SEA SURFACE TEMPERATURE RESPONSE TO TYPHOON MORAKOT(2009) AND ITS INFLUENCE

While previous studies indicate that typhoons can decrease sea surface temperature(SST) along their tracks, a few studies suggest that the cooling patterns in coastal areas are different from those in the open sea. However, little is known about how the induced cooling coupled with the complex ocean circulation in the coastal areas can affect tropical cyclone track and intensity. The sea surface responses to the land falling process of Typhoon Morakot(2009) are examined observationally and its influences on the activity of the typhoon are numerically simulated with the WRF model. The present study shows that the maximum SST cooling associated with Morakot occurred on the left-hand side of the typhoon track during its landfall. Numerical simulations show that, together with the SST gradients associated with the coastal upwelling and mesoscale oceanic vortices, the resulting SST cooling can cause significant difference in the typhoon track, comparable to the current 24-hour track forecasting error. It is strongly suggested that it is essential to include the non-uniform SST distribution in the coastal areas for further improvement in typhoon track forecast.     

SENSITIVITY OF SEA SURFACE TEMPERATURE RETRIEVAL TO SEA SURFACE EMISSIVITY

To estimate sea surface temperature(SST)with high accuracy from radiometrie measure-ments,it is no longer acceptable to assume that sea surface emissivity is unity or any other con-stant.This note presents an investigation of the desirable emissivity accuracy in relation to re-trieval.It was found that 1% error in surface emissivity can cause up to 0.7 K error in the re-trieved SST,although this sensitivity is often reduced to about 0.5 K on average because of thedownward atmospheric radiation at surface partially compensates for the emissivity error.Since thedownward atmospheric radiation ratio is controlled to a large extent by the integrated water vaporin the atmosphere and,secondarily,by view angle,the sensitivity of SST retrieval to surface emis-sivity has been computed as a function of these two parameters.     

不同海表面温度对南海台风“杜鹃”的影响试验

采用水平分辨率0.25 °×0.25 °的日平均和周平均的卫星微波成像仪(TMI)和卫星微波辐射计(AMSRE)的海温资料(TMI-AMSRE SST)作为下强迫源,利用中尺度数值模式MM5对南海过境台风"杜鹃"进行了模拟.试验结果表明:台风中心附近SST的差异会导致大气风场的差异,从而使模式对SST有比较快速而且明显的响应;不同的SST对台风的强度和路径都有一定的影响,而对台风降水和台风中心附近潜热通量有明显的影响;不同SST对台风的影响主要是通过改变海-气潜热通量来实现的.     

SPATIAL VARIATION OF WINTER SEA SURFACE TEMPERATURE IN NORTH PACIFIC AND ITS RELATION TO ATMOSPHERIC OSCILLATION MODES

The spatial variation of sea surface temperature anomalies(SSTA) in the North Pacific Ocean during winter is investigated using the EOF decomposition method.The first two main modes of SSTA are associated with Pacific Decadal Oscillation(PDO) mode and North Pacific Gyre Oscillation(NPGO) mode,respectively.Moreover,the first mode(PDO) is switched to the second mode(NPGO),a dominant mode after mid-1980.The mechanism of the modes’ transition is analyzed.As the two oceanic modes are forced by the Aleutian Low(AL) and North Pacific Oscillation(NPO) modes,the AR-1 model is further used to examine the possible effect and mechanism of AL and NPO in generating the PDO and NPGO.The results show that compared to the NPO,the AL plays a more important role in generating the NPGO mode since the 1970s.Likewise,both the AL and NPO affect the PDO mode since the 1980s.     

A NUMERICAL STUDY OF THE INFLUENCE OF SEA SURFACE TEMPERATURES WITH DIFFERENT TEMPORAL RESOLUTIONS ON TYPHOON DUJUAN OVER THE SOUTH CHINA SEA

Daily and weekly sea surface temperature data of Tropical Rainfall Measuring Mission (TRMM) Microwave Imager and Advanced Microwave Scanning Radiometer-Earth Observing System sensors are used as forcing of the underlying sea surface in the mesoscale numerical model to simulate Typhoon Dujuan that moved across the South China Sea in 2003. The numerical results show that different SSTs near the typhoon center result in differences in the atmospheric wind field, indicating that the model has a fast and obvious response to SSTs. Different SST influences the intensity and track of Dujuan to some degree and has significant impacts on its precipitation and latent heat flux near the eye. The SST influence on Dujuan is mainly fulfilled by changing the latent heat flux between the ocean surface and the atmosphere above.     

MECHANISM FOR THE SUSTAINING ATMOSPHERIC RESPONSE TO WARM WINTERTIME SEA SURFACE TEMPERATURE ANOMALY IN THE KUROSHIO EXTENSION

Sensitivity experiments with atmospheric general circulation model CAM3 have been performed to investigate the atmospheric response to warm wintertime sea surface temperature anomalies in the Kuroshio Extension (KE). Mechanism for the sustaining abnormal atmospheric response to sea surface temperature anomaly (SSTA) is revealed. It is found that the warm wintertime SSTA in KE leads to soil moisture changes across the Asia continent. The abnormal soil moisture may possibly be one of the reasons for the sustaining of abnormal atmospheric response intrigued by SSTA. Oscillations of perturbations intrigued by warm wintertime SSTA in KE, which have similar frequencies with that of intrinsic atmospheric oscillations, are superposed on the atmospheric oscillations and propagate with primary periodic oscillation of the atmosphere. These SSTA-intrigued oscillations are coupled with natural atmospheric oscillation and finally become parts of it. This is probably another reason for the sustaining of abnormal atmospheric response to SSTA in KE.     

PRELIMINARY ANALYSIS ON THE INTENSITY AND STRUCTURE OF TYPHOON MORAKOT (2009) DURING ITS LANDING PROCESS

NCEP GFS(Global Forecast System)analytical data(available 4 times per day),satellite cloud image data and real-time observations of path and intensity of Typhoon Morakot are employed to investigate the variation of synoptic dynamics in its intensity and structure before and after the landing.This study intends to offer some hints for the forecast of intensity and structure of typhoons.Results show that in the tangential direction,the averaged asymmetry amplitude of wind on the radius of a large-value center of the low-level wind can be used as an important parameter for diagnosing the intensity of typhoons.Besides,the maximum of the upper dry potential vorticity in Morakot’s center tends to extend downward along the intensive gradient of tangential wind situated on the inner side of a large-value center of the low-level tangential wind.Additionally,the radial advection of the tangential wind determines the variation of tangential wind in conjunction with the vertical transmission of the tangential wind,the inertial centrifugal force and the Coriolis force.These four items are dominant in the motion equation of tangential wind based on a cylindrical coordinate without the effects of friction and turbulence.Moreover,the low-level convergence center of the typhoon has a tendency of shifting and developing along the intensive gradient of the tangential wind in the tangential direction.     

北极海冰与赤道东太平洋海温的相互影响及其与El Nino的联系

对北极海冰面积与赤道东太平洋海温作了交叉相关分析，揭示了北极海冰与赤道东太平洋海温间相互影响的关系及其时空特点，并讨论了Ｅｌ Ｎｉｎｏ与北极海冰间的联系。结果表明，北极Ⅴ－Ⅵ区冬季海冰面积能够对以后冬季的赤道东太平洋海温产生持续的影响，而北极Ⅶ区夏秋季的海冰面积能够影响来年夏秋季的海温；赤道东太平洋海温对海冰的影响表现为前期海温能够影响北极Ｉ区春季海冰的消融。此外，前期北极海冰状况对Ｅｌ Ｎｉｎｏ     

SENSITIVITY OF PRECIPITATION TO SEA SURFACE TEMPERATURE AND ITS DIURNAL VARIATION: A PARTITIONING ANALYSIS BASED ON SURFACE RAINFALL BUDGET

The sensitivity of precipitation to sea surface temperature(SST) and its diurnal variation is investigated through a rainfall partitioning analysis of two-dimensional cloud-resolving model experiments based on surface rainfall budget.For all experiments,the model is set up using zero vertical velocity and a constant zonal wind and is integrated over 40 days to reach quasi-equilibrium states.The 10-day equilibrium grid-scale simulation data and a time-invariant SST of 29°C are used in the control experiment.In the sensitivity experiments,time-invariant SSTs are 27°C and 31°C with an average value of 29°C when the minimum and maximum values of diurnal SST differences are 1°C and 2°C,respectively.The results show that the largest contribution to total rainfall is from the rainfall with water vapor convergence and local atmospheric drying and hydrometeor gain/divergence(~30%) in all experiments.When SST increases from 27°C to 29°C,the contribution from water vapor convergence decreases.The increase of SST reduces the contribution of the rainfall with water vapor convergence primarily through the decreased contribution of the rainfall with local atmospheric drying and hydrometeor gain/divergence and the rainfall with local atmospheric moistening and hydrometeor loss/convergence.The inclusion of diurnal variation of SST with the diurnal difference of 1°C decreases the rainfall contribution from water vapor convergence primarily through the decreased contribution of the rainfall with local atmospheric moistening and hydrometeor loss/convergence.The contribution of the rainfall from water vapor convergence is barely changed as the diurnal difference of SST increases from 1°C to 2°C.     

VARIATIONS OF SEA SURFACE TEMPERATURE AND SEA LEVEL PRESSURE OVER PACIFIC AND INDIAN OCEANS AND THEIR INFLUENCE ON COLD SUMMER IN EAST ASIA

An EOF analysis was performed to investigate the variations of sea surface temperature (SST) of Pacific and Indian Oceans.Result shows that the distribution of SST anomaly exhibits a distinct anticorrelation pattern between Northwest and Southeast Pacific,as well as between Northwest Pacific and the Arabian Sea.It also shows that the sea level pressure (SLP) anomaly between North Pacific and North Indian Oceans is of a seesaw pattern,which we named the North Paci-fic and North Indian Ocean Oscillation (PIO).Such a phenomenon is closely correlated with the cold summer in East Asia.     

RESPONSES OF SUMMER RAINFALL OVER CHINA TO ANOMALIES OF SEA SURFACE TEMPERATURE IN THE NORTH PACIFIC

The temporal and spatial responses of summer(June—August)rainfall (SR) over China to anomalies of sum-mer sea surface temperature (SST) in the North Pacific is investigated by using regression analysis and prin-cipal component analysis in this paper.The results show that the responses of SR over China to SST anomalyin the eastern North Pacific in middle latitudes are more sensitive than the others.The strong response areasof SR to SST are generally located in the Changjiang River valley,west:rn North of China,and South China.The spatial pattern of responses of SR to SST manifests in the spatial pattern of the first principal componentnamely,the domain shape with negative-positive-negative anomalies along south-north direction,thus the signof anomalies in the Changjiang River valley is contrary to that of its North or South.     

TROPICAL SEA SURFACE TEMPERATURE ANOMALY AND INDIAN SUMMER MONSOON

The time series of the sea surface temperature(SST)anomaly,covering the eastern(western)equatorial Pacific,central Indian Ocean,Arabian Sea.Bay of Bengal and South ChinaSea(SCS),have been analyzed by using wavelet transform.Results show that there exists sameinterdeeadal variability of SST in the tropical Pacific and tropical Indian Ocean,and also show thatthe last decadal abrupt change occurred in the 1970s.On the interannual time scale,there is asimilar interannual variability among the equatorial central Indian Ocean and the adjacent three seabasins(Arabian Sea.Bay of Bengal and South China Sea).but the SST interannual changes of theIndian Ocean lagged 4—5 months behind that of the equatorial central-east Pacific.Meanwhile,the interannual variability and long-range change between SST anomaly and Indian summermonsoon rainfall in recent decades have been explained and analyzed.It indicates that there existeda wet(dry)period in India when the tropical SST was lower(higher)than normal,but there wasa lag of phase between them.     

影响我国热带气旋活动的气候特征及其与太平洋海温的关系

利用1956~2000年的热带气旋(简称TC,下同)资料对影响我国TC活动的气候特征进行了初步的统计分析,结果发现影响我国的TC活动具有明显的阶段性特征,1960年代影响我国的TC数明显偏少,而后进入偏多期,1990年代又相对偏少。影响我国的TC强度多集中于980~999 hPa,华东的闽、浙一带TC登陆比华南晚,但强度较大。在此基础上通过对影响我国的TC年个数与太平洋海温场进行相关分析,发现两个相关较密切的区域: 西太平洋暖池(120~150 E, 10~20 N)正相关区、赤道中东太平洋(180 ~90 W, 10 S~5 N)负相关区,这两个相关区具有较好的持续性。进一步分析影响我国的TC在El Ni駉年与La Ni馻年的气候特征发现,El Ni駉年影响我国的TC数较少,但强度较大,La Ni馻年则相反,影响我国TC多年和少年对应的太平洋海温距平分布形势分别与La Nia年和El Nio年的海温距平分布形势类似。     

太平洋-印度洋海温异常模态及其指数定义的初步研究

通过对太平洋地区海温场的经验正交函数分解,分析其第一特征向量场发现,春夏秋冬四季在印度洋中西部大范围地区、赤道中东太平洋的大范围地区的海面温度西太平洋地区的相反,这一特征向量在四个季节的方差贡献率均大于33%,说明这一海温场的空间分布结构是稳定的,我们称之为太平洋-印度洋海温异常模态.并对这种模态经过认真分析比较和研究后定义了该模态的指数.     

EFFECTS OF SEA SURFACE TEMPERATURE AND ITS DIURNAL VARIATION ON DIURNAL VARIATION OF RAINFALL: A PARTITIONING ANALYSIS BASED ON SURFACE RAINFALL BUDGET

The effects of sea surface temperature(SST) and its diurnal variation on diurnal variation of rainfall are examined in this study by analyzing a series of equilibrium cloud-resolving model experiments which are imposed with zero large-scale vertical velocity.The grid rainfall simulation data are categorized into eight rainfall types based on rainfall processes including water vapor convergence/divergence,local atmospheric drying/moistening,and hydrometeor loss/convergence or gain/divergence.The rainfall contributions of the rainfall types with water vapor convergence are insensitive to the increase in SST from 27°C to 29°C during the nighttime,whereas they are decreased during the daytime.The rainfall contributions of the rainfall types with water vapor convergence are decreased as the SST increases from 29°C to 31°C but the decreases are larger during the nighttime than during the daytime.The rainfall contributions of the rainfall types with water vapor convergence are decreased by the inclusion of diurnal variation of SST with diurnal difference of 1°C during the nighttime,but the decreases are significantly slowed down as the diurnal difference of SST increases from 1°C to 2°C.The rainfall contributions of the rainfall types with water vapor convergence are insensitive to the inclusion of diurnal variation of SST during the daytime.     

ON THE RELATIONSHIPS BETWEEN THE UNUSUAL TRACK OF TYPHOON MORAKOT (0908) AND THE UPPER WESTERLY TROUGH

In this paper,by carrying out sensitivity tests of initial conditions and diagnostic analysis of physical fields,the impact factors and the physical mechanism of the unusual track of Morakot in the Taiwan Strait are discussed and examined based on the potential vorticity(PV)inversion.The diagnostic results of NCEP data showed that Morakot’s track was mainly steered by the subtropical high.The breaking of a high-pressure zone was the main cause for the northward turn of Morakot.A sensitivity test of initial conditions showed that the existence of upper-level trough was the leading factor for the breaking of the high-pressure zone.When the intensity was strengthened of the upper-level trough at initial time,the high-pressure zone would break ahead of time,leading to the early northward turn of Morakot.Conversely,when the intensity was weakened,the breaking of the high-pressure zone would be delayed.Especially,when the intensity was weakened to a certain extent,the high-pressure zone would not break.The typhoon,steered by the easterly flow to the south of the high-pressure zone,would keep moving westward,with no turn in the test.The diagnostic analysis of the physical fields based on the sensitivity test revealed that positive vorticity advection and cold advection associated with the upper-level trough weakened the intensity of the high-pressure zone.The upper-level trough affected typhoon’s track indirectly by influencing the high-pressure zone.     

中国西北春季降水与太平洋海温相关特征的研究

应用奇异值分解对中国西北3-5月的降水资料与太平洋海温进行了相关分析。结果表明：北太平洋与中国西北春季降水有较好的相关,在太平洋海温3-5月呈El Nino峰值位相时,除青海高原外,中国西北地区降水偏水;而当12-2月的海温呈El Nino成熟位相型时,次年西北地区3-5月的降水偏多;而当北太平洋海温12-2月呈La Nina型时,次年青海高原区3-5月的降水偏少。太平洋海温的西风漂流区、黑潮区、加利福尼亚海流区和北赤道海流区的海温的变化与中国西北地区3-5月的降水之间有明显的相关关系。其中黑潮区12-2月的海温与新疆北部的降水呈反相变化,即当黑潮区的海温低时,西北地区的新疆北部降水偏多。中国西北地区3-5月的降水对太平洋温变化的响应区主要的新疆北部、青海高原及其东北侧、沙漠盆地、河西西部等地。     

最优决策法在台风路径集成预报中的运用

关于台风路径的预报方法很多，但没有一种方法预报的准确性占绝对优势，为此提出了一种集成方法——最优决策法，将几种性能较好的预报方法进行集成。经初步预报试验，效果较好，其预报误差均比其它方法小。     

海温及其变化对南海夏季风爆发的影响

文中利用 1 5a(1 982～ 1 996)的 NOAA射出长波辐射 (OLR)、NCEP/NCAR的风场和海表温度 (SST)再分析网格点资料研究了南海、太平洋和印度洋海温及其变化对南海夏季风爆发的影响。首先发现爆发时南海区域平均的海表温度高于 2 9℃。季风爆发的时间与南海南部SST年循环最高值出现的时间基本一致。冬春季海表增暖是环流场突变的基础。SST超前于低层西风和对流的增强而升高 ,从而造成季节性的大气条件不稳定增大。通过暖池移动过程 ,考察了南海夏季风爆发期间 SST场、风场和 OLR场的演变特征。季风环流的变化是对海表增温强迫的响应。最大暖水轴在 1 0°N出现有利于 ITCZ在南海建立。南海 -西太平洋增温时 ,具有很强的纬向不均匀性 ,而印度洋则比较均匀。南海深对流的爆发与 SST纬向梯度有关。南海夏季风爆发的年际变化与 SST异常有关系。季风爆发偏晚年和偏早年冬春季 SST正、负距平区的符号相反。偏晚年的 SSTA分布呈 El Nino型 ,偏早年的 SSTA分布如同 L aNina型。不同类型的 SST异常对季风环流的影响不同。在 El Nino型强迫下 ,西太平洋副热带高压比常年偏南、偏西 ,东风在南海维持的时间较长 ,赤道西风出现的时间晚 ,南海地区对流活动受到抑制 ,故南海季风爆发偏晚。反之 ,爆发偏早     

TROPICAL SEA SURFACE TEMPERATURE ANOMALY AND INDIAN SUMMER MONSOON*

The time series of the sea surface temperature(SST) anomaly,covering the eastern (western) equatorial Pacific,central Indian Ocean,Arabian Sea.Bay of Bengal and South China Sea(SCS),have been analyzed by using wavelet transform.Results show that there exists same interdeeadal variability of SST in the tropical Pacific and tropical Indian Ocean,and also show that the last decadal abrupt change occurred in the 1970s.On the interannual time scale,there is a similar interannual variability among the equatorial central Indian Ocean and the adjacent three sea basins(Arabian Sea.Bay of Bengal and South China Sea).but the SST interannual changes of the Indian Ocean lagged 4-5 months behind that of the equatorial central-east Pacific.Meanwhile,the interannual variability and long-range change between SST anomaly and Indian summer monsoon rainfall in recent decades have been explained and analyzed.It indicates that there existed a wet(dry) period in India when the tropical SST was lower(higher)than normal,but there was a lag of phase between them.