An extreme drought occurred over Southeast China (SEC) in August 2019. We demonstrate synergistic effects of midlatitude and tropical circulation on this extreme event and highlight the impacts of the coupling and locking of two cyclones at different latitudes, which are otherwise ignored. We propose the relaying roles of the Tibetan Plateau (TP) and western North Pacific in connection with the tropical convection and SEC precipitation. The equivalent-barotropic anticyclone over the TP and lower-tropospheric cyclone over the western North Pacific both resulted from the positive Indian Ocean dipole and El Nino Modoki. The equivalent-barotropic cyclone over Northeast China originated from the dispersion of Rossby waves upstream along the subtropical waveguide associated with the North Atlantic tripole sea surface temperature anomaly pattern and the Rossby wave response to the TP precipitation deficiency. Further, they jointly contributed to this drought by inducing strong northerly wind anomalies in the entire troposphere over East China. These anomalous northerly winds led to decreased warm moisture from the south and substantial sinking motions, which inhibited the occurrence of the SEC local convection and precipitation. The SEC precipitation is closely related to convection over the Maritime Continent from a climate perspective. This relationship is verified by observations, linear baroclinic model experiments, and general circulation model sensitivity experiments with and without the TP, in which precipitation anomalies over the southern TP and Philippine Sea play important bridge roles. The results will advance the prediction of the SEC extreme drought events.

Several probabilistic forecast methods for heatwave (HW) in extended-range scales over China are constructed using four models (ECMWF, CMA, UKMO, and NCEP) from the Subseasonal-to-Seasonal (S2S) database. The methods include four single-model ensembles (SME; ECMWF, CMA, UKMO, and NCEP), multi-model ensemble (MME), and Bayesian model averaging (BMA). The construction and verification of reforecasts are implemented by a defined heat wave index (HWI) which is not only able to reflect the actual occurrence of heatwaves, but also to facilitate forecast and verification. The performance is measured by traditional verification method at each grid point of the 105°E to 132°E; 20°N to 45°N domain for the July, August, and September (JAS) of 1999–2010. For deterministic evaluations of HWI forecast, BMA shows a better pattern correlation coefficient than SME and MME and comparable equitable threat score (ETS) with ECMWF and MME. The good performance of ECMWF and MME take advantage of setting the percentile thresholds for forecasting HW. For the probabilistic forecast, the Brier score of BMA is comparable (superior) to that of MME and ECMWF at short (long) lead-time. BMA also demonstrates an improvement on the reliability of probabilistic forecast, indicating that BMA method is a useful tool for an extended-range forecast of HW. Meanwhile, in the real-time extended-range probabilistic forecast, the beginning date, end date, and probability of HW event can be predicted by the HWI probabilistic forecast of BMA.

基于1979～2018年观测的向外长波辐射(outgoing longwave radiation,OLR)资料和其他多种再分析资料,发现西太平洋暖池对流存在3类显著的月际变化.第一类为OLR在6月和8月为负异常而7月为正异常;第二类与第一类完全相反;第三类为OLR在6～7月为正异常,8月为负异常.3类月际变化与ENSO循环的背景有关,前两类发生在较弱的La Ni?a年和El Ni?o发展年,与春季暖池海温异常有关.当前一个月海温偏高时,后一个月对流偏强,造成局地海温降低,偏低的海温又反过来抑制了后一个月的对流发展,因此暖池地区局地海气相互作用在这两类月际变化中起到关键作用.与前两类不同的是,第三类月际变化发生在El Ni?o衰减年,与春季热带印度洋海温偏高有关.热带印度洋海温偏高造成印度附近对流在6～7月间增强,通过东传Kelvin波抑制了暖池对流发展.同时,印度附近对流偏强造成8月印度洋海温降低和对流减弱,对暖池对流的影响因而减弱.另一方面,6～7月暖池对流偏弱造成8月暖池海温升高,结果造成暖池对流增强.因此,第三类月际变化受到热带印度洋强迫以及暖池地区局地海气相互作用的共同影响.

Abstract With the tropical cyclone (TC) size parameter defined as the radius of 17‐m·s−1 oceanic surface wind, 225 TC cases were recorded in the western North Pacific during 2000–2009 based on the QuikSCAT near‐surface wind vector database and the best‐track dataset. In accordance with the symmetry index (the ratio of minimum and maximum quadrant sizes), the TCs were classified into symmetric and asymmetric structures. The asymmetric TCs were divided into four types: the northeast, southeast, southwest, and northwest. The spatio‐temporal characteristics of these TC types were further investigated. The monthly variation and the spatial distribution of maximum quadrant TC size exhibited significant differences among the four types. By contrast, the quadrant size of the symmetric TCs from June to November showed few changes (2.6°–2.7° latitude). It was found that the TC lifetime is an important factor affecting the quadrant TC size because of its close relationship with the activities of the western Pacific subtropical high. In addition, the climatological mean circulation also has a notable influence on the quadrant TC size through superposition of prominent background wind. Symmetric TCs are more likely to occur in the oceanic region where the background low‐level wind is the weakest.

基于1979~2013年多种再分析资料，合成分析了El Nino发展年和La Nina年东亚夏季风的季节内变化。结果表明，东亚夏季风在两种情况下呈现出不同的季节内变化特征。在El Nino发展年，初夏期间高纬度地区出现偏北风异常，造成东亚地区位势高度场偏低，西太平洋副热带高压偏东，但均不显著。盛夏期间，El Nino强迫造成中太平洋对流增强，副热带西太平洋出现气旋异常，位势高度显著降低，副热带高压明显偏东。与此不同的是，La Nina年春季暖池海温偏高，造成夏季对流偏强，西太平洋地区位势高度场偏低，副热带高压减弱东退。此外，La Nina年东亚夏季风的季节内变化较为复杂，6月异常较弱，7月达到最强，8月又开始减弱。因此，虽然El Nino发展年和La Nina年夏季平均副高异常有一定的相似性，但季节内变化则有很大差异，其成因也完全不同。

Evident sea surface temperature anomalies (SSTAs) were not observed over the Pacific and Indian Oceans in the summer and the preceding winter and spring of 1980 and 1981.Yet intraseasonal variation of the East Asian summer monsoon (EASM) circulation exhibited significant anomalies in both years with large differences. The western Pacific subtropical high (WPSH) experienced a much earlier first jump and an obviously later second jump in 1980, while a near-normal first jump and a notably earlier second jump were found in 1981. It was also noted that the jump processes in both years were influenced by different factors. In 1980, both jumps were induced by the enhancement of tropical western Pacific convection. In 1981, however, both jumps were attributed to the phase-locking of the poleward propagation of Rossby wave trains induced by the intensified convection in the tropical western Pacific and the eastward propagation of Rossby waves in the middle and high latitudes. Compared with the jump processes, the maintenance of stable circulations during pre- and post-jumps was more important. Due to the cooperation of the southern hemispheric circulation and circulation in the middle and high latitudes of the Northern Hemisphere at various periods, the extent of the WPSH anomaly in the summer of 1980 was comparable to that in strong El Ni?o decay years like 1983 and 1998. Specifically, the abnormally strong WPSH in June and August played a much more important role in the remarkable anomaly of summer-mean WPSH. Note that the stronger than normal WPSH in June and August was related to the enhancements of the Mascarene high (MH) and the Australian high (AH), respectively. In addition, the WPSH tended to shift southward in July and August since the blocking high in the Okhotsk Sea persisted for a long time. In contrast, the MH and AH in the summer of 1981 were relatively weak, resulting in a weak influence on the WPSH. Instead, the intraseasonal variation of circulation in the middle and high latitudes played a leading role in the rapid northward advance and subsequent southward retreat of WPSH. In particular, the prevalence of meridional circulation in August led to a weaker WPSH that shifted eastward. As a result, the whole summer-mean WPSH tended to be weaker than normal as well. The case study showed that in the absence of SSTA forcing, special attention should be paid to the influence of the southern hemispheric circulation and circulation in the middle and high latitudes of the Northern Hemisphere on the intraseasonal evolution of the WPSH and associated EASM circulation. On the other hand, both factors are difficult to be used in the seasonal prediction due to their relatively short periods of maintenance.

Based on various reanalysis datasets during 1979-2013, we compare the intraseasonal variation of the East Asian summer monsoon (EASM) between El Ni?o developing years and La Ni?a years. It is shown that the EASM exhibits different features in the intraseasonal variation under the two situations. During the early summer of El Ni?o developing years, there exist weak northerly anomalies in the high latitudes, which induce negative geopotential height anomalies over East Asian and a slightly eastward retreat of the western Pacific subtropical high (WPSH). During the late summer, a cyclonic anomaly is induced in the subtropical western Pacific with enhanced convection in the central Pacific due to El Ni?o forcing. As a result, geopotential height reduces significantly and the WPSH tends to retreat eastward remarkably. In La Ni?a years, however, the warm pool convection in the summer clearly enhances due to warmer sea surface temperature in the western Pacific in the spring. Accordingly, geopotential height reduces and the WPSH tends to retreat eastward. Besides, the intraseasonal variation of the East Asian summer monsoon exhibits complicated characters in La Ni?a years with the strongest anomaly occurring in July and relatively weak anomaly occurring in June and August. Although the summer mean WPSH in El Ni?o developing years is somewhat similar to that in La Ni?a years, there is a significant discrepancy in the intraseasonal variation. More importantly, the physical mechanisms for the intraseasonal variation are completely different.

本文对比分析了1998年和2016年这两个强El Nino衰减年东亚夏季风的季节内变化。结果表明，在6~7月期间，由于热带印度洋海温偏高、对流偏强，造成西太平洋暖池对流偏弱，西太平洋副热带高压（副高）偏西偏强，长江流域降水偏多，华南偏少，东亚夏季风异常具有典型的El Nino衰减年特征。但两年的8月份有很大差异，虽然1998年8月与6~7月相似，但2016年8月份则完全不同。受乌拉尔地区异常反气旋的影响，源自西伯利亚东部的北风异常穿越东亚并直抵暖池地区，造成副高分裂并减弱东退，同时激发暖池对流发展，而对流的发展则进一步促使副高减弱。因此，2016年8月东亚夏季风异常与1998年8月相反，中国北方夏季降水异常也呈现很大差异。另外，1998年热带大西洋偏暖，并通过热带环流变化影响到东亚夏季风异常，其强迫作用与热带印度洋类似。而2016年大西洋海温异常较弱，对东亚夏季风影响也较弱。因此，El Nino对东亚夏季风的影响不仅与其强度有关，还与El Nino衰减之后造成的印度洋和大西洋海温异常有关。本文的分析结果表明，即使在强El Nino衰减年夏季，由于El Nino之间的个性差异以及其他因子的影响，东亚夏季风季节内变化仍然能呈现出显著差异，特别是在8月份。因此，在预测东亚夏季风异常时，宜将6~7月和8月分别考虑。此外，为进一步提高东亚夏季风预测水平，除传统的季度预测外，还需要进一步加强季节内尺度的预测。

Based on the NCEP Reanalysis 2 data,the variation of geopotential heights during the rebuilding process of the Meiyu over the Yangtze River valley in July 1998 is analyzed by using the linear geopotential equation.It is revealed that the rebuilding of the Meiyu is due to the southward retreat and enhancement of the western Pacific subtropical high (WPSH),which is mainly characterized by respective decrease and increase of geopotential heights in the northern and southem part of the WPSH.This process consists of two stages,i.e.,the first stage of decreasing and southwardretreating,and the second stage of increasing and maintaining.Moreover,the weather systems in high and low latitudes play a collective role in the adjustment process.The vorticity-related geostrophic mode and the friction divergence are the two most important items.Specifically,without considering the boundary effect,the former's contribution order is 101 gpm and the latter's is 100 gpm,yet with the contribution constrained to the lower level of the troposphere.Due to deep depression troughs induced by Rossby waves in the middle and high latitudes,the geopotential height to the north of WPSH tends to decrease persistently under the impact of a vorticity-related geostrophic mode.Similarly,it is also this item that makes geopotential heights to the south turn to increase after reaching a minimum because of the northward movement of an equatorial anticyclone.Through positive feedback,the friction divergence item tends to accelerate the aforementioned decrease and increase trends.

During summertime,the WPSH (western Pacific subtropical high) exhibits two northward jumps.The first jump signals the termination of pre-flood period in southern China and the start of the Meiyu over the Yangtze-Huaihe River valley;the second jump indicates the termination of the Meiyu and the start of rainy season in northern China.Based on the fast Fourier transformation and composite analysis of observational and reanalysis data,the authors investigated the impact of the intraseasonal oscillations (ISOs) on various timc-scales on the northward jumps in the normal and abnormal years.The dominant periods of the ISOs are different in normal and abnormal years,i.e.10-20 days and quasi-60 days in normal years,30-60 days in abnormal years of the first jump and earlier years of the second jump,10-20 days and 30-60 days in later years of the second jump.During the annual cycle,the ISOs tend to propagate northward in the East Asian-tropical northwestern Pacific region,leading to the northward jump of the WPSH.With the northeastward propagation of the ISOs over the Indian monsoon region,a more remarkable first jump is observed due to the eastward extension of the westerly.By contrast,the biweekly oscillation of the warm pool convection triggered by the cold air invasion from the Australian high plays an important role in a more evident second jump during the normal and later years.The sea surface temperature anomaly over the northwestern Indian Ocean in the preceding spring leads to the phase migration of local ISOs and associated abnormal first jump.Besides,the time-scale and amplitude of the ISOs in the warm pool can be regulated by ENSO,resulting in the abnormal second jump.

Based on air temperature from ECMWF data, three groups of training period schemes that include original schemes, improved schemes and application schemes are designed to test and compare results of daily maximum and minimum temperature of national stations in Fujian Province twice per day from 2014 to 2015. For the quasi-symmetrical mixed running training period (QSRTP) method, several days of current year before forecasting day and equal numbers of days in last year after are adopted separately to compose an initial 1-year or multi-year dataset. When using multiple-year data, lengths of the optimal training period every year are different or the same in each scheme. From three original schemes, it is found that the methods of QSRTP are much better than other running training period methods and traditional fixed period classification, and the number of optimal training day is more stable. The QSRTP with 2-year data shows better performance than that with 1-year data for comparison of improvement schemes, and different training period lengths lead to better performance in 2-year evaluation schemes. Considering model version updating and the continuity of weather, lengths of the optimal training period before the first year are slightly longer than that in the second year. Similar to the traditional method, the optimal training period of original and improved schemes are obtained after verification rather than before forecast. Three application schemes with different terms of evaluation are designed to test the stability, the usability and seasonal patterns of the optimal training period. Based on 1-year evaluation, total samples of the training period are stable with the best forecasting score. In terms of the monthly evaluation, the best period has no significant patterns with a relatively low score. For 10-day evaluation, the best period varies greatly, but when the forecasting time becomes shorter, the forecast quality becomes better. When the weather changes suddenly, the optimal training days will be quite different. Hindcasting experiment of 2015 suggests that the MOS forecast for daily maximum and minimum temperature using the optimal training period of last year has a much higher score than the original ECMWF products, it is better than the subjective forecast, and the forecast absolute deviation is significantly reduced. With the accumulation of data in the future, the forecast quality would be improved greatly, indicating that the method of multi-year QSRTP has an important application prospect on the daily operation.

On the basis of more than 200-year control run, the performance of the climate system model of Chinese Academy of Sciences（CAS-ESM-C） in simulating the El Ni?o-Southern Oscillation（ENSO） cycle is evaluated, including the onset, development and decay of the ENSO. It is shown that, the model can reasonably simulate the annual cycle and interannual variability of sea surface temperature（SST） in the tropical Pacific, as well as the seasonal phase-locking of the ENSO. The model also captures two prerequisites for the El Ni?o onset, i.e., a westerly anomaly and a warm SST anomaly in the equatorial western Pacific. Owing to too strong forcing from an extratropical meridional wind, however, the westerly anomaly in this region is largely overestimated. Moreover, the simulated thermocline is much shallower with a weaker slope. As a result, the warm SST anomaly from the western Pacific propagates eastward more quickly, leading to a faster development of an El Ni?o. During the decay stage, owing to a stronger El Ni?o in the model, the secondary Gill-type response of the tropical atmosphere to the eastern Pacific warming is much stronger, thereby resulting in a persistent easterly anomaly in the western Pacific. Meanwhile, a cold anomaly in the warm pool appears as a result of a lifted thermocline via Ekman pumping. Finally, an El Ni?o decays into a La Ni?a through their interactions. In addition, the shorter period and larger amplitude of the ENSO in the model can be attributed to a shallower thermocline in the equatorial Pacific, which speeds up the zonal redistribution of a heat content in the upper ocean.

Based on a simulation using a newly developed climate system model（Chinese Academy of Sciences-Earth System Model-Climate system component, CAS-ESM-C）, the author investigated the Aleutian Low- Icelandic Low Seesaw（AIS） and its decadal variation. Results showed that the CAS-ESM-C can reasonably reproduce not only the spatial distribution of the climatology of sea level pressure（SLP） in winter, but also the AIS and its decadal variation. The period 496–535 of the integration by this model was divided into two sub-periods: 496–515（P1） and 516–535（P2） to further investigate the decadal weakening of the AIS. It was shown that this decadal variation of the AIS is mainly due to the phase transition of the Pacific Decadal Oscillation（PDO）, from its positive phase to its negative phase. This transition of the PDO causes the sea surface temperature（SST） in the equatorial eastern（northern） Pacific to cool（warm）, resulting in the decadal weakening of mid-latitude westerlies over the North Pacific and North Atlantic. This may be responsible for the weakening of the inverse relation between the Aleutian Low（AL） and the Icelandic Low（IL）.

This study focuses on the intraseasonal variation of the East Asian summer monsoon (EASM) simulated by IAP AGCM 4.0, the fourth-generation atmospheric general circulation model recently developed at the Institute of Atmospheric Physics, Chinese Academy of Sciences. In general, the model simulates the intraseasonal evolution of the EASM and the related rain belt. Besides, the model also simulates the two northward jumps of the western Pacific subtropical high (WPSH), which are closely related to the convective activities in the warm pool region and Rossby wave activities in high latitudes. Nevertheless, some evident biases in the model were found to exist. Due to a stronger WPSH, the model fails to simulate the rain belt in southern China during May and June. Besides, the model simulates a later retreat of the EASM, which is attributed to the overestimated land-sea thermal contrast in August. In particular, the timing of the two northward jumps of the WPSH in the model is not coincident with the observation, with a later jump by two pentads for the first jump and an earlier jump by one pentad for the second, i.e., the interval between the two jumps is shorter than the observation. This bias is mainly ascribed to a shorter oscillating periodicity of convection in the tropical northwestern Pacific.

Based on the pentad mean ridgeline index of the western Pacific subtropical high (WPSH), the authors identified the two northward jumps of the WPSH from 1979 to 2008 and revealed their associations with the tropical SST anomalies. The authors show that the northward jumps, especially the second jump, exhibited remarkable interannual variability. In addition, the authors find that the two northward jumps were mutually independent and were influenced by the SST anomalies in the different regions of the tropical Pacific. The first jump was positively correlated with the SST anomalies in the tropical central Pacific from the preceding winter to June. In contrast, the second jump was positively related to ENSO in the preceding winter, but this correlation tended to weaken with the decay of ENSO and disappeared in July. Instead, a positive correlation was found in the Indian Ocean. We therefore suggest that ENSO plays an indirect role in the second jump through the capacitor effect of the Indian Ocean.

Based on routine meteorological data and the method of the normalized finite temporal variation(NFTV),the evolution features of East Asian summer monsoon(EASM) circulation are analyzed.It is found that there exist two distinct subseasonal abrupt changes in East Asia during the summertime,characterized primarily by two eastward contractions and northward jumps of the western Pacific subtropical high(WPSH),one of which is in middle June and the other is in late July.Because of the close relationship between the WPSH and rainfall distributions in East Asia,the rain belt in East Asia exhibits two obvious northward jumps in the seasonal evolution as well,corresponding to the beginning of the Meiyu period from the Yangtze-Huaihe valley to Japan and the rainy season in North China and Northeast China,respectively.Compared with the first jump,the second jump of the WPSH is much more evident.The first jump is mainly caused by the enhancement of the convective activities in the South China Sea(SCS) while the second jump is influenced by both the convective activities over the western Pacific warm pool(WPWP) and the circulation systems in high latitudes.Through the phase-locking of the northeastward propagation of the Rossby wave trains from the WPWP and the downstream propagation of the Rossby waves in high latitudes,both the convective activities over the WPWP and the circulation systems in high latitudes play a key role in the second northward jump of the WPSH.In addition,the interactions between the WPSH and the release of the latent heat on its western edge lead to the intraseasonal low-frequency oscillation of the WPSH.The analyses of NFTV indicate that the adjustment of the low-level circulation in high latitudes tends to weaken gradually with the seasonal evolution,which is related to the temperature difference between middle and high latitudes.By contrast,the adjustment of the high-level circulation tends to intensify with the altitude during the latter half of the summertime.Besides,the evolution of the similarity also shows that the atmospheric circulation in East Asia exhibits a distinctly different state after the second jump of the WPSH.The Southern Hemisphere circulation plays an important role in the enhancement of the convective activities in the SCS and the WPWP.In middle June,the enhancement of the convective activities in these two regions is due to the intensification and eastward extension of the westerly on the western edge of the SCS,which is deeply involved with the Mascarene high(MH).In middle July,the enhancement of the Australian high(AH) leads to the intensification of the cross-equatorial flow on its northeastern edge,and a large amount of cold air from the Southern Hemisphere invades into the warm pool region,which increases the atmospheric instability and the low-level convergence over this region.As a result,the convective activities in the warm pool are enhanced.In the first half of the summertime,however,the relationship between the intensity of the AH and the cross-equatorial flow on its northeastern edge can be modulated by the convective activities in the WPWP,resulting in an opposite trend between the AH and the associated cross-equatorial flow.During the austral wintertime,the AH tends to weaken with the low-frequency oscillation,which is affected by both the surface temperature in Australia and the energy dispersion of the upstream MH.The weakening trend of the AH is influenced by the former while the MH plays a dominant role in the low-frequency oscillation of the AH.

根据1958—2002年的再分析资料,研究了夏季西太平洋副高的年代际变化及其与南半球环流变化的联系.20世纪70年代末之后,副高偏向西南,强度增强,但这种变化趋势在6—8月间逐渐趋弱.从全球范围看,最显著的变化发生在南半球,南极涛动趋于正位相,索马里急流偏强,致使副高偏向西南,强度增强.同时,由于副高在夏季期间向北移动,南半球环流变化对副高的影响在6—8月间逐渐减弱,这与副高的变化趋势相吻合.