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1.
The paper examines different impacts of eastern Pacific warm/cold (EPW/EPC) and central Pacific warm/cold (CPW/CPC) events on tropical cyclones (TCs) in the western North Pacific (WNP) by considering the early season of April–June (AMJ), the peak season of July–September (JAS) and the late season of October–December (OND). During AMJ, EPW (EPC) is associated with a significant increase of the TC genesis number in the southeastern (southwestern) sub-region of the WNP, but no class of El Niño-Southern Oscillation (ENSO) events shows a significant change in the TC lifetime and intensity. During JAS, EPW corresponds to an increase (decrease) of the TC genesis number in the southeastern (northwestern) sub-region, but CPW shows no significant change. EPC increases the TC genesis in the northwestern and northeastern sub-regions and decreases the genesis in the southwestern sub-region, whereas CPC suppresses the genesis in the southeastern sub-region. Both the lifetime and intensity of TCs are increased in EPW, but only a shortened lifetime is seen for CPC. During OND, EPW reduces the TC genesis in the southwestern and northwestern sub-regions, whereas CPW enhances the genesis in the southeastern sub-region. Over the South China Sea, CPW and CPC show a significant decrease and increase of the TC genesis, respectively. The TC lifetime is significantly longer in both EPW and CPW and shorter in EPC, and TCs tend to be more (less) intense in EPW (CPC). All of these variations are consistent with the development of ENSO-related SST anomalies during different seasons and are supported by distributions of the genesis potential index—a combination of large-scale oceanic and atmospheric factors that affect TC activity. TCs in the WNP mainly take the straight westward, northwestward and recurving tracks. During AMJ of EPW years, the TC steering flow patterns favor the recurving track and suppress the straight westward and northwestward tracks. During JAS, EPW is associated with the steering flows that are unfavorable for TCs to move northwestward or westward, whereas CPW favors the northwestward track and suppresses the straight westward track. The steering flow patterns during OND are similar to those during JAS, except that EPC may increase the possibility of the northwestward track.  相似文献   

2.
This study reveals homogeneous sub-regions over the poorly studied area of western equatorial Africa (10S?C7N and 7E?C30E). Monthly totals of 141 stations covering the period 1955?C1984 are used. The stations are grouped based on the similarity of their interannual rainfall variability. In addition to annual totals, four different seasons are examined separately for regionalization, an approach that has lacked in previous studies. The four 3-month seasons are defined as follows: January?CFebruary?CMarch (JFM), April?CMay?CJune (AMJ), July?CAugust?CSeptember (JAS), and October?CNovember?CDecember (OND). Two different algorithms are applied and compared: the rotated principal component analysis (RPCA) in conjunction with Ward's method, and the RPCA in conjunction with k-means method. The principal components that explain about 65% of total variance are retained and then varimax rotated. The corresponding scores are utilized as input for cluster analysis. Using Ward's method, five sub-regions are recognized for AMJ, JAS and OND and 4 sub-regions for JFM and annual data. The regions are geographically well distributed over the area and consist of roughly the same number of stations. The F-test is used to evaluate the homogeneity of each sub-region. The results show that all sub-regions are strongly homogeneous. Assuming the same number of clusters, the k-means method provides comparable spatial patterns with those of Ward's method. However, there are some differences, which are more evident in JAS and OND. Like Ward's method, the values of F-ratio for the k-means algorithm also confirm the homogeneity of all seasons/sub-regions. The interannual variability of rainfall for each season/sub-region is also provided and compared.  相似文献   

3.
Summary This paper investigates the warming trend and interannual variability of surface air temperatures in the Malaysian region during the period 1961–2002. The trend analyses show that most regions in Malaysia experience warming over the period at comparable rates to those in regions surrounding the Bay of Bengal. The regions of Peninsular Malaysia and northern Borneo experience warming rates of between 2.7–4.0 °C/100 years. However, the warming rates are lower in the south-western region of Borneo. The interannual variability of Malaysian temperature is largely dominated by the El Ni?o-Southern Oscillation (ENSO). Regardless of the warming trends, all regions in Malaysia experience uniform warming during an El Ni?o event, particularly during the October–November–December (OND) and the January–February–March (JFM) periods. This uniform warming is associated with the latent heat released from the central eastern Pacific region and forced adiabatic subsidence in the Maritime Continent during an El Ni?o event. During its early development period i.e. during the July–August–September (JAS) season, the El Ni?o’s impact on the Malaysian temperatures is relatively weak compare to its influence during the OND and JFM seasons. However, the warming continues to the April–May–June (AMJ) season although during this period the anomalous conditions in the eastern central Pacific have begun or have returned to normal. The Indian Ocean Dipole (IOD) mode exerts an influence on Malaysian temperatures. When it co-occurs with ENSO, it tends to weaken the ENSO influence particularly during an OND period. However, it appears to have an appreciable influence only during an AMJ period when it occurs in the absence of an ENSO event. Despite the strong influence of the ENSO, the warming rates during the 42-year period appears to be least affected by interannual variability.  相似文献   

4.
 NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude 17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics. The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component. The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR reanalyses, especially for the 3–5-day regime over the Atlantic. Received: 28 May 1998 / Accepted: 2 May 1999  相似文献   

5.
李欣  朱伟军 《气象科学》2019,39(2):143-152
基于1971—2016年NCEP/NCAR(美国环境预报中心和国家大气研究中心)的逐日再分析资料及NCPC(美国国家海洋和大气管理局气候预报中心)的海温、大气环流及海洋指数等资料通过多尺度能量分析(MS-EVA)等方法,把冬季北半球风暴轴看做一整体,分析了风暴轴区域多尺度的能量变化特征及其可能机制。主要结论概括如下:(1)多年气候平均状态下,风暴轴的动能来源主要表现为在风暴轴中上游先由低频尺度向天气尺度输送有效位能,随后在风暴轴主体区再由天气尺度有效位能转换为天气尺度动能,其中风暴轴西端可直接由低频尺度向天气尺度输送动能。(2)北半球三大风暴轴联合EOF结果表明:第一模态下,主要体现了北西伯利亚风暴轴与北太平洋风暴轴强度的减弱(增强),同时伴随着北大西洋风暴轴位置北抬(南压);第二模态下,主要体现了北西伯利亚风暴轴强度减弱(增强),同时北太平洋风暴轴位置北抬(南压)中东部强度增强(减弱),而北大西洋风暴轴位置南压(北抬)。(3)回归分析表明:北半球风暴轴异常在不同模态下与低频尺度环流联系密切。低频尺度波动可通过海温及西风急流等异常变化先影响风暴轴区域多尺度间的能量转换,进而影响风暴轴整体的异常变化。  相似文献   

6.
冬季北大西洋风暴轴的东西变化及其能量诊断   总被引：6，自引：4，他引：2  
利用NCEP/NCAR再分析资料,定义一个风暴轴经度指数,基于这个指数做合成分析,对冬季北大西洋风暴轴63 a(1948-2010年)的东西变化特征及其能量平衡差异进行了诊断。主要结论如下:(1)北大西洋风暴轴存在明显地东扩和西退。当风暴轴向东扩展时,天气尺度瞬变波可以向下游发展至乌拉尔山以东的亚洲上空;风暴轴西退时,天气尺度瞬变波活动范围向西收缩到15°W以西的大洋上空。(2)能量诊断表明,当风暴轴向东扩展时,涡动动能在高纬度的大西洋东部及西欧上空明显增强。在0°以西的区域,涡动动能的增强主要归因于能量斜压转换过程的增强;而在0°以东区域,涡动动能的增强可能与涡动非地转位势通量引起的"下游发展效应"增强有关。风暴轴向西收缩时,变化相反。  相似文献   

7.
This study analyzes projected changes in seasonal extreme precipitation intensity and dry spell length in the investigation area (Côte d’Ivoire) under RCP4.5 and RCP8.5 forcing scenarios. To this end, a multi-model ensemble of fourteen CORDEX-Africa regional climate model simulations is used during the three stages of the West African Monsoon (WAM) season (April–June (AMJ), July–September (JAS), and October–December (OND)). The results indicate that Côte d’Ivoire is subject to a robust increase of cumulative intensity of precipitation associated with an amplification of extreme precipitation events during the WAM. In particular during JAS, a substantial increase in extreme precipitation reaching up to 50–60% compared to the reference mean value prevails in the western and coastal areas in the far future and under the RCP8.5 scenario. In addition, AMJ season is dominated by an increase in dry spell length of about 12% and 17% in the near future and 20% and 30% in the far future in the entire country under RCP4.5 and RCP8.5 scenarios, respectively, albeit considerable uncertainties. OND considered as the post-monsoon season is mostly characterized by a robust decrease in dry spell length more marked in the southwest in the RCP8.5 scenario during the far future. These results suggest that agricultural production and particularly cocoa plantations in the southwestern regions could be at the risk of flooding events and that water stress remains a threat for cocoa, coffee, and other cash crop plantations in the eastern regions.  相似文献   

8.
Ensembles of 1-year-long experiments with a relatively high-resolution ECMWF model were conducted in order to investigate the impact of modified land surface properties on local, regional and large-scale atmospheric circulations. The modifications consisted of changes to land cover and increased albedo over the northern part of South America. In many respects the experimental design resembles the setting of classical deforestation experiments. The local model response to imposed modifications, which includes a reduction in precipitation as well as in evaporation and an increase in surface temperature, was found to be stronger in dry (July–September, JAS) than in wet (January–March, JFM) season, and in the ensemble with higher albedo value. Local drying is discussed in terms of locally generated overturning that resembles a direct thermal circulation. The effects of this circulation seem to be dominant over the reduction in large-scale moisture supply from the adjacent ocean. On large scales, changes to the Pacific branch of the Walker circulation lead, through modified divergent flow, to a tropics-wide impact on precipitation. In addition to South America, the largest changes are seen in the south Pacific convergence zone in JFM, while the impact on the Atlantic inter-tropical convergence zone is stronger in JAS. In the extratropics, there is little change in precipitation. In the upper troposphere, a distinctive teleconnection wave-pattern could be seen in the Pacific/North American region during JFM. A notable feature in the upper-air model response in JAS is a wave train extending from South America, over the northern Atlantic into Europe. With regard to the interaction between the land surface response and model systematic errors, our results suggest that the erroneous shift of the downward branch of the Pacific/South American Walker circulation is likely to be a cause, rather than a consequence, of the rainfall deficit over South America in the model climatology.  相似文献   

9.
Summary  The Mediterranean basin experiences considerable cyclone activity mostly during fall, winter and spring and diminished activity during summer. In this study we present results of synoptic disturbance track analysis for two contrasting winter months and two, near average, summer months over the eastern Mediterranean. The surface and 500 hPa disturbance tracks were subjectively analyzed from two points of view. First, looking at tracks of conventionally defined cyclone centers (eddies) based on actual pressure and height distribution and second, looking at tracks of transient cyclonic disturbances (TRADs), defined as centers of negative deviations from the time mean. The second type of analysis demonstrated a considerable increase in the number of detectable tracks. Over the Mediterranean and vicinity the ratio between the number of surface TRAD tracks to cyclone tracks is, about 2, whereas at 500 hPa the ratio is much higher, about 5. However, the average life span of transient disturbances was only slightly longer than that of conventional cyclones (mainly at 500 hPa). At the surface and at 500 hPa about 50% of the cyclone tracks coincided to a certain extent with TRAD tracks. In summer, when conventional analysis over the eastern Mediterranean yields mostly quasi‐stationary low pressure centers associated with the Persian Gulf Trough, we detected clear signs of transient disturbances. Some interpretations of the differences between cyclones and TRADs in terms of weather in the eastern Mediterranean are also made. Received January 19, 1999Revised June 23, 1999  相似文献   

10.
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.  相似文献   

11.
The present paper uses an atmospheric general circulation model to explore large-scale atmospheric response to various El Niño-Southern Oscillation events associated with tropical cyclone (TC) activity in the western North Pacific. The simulated response is basically consistent with and confirms the observed results. For eastern Pacific warm (EPW) event, anomalously wet ascent occurs over the tropical central/eastern Pacific and dry descent is over the western Pacific. This Walker circulation is associated with anomalous low-level convergence, reduced vertical wind shear (VWS), and enhanced genesis potential index (GPI) in the southeast sub-region. These are consistent with the observed increase of the TC formation in the southeast sub-region but decrease in the northwest sub-region during July–September (JAS) and the increase in the southwest and northwest sub-regions during October–December (OND). In addition, the strong westerly anomalies of the TC steering flow prevail in the East Asian coast, suppressing the TC northwestward or westward tracks. For eastern Pacific cold (EPC) event, all of the simulated variables show almost a mirror image of EPW. For central Pacific warm event, the anomalous Walker circulation shifts westward because of the westward shift of the maximum SST anomaly forcing. The anomalous subsidence associated with the western branch of the Walker circulation during OND shifts northward to the South China Seas, resulting in a decrease of the TC genesis there. The TC steering flow patterns during JAS are favorable for TCs to make landfall over Japan and Korea. Compared with EPC, the descending motion in the central/eastern Pacific is much stronger for central Pacific cold (CPC) event, accompanied by more enhanced VWS and reduced GPI in the southeast sub-region. Therefore, CPC provides a more adverse environment to the TC formation there during JAS and OND, consistent with the observed decrease of TC formation there. Moreover, the easterly anomalies of the TC steering flow dominate the tropics during JAS, enhancing TC activity in the east coast of China. Additionally, the convection over the western Pacific moves to the South China Sea during OND, favoring the TC genesis there.  相似文献   

12.
Abstract

Six‐hourly surface wind analyses over the North Pacific Ocean covering the 10‐year period 1969–78 are used to describe synoptic storm activity in terms of parameters that are directly related to the atmospheric forcing of the ocean. The cube of the atmospheric friction velocity, u3 * and the curl of the surface wind stress, curl τ, are used because of their relationship to turbulent vertical mixing and Ekman pumping in the ocean, respectively. In an attempt to isolate synoptic disturbances from mean fields, the time series of surface wind components at each individual grid point are partitioned into “high‐pass” (periods shorter than 10 days) and “low‐pass” (periods longer than 10 days) components by means of conventional filtering procedures. The two quantities u3 * and curl τ are then calculated from (a) the high‐pass filtered wind components only, (b) a combination of the filtered wind components that include the interaction between the high‐ and low‐pass fields, and (c) the unfiltered wind components. These quantities describe the atmospheric forcing of the ocean that is attributable to (a) synoptic storm activity by itself, (b) synoptic storm activity in the presence of the low‐pass (mean) flow, and (c) the total spectrum of wind forcing, respectively.

Maps of the long‐term (10‐year) monthly mean u3 * calculated from (a) and (b) are coherent across the mid‐latitude North Pacific and appear to coincide with the normal seasonal evolution of synoptic storm activity in that region. In mid‐latitudes, the values of u3 * calculated from (a) and (b) are 27 and 83%, respectively, of the value of u3 * itself. Thus, a major fraction of the production of turbulent energy available for mixing in the upper layers of the ocean comes from synoptic disturbances with a period shorter than 10 days. Maps of the long‐term monthly mean wind stress curl are quite different in that the mean wind stress curl calculated from (a) is essentially negligible. However, the mean curl calculated from (b) closely resembles the pattern of total curl (c), but with a magnitude of only 41% of (c). Thus, synoptic disturbances with a period shorter than 10 days are also responsible for a significant fraction of the Ekman pumping of the ocean.

Future studies with these data will attempt to determine whether a relationship exists between synoptic storm activity, as measured by the parameters developed in this study, and large‐scale sea‐surface temperature anomalies.  相似文献   

13.
Synoptic activity over the Northern Hemisphere is evaluated in ensembles of ECHAM5/MPI-OM1 simulations for recent climate conditions (20C) and for three climate scenarios (following SRES A1B, A2, B1). A close agreement is found between the simulations for present day climate and the respective results from reanalysis. Significant changes in the winter mid-tropospheric storm tracks are detected in all three scenario simulations. Ensemble mean climate signals are rather similar, with particularly large activity increases downstream of the Atlantic storm track over Western Europe. The magnitude of this signal is largely dependent on the imposed change in forcing. However, differences between individual ensemble members may be large. With respect to the surface cyclones, the scenario runs produce a reduction in cyclonic track density over the mid-latitudes, even in the areas with increasing mid-tropospheric activity. The largest decrease in track densities occurs at subtropical latitudes, e.g., over the Mediterranean Basin. An increase of cyclone intensities is detected for limited areas (e.g., near Great Britain and Aleutian Isles) for the A1B and A2 experiments. The changes in synoptic activity are associated with alterations of the Northern Hemisphere circulation and background conditions (blocking frequencies, jet stream). The North Atlantic Oscillation index also shows increased values with enhanced forcing. With respect to the effects of changing synoptic activity, the regional change in cyclone intensities is accompanied by alterations of the extreme surface winds, with increasing values over Great Britain, North and Baltic Seas, as well as the areas with vanishing sea ice, and decreases over much of the subtropics.  相似文献   

14.
北太平洋风暴轴的三维空间结构   总被引：9，自引：1，他引：8  
傅刚  毕玮  郭敬天 《气象学报》2009,67(2):189-200
文中利用最新的0.5°×0.5°分辨率QuikSCAT(QuikBird Satellite Microwave Scatterometer Sea Winds Data)海面风场资料、NCEP(National Center for Environmental Prediction)的10 m高度风场资料和全球客观再分析资料,对1999-2005年冬季(1月)和夏季(7月)北太平洋风暴轴的三维空间结构进行了分析,发现冬季北太平洋风暴轴的强度较强,呈明显的纬向拉伸带状分布特征,位置偏南.夏季北太平洋风暴轴的强度较弱,位置偏北.根据不同高度上位势高度方差的水平分布特征,绘制了北太平洋风暴轴的三维结构示意图.利用高分辨率QuikSCAT资料对风暴轴特征的刻画更为细致,不但验证了Nakamu-ra在南大洋发现的双风暴轴现象,而且还发现在北太平洋和北大西洋下层分别存在"副热带风暴轴"和"副极地风暴轴"两个风暴轴.对1999-2005年冬季北太平洋气旋和反气旋的移动路径进行的统计分析,为北太平洋"双风暴轴"的存在提供了强有力的证据.  相似文献   

15.
北半球温带气旋活动和风暴路径的年代际变化   总被引：12，自引：3，他引：9  
基于欧洲中心再分析数据ERA40的海平面气压场和高度场,本文分别采用拉格朗日和欧拉方法研究分析了1958～2001年北半球的不同季节温带气旋活动和风暴路径的年代际变化,以及可能的原因.以客观判定和追踪温带气旋为基础的拉格朗日方法得到了北半球的两个温带气旋主要活动中心,即北太平洋地区和北大西洋/北美地区,同时以500 hPa位势高度天气尺度滤波方差为基础的欧拉方法得到了同主要气旋活动中心相吻合的两条风暴轴.研究表明,44年中北大西洋/北美地区温带气旋活动北移加强,以春季最为显著.风暴轴也同样存在着向极移动并加强的特征,并且温带气旋和风暴路径两者移动趋势的相关性很高.作为一个典型地区,北大西洋/北美地区的气旋活动体现了风暴路径的北移,以及温带地区向极地的扩展.但有意思的是北太平洋的情况完全不同,即北太平洋地区的温带气旋活动和风暴轴向低纬度偏移并加强,以春季的南移趋势最为显著.对于此结论,两种方法也有很高的统计相关性.虽然大量研究表明北半球整体上呈现出风暴路径北移的变化特征,但对于具体地区情况有明显差异.另外,400 hPa最大Eady增长率和气旋活动频率的经验正交展开函数 (EOF) 第一模态的空间分布和时间序列非常相似,北太平洋地区和北大西洋地区风暴路径相反的变化趋势很可能同其大气斜压性的同位相的变化有着密切的关系.这也从另一个方面支持了本文对温带气旋和风暴路径年代际变化的分析.  相似文献   

16.
广东前汛期锋面强降水和后汛期季风强降水特征对比分析   总被引：3，自引：7，他引：3  
应用近二十年的历史观测资料和EC再分析资料,对由锋面和季风槽两种不同天气系统影响下广东发生的两组暴雨过程的天气形势、降水/短时强降水落区及其对流活动和物理量特征进行了诊断分析和对比分析。结果表明:无论是前汛期锋面降水还是后汛期季风降水,珠三角（珠江三角洲）地区都是次中心,有大到暴雨量级降水。珠三角地区也是小时雨量≥50 mm的短时强降水高发区。前汛期锋面对流活动的抬升凝结高度约在900~850 hPa,南北方向的温度梯度提供了斜压不稳定能量,0~3 km强的风垂直切变使对流易于维持和发展;对流区有较强的水汽通量辐合;风暴相对螺旋度较大,对流的旋转性和沿着旋转方向的移动特征明显。相对而言,后汛期季风强降水对流凝结高度更低,对流活动具有正压的热带对流性质,可在弱的水汽通量辐合和垂直风切变环境中维持,但对流强度不如前汛期。以上结论可为同类天气的短期和短临主客观预报提供预报思路和依据。   相似文献   

17.
Two methods for identifying mid-latitude synoptic time scale variability have been applied to data from the first United Kingdom Meteorological Office (UKMO) coupled ocean-atmosphere model experiments with present day and gradually increasing CO2 concentrations. In the first the standard deviation of the time filtered mean sea level pressure field is taken to identify the location of the storm track and in the second individual cyclones are identified using synoptic criteria. The results have been compared with data from a 10 year archive of UKMO analysis. In the enhanced CO2 experiment the changes in storminess identified by the two methods have been compared with changes in mean and maximum winds with special emphasis on the North Atlantic. The relative utility of the different measures for predicting potentially damaging synoptic events is discussed. Received: 3 May 1995 / Accepted: 7 November 1995  相似文献   

18.
We evaluate the performance of GAMIL1.1.1 in a 27-year forced simulation of the summer intraseasonal oscillation (ISO) over East Asia (EA)-western North Pacific (WNP). The assessment is based on two measures: climatological ISO (CISO) and transient ISO (TISO). CISO is the ISO component that is phase-locked to the annual cycle and describes seasonal march. TISO is the ISO component that varies year by year. The model reasonably captures many observed features of the ISO, including the stepwise northward advance of the rain belt of CISO, the dominant periodicities of TISO in both the South China Sea-Philippine Sea (SCS-PS) and the Yangtze River Basin (YRB), the northward propagation of 30--50-day TISO and the westward propagation of the 12--25-day TISO mode over the SCS-PS, and the zonal propagating features of three major TISO modes over the YRB. However, the model has notable deficiencies. These include the early onset of the South China Sea monsoon associated with CISO, too fast northward propagation of CISO from 20oN to 40oN and the absence of the CISO signal south of 10oN, the deficient eastward propagation of the 30--50-day TISO mode and the absence of a southward propagation in the YRB TISO modes. The authors found that the deficiencies in the ISO simulation are closely related to the model's biases in the mean states, suggesting that the improvement of the model mean state is crucial for realistic simulation of the intraseasonal variation.  相似文献   

19.
Two methods for identifying mid-latitude synoptic time scale variability have been applied to data from the first United Kingdom Meteorological Office (UKMO) coupled ocean-atmosphere model experiments with present day and gradually increasing CO2 concentrations. In the first the standard deviation of the time filtered mean sea level pressure field is taken to identify the location of the storm track and in the second individual cyclones are identified using synoptic criteria. The results have been compared with data from a 10 year archive of UKMO analysis. In the enhanced CO2 experiment the changes in storminess identified by the two methods have been compared with changes in mean and maximum winds with special emphasis on the North Atlantic. The relative utility of the different measures for predicting potentially damaging synoptic events is discussed.  相似文献   

20.
Responses of global ocean circulation and temperature to freshwater runoff from major rivers were studied by blocking regional runoff in the global ocean general circulation model(OGCM)developed at the Massachusetts Institute of Technology.Runoff into the tropical Atlantic,the western North Pacific,and the Bay of Bengal and northern Arabian Sea were selectively blocked.The blocking of river runoff first resulted in a salinity increase near the river mouths(2 practical salinity units).The saltier and,therefore,denser water was then transported to higher latitudes in the North Atlantic,North Pacific,and southern Indian Ocean by the mean currents.The subsequent density contrasts between northern and southern hemispheric oceans resulted in changes in major ocean currents.These anomalous ocean currents lead to significant temperature changes(1°C-2°C)by the resulting anomalous heat transports.The current and temperature anomalies created by the blocked river runoff propagated from one ocean basin to others via coastal and equatorial Kelvin waves.This study suggests that river runoff may be playing an important role in oceanic salinity,temperature,and circulations;and that partially or fully blocking major rivers to divert freshwater for societal purposes might significantly change ocean salinity,circulations,temperature,and atmospheric climate.Further studies are necessary to assess the role of river runoff in the coupled atmosphere-ocean system.  相似文献   

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