Autumn Tropical Cyclones over the Western North Pacific during 1949-2016: A Statistical Study

We used tropical cyclone (TC) best track data for 1949–2016, provided by the Shanghai Typhoon Institute, China Meteorological Administration (CMA-STI), and a TC size dataset (1980-2016) derived from geostationary satellite infrared images to analyze the statistical characteristics of autumn TCs over the western North Pacific (WNP). We investigated TC genesis frequency, location, track density, intensity, outer size, and landfalling features, as well as their temporal and spatial evolution characteristics. On average, the number of autumn TCs accounted for 42.1% of the annual total, slightly less than that of summer TCs (42.7%). However, TCs classified as strong typhoons or super typhoons were more frequent in autumn than in summer. In most years of the 68-yr study period, there was an inverse relationship between the number of autumn TCs and that of summer TCs. The genesis of autumn TCs was concentrated at three centers over the WNP: the first is located near (14°N, 115°E) over the northeastern South China Sea and the other two are located in the vast oceanic area east of the Philippines around (14°N, 135°E) and (14°N, 145°E), respectively. In terms of intensity, the eight strongest TCs during the study period all occurred in autumn. It is revealed that autumn TCs were featured with strong typhoons and super typhoons, with the latter accounting for 28.1% of the total number of autumn TCs. Statistically, the average 34-knot radius (R34) of autumn TCs increased with TC intensity. From 1949 to 2016, 164 autumn TCs made landfall in China, with an average annual number of 2.4. Autumn TCs were most likely to make landfall in Guangdong Province, followed by Hainan Province and Taiwan Island.

     

Tropical cyclone genesis frequency over the western North Pacific simulated in medium-resolution coupled general circulation models

This study examines the tropical cyclone (TC) genesis frequency over the western North Pacific simulated in atmosphere–ocean coupled general circulation models from the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3. We first evaluate performances of eight models with atmospheric horizontal resolution of T63 or T106 by analyzing their daily-mean atmospheric outputs of twentieth-century climate simulations available from the Program for Climate Model Diagnosis and Intercomparison database. The genesis frequency is validated against the best-track data issued by the Japan Meteorological Agency. Five of the eight models reproduce realistic horizontal distribution of the TC genesis with a large fraction over the 10°–20°N, 120°–150°E area. These five high-performance models also realistically simulate the summer–winter contrast of the frequency. However, detailed seasonal march is slightly unrealistic; four of the models overestimate the frequency in the early season (May–June) while all of them underestimate the frequency in the mature season (July–September). Reasons for these biases in the seasonal march for the five high-performance models are discussed using the TC genesis potential (GP) index proposed by Emanuel and Nolan (in Am Meteor Soc, pp 240–241, 2004). The simulated GP has seasonal biases consistent with those of the TC genesis frequency. For all five models, the seasonal biases in GP are consistent with those in environmental lower-tropospheric vorticity, vertical wind shear, and relative humidity, which can be attributed to the simulated behavior of monsoon trough. The observed trough migrates northward from the equatorial region to reach the 10°–20°N latitudinal band during the mature season and contributes to the TC frequency maximum, whereas the simulated trough migrates northward too rapidly and reaches this latitude band in the early season, leading to the overestimation of the TC genesis frequency. In the mature season, the simulated trough reaches as far as 15°–25°N, accompanied by a strong vertical shear south of the trough, providing an unfavorable condition for TC genesis. It is concluded that an adequate simulation of the monsoon trough behavior is essential for a better reproduction of the TC frequency seasonal march.     

More intensive summer tropical cyclone near 30°N of East Asia

The present study revealed that a climate regime shift occurred during the 1988–1991 period involving changes in tropical cyclone (TC) intensity (central pressure, maximum sustained wind speed) during the summer near 30°N in East Asia. Climatologically, TC intensity at 110°–125°E near 30°N (over Mainland China) is the weakest at that latitude while the strongest is found at 125°–130°E (over Korea). The TC intensity during the 1991–2015 (91–15) period had strengthened significantly compared to that of the 1965–1988 (65–88) period. The strengthening was due to a significantly lower frequency of TCs that passed through Mainland China during the 91–15 period. This lower frequency of was due to anomalous northeasterlies blown from the anomalous anticyclonic circulation located over continental East Asia and that had strengthened along the coast. Instead, TCs mainly followed a path from eastern regions in the subtropical western North Pacific to Korea and Japan via the East China Sea due to anomalous cyclonic circulations that had strengthened in the western North Pacific. In addition, low vertical wind shear had formed along the mid-latitude region in East Asia and along the main TC track in the 91–15 period, and most regions in the western North Pacific experienced a higher sea surface temperature state during the 91–15 period than in the previous period, indicating that a favorable environment had formed to maintain strong intensities of TCs at the mid–latitudes. The characteristics of TCs at the lower latitudes caused a strong TC intensity at the time of landfall in Korea and a gradual shifting trend of landing location from the western to southern coast in recent years.     

西北太平洋与南海热带气旋活动季节变化的差异及可能原因

利用1945~2011年美国联合台风预警中心(JTWC)西北太平洋热带气旋资料,研究了南海(5°N~25°N,110°E~120°E)与西北太平洋(5°N~25°N,120°E~180°)热带气旋生成位置、生成频数、强度和持续时间的季节变化差异及其成因。从热带气旋路径穿越经度带频数的角度,探讨了ENSO对气旋活动年际变化的影响。结果表明,南海热带气旋活动显著地受季风调控。在南海冬季风作用下,1~4月热带气旋生成于10°N以南且频数较少、强度较弱,这主要是低层气旋式相对涡度和弱东风切变区偏南造成的。相反,受夏季风影响,6~9月是热带气旋生成最多、最频繁的季节,大都生成于南海北部17°N附近。在5月(10月)的季节转换期,生成位置大幅度北进(南撤)且生成频数显著增加(减少),取决于风速垂直切变及中层的相对湿度的急剧转变。11、12月两海域热带气旋生成于10°N以南主要归因于其上空中层大气相对湿度较北部偏大。在西北太平洋,热带气旋生成的季节变化没有南海显著,只在7月有一次明显的变化,7~10月是热带气旋活动的"盛期"。在强度上,西北太平洋大部分区域全年均为弱东风切变,因此热带气旋以台风为主且持续时间长;但南海多为热带风暴。ENSO事件使得不同季节热带气旋生成区域和气旋路径地理位置发生显著变化。在El Nio事件期间,穿越南海所在经度带路径频数为负距平,而西北太平洋经度带为正距平;在La Nia事件期间,情况相反。     

西北太平洋热带气旋频数的气候变化及其与环境要素间的联系

使用Emanuel和Nolan完善的潜在生成指数(GPI)的计算方法,利用美国联合台风警报中心提供的热带气旋(TC)资料和欧洲中期数值天气预报中心提供的全球ERA-40再分析资料,比较了1970-2001年西北太平洋海域的TC生成频数和GPI的气候特征,分析了包含于GPI中的环境要素对西北太平洋TC频数年代际变化空间分布的影响.结果表明:GPI能近似地表述西北太平洋TC频数的季节变化和空间分布.各环境要素对TC、较弱类TC和较强类TC生成频数的影响有显著差异,相对湿度随着TC强度的增强而减弱,风速垂直切变则相反.西北太平洋TC频数年代际变化空间分布的正异常主要分布于130°E以东,(15°N,140°E)附近最大的正异常频数中心主要受绝对涡度和相对湿度正异常变化的影响;负的风速垂直切变和正的相对湿度异常变化引起了(10～15°N,160°E)附近的TC频数正异常.     

2010年西北太平洋与南海热带气旋活动异常的成因分析

利用中国气象局热带气旋（TC）资料、NCEP/NCAR 再分析资料和美国 NOAA 向外长波辐射（OLR）等资料,分析了2010年西北太平洋（WNP）及南海（SCS）热带气旋活动异常的可能成因,讨论了同期大气环流配置和海温外强迫对TC生成和登陆的动力和热力条件的影响。结果表明,2010年生成TC频数明显偏少,生成源地显著偏西,而登陆TC频数与常年持平。导致7～10月TC频数明显偏少的大尺度环境场特征为：副热带高压较常年异常偏强、西伸脊点偏西,季风槽位置异常偏西,弱垂直风切变带位置也较常年偏西且范围偏小,南亚高压异常偏强,贝加尔湖附近对流层低高层均为反气旋距平环流,这些关键环流因子的特征和配置都不利于 TC 在WNP的东部生成。影响TC活动的外强迫场特征为：2010年热带太平洋经历了El Ni?o事件于春末夏初消亡、La Ni?a事件于7月形成的转换;7～10月,WNP海表温度维持正距平,140°E以东为负距平且对流活动受到抑制;暖池次表层海温异常偏暖,对应上空850 hPa为东风距平,有利于季风槽偏西和TC在WNP的西北侧海域生成。WNP海表温度和暖池次表层海温的特征是2010年TC生成频数偏少、生成源地异常偏西的重要外强迫信号。有利于7～10月热带气旋西行和登陆的500 hPa风场特征为：北太平洋为反气旋环流距平,其南侧为东风异常,该东风异常南缘可到25°N,并向西扩展至中国大陆地区;南海和西北太平洋地区15°N以南的低纬也为东风异常;在这样的风场分布型下,TC容易受偏东气流引导西行并登陆我国沿海地区。这是2010年生成TC偏少但登陆TC并不少的重要环流条件。     

IMPACT OF CONVECTION OVER THE SOUTH CHINA SEA ON TROPICAL CYCLONE MOTION OVER THE WESTERN NORTH PACIFIC DURING SUMMER MONSOON

The intraseasonal oscillation(ISO) of the South China Sea(SCS, 105-120°E, 5-20°N) convection and its influences on the genesis and track of the western North Pacific(WNP) tropical cyclones(TCs) were explored, based on the daily average of NCEP/NCAR reanalysis data, the OLR data and the western North Pacific tropical cyclone best-track data from 1979 to 2008. The mechanism of the influences of ISO on TC movement and the corresponding large-scale circulation were discussed by a trajectory model. It was found as follows.(1) During the SCS summer monsoon, the SCS convection exhibits the ISO features with active phases alternating with inactive phases. The monsoon circulation patterns are significantly different during these two phases. When the SCS convection is active(inactive), the SCS-WNP monsoon trough stretches eastward(retreats westward) due to the activity(inactivity) of SCS monsoon, and the WNP subtropical high retreats eastward(stretches westward), which enhances(suppresses) the monsoon circulation.(2) The amount of TC genesis in the active phase is much more than that in the inactive phase. A majority of TCs form west of 135 °E during the active phases but east of 135 °E in the inactive phases.(3) The TCs entering the area west of 135 °E and south of 25 °N would move straight into the SCS in the active phase, or recurve northward in the inactive phase.(4) Simulation results show that the steering flow associated with the active(inactive)phases is in favor of straight-moving(recurving) TCs. Meanwhile, the impacts of the locations of TC genesis on the characteristics of TC track cannot be ignored. TCs that occurred father westward are more likely to move straight into the SCS region.     

Enhanced western North Pacific tropical cyclone activity in May in recent years

The tropical cyclone (TC) power dissipation index (PDI) in May over the western North Pacific (WNP) region shows a remarkable increase from the pre-1999 years (1979–1999) to the post-1999 years (2000–2011). Both increased TC numbers and enhanced TC intensity contributed to the change in the PDI. The averaged TC number in May increased from 1.05 per year in the pre-1999 years to 1.75 per year in the post-1999 years. In particular, the number of intense typhoon goes up from 0.14 per year to 0.83 per year, implying a sharp increase of TC intensity. Examination of the large scale background circulation in May shows that the epochal increase of TC number is caused by a significant increase of the genesis potential index (GPI), which has increased by about 33 % from the first (1979–1998) to the second (1999–2011) epoch over the TC genesis region (110°E–160°E, 5°N–20°N). The higher TC intensity is related to the increased maximum potential intensity and reduced TC ambient vertical wind shear in the second epoch. These decadal changes in background conditions over the WNP are the results of the enhanced summer monsoon in May over the both South Asia and South China Sea.     

Impact of the Spring SST Gradient between the Tropical Indian Ocean and Western Pacific on Landfalling Tropical Cyclone Frequency in China

The present study identifies a significant influence of the sea surface temperature gradient(SSTG) between the tropical Indian Ocean(TIO; 15°S-15°N, 40°-90°E) and the western Pacific warm pool(WWP; 0°-15°N, 125°-155°E) in boreal spring on tropical cyclone(TC) landfall frequency in mainland China in boreal summer. During the period 1979-2015, a positive spring SSTG induces a zonal inter-basin circulation anomaly with lower-level convergence, mid-tropospheric ascendance and upper-level divergence over the west-central TIO, and the opposite situation over the WWP, which produces lower-level anomalous easterlies and upper-level anomalous westerlies between the TIO and WWP. This zonal circulation anomaly further warms the west-central TIO by driving warm water westward and cools the WWP by inducing local upwelling, which facilitates the persistence of the anomaly until the summer. Consequently, lower-level negative vorticity, strong vertical wind shear and lower-level anticyclonic anomalies prevail over most of the western North Pacific(WNP), which decreases the TC genesis frequency. Meanwhile, there is an anomalous mid-tropospheric anticyclone over the main WNP TC genesis region,meaning a westerly anomaly dominates over coastal regions of mainland China, which is unfavorable for steering TCs to make landfall in mainland China during summer. This implies that the spring SSTG may act as a potential indicator for TC landfall frequency in mainland China.     

西北太平洋热带气旋强度变化的统计特征

用中国气象局整编的1949-2003年共55年的《台风年鉴》和《热带气旋年鉴》资料,依据平均值与标准差的数学涵义,给出了TC突然增强、缓慢增强、强度稳定、缓慢减弱和突然减弱的标准,分析了西北太平洋热带气旋(TC)强度变化的年代际、年际、月际、日变化和区域分布的基本特征。结果表明:(1)1960年代以前,T℃的年平均增强或减弱幅度较小。(2)在TC出现较为频繁的夏秋季节,8月份TC强度变幅较小。TC在14时(北京时,下同)最易发展,20时最易减弱;08时TC增强速度最快,02时最慢;02时TC减弱速度最快, 20时最慢。(3)TC频数和增强TC频数的高值区位于海南岛以东的南海北部中国近海区域和菲律宾以东洋面,减弱类TC频数极值区在吕宋岛及其东部海域、海南岛以西的北部湾、广东沿岸。(4)TC突然增强不出现在30°N以北的中高纬地区和0—5°N的低纬地区。TC突然减弱多出现在125°E以西的中国近海大范围海域,在0～5°N的低纬地区基本不出现。     

Boreal summer quasi-monthly oscillation in the global tropics

The boreal summer intraseasonal oscillation (ISO) in the global tropics is documented here using a 7-year suite (1998–2004) of satellite measurements. A composite scenario was made of 28 selected events with reference to the oscillation in the eastern equatorial Indian Ocean (EIO), where the oscillation is most regular and its intensity is indicative of the strength of the subsequent northward propagation. The average oscillation period is about 32 days, and this quasi-monthly oscillation (QMO) is primarily confined to the tropical Indian and Pacific Oceans. Topics that were investigated are the partition of convective versus stratiform clouds, the vertical structure of precipitation rates, and the evolution of cloud types during the initial organization and the development of intraseasonal convective anomalies in the central Indian Ocean. During the initiation of the convective anomalies, the stratiform and convective rains have comparable rates; the prevailing cloud type experiences a trimodal evolution from shallow to deep convection, and finally to anvil and extended stratiform clouds. A major northwest/southeast-slanted rainband forms as the equatorial rainfall anomalies reach Sumatra, and the rainband subsequently propagates northeastward into the west Pacific Ocean. The enhanced precipitation in the west Pacific then rapidly traverses the Pacific along the Intertropical Convergence Zone, meanwhile migrating northward to the Philippine Sea. A seesaw teleconnection in rainfall anomalies is found between the southern Bay of Bengal (5–15°N, 80–100°E) and the eastern Pacific (5–15°N, 85–105°W). Local sea-surface temperature (SST)-rainfall anomalies display a negative simultaneous correlation in the off-equatorial regions but a zero correlation (quadrature phase relationship) near the equator. We propose that atmosphere–ocean interaction and the vertical monsoon easterly shear are important contributors to the northeastward propagation component of the intraseasonal rainband. The observed evidence presented here provides critical information for validating the numerical models, and it supports the self-induction mechanism theory for maintenance of the boreal summer ISO.     

Analyzing the genesis, intensity, and tracks of western Mediterranean cyclones

Midlatitude cyclones are analyzed on a selected region covering most of southern Europe and the western part of the Mediterranean Sea (35–50°N, 10°W–25°E). On the basis of mean sea level pressure fields of the ECMWF (European Centre for Medium-range Weather Forecast) Reanalysis Dataset (ERA-40), detailed evaluation of Mediterranean cyclones is accomplished for the period between 1957 and 2002 on a 1° horizontal resolution grid. Cyclone centers are identified and their paths are tracked with a 6-h time step (using 00 UTC, 06 UTC, 12 UTC, and 18 UTC). Decadal, annual, and seasonal statistical analysis of cyclone tracks includes the study of the genesis, frequency, and activity of the Mediterranean cyclones as well as the variability of cyclone tracks. The results suggest that the cyclone frequency in the western Mediterranean region increased in summer and autumn, and decreased in winter and spring. A special belt-shaped area is identified, which plays a special role in cyclogenesis, and also, the cyclone tracks often remain within this belt. An overall decreasing trend is detected in winter and spring in the entire Mediterranean belt, while cyclone frequency increased in autumn. The largest positive and negative trend coefficients are identified in summer.     

The effects of topography on the summer atmospheric energetics of the Northern Hemisphere in a low-resolution global spectral model

An analysis is made of the effects of topography on the summer atmospheric energetics of the Northern Hemisphere in a low-resolution global spectral model. The numerical mode! is a global, spectral, primitive equation model with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. The model includes comparatively full physical processes. Each term of the energy budget equations is calculated in four specific latitudinal belts (81.11°S–11.53°S; 11.53°S–11.53°N; 11.53°N–46.24°N; 46.24°N–81.11°N) from a five-year simulation with mountains and a one-year simulation without mountains, respectively. Differences between them are compared and statistically tested. The results show that synoptical scale waves transport available potential energy and kinetic energy to long waves and increase conversion from available potential energy of the zonal flow to eddy's and from the eddy kinetic energy to the zonal kinetic energy in region 3 (11.53°N-46.24°N) due to mountains; topography intensifies the atmospheric baroclinity in region 3, consequently the baroclinic conversion of atmosphere energy is increased. The seasonal characteristics associated with the summer atmospheric energy source in region 3 are caused by seasonal variation of the solar radiation and the land-ocean contrasts and independent of topographic effects. The mechanism of topographic effects on the increase of long wave kinetic energy is also discussed.     

热带大气季节内振荡与西北太平洋热带气旋活动的季节预测:统计事实研究

应用NOAA气候预测中心提供的热带大气季节内振荡(MJO)客观业务指数及中国气象局上海台风研究所提供的西北太平洋热带气旋(TC)最佳路径资料集,定量统计榆验了MJO对夏季西北太平洋TC活动的调制作用.结果表明:MJO对TC的生成、强度、路径和登陆活动都有显著的调节作用.当高空辐合中心位于120°E～160°E(MJO位相3～5)时,西北太平洋TC生成偏少,且生成位置偏北;而当高空辐合中心位于10°W～70°E(MJO位相8～10)时,西北太平洋TC生成偏多,且生成位置偏南;随着TC强度加强,能达到显著调节作用的MJO位相逐渐减少,当高空辐合辐散中心位于70°E(MJO位相10)时,对TC强度调制最显著.在路径调节方面,MJO位相1～4和10时,TC活跃于菲律宾以东的西北太平洋上,主要路径为西北偏北行,可能登陆华东、华北;而位相5～8时,TC主要活跃在菲律宾附近及以西到南海,以偏西行路径为主,可能登陆华南.MJO对登陆华南TC也有显著影响.该定量统计检验结果可为TC活动季节内预测提供依据.     

Interannual variability of cut-off low systems over the European sector: The role of blocking and the Northern Hemisphere circulation modes

Summary An earlier developed multidecadal database of Northern Hemisphere cut-off low systems (COLs), covering a 41 years period (from 1958 to 1998) is used to study COLs interannual variability in the European sector (25°–47.5° N, 50° W–40° E) and the major factors controlling it. The study focus on the influence on COLs interannual variability, of larger scale phenomena such as blocking events and other main circulation modes defined over the Euro-Atlantic region. It is shown that there is a very large interannual variability in the COLs occurrence at the annual and seasonal scales, although without significant trends. The influence of larger scale phenomena is seasonal dependent, with the positive phase of the NAO favoring autumn COL development, while winter COL occurrence is mostly related to blocking events. During summer, the season when more COLs occur, no significant influences were found.     

南海与西北太平洋地区夏季热带气旋潜在生成指数的改进

现阶段使用的热带气旋潜在生成指数(Genesis Potential Index,GPI)在气候场的空间分布上能很好地拟合热带气旋的生成情况,但在热带气旋的年际变化拟合上效果很差。本研究考虑了相对涡度在热带气旋年际变化拟合上的重要作用,并以此为出发点,尝试改善GPI在西北太平洋地区的拟合效果。基于对1979—2011年美国联合飓风警报中心提供的热带气旋最佳路径数据和NCEP/NCAR再分析资料数据集的研究,将之前GPI中的绝对涡度项替换为修正过的相对涡度项。科氏力项仍然保留;将南海(100°～120°E,5°～25°N)与西北太平洋地区(120°～180°E,5°～40°N)热带气旋生成的差异性也纳入了考量,并在这两个区域分别构建GPI公式,改善了对热带气旋生成的气候分布模拟。除此之外,较之已存的GPI指数,改进后的GPI还很大程度提高了GPI对热带气旋生成年际变化的拟合效果,特别是对弱热带气旋年际变化的拟合效果有了显著提升。     

ENERGY BALANCE IN 40－50 DAY PERIODIC OSCILLATION OVER THE ASIAN SUMMER MONSOON REGION DURING THE 1979 SUMMER

Based on calculations of data from FGGE Level III b, a discussion is made of the energy balance in the 40－50 day periodic oscillation over the Asian monsoon region during the 1979 summer. It is found that the main source of 40－50 day periodic perturbation is the monsoon region extending from central South Asia to Southeast Asia. In the upper layer over the North Pacific subtropical area (10－20oN, 150oE－150oW) pres-sure work turns into kinetic energy that maintains 40－50 day periodic perturbation associated with the variation in position and intensity of the mid-Pacific trough. The mean energy budget in the three-dimensional space (0－30oE, 30oE－150oW, 100－1000 hPa) indicates that the 40－50 day periodic perturbation transports kinetic energy to a seasonal mean and a transient perturbation wind field.     

Tropical Cyclone Genesis Potential Index over the Western North Pacific Simulated by LASG/IAP AGCM

Tropical cyclone genesis potential index(GPI) is a useful metric for gauging the performance of global climate models in the simulation of tropical cyclone(TC) genesis.The performance of LASG/IAP AGCM GAMIL2.0 in the simulation of GPI over the western North Pacific(WNP) is assessed in this paper.Since GPI depends on large scale environmental factors including low-level vorticity at 850 hPa,relative humidity at 700 hPa,vertical wind shear between 850 and 200 hPa,maximum potential intensity(MPI),and vertical velocity,the bias of GPI simulation is discussed from the perspective of thermal and dynamical factors.The results are compared with the ECMWF reanalysis data(ERA40).The analyses show that both the climatological spatial pattern and seasonal cycle of GPI over the WNP are reasonably simulated by GAMIL2.0,but due to the overestimation of relative humidity,the simulated GPI extends to 170°E,about 10°east to that in the reanalysis data.It is demonstrated that the bias in the simulation of monsoon trough,which is about 5°north to the reanalysis,leads to an overestimation of GPI during May-June and September-October,but an underestimation during July-August.Over the WNP,the response of GPI to ENSO is well captured by GAMIL2.0,including the eastward(westward) shift of TC genesis location during El Nin o(La Nin a) years.However,the anomalous convective center associated with El Nin o shifts westward about 20°in comparison to ERA40,which leads to the biases in both vertical velocity and relative humidity.These eventually result in the westward deflection of the boundary between the positive and negative GPI centers along 20°-30°N.The results from this study provide useful clues for the future improvement of GAMIL2.0.     

Seasonal variations of net CO2 exchange in European Arctic ecosystems

Summary  The carbon dioxide exchange in arctic and subarctic terrestrial ecosystems has been measured using the eddy-covariance method at sites representing the latitudinal and longitudinal extremes of the European Arctic sea areas as part of the Land Arctic Physical Processes (LAPP) project. The sites include two fen (Kaamanen and Kevo) and one mountain birch ecosystems in subarctic northern Finland (69° N); fen, heathland, and snowbed willow ecosystems in northeastern Greenland (74° N); and a polar semidesert site in Svalbard (79° N). The measurement results, which are given as weekly average diurnal cycles, show the striking seasonal development of the net CO₂ fluxes. The seasonal periods important for the net CO₂ fluxes, i.e. winter, thaw, pre-leaf, summer, and autumn can be identified from measurements of the physical environment, such as temperature, albedo, and greenness. During the late winter period continuous efflux is observed at the permafrost-free Kaamanen site. At the permafrost sites, efflux begins during the thaw period, which lasts about 3–5 weeks, in contrast to the Kaamanen site where efflux continues at the same rate as during the winter. Seasonal efflux maximum is during the pre-leaf period, which lasts about 2–5 weeks. The summer period lasts 6 weeks in NE Greenland but 10–14 weeks in northern Finland. During a high summer week, the mountain birch ecosystem had the highest gross photosynthetic capacity, GP _max, followed by the fen ecosystems. The polar semidesert ecosystem had the lowest GP _max. By the middle of August, noon uptake fluxes start to decrease as the solar elevation angle decreases and senescence begins within the vascular plants. At the end of the autumn period, which lasts 2–5 weeks, topsoil begins to freeze at the end of August in Svalbard; at the end of September at sites in eastern Greenland; and one month later at sites in northern Finland. Received March 1, 2000 Revised October 2, 2000     

西北太平洋季风槽的季节和年际变化特征及其与热带气旋生成大尺度环境因子的联系

利用ERA-Interim再分析资料分析了夏秋季西北太平洋季风槽的气候特征以及季节和年际变化特征及其对西北太平洋热带气旋和台风(TCs)生成大尺度环境因子的影响。研究结果表明了西北太平洋季风槽有很明显的季节变化,在6~7月,季风槽和强对流活动区在5°N~15°N的南海和西北太平洋西侧上空,并逐渐东伸;到了8~9月,季风槽和强对流活动区向北移动、并向东扩展,一般位于10°N~20°N的南海和西北太平洋西侧、中部上空,有的年份可东伸到西北太平洋东侧,强度加强;到了10~11月,季风槽迅速减弱,并成为涡旋,强对流活动区也向南移和向西收缩。同时,研究还表明了西北太平洋季风槽有明显的年际变化。在季风槽强的年份,季风槽和强对流活动区可以从南海经西北太平洋西侧和中部东伸到西北太平洋的东侧上空;而在季风槽弱的年份,季风槽和强对流活动区主要位于南海和西北太平洋西侧和中部上空,季风槽强度的年际变化对它的季节变化也有重要影响。此外,研究还表明了随着季风槽的季节和年际变化,西北太平洋TCs生成的大尺度环境因子分布也发生很明显的变化。