计和POCM模式输出的海平面高度（SSH）资料、Levitus气候平均温盐资料、NCEP/NCAR再分析纬向流资料以及沿137oE剖面的海流资料。分析结果表明,在年平均和月平均意义下,向东的副热带逆流（STCC）位于18oN～25oN、130oE～157oW间150 m以内的海洋上层,其强度大约为0.02～0.04 m/s,在西太平洋夏季（7月）最强,冬季（1月）最弱。在STCC以下650 m以上,还存在一支向西的流,流速也约为0.02～0.04 m/s。在STCC区域内,SSH有明显的准90天振荡。在天气时间尺度上,一些气旋和反气旋涡出现在STCC海域,使该海域的流在方向和强度上变化很大。     

热带西太平洋风应力异常在ENSO循环中作用的数值试验

利用一个中等复杂程度的海气耦合模式来研究热带西太平洋地区风应力异常对ENSO循环的影响,并以1997/1998年的El Nino事件为例,分析了模式结果中热带西太平洋地区风应力异常对El Nino事件形成影响的动力学及热力学作用.结果表明,热带西太平洋地区的风应力异常对ENSO循环有很重要的作用.在耦合模式中,当热带西太平洋地区的风应力异常由观测给定时,耦合模式基本上可以模拟出自1971年以来的EI Nino事件,观测与模拟的Nino 3区海面温度异常(SSTA)的相关系数可达0.63.赤道西太平洋地区的风应力异常可加强该地区大气的辐合,从而加强了大气的加热场,进而加强了海气相互作用的不稳定.赤道西太平洋地区西风异常激发出来的Kelvin波及水平平流对EI Nino事件初始阶段的发展起重要作用;海气相互作用及东边界的反射对EI Nino事件的发展及维持起重要作用.     

异常东亚冬季风对赤道西太平洋纬向风异常的影响

ENSO由于经常带来大范围的气候异常而倍受关注,赤道西太平洋的纬向风异常对ENSO的发生起着重要作用.首先对赤道西太平洋纬向风异常的发生进行研究,其结果清楚地表明它同东亚冬季风的异常有密切关系.异常强(弱)的东亚冬季风不仅能激发产生赤道西太平洋的西(东)风异常,而且还将在菲律宾以东的西太平洋上导致一个气旋性(反气旋性)环流的形成.研究还表明,上述异常流型产生的物理过程是异常东亚冬季风所造成的气压形势的动力影响,因为在强(弱)东亚冬季风影响下,赤道西太平洋地区会形成向西(东)的气压梯度,即( )p/( )x＜0(( )p/( )x＞0)的异常气压形势.通过赤道太平洋的海-气相互作用,赤道西太平洋的持续西(东)风异常则将进一步导致El Nino(La Ni(n)a)事件的发生.     

中东太平洋ITCZ对两类厄尔尼诺SST异常的敏感性试验

利用MPAS-A(The Model for Prediction Across Scales-Atmosphere)模式设计了中东太平洋热带辐合带CEP-ITCZ(Intertropical Convergence Zone over Central and Eastern Pacific)对两类厄尔尼诺SST(Sea Surface Temperature)异常的敏感性试验,通过试验结果与两类厄尔尼诺年实际大气异常的对比,初步解释了CEP-ITCZ在两类厄尔尼诺年产生不同异常的可能原因。通过CP-EL试验发现,热带太平洋SST异常的第一模态会使中东太平洋低层风场辐合增强,但对辐合带的位置影响不大,与中部型厄尔尼诺对CEP-ITCZ的影响基本一致。通过EP-EL试验发现,热带太平洋SST异常的第二模态会使中东太平洋低层风场产生较大异常,辐合带中心向南移动,辐合带明显减弱增宽,与东部型厄尔尼诺对CEP-ITCZ的影响基本一致。     

Response of the Subtropical Gyre Circulation in the North Pacific Ocean to CO2 Quadrupling

ABSTRACT

This study investigates the response of the subtropical gyre circulation in the North Pacific Ocean to quadrupled CO₂ using the Community Earth System Model, version 1 (CESM1). In particular, an overriding technique is applied to isolate and quantify the effects of wind stress and thermal warming caused by CO₂ emissions. Results show that, in response to the increase in CO₂, the total mass transport in the subtropical gyre is reduced by approximately 11%. This reduction results mainly from negative anomalies of the wind stress curl over the subtropical region, with a smaller contribution from the thermal warming effect. Furthermore, a detailed analysis finds that the change in the subtropical gyre is baroclinic in nature [i.e., the gyre appears to be spin-up in the upper ocean (above 300?m) but spin-down in the lower thermocline (from 300 to 1500?m)]. This reversal between the upper ocean and lower thermocline is a result of the thermal warming effect, which intensifies ocean stratification, hindering the transfer of momentum from the upper layers to the lower layers and leading to an acceleration of the gyre in the upper ocean but a deceleration in the lower thermocline. Another feature of the response of the subtropical gyre to quadrupled CO₂ is the respective poleward and equatorward movements of its northern and southern boundaries, which is a result of the change in the zero wind stress curl lines.     

An Empirical Tropical Instability Wave-induced Wind Stress Model in the Equatorial Pacific and its Incorporation into the Ocean Model

Abstract

Satellite observations revealed that there is a close relationship between perturbations of sea surface temperature (SST) and wind stress (τ) induced by tropical instability waves (TIWs; SST_TIW and τ _TIW). Using the empirical relationship observed between TIW-induced wind stress divergence (curl) and downwind (crosswind) SST gradients, this study establishes a TIW-induced wind stress field perturbation model τ _TIW?=?F(SST). This empirical model solves τ _TIW from the TIW-induced wind stress divergence and curl, which are estimated from the downwind and crosswind SST gradients. This empirical τ _TIW?=?F(SST) model can be incorporated into the ocean model to take into account the effect of τ _TIW. By comparing two experiments with and without the τ _TIW effect, this study demonstrates that τ _TIW has a substantial effect on the equatorial Pacific heat budget and induces the long-term mean SST to exhibit a 0.2°C difference, which is consistent with previous studies.     

北太平洋副热带海区的两支东向逆流

分析SODA同化资料(1950～1999年)所描述的北太平洋副热带海区表层一次表层的东向逆流的空间分布和季节变化特征.从气候平均场看,北太平洋副热带的东向流主要包括东、西两部分,其中,太平洋中部夏威夷岛西侧的东支逆流位置偏南,强度夏季最大,春季最弱.海表风应力的旋度异常产生的Ekman抽吸是东支逆流形成的主要原因.位于西太平洋的西支逆流位置偏北,春季一夏季强度较大.西支逆流的形成和季节变化可以由副热带模态水的辐合解释.     

IAP/LASG海洋环流模式对风应力的响应

为了检验第三代IAP/LASG全球海洋环流模式在模拟20世纪80年代热带太平洋年际变化方面特别是ElNino和LaNina事件的能力,作者使用了三种风应力资料强迫该模式.然后,将模拟的海温同NCEP海洋同化资料相比较.分析表明,在赤道附近,模式对风应力的响应在SST分布、温跃层的描绘以及突出E1Nino和LaNina事件方面同NCEP海洋同化资料有许多共同点.然而,在描绘l0°N以北和l0°S以南的SST异常以及描写季节循环方面,响应是不够精确的.另外,模拟的温度异常滞后于NCEP海洋同化数据两个月左右.     

A Reconstructed Wind Stress Dataset for Climate Research over the Tropical Pacific during a 153-Year Period

There are close relationships between the sea surface temperature (SST) and the surface wind over the tropical Pacific.To study the past climate variability over the tropical Pacific,the long-term monthly wind stress anomalies over the tropical Pacific for the period of 1856–2008 are reconstructed with an SVD (singular value decomposition)-based statistical atmospheric model,where the wind stress anomalies are slave and directly correspond to the SST anomalies.The verification results show that the reconstructed wind stress data have high correlations and a small root mean square (RMS) error with the three reanalysis/simulated surface wind datasets from the last 50 years.In addition,the simulated SST anomalies from an intermediate oceanic model (IOM),which is forced by the reconstructed wind stress,can simulate the realistic interannual and decadal variability of the ENSO (El Nio-Southern Oscillation);this indicates that this new long-term wind stress dataset is useful for various climate studies,especially for the large-scale interannual and decadal variability.     

Remote effects of tropical cyclone wind forcing over the western Pacific on the eastern equatorial ocean

An ocean general circulation model （OGCM） is used to demonstrate remote effects of tropical cyclone wind （TCW） forcing in the tropical Pacific. The signature of TCW forcing is explicitly extracted using a locally weighted quadratic least=squares regression （called as LOESS） method from six-hour satellite surface wind data; the extracted TCW component can then be additionally taken into account or not in ocean modeling, allowing isolation of its effects on the ocean in a clean and clear way. In this paper, seasonally varying TCW fields in year 2008 are extracted from satellite data which are prescribed as a repeated annual cycle over the western Pacific regions off the equator （poleward of 10°N/S）; two long-term OGCM experiments are performed and compared, one with the TCW forcing part included additionally and the other not. Large, persistent thermal perturbations （cooling in the mixed layer （ML） and warming in the thermocline） are induced locally in the western tropical Pacific, which are seen to spread with the mean ocean circulation pathways around the tropical basin. In particular, a remote ocean response emerges in the eastern equatorial Pacific to the prescribed off-equatorial TCW forcing, characterized by a cooling in the mixed layer and a warming in the thermocline. Heat budget analyses indicate that the vertical mixing is a dominant process responsible for the SST cooling in the eastern equatorial Pacific. Further studies are clearly needed to demonstrate the significance of these results in a coupled ocean-atmosphere modeling context.     

赤道波动转换在西北太平洋热带气旋生成中的作用

This study associates tropical cyclone (TC) activity over the western North Pacific (WNP) with the equatorial wave transition from an interannual viewpoint, revealing that the tropical cyclogenesis mean location may be modulated by a longitudinal shift in the transition of Mixed Rossby-gravity (MRG) waves to off-equatorial tropical depression (TD) disturbances from year to year. To a large extent, the wave transition is attributable to the monsoon trough in response to the thermal state of the warm pool (WP) over the WNP. During the cold state years in the WP, the basic flow confluence region associated with the monsoon trough penetrates eastward, leading to an eastward shift in the location of the wave transition. Such an environment, in which wave accumulation and energy conversion occur, is favorable for tropical cyclogenesis; as a result, the averaged cyclogenesis location moves eastward. The condition is reserved during the warm years in the WP, resulting in the prominent westward-retreating mean TC formation. Citation: Chen, G. H., and R. H. Huang, 2008: Role of equatorial wave transitions in tropical cyclogenesis over the western north Pacific, Atmos. Oceanic Sci. Lett., 1, 64-68     

Impacts of an improved low-level cloud scheme on the eastern Pacific ITCZ-cold tongue complex

A statistically-based low-level cloud parameterization scheme is introduced, modified, and applied in the Flexible coupled General Circulation Model (FGCM-0). It is found that the low-level cloud scheme makes improved simulations of low-level cloud fractions and net surface shortwave radiation fluxes in the subtropical eastern oceans off western coasts in the model. Accompanying the improvement in the net surface shortwave radiation fluxes, the simulated distribution of SSTs is more reasonably asymmetrical about the equator in the tropical eastern Pacific, which suppresses, to some extent, the development of the double ITCZ in the model. Warm SST biases in the ITCZ north of the equator are more realistically reduced, too. But the equatorial cold tongue is strengthened and extends further westward, which reduces the precipitation rate in the western equatorial Pacific but increases it in the ITCZ north of the equator in the far eastern Pacific. It is demonstrated that the low-level cloud-radiation feedback would enhance the cooperative feedback between the equatorial cold tongue and the ITCZ. Based on surface layer heat budget analyses, it is demonstrated that the reduction of SSTs is attributed to both the thermodynamic cooling process modified by the increase of cloud fractions and the oceanic dynamical cooling processes associated with the strengthened surface wind in the eastern equatorial Pacific, but it is mainly attributed to oceanic dynamical cooling processes associated with the strengthening of surface wind in the central and western equatorial Pacific.     

QuikSCAT和ASCAT卫星反演风场在中国南海北部的适用性研究

对QuikSCAT和ASCAT原始轨道10 m反演风场与浮标资料在中国南海北部的统计检验分析结果表明:两套卫星资料在中国南海北部具有较好的适用性,QuickSCAT反演风速偏高0.46 m·s~(-1),ASCAT反演风速在近海偏高0.45 m·s~(-1),在开阔海域偏高0.07 m·s~(-1)。超过半数的QuickSCAT反演风向误差30°。在近岸海域,ASCAT反演风向误差30°的超过56%,在开阔海域,误差绝对值30°的达到64%。小风时卫星反演风速偏大,大风时卫星反演风速明显偏小,且白天的偏差大于夜间;在5~10 m·s~(-1)风速条件下,两者的一致性较好。用WRF模式模拟的近海风能资源存在高估的可能,卫星资料对近海风能资源评估是个有益的补充,本文对卫星反演风场误差的分析结果也可以为卫星反演风场的资料同化提供参考。     

冬季北太平洋优势气候模态的转移

采用EOF分解和小波分析,并引入相似度,分析冬季北太平洋的两个主要气候模态,即太平洋年代际振荡（PDO）模态与北太平洋涡旋振荡（NPGO）模态,及其结构特征随时间的演变。结果表明：1988/1989年的气候转移后,冬季海温距平（SSTA）优势模态为NPGO模态的年份越来越多,这种优势气候模态的转移现象表现出准18 a的年代际周期;SSTA与NPGO模态相似度的20年滑动平均在20世纪80年代中后期之后超过了SSTA与PDO模态的相应值,这表明此后SSTA的优势模态发生了转移,由PDO型转换为NPGO型;对典型时间段SSTA的合成分析显示,其优势模态由1988年前的PDO型转变为之后的NPGO型。     

赤道东太平洋海温正异常对西太平洋副高影响的数值模拟研究

利用LASG九层大气环流谱模式及IAP两层大气环流模式，模拟研究了不同持续时间的赤道东太平洋海表温度正异常(海表温度异常的持续时间分别为1月份，1～2月份，1～4月份及1～8月份，其他月份为气候SST)对西太平洋副高的影响。结果表明，尽管海表温度异常的持续时间不同，但其引起的西太平洋副高的异常演变及其分布却十分相似；同时，季风区的异常降水(进而异常潜热释放)随时间的演变及其分布也存在一定的相似性(对应于不同持续时间的赤道东太平洋的海表温度正异常，5月份印度洋至西太平洋地区都表现出赤道辐合带北移偏晚的特征)；季风区降水的这种变化同西太平副高的异常是一致的，从而揭示出这两种现象有可能存在着某种联系。结果还表明，导致这种大气响应场对赤道东太平洋海表温度异常持续时间不敏感的一个重要原因是大气内部过程的影响:中纬大气的内部Rossby波源维持了热带地区激发的扰动在中高纬的存在，同时大气内部Rossby波源对赤道太平洋地区的海表温度异常持续时间表现出不敏感性，正是由于这种不敏感性才导致了响应场对赤道太平洋地区海表温度异常持续时间的不敏感性。模拟结果还表明，在夏季赤道东太平洋存在海表温度正异常的情况，尽管大气内部动力过程的作用十分重要，但夏季赤道东太平洋海表温度正异常对夏季西太平洋副高的影响却明显存在，因此，基于赤道太平洋地区海表温度异常的夏季西太平洋副高的可预报性受到赤道东太平洋海表温度正异常及大气内部动力过程的双重影响。模式的依赖性研究表明，模拟结果具有一定的普遍性。     

Assessing the Influence of the ENSO on Tropical Cyclone Prevailing Tracks in the Western North Pacific

Using a statistical model for simulating tropical cyclone (TC) formation and a trajectory model for simulating TC tracks, the influence of the El Nino-Southern Oscillation (ENSO) on the peak-season (July-September) TC prevailing tracks in the western North Pacific basin is assessed based on 14 selected El Nino and 14 selected La Nina years during the period 1950-2007. It is found that the combination of statistical formation model and a trajectory model can simulate well the primary features of TC prevai...