Interannual variability in the onset of the summer monsoon over the Eastern Bay of Bengal

Summary Climatological characteristics associated with summer monsoon onset over the eastern Bay of Bengal (BOB) are examined in terms of the westerly-easterly boundary surface (WEB). The vertical tilt of the WEB depends on the horizontal meridional temperature gradient (MTG) near the WEB, under the constraint of the thermal wind balance. The switch in the WEB tilt firstly occurs between 90 and 100°E during the first pentad of May. At this time the 850 hPa ridgeline splits over the BOB and heavy rainfall commences over the eastern BOB, indicating the onset of the BOB summer monsoon (BOBSM). The area-averaged MTG (200–500 hPa) is proposed as an index to define the BOBSM onset. A comparison of the onset determined by the MTG, 850 hPa zonal wind, and outgoing longwave radiation (OLR) shows that the MTG index is the most effective in characterizing the interannual variability of the BOBSM onset. Strong precursor signals are found prior to an anomalous BOBSM onset. Composite results show that early (late) BOBSM onset follows excessive (deficient) rainfall over the western Pacific and anomalous lower tropospheric cyclonic circulation which extends zonally from the northern Indian Ocean into the western Pacific, and strong (weak) equatorial westerly anomalies in the preceding winter and spring. Prior to an early (late) BOBSM onset, significant positive (negative) thickness anomalies exist around the Tibetan Plateau, accompanied by anomalous upper tropospheric anticyclonic (cyclonic) circulation. The interannual variations of the BOBSM onset are significantly correlated with anomalous sea surface temperature related to ENSO. These occurs through changes in the Walker circulation and local Hadley circulation, leading to middle and upper tropospheric temperature anomalies over the Asian sector. The strong precursor signals around the Tibetan Plateau may be partly caused by local snow cover anomalies, and an early (late) BOBSM onset is preceded by less (more) snow accumulation over the Tibetan Plateau during the preceding winter.     

Influence of PDO on South Asian summer monsoon and monsoon–ENSO relation

This study has investigated the possible relation between the Indian summer monsoon and the Pacific Decadal Oscillation (PDO) observed in the sea surface temperature (SST) of the North Pacific Ocean. Using long records of observations and coupled model (NCAR CCSM4) simulation, this study has found that the warm (cold) phase of the PDO is associated with deficit (excess) rainfall over India. The PDO extends its influence to the tropical Pacific and modifies the relation between the monsoon rainfall and El Niño-Southern Oscillation (ENSO). During the warm PDO period, the impact of El Niño (La Niña) on the monsoon rainfall is enhanced (reduced). A hypothesis put forward for the mechanism by which PDO affects the monsoon starts with the seasonal footprinting of SST from the North Pacific to the subtropical Pacific. This condition affects the trade winds, and either strengthens or weakens the Walker circulation over the Pacific and Indian Oceans depending on the phase of the PDO. The associated Hadley circulation in the monsoon region determines the impact of PDO on the monsoon rainfall. We suggest that knowing the phase of PDO may lead to better long-term prediction of the seasonal monsoon rainfall and the impact of ENSO on monsoon.     

太平洋海温异常对其上空环流影响的分析及数值试验

本文首先用《逐月全球气候资料》中的实测风资料对80年代的两次El Nino。事件(1982—1983年和1986—1987年)太平洋地区上空环流的变化情况进行了分析研究,然后用一个九层原始方程模式对其进行数值模拟试验. 观测资料分析及数值模拟结果皆表明,赤道东太平洋的这两次海温异常都使其上空的环流发生明显的变化,但由于两次海温异常的强度、范围和延续时间有所不同,对环流的影响也不相同.本文将简要地介绍这两次El Nio期间太平洋上空环流的演变特征及数值试验的结果.     

Biases in AMIP model simulations of the east China monsoon system

 AMIP model simulations of the east China (5–50°N; 105–122°E) monsoon system are analyzed to study coherent relationships between rainfall and wind annual cycle biases. A comparison with observed interannual variability patterns is carried out to identify the physical processes that explain the biases. The analyses show that poleward displacement of the simulated east Asian jet stream causes the ascending branch of the jet-induced transverse circulation to move north and, as a consequence, produces negative (positive) rainfall biases occur in central (northeast) China. The model simulations show decreased southwesterly flow and ITCZ rainfall over the South China Sea when weaker (versus observations) summer Hadley and Walker circulations are present. This results from diminished model tropical disturbance activity, and highlights the importance of air-sea interactions. In addition, during October–January, intensified model low-level easterlies enhance moisture transport and produce positive local rainfall biases over central and northeast China. Biases in the east China monsoon system are concurrently reflected in the planetary circulation. Enhanced northeast China rainfall results from increased surface pressure over the North Pacific and an amplified zonal pressure gradient along the east China coast. This bias pattern is associated with differences in model representations of topography. On the other hand, the South China Sea experiences an extensive elongated meridional rainfall bias dipole structure that straddles the equator. This is accompanied by a baroclinic vertical pattern over the tropics as well as a barotropic wave train that extends from Australia to the Antarctic, where the teleconnection is likely a direct atmospheric response to tropical convective heating. Received: 20 June 2000 / Accepted: 17 September 2000     

1991 年夏暴雨期间Walker 环流和Hadley 环流的特征

该文分析了1991年7月中国江淮暴雨洪水期间赤道太平洋沃克环流及沿115°E哈得莱环流的逐日演变特征。结果发现在暴雨时期赤道东风减弱，西风加强，反哈得莱环流（即季风环流）在中国东部上空加强。在暴雨期间，高低空散度风场的分析指出，在低空出现明显的散度风越赤道气流，而在对流层上部出现散度风的流出。     

Intraseasonal oscillations of the monsoon circulation over South Asia

The space–time structure of the three-dimensional circulation over the South Asian monsoon region has been studied using the ERA-40 reanalysis of the European Centre for Medium-Range Weather Forecasts. Applying multi-channel singular spectrum analysis on combined daily values of horizontal winds and pressure vertical velocity at ten vertical levels for the period 1958–2001, two leading intraseasonal nonlinear oscillations were extracted. The first oscillation has an average period of 50?days and propagates northeastward from the Indian Ocean to the Indian subcontinent. The second oscillation has a period of 30?days and propagates northwestward from the West Pacific to the Indian region. Both the oscillations exhibit the oscillatory and propagation features at all vertical levels from 1,000 to 100?hPa. The two oscillations correspond well with similar oscillations found in outgoing longwave radiation and precipitation in earlier studies. The wind oscillations also account for the active and break phases of the Indian monsoon. The vertical structures and propagation of specific humidity and temperature are found to be consistent with those of the winds in each oscillation. The structure and movement of regional Hadley and Walker circulations have also been described. The analyses provide further strong evidence for the existence of two distinct monsoon intraseasonal oscillations.     

东亚夏季风降水年代际变异模态及其与太平洋年代际振荡的关系

利用全球陆地月平均降水资料、英国气象局哈德莱中心的月平均海表温度距平(SSTA)资料及NCEP/NCAR再分析大气环流资料,探讨东亚夏季风降水年代际变率及其与太平洋年代际振荡(PDO)的联系。研究指出:东亚夏季风降水年代际变异模态以及PDO均在1976年前后呈现显著的年代际转折,并且东亚夏季风降水与PDO在年代际尺度上具有较好的相关关系。PDO能够在对流层低层激发出与年代际东亚夏季风环流较为相似的大气环流异常特征,表明东亚夏季风环流的年代际变化可能受大气外强迫因子PDO在对流层低层的外源强迫作用影响,最终导致东亚夏季风降水发生年代际变化。     

Simulation of Indian summer monsoon circulation and rainfall using RegCM3

Summary The Regional Climate Model RegCM3 has been used to examine its suitability in simulating the Indian summer monsoon circulation features and associated rainfall. The model is integrated at 55 km horizontal resolution over a South Asia domain for the period April–September of the years 1993 to 1996. The characteristics of wind at 850 hPa and 200 hPa, temperature at 500 hPa, surface pressure and rainfall simulated by the model over the Indian region are examined for two convective schemes (a Kuo-type and a mass flux scheme). The monsoon circulation features simulated by RegCM3 are compared with those of the NCEP/NCAR reanalysis and the simulated rainfall is validated against observations from the Global Precipitation Climatology Centre (GPCC) and the India Meteorological Department (IMD). Validation of the wind and temperature fields shows that the use of the Grell convection scheme yields results close to the NCEP/NCAR reanalysis. Similarly, the Indian Summer Monsoon Rainfall (ISMR) simulated by the model with the Grell convection scheme is close to the corresponding observed values. In order to test the model response to land surface changes such as the Tibetan snow depth, a sensitivity study has also been conducted. For such sensitivity experiment, NIMBUS-7 SMMR snow depth data in spring are used as initial conditions in the RegCM3. Preliminary results indicate that RegCM3 is very much sensitive to Tibetan snow. The model simulated Indian summer monsoon circulation becomes weaker and the associated rainfall is reduced by about 30% with the introduction of 10 cm of snow over the Tibetan region in the month of April.     

Reconstruction of Northern Hemisphere 500 hPa geopotential heights back to the late 19th century

Summary In this study the authors have developed a statistical method and have reconstructed Northern Hemisphere 500 hPa heights back to the late 19th century using one temperature and three sea level pressure (SLP) data sets. First, the relationship between ERA40 500 hPa heights and surface temperature and SLP was screened using stepwise multiple regression based on the calibration period of 1958–2002 (1998/2000 according to the availability of SLP data). All selected predictors (temperature and SLP) were significant and their variance contribution was greater than 1%. On average, there were 8.1 variables retained in the final regression equations. Second, the regression equations were applied to compute the 500 hPa height through to the late 19th century for the whole Northern Hemisphere. As the SLP and temperature coverage improved over time, the number of predictors decreased by about 1 in the most recent periods, and the root mean squared error decreased by about 0.8 m. A leave-one-out cross-validation method was used to test the skill and stability of the regression models. The reduction of error during the cross-validation period of 1958–1997 varied from 0.33 to 0.56, depending on the SLP data. Reconstructions were also checked using NCEP/NCAR 500 hPa heights from January 1949 to December 1957, and compared with the historical reconstruction over Europe. Reconstructions show high consistency with these independent data sets. Generally, the reconstruction provides a valuable opportunity to analyze, as well as to validate climate simulations of the variability in free atmosphere circulations over the past one hundred years.     

Vertical circulation in the tropical atmosphere during extreme El Niño-Southern Oscillation events

Scenarios for the development of large-scale vertical circulation anomalies during warm and cold phases of El Niño-Southern Oscillation are generalized based on the NCEP/NCAR reanalysis data for 1958-1998. Composite models of the cells of vertical circulation in the monsoon and trade-wind regions of the tropical Pacific are obtained for the first time for El Niño and La Niña separately. An unprecedented shift of the ascending branch of the zonal Walker circulation from the “maritime continent” of Indonesia to the east, to the central and eastern Pacific, was observed during the warm phase over the tropical Pacific; this shift was accompanied by an abrupt increase in the tropical cyclogenesis activity in the southern Pacific zone of convergence. On the contrary, during the cold phase, the ascending motions in the region of the summer Australian monsoon are subject to abrupt intensification. The reconstruction of the vertical meridional circulation during the warm phase manifested itself in the almost complete disappearance of the Hadley classic circulation over the central Pacific, characteristic of the trade-wind intertropical convergence zone (ITCZ), and in its replacement by the latitudinal monsoon circulation typical of the ITCZ over the Indian Ocean. During a cold phase, the Hadley circulation is both restored and intensified.     

Intraseasonal variability of the Tropical Easterly Jet

Summary By analyzing 12-year (1979–1990) 200 hPa wind data from National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis, we demonstrate that the intraseasonal time scale (30–60 days) variability of the Tropical Easterly Jet (TEJ) reported in individual case studies occurs during most years. In the entrance region (east of ∼70° E), axis of the TEJ at 200 hPa is found along the near equatorial latitudes during monsoon onset/monsoon revivals and propagates northward as the monsoon advances over India. This axis is found along ∼5° N and ∼15° N during active monsoon and break monsoon conditions respectively. Examination of the European Centre for Medium Range Weather Forecasts reanalysis wind data also confirms the northward propagation of the TEJ on intraseasonal time scales. During the intraseasonal northward propagations, axis of the TEJ is found about 10°–15° latitudes south of the well-known intraseasonally northward propagating monsoon convective belts. Because of this 10°–15° displacement, axis of the TEJ arrives over a location about two weeks after the arrival of the monsoon convection. Systematic shifting of the locations by convection, low level monsoon flow and TEJ in a collective way during different phases of the monsoon suggests that they all may be related.     

东亚季风指数及其与大尺度热力环流年际变化关系

将东西向海平面气压差与低纬度高、低层纬向风切变相结合 ,定义了东亚季风指数 ,该季风指数较好地反映了东亚冬、夏季风变化。其中 ,夏季风指数年际异常对西太平洋副热带高压南北位置变化和长江中下游旱涝具有较强的反映能力。分析表明 :东亚夏季风年际变化与印度洋 -西太平洋上空反 Walker环流及夏季越赤道南北半球间的季风环流呈显著正相关关系。在强、弱异常东亚夏季风年份 ,异常的 Walker环流在西太平洋上的辐合 (辐散 )中心在垂直方向不重合 ,高层 ( 2 0 0 h Pa)速度势与东亚夏季风显著相关区域位于西北太平洋上 ,该异常环流的高层的辐合 (辐散 )通过改变低层空气质量而影响夏季 50 0 h Pa西北太平洋副热带高压。采用 SVD分析进一步发现 :与海温耦合的异常 Walker环流在西太平洋上空的上升支表现出南北半球关于赤道非对称结构 ,亚澳季风区受该异常 Walker环流控制。因而 ,东亚季风与热带海气相互作用可直接通过这种纬向非对称的 Walker环流发生联系。     

The impact of tropical western Pacific convection on the North Pacific atmospheric circulation during the boreal winter

In this study, we investigate the impact of atmospheric convection over the western tropical Pacific (100–145°E, 0–20°N) on the boreal winter North Pacific atmosphere flow by analyzing National Center for Environmental Prediction Reanalysis 1, Extended Reconstructed Sea Surface Temperature and Global Precipitation Climatology Project data. The western tropical Pacific convection is not only the main energy source driving the local Hadley and Walker circulations, but it also significantly influences North Pacific circulation, by modifying a mid-latitude Jet stream through the connection with the local Hadley circulation. On the one hand, this strong convection leads to a northward expansion of local Hadley cells simultaneous with a northward movement of the western North Pacific jet because of the close correlation between the Jet and Hadley circulation boundaries. On the other hand, this strong convection also intensifies tropical Pacific Walker circulation, which reduces the eastern Pacific sea surface temperature, resembling a La Nina state through the enhanced equatorial upwelling. The cooling of the eastern tropical Pacific has an inter-tropical convergence zone located further north; thus, the local Hadley circulation moves northward. As a result, the jet axis over the eastern North Pacific, which also corresponds to the boundary of the local Hadley circulation, moves to higher latitude. Consequently, this northward movement of the Jet axis over the North Pacific is reflected as a northwest–southeast dipole sea level pressure (SLP) pattern. The composite analysis of SLP over the North Pacific against the omega (Ω) (Pa/s) at 500 hPa over the western tropical Pacific actually reveals that this northwest-southeast dipole structure is attributed to the intensified tropical western Pacific convection, which pushes the Pacific Jet to the north. Finally we also analyzed south Pacific for the austral winter as did previously to North Pacific, and found that the results were consistent.     

准四年周期的北半球夏季遥相关与东亚夏季风环流的相互关系

利用ＮＭＣ的２００ｈＰａ和８５０ｈＰａ风场资料研究了北半球夏季遥相关和东亚夏季风异常环流在准４年时间尺度上的相互关系。研究发现当北半球夏季出现积雪强迫型遥相关时,东亚－西太平洋地区既存在异常纬向环流,也存在异常经向环流;而当出现东亚太平洋型遥相关时,东亚－西太平洋地区以异常纬向环流为主。分析还发现源于北太平洋的异常涡旋在向南传播的过程中,先取西南路径,在到达菲律宾东部以后折向东南。     

冬夏东亚季风环流对太平洋热状况的响应

冬夏隔季韵律关系一直是我国长期天气预报和短期气候预测的一个重要依据,然而迄今为止对它们之间的物理过程及成因机理并不十分清楚。利用NCEP/NCAR全球2.5°×2.5°网格月平均再分析资料,研究1951~2000年冬夏东亚季风环流异常变化与太平洋海面温度(SST)的关系及对关键海温区响应机理。研究指出:冬夏东亚季风环流隔季韵律关系及其年际变化与赤道东太平洋海面温度异常(SSTA)变化密切相关,冬季赤道东太平洋出现La Ni~na(El Ni~no)型的SST分布,有利冬、夏东亚季风环流加强(减弱),其影响过程通过赤道Walker环流强(弱)以及东亚地区Hadley环流强(弱)过程完成。冬季赤道东太平洋海温变化是冬、夏东亚环流季节以及年际变化的一个重要外强迫因子。     

CMIP5 simulated change in the intensity of the Hadley and Walker circulations from the perspective of velocity potential

Based on the simulations of 31 global models in CMIP5, the performance of the models in simulating the Hadley and Walker circulations is evaluated. In addition, their change in intensity by the end of the 21 st century(2080–2099) under the RCP4.5 and RCP8.5 scenarios, relative to 1986–2005, is analyzed from the perspective of 200 h Pa velocity potential.Validation shows good performance of the individual CMIP5 models and the multi-model ensemble mean(MME) in reproducing the meridional(zonal) structure and magnitude of Hadley(Walker) circulation. The MME can also capture the observed strengthening tendency of the winter Hadley circulation and weakening tendency of the Walker circulation. Such secular trends can be simulated by 39% and 74% of the models, respectively. The MME projection indicates that the winter Hadley circulation and the Walker circulation will weaken under both scenarios by the end of the 21 st century. The weakening amplitude is larger under RCP8.5 than RCP4.5, due to stronger external forcing. The majority of the CMIP5 models show the same projection as the MME. However, for the summer Hadley circulation, the MME shows little change under RCP4.5 and large intermodel spread is apparent. Around half of the models project an increase, and the other half project a decrease. Under the RCP8.5 scenario, the MME and 65% of the models project a weakening of the summer southern Hadley circulation.     

The Association Between Circulation Anomaliesin the Mid-Troposphere and Area Burnedby Wildland Fire in Canada

Summary  There is evidence that the area burned by wildland fire has increased in certain regions of Canada in recent decades. One cause for this increase is changes in the mid-tropospheric circulation at 500 hPa over northern North America. This study examines the physical links between anomalous mid-tropospheric circulation over various regions of Canada and wildland fire severity. Analysis of monthly and seasonal burned areas for the period 1953 to 1995 reveals a bimodal distribution with distinct low and extreme high burned area years. The high/low burned area years coincide with positive/negative 500 hPa height anomalies over north-western, western, west-central and east-central Canada. Total area burned and the 500 hPa height anomaly data are analyzed for statistical relationships using the Spearman rank correlation non-parametric measure. Results for the May to August fire season indicate statistically significant correlations between regional total area burned and clusters of anomalous 500 hPa geopotential height values immediately over, and immediately upstream of the affected region. For the north-western and west-central regions, significantly correlated clusters are found in the central Pacific as well, providing evidence of the influence of a teleconnection structure on the summer climate of western and north-western North America. Two sample comparison tests show statistically significant differences in both the means and variances of the fire data populations during negative and positive phases of mid-tropospheric flow, and the means of the height anomaly populations during extremely high and extremely low area burned seasons. Increases in regional total area burned are related to increases in mean 500 hPa heights, taken from the significantly correlated clusters of height values, between two successive periods 1953–74 and 1975–95. For Canada as a whole, the five lowest area burned seasons all occurred during the early period, while the five highest seasons occurred during the later period. The difference in the geopotential height fields between the two periods identifies an increase in 500 hPa heights over most of Canada with an amplification of the western Canada ridge and an eastward shifted Canadian Polar Trough (CPT). Received October 19, 1998     

热带太平洋低层环流主模态与东亚大气环流的可能联系

利用1958—2001年NCEP/NCAR再分析资料,探讨了热带太平洋(100°E~60°W,10°S~10°N)10 m风场的时空变化特征及其与东亚大气环流的可能联系。结果表明:1)热带太平洋风场异常存在两种主模态,第一模态对应中西太平洋一致的西(东)风异常,关于赤道呈准对称分布,与ENSO(El Nio-Southern Oscillation)暖(冷)位相时风场的分布对应;第二模态则关于赤道呈反对称分布,西北太平洋存在显著的反气旋(气旋)式环流,中太平洋异常西风不再位于赤道上,而是南移到了10°S左右,对应ENSO暖(冷)位相向相反位相转换时的风场分布特征。2)两模态时间系数的主振荡周期不同,与ENSO循环的位相关系也不同。研究发现,当两模态呈正(负)位相分布时,贝加尔湖南侧(South to Lake Baikal,SLB)容易发生持续的高压(低压)异常环流。3)两模态与SLB异常环流的联系途径不同。第一模态正位相对应热带中东太平洋大范围暖海温引起的二极型Walker环流异常,SLB异常高压不仅能通过东亚沿岸北风和南海低槽的作用促进第一模态的前期发展,还对其后期维持起重要作用。负位相时,情况相反。该环流系统既与热带中东太平洋大范围垂直运动有关,还与邻近的中国东南沿海低层异常辐合有关;第二模态则对应热带西太平洋及东印度洋为主、大西洋为辅的暖海温引起的热带四极型Walker环流异常。此时热带西太平洋到东印度洋局地偏强的经圈Hadley环流可能是SLB异常环流维持的主要原因。     

PREVIOUS CONVECTION ANOMALY IN AUTUMN AND WINTER ASSOCIATED WITH THE GENERAL CIRCULATION OVER EAST ASIA IN WINTER AND SPRING AND APRIL PRECIPITATION IN SHANDONG

Using OLR and 850 hPa and 200 hPa wind fields data (1979 – 2006), this paper diagnoses the characteristics of convection over the tropical area in preceding autumns and winters in association with April precipitation anomalies in Shandong province. It is found that preceding convection anomalies over the Western Pacific Warm Pool in December have close relationships with the April precipitation in Shandong. Further analysis of the relationship with the general circulation over the East Asia shows that the convection anomaly over the Western Pacific Warm Pool has close relationships with the Main East Asian Trough, the Hadley cell over East Asia and the Walker cell. The characteristics of East Asian atmospheric circulation anomalies accompanied with stronger (weaker) convection are consistent with those of less (more) April precipitation anomalies in Shandong. Therefore, the convection anomaly over the tropics in December may be an important indicator for April precipitation in Shandong.