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A cold vortex occurred over the northeastern Tibetan Plateau (TP) on 27 April 2018 and subsequently brought excessive rainfall to the spring farming area in southern China when moving eastward. This study investigates the genesis mechanism of the cold TP vortex (TPV) by diagnosing reanalysis data and conducting numerical experiments. Results demonstrate that the cold TPV was generated in a highly baroclinic environment with significant contributions of positive potential vorticity (PV) forcing from the tropopause and diurnal thermodynamic impact from the surface. As a positive PV anomaly in the lower stratosphere moved towards the TP, the PV forcing at the tropopause pushed the tropospheric isentropic surfaces upward, forming isentropic-isplacement ascent and reducing static stability over the TP. The descent of the tropopause over the TP also produced a tropopause folding over the northeastern TP associated with a narrow high-PV column intruding downwards over the TPV genesis site, resulting in ascending air in the free atmosphere. This, in conjunction with the descending air in the valley area during the night, produced air stretching just at the TPV genesis site. Because the surface cooling at night increased the surface static stability, the aforementioned vertical air-stretching thus converted the produced static stability to vertical vorticity. Consequently, the cold TPV was generated over the valley at night.

On 20 July 2021, northern Henan Province in China experienced catastrophic flooding as a result of an extremely intense rainstorm, with a record-breaking hourly rainfall of 201.9 mm during 0800–0900 UTC and daily accumulated rainfall in Zhengzhou City exceeding 600 mm (“Zhengzhou 7.20 rainstorm” for short). The multi-scale dynamical and thermodynamical mechanisms for this rainstorm are investigated based on station-observed and ERA-5 reanalysis datasets. The backward trajectory tracking shows that the warm, moist air from the northwestern Pacific was mainly transported toward Henan Province by confluent southeasterlies on the northern side of a strong typhoon In-Fa (2021), with the convergent southerlies associated with a weaker typhoon Cempaka (2021) concurrently transporting moisture northward from South China Sea, supporting the rainstorm. In the upper troposphere, two equatorward-intruding potential vorticity (PV) streamers within the planetary-scale wave train were located over northern Henan Province, forming significant divergent flow aloft to induce stronger ascending motion locally. Moreover, the converged moist air was also blocked by the mountains in western Henan Province and forced to rise so that a deep meso-β-scale convective vortex (MβCV) was induced over the west of Zhengzhou City. The PV budget analyses demonstrate that the MβCV development was attributed to the positive feedback between the rainfall-related diabatic heating and high-PV under the strong upward PV advection during the Zhengzhou 7.20 rainstorm. Importantly, the MβCV was forced by upper-level larger-scale westerlies becoming eastward-sloping, which allowed the mixtures of abundant raindrops and hydrometeors to ascend slantwise and accumulate just over Zhengzhou City, resulting in the record-breaking hourly rainfall locally.

Based on a simple coupled Lorenz model, we investigate how to assess a suitable initial perturbation scheme for ensemble forecasting in a multiscale system involving slow dynamics and fast dynamics. Four initial perturbation approaches are used in the ensemble forecasting experiments: the random perturbation (RP), the bred vector (BV), the ensemble transform Kalman filter (ETKF), and the nonlinear local Lyapunov vector (NLLV) methods. Results show that, regardless of the method used, the ensemble averages behave indistinguishably from the control forecasts during the first few time steps. Due to different error growth in different time-scale systems, the ensemble averages perform better than the control forecast after very short lead times in a fast subsystem but after a relatively long period of time in a slow subsystem. Due to the coupled dynamic processes, the addition of perturbations to fast variables or to slow variables can contribute to an improvement in the forecasting skill for fast variables and slow variables. Regarding the initial perturbation approaches, the NLLVs show higher forecasting skill than the BVs or RPs overall. The NLLVs and ETKFs had nearly equivalent prediction skill, but NLLVs performed best by a narrow margin. In particular, when adding perturbations to slow variables, the independent perturbations (NLLVs and ETKFs) perform much better in ensemble prediction. These results are simply implied in a real coupled air-sea model. For the prediction of oceanic variables, using independent perturbations (NLLVs) and adding perturbations to oceanic variables are expected to result in better performance in the ensemble prediction.

An extreme rainfall event occurred over the middle and lower reaches of the Yangtze Basin (MLY) during the end of June 2016, which was attributable to a Tibetan Plateau (TP) Vortex (TPV) in conjunction with a Southwest China Vortex (SWCV). The physical mechanism for this event was investigated from Potential Vorticity (PV) and omega perspectives based on MERRA-2 reanalysis data. The cyclogenesis of the TPV over the northwestern TP along with the lower-tropospheric SWCV was found to involve a midtropospheric large-scale flow reconfiguration across western and eastern China with the formation of a high-amplitude Rossby wave. Subsequently, the eastward-moving TPV coalesced vertically with the SWCV over the eastern Sichuan Basin due to the positive vertical gradient of the TPV-related PV advection, leading the lower-tropospheric jet associated with moisture transport to intensify greatly and converge over the downstream MLY. The merged TPV-SWCV specially facilitated the upper-tropospheric isentropic-gliding ascending motion over the MLY. With the TPV-embedded mid-tropospheric trough migrating continuously eastward, the almost stagnant SWCV was re-separated from the overlying TPV, forming a more eastward-tilted high-PV configuration to trigger stronger ascending motion including isentropic-gliding, isentropic-displacement, and diabatic heating-related ascending components over the MLY. This led to more intense rainfall. Quantitative PV diagnoses demonstrate that both the coalescence and subsequent re-separation processes of the TPV with the SWCV were largely dominated by horizontal PV advection and PV generation due to vertically nonuniform diabatic heating, as well as the feedback of condensation latent heating on the isentropic-displacement vertical velocity.

Interannual variations of the Bay of Bengal summer monsoon (BOBSM) onset in association with El Niño-Southern Oscillation (ENSO) are reexamined using NCEP1, JRA-55 and ERA20C atmospheric and Hadley sea surface temperature (SST) reanalysis datasets over the period 1900–2017. Decadal changes exist in the dependence of the BOBSM onset on ENSO, varying with the Pacific Decadal Oscillation (PDO). A higher correlation between the BOBSM onset and ENSO arises during the warm PDO epochs, with distinct late (early) onsets following El Niño (La Niña) events. In contrast, less significant correlations occur during the cold PDO epochs. The mechanism for the PDO modulating the ENSO-BOBSM onset relationship is through the variations in SST anomaly (SSTA) patterns. During the warm PDO epochs, the superimpositions of the PDO-related and ENSO-related SSTAs lead to the SSTA distribution of an El Niño (La Niña) event exhibiting significant positive (negative) SSTAs over the tropical central-eastern Pacific and Indian Ocean along with negative (positive) SSTAs, especially over the tropical western Pacific (TWP), forming a strong zonal interoceanic SSTA gradient between the TWP and tropical Indian Ocean. Significant anomalous lower tropospheric easterlies (westerlies) together with upper-tropospheric westerlies (easterlies) are thus induced over the BOB, favoring an abnormally late (early) BOBSM onset. During the cold PDO epochs, however, the superimpositions of PDO-related SSTAs with El Niño-related (La Niña-related) SSTAs lead to insignificant SSTAs over the TWP and a weak zonal SSTA gradient, without distinct circulation anomalies over the BOB favoring early or late BOBSM onsets.

ABSTRACTA regional potential vorticity (PV) intrusion (PVI) (RPVI) index, defined as the sum of the numbers of grids containing PVI within a certain area for each day, is used to reflect the day-to-day PVI variability over northeastern China during winter from 1979 to 2016. The synoptic-scale PVI variations and resultant surface air temperature (SAT) anomalies are identified by comparing the high and low RPVI index cases. In high (low) RPVI cases, significantly strong positive (negative) PV anomalies are found in the stratospheric midlatitudes, which intrude downward mostly within 90°–110°E into the upper troposphere to reach around 300 hPa and extend eastward to the east of 120°E, forcing an anomalous cyclonic (anticyclonic) circulation in the middle and lower troposphere over East Asia, with the anomalous northerlies (southerlies) of the forced lower-tropospheric cyclone (anticyclone) leading to significant negative (positive) SAT anomalies of less (greater) than −0.9°C (0.9°C), especially over northeastern China. In the stratosphere, the positive (negative) midlatitude PV anomalies over northern China are actually associated with a weakening (strengthening) of the polar vortex over the Eurasian continent for the high (low) RPVI cases, resulting mostly from positive (negative) barotropic vorticity anomalies associated with static stability due to the meridional shear of anomalous zonal winds on the southern side of the anomalous Eurasian anticyclone (cyclone).

ABSTRACTThis study investigated the contributions of mid–high-latitude circulation anomalies to the extremely hot summer (July and August; JA) of 2018 over Northeast Asia (NEA). The JA-mean surface air temperature in 2018 was 1.2°C higher than that of the 1979–2018 climatology, with the amplitude of such an anomaly almost doubling the interannual standard deviation, making 2018 the hottest year during the analysis period 1979–2018. The abnormal warming over NEA was caused by a local positive geopotential height anomaly reaching strongest intensity in JA 2018. Further investigation suggested that the upper-tropospheric circulation anomalies over northern Europe and the Caspian Sea were crucial to forming this NEA circulation anomaly through initiating downstream wave trains. Particularly, the geopotential heights over these two regions were concurrently at their highest in JA 2018, and therefore jointly contributed to the profound circulation anomaly over NEA and the hottest summer on record. Due to these two teleconnection patterns, the temperature anomalies in NEA are closely related to those in both northern Europe and the Caspian Sea, where the similarly extreme warming also happened in 2018.

Sea surface temperature (SST) in the midlatitude south Indian Ocean (MSIO) exhibits prominent multidecadal fluctuations that have profound climate impacts for regions around the Indian Ocean. Observational analysis suggests that these multidecadal fluctuations can be explained by remote forcing from South Atlantic multidecadal variability. A suite of Atlantic Pacemaker experiments performs well in reproducing the observed MSIO SST multidecadal variation and its association with the South Atlantic. This transbasin covariability can be described by an atmospheric bridge mechanism, in which South Atlantic multidecadal variability‐related SST warming weakens regional meridional circulation over the South Atlantic, suppressing convection over tropical Africa. The reduced diabatic heating then drives a cold atmospheric Kelvin wave that penetrates into the tropical Indian Ocean, leading to an anomalous cyclonic circulation there that weakens the westerlies over the MSIO and subsequently triggers the warming of MSIO. This initial warming is further amplified by local SST‐water vapor positive feedback over the MSIO. The midlatitude south Indian Ocean (MSIO) plays a critical role in global energy distribution and is one of the major windows through which the heat penetrates into deep ocean layers during the global warming hiatus periods. The MSIO sea surface temperature (SST) exhibits pronounced decadal variations, which have widespread climate impacts, affecting the East Asian summer monsoon and Indian monsoon rainfall. However, the underlying mechanism governing this decadal variability remains unclear. In this paper, we find that decadal fluctuation in MSIO SST is remotely synchronized by South Atlantic multidecadal variability (SAMV) based on both observational products and Atlantic Pacemaker experiments. This transbasin covariability is triggered by an atmospheric bridge emanating from the South Atlantic and further amplified by local positive air‐sea feedback over the MSIO. This new insight into SAMV‐MSIO SST connection not only extends our knowledge of the climatic impacts of SAMV but also has implications for interpreting the decadal variability in the East Asian summer monsoon. A multidecadal transbasin SST teleconnection is identified between the South Atlantic and the midlatitude south Indian Ocean (MSIO)Decadal fluctuation in MSIO SST is remotely synchronized by South Atlantic multidecadal variabilityThis transbasin teleconnection is initiated by an atmospheric bridge from the South Atlantic and is amplified by a local air‐sea feedback

Water isotopes‐climate correlations are used to reconstruct paleoclimate from various natural archives. Intra‐seasonal oscillation is one of the major atmospheric patterns that modulate the ratio of precipitation isotope in low latitudes, yet their spatial and temporal distribution patterns are unclear. Here we presented a detailed analysis of how the intra‐seasonal oscillations (MJO, Madden‐Julian Oscillation, and BSISO, Boreal Summer Intra‐seasonal Oscillation) modulate precipitation δ18O and vapor δ18O over the Asian monsoon region and the equatorial region. This analysis found consistent intra‐seasonal variations between the ISO and rain δ18O. We interpret it as the ISO regulating the active and inactive convective systems on the regional scale, leading to the intra‐seasonal oscillations in precipitation δ18O, with amplitudes from 4‰ up to 15‰. The MJO (BSISO) leads to eastward (northeastward) propagation of δ18O intra‐seasonal oscillations in the Asian equatorial (monsoon) region, coinciding with the spatial patterns of OLR oscillations, reflecting the response of precipitation/vapor δ18O to the oscillation of large‐scale convective activity or accumulated depletion of regional precipitation. We proved that the amplitudes of intra‐seasonal variation of precipitation δ18O are comparable to seasonal variations, and both of them are the main patterns for precipitation δ18O in study regions. Our results are conducive to the accurate interpretation of δ18O in climate proxies such as precipitation, ice cores, tree cellulose, cave stalagmites, et al. on the intra‐seasonal scale in this region. Periodicity is a phenomenon in which similar characteristics or states of things reappear steadily and continuously. The Tropical Intra‐seasonal Oscillation (Andersen et al., 2004, https://doi.org/10.1038/nature02805) is the reappearance of a large‐scale envelope of more abundant convective rain systems moving eastward in the tropical regions with a typical oscillatory period of 30–60 days. The ratios of H218O and HDO vary during the cycle of continuous circulation of water in the atmosphere, and therefore are widely used as proxies for climate records. We used the measured data of precipitation or vapor isotopes to reveal that the propagation of ISO can be recorded by the ratio of H218O covering Asian tropical and monsoon regions. Furthermore, the increased (decreased) ISO intensity leads to the increased (decreased) amplitude of convection intensity, which results in the increased (decreased) amplitude of the H218O ratio in precipitation or vapor. The intra‐seasonal oscillations are one of the main patterns of precipitation δ18O covering the Asian monsoon and equatorial regionsThe stronger δ18O intra‐seasonal oscillation corresponds to the season with active convective activities, more rainfall, and low‐level δ18OThe stronger regional convective intensity leads to the cumulative effect in the depletion of rain δ18O during the convective phases of ISO The intra‐seasonal oscillations are one of the main patterns of precipitation δ18O covering the Asian monsoon and equatorial regions The stronger δ18O intra‐seasonal oscillation corresponds to the season with active convective activities, more rainfall, and low‐level δ18O The stronger regional convective intensity leads to the cumulative effect in the depletion of rain δ18O during the convective phases of ISO

This study investigates the structure and propagation of intraseasonal sea surface temperature (SST) variability in the South China Sea (SCS) on the 30–60-day timescale during boreal summer (May–September). TRMM-based SST, GODAS oceanic reanalysis and ERA-Interim atmospheric reanalysis datasets from 1998 to 2013 are used to examine quantitatively the atmospheric thermodynamic and oceanic dynamic mechanisms responsible for its formation. Power spectra show that the 30–60-day SST variability is predominant, accounting for 60% of the variance of the 10–90-day variability over most of the SCS. Composite analyses demonstrate that the 30–60-day SST variability is characterized by the alternate occurrence of basin-wide positive and negative SST anomalies in the SCS, with positive (negative) SST anomalies accompanied by anomalous northeasterlies (southwesterlies). The transition and expansion of SST anomalies are driven by the monsoonal trough–ridge seesaw pattern that migrates northward from the equator to the northern SCS. Quantitative diagnosis of the composite mixed-layer heat budgets shows that, within a strong 30–60-day cycle, the atmospheric thermal forcing is indeed a dominant factor, with the mixed-layer net heat flux (MNHF) contributing around 60% of the total SST tendency, while vertical entrainment contributes more than 30%. However, the entrainment-induced SST tendency is sometimes as large as the MNHF-induced component, implying that ocean processes are sometimes as important as surface fluxes in generating the 30–60-day SST variability in the SCS. 本文研究了夏季5-9月南海海表温度(SST)30-60天季节内变率的结构和传播特征. 基于1998-2013年TRMM的海表温度, GODAS和ERA-interim的海洋和大气再分析资料, 定量地分析了大气的热力强迫和海洋的动力过程在海表温度30-60天变率形成中的作用. 功率谱分析发现, 南海SST存在显著的 30-60天季节内振荡; 这种30-60天变率所占的10-90天季节内变率总方差在南海大部海域均超过60%. 较强的30-60天SST振荡事件的合成分析表明, 南海SST 30-60天变率的主要特征是海盆尺度的正、负SST异常的交替出现, 同时, 正(负)SST异常伴随着东北(西南)风异常, 并且SST异常的转换和扩张是受自赤道向南海北部移动的季风槽脊所驱动的. 通过混合层热力收支方程的定量诊断发现, 在较强的30-60天SST变率事件中, 混合层净的热通量对海表温度趋势变化的贡献超过60%, 而海洋的垂直夹卷对海表温度趋势变化的贡献也超过了30%. 尽管大气的热力强迫是影响SST异常变化的主要因子. 然而, 个例分析发现, 在某些30-60天SST振荡事件中, 海洋的垂直夹卷效应与混合层净热通量强迫对SST趋势的贡献是同样的. 这表明, 海洋动力过程和海表热通量对于30-60天SST异常变化的影响在某些情况下是同等重要的.

Prediction of the rapid intensification (RI) of tropical cyclones (TCs) remains challenging. In this paper, by using information from the early period following TC formation, the rapid intensification warning index (RIWI) is developed based on the analog method. A 10‐year cross‐validation and data from Hurricane Ida (2021) are used to verify its potential application. Results show that the RIWI can efficiently discriminate between RI and non‐RI storms and has a significant positive correlation with the lifetime maximum intensity (LMI) of the TCs. By using this index, an early warning can be issued ∼30 hr before the onset of RI, which is much earlier than the predictions made using the probabilistic Statistical Hurricane Intensity Prediction System RI index. In addition, by using the RIWI as a predictor, the prediction of LMI provides an early estimate of TC severity. Variations in the intensity of tropical cyclones (TCs) are difficult to forecast, especially the occasional TCs that develop quickly over a period of couple of days; that is, rapid intensification (RI). Recent studies have demonstrated that it is easier to forecast RI likelihood than RI timing. Thus, instead of attempting to forecast the exact timing of RI, it may be more useful to predict whether RI will occur during the lifetime of a TC. In this paper, by using information from the early stage of TCs, we use the analog method to develop the RI warning index (RIWI). Then, using an appropriate threshold, it can be used to predict whether RI will occur during the lifetime of a TC. Furthermore, an important TC parameter, the lifetime maximum intensity (LMI) of the TC, has a significant positive correlation with its RIWI. The application of RIWI to Hurricane Ida (2021) is also presented in this paper and shows that an early warning can be issued ∼30 hr before the onset of RI. By using the information of tropical cyclones (TCs) early stage, a rapid intensification warning index (RIWI) is constructed based on the analog methodThe RIWI can distinguish between rapid intensification (RI) and non‐RI storms during the early stages of TCsThere is a significant positive correlation between a TC's RIWI and its lifetime maximum intensity By using the information of tropical cyclones (TCs) early stage, a rapid intensification warning index (RIWI) is constructed based on the analog method The RIWI can distinguish between rapid intensification (RI) and non‐RI storms during the early stages of TCs There is a significant positive correlation between a TC's RIWI and its lifetime maximum intensity

Recent advances in the bridging roles played by the Tibetan Plateau (TP) are reviewed in terms of the remote influence of circulation anomalies over the North Atlantic Ocean on Asian monsoon and El Niño-Southern Oscillation (ENSO) events, and in a clear link between the tropical oceans and Asian climate anomalies. The authors firstly introduce how the winter and spring anomalies in the North Atlantic Ocean affect the seasonal transition over the South Asian monsoon region and subsequent ENSO events on the interannual timescale. A distinct negative sensible heating-baroclinic structure in May over the TP is found to provide an intermediate bridging effect in this Atlantic-Asian-Pacific connection. In summer, the North Atlantic Oscillation is significantly correlated with the variations of East China summer rainfall, and it is the TP's latent heating that plays the bridging role within. On the other hand, such a TP bridging effect also exists in the connection from the tropical oceans to extreme precipitation events over eastern China in summer, and from the midlatitude wave train to the biweekly oscillation of South China rainfall in spring.

A timescale decomposed threshold regression (TSDTR) downscaling approach to forecasting South China early summer rainfall (SCESR) is described by using long-term observed station rainfall data and NOAA ERSST data. It makes use of two distinct regression downscaling models corresponding to the interannual and interdecadal rainfall variability of SCESR. The two models are developed based on the partial least squares (PLS) regression technique, linking SCESR to SST modes in preceding months on both interannual and interdecadal timescales. Specifically, using the datasets in the calibration period 1915–84, the variability of SCESR and SST are decomposed into interannual and interdecadal components. On the interannual timescale, a threshold PLS regression model is fitted to interannual components of SCESR and March SST patterns by taking account of the modulation of negative and positive phases of the Pacific Decadal Oscillation (PDO). On the interdecadal timescale, a standard PLS regression model is fitted to the relationship between SCESR and preceding November SST patterns. The total rainfall prediction is obtained by the sum of the outputs from both the interannual and interdecadal models. Results show that the TSDTR downscaling approach achieves reasonable skill in predicting the observed rainfall in the validation period 1985–2006, compared to other simpler approaches. This study suggests that the TSDTR approach, considering different interannual SCESR-SST relationships under the modulation of PDO phases, as well as the interdecadal variability of SCESR associated with SST patterns, may provide a new perspective to improve climate predictions.

Anomalous warming occurred in the equatorial central-eastern Pacific in early May 2014, attracting much attention to the possible occurrence of an extreme El Niño event that year because of its similarity to the situation in early 1997. However, the subsequent variation in sea surface temperature anomalies (SSTAs) during summer 2014 in the tropical Pacific was evidently different to that in 1997, but somewhat similar to the situation of the 1990 aborted El Niño event. Based on NCEP (National Centers for Environmental Prediction) oceanic and atmospheric reanalysis data, the physical processes responsible for the strength of El Niño events are examined by comparing the dominant factors in 2014 in terms of the preceding instability of the coupled ocean–atmosphere system and westerly wind bursts (WWBs) with those in 1997 and 1990, separately. Although the unstable ocean–atmosphere system formed over the tropical Pacific in the preceding winter of 2014, the strength of the preceding instability was relatively weak. Weak oceanic eastward-propagating downwelling Kelvin waves were forced by the weak WWBs over the equatorial western Pacific in March 2014, as in February 1990. The consequent positive upper-oceanic heat content anomalies in the spring of 2014 induced only weak positive SSTAs in the central-eastern Pacific–unfavorable for the subsequent generation of summertime WWB sequences. Moreover, the equatorial western Pacific was not cooled, indicating the absence of positive Bjerknes feedback in early summer 2014. Therefore, the development of El Niño was suspended in summer 2014.

The mei-yu season (June–July) rainfall over the mei-yu monitoring domain (MMD) in the Yangtze–Huaihe Basin has shown an increasing trend in recent decades. This study examines the dominant factors responsible for this increasing trend for the period 1979–2020 based on station-observed rainfall and ERA5 reanalysis datasets from the perspective of changes in atmospheric circulation. Although significantly increasing trends exist in the mei-yu season rainfall over the entire MMD, the magnitude of the trends is slightly larger over the eastern MMD (EMMD) than over the western MMD (WMMD). Quantitative diagnoses demonstrate that the relative contributions of anomalous evaporation and moisture advection to the increasing rainfall trend are different between the EMMD and WMMD. The increasing rainfall trend over the WMMD (EMMD) is attributable to increased evaporation (enhanced vertical moisture advection), which is dependent on an anomalous cyclonic circulation in the middle-lower troposphere over the MMD. Such an anomalous cyclone on the northwestern side of the climatological western North Pacific subtropical high facilitates an increase in moisture divergence above 600 hPa over the EMMD, leading to enhanced vertical moisture advection in conjunction with strengthened moisture convergence at 850 hPa. By contrast, the anomalous cyclone favors increasing local evaporation over the WMMD.

Significant 25–60‐day intraseasonal oscillations (ISOs) in rainfall over the middle‐lower reaches of the Yangtze River (MLYR) are found for eight summers during 1979–2020. These 25–60‐day ISOs of MLYR rainfall account for 66.7% of persistently severe floods (PSFs) during 1979–2020, indicating their major role in inducing PSFs and the potential for extended‐range forecasting of PSFs. The 25–60‐day ISO in MLYR rainfall results from the synergistic effect of tropical and midlatitude ISOs. During the eight identified summers, more barotropic energy is transferred from summer‐mean to 25–60‐day circulation, thus forming a midlatitude ISO wave train, which cooperates with the tropical ISO in triggering twin vertical cells. Phase locking of ascending branch from the vertical cell induced by the tropical ISO with that from the reversed vertical cell triggered by the midlatitude ISO brings about the wet phase of 25–60‐day ISO of MLYR rainfall. Meiyu rainfall over the middle‐lower reaches of Yangtze River (MLYR) is characterized by strong intraseasonal oscillations (ISOs, with the period of 10–90 days). While the quasi‐biweekly (10–20‐day) oscillation of MLYR rainfall has been well acknowledged, we found that significant 25–60‐day ISOs of rainfall also exist in eight summers of 1979–2020. These 25–60‐day ISOs of rainfall further induced persistently severe floods (PSFs), leading to devastating damage to social and economic development. During the summers of 1979–2020, 66.7% of PSFs were associated with the 25–60‐day ISO in MLYR rainfall, indicating that the 25–60‐day rainfall ISO is a typical driver of PSF, thus providing the potential for extending the forecast lead time of PSF. The 25–60‐day ISO in MLYR rainfall results from the coordinated influence of atmospheric ISOs from both tropics and midlatitudes. The tropical ISO induces a vertical cell between the MLYR and South China Sea. As more kinetic energy is transferred from summer‐mean to 25–60‐day circulation, a 25–60‐day ISO wave train forms in the midlatitudes, which induces a reversed vertical cell. Phase locking of the ascending branch from these twin vertical cells provides the essential dynamical condition for the wet phase of 25–60‐day ISO of MLYR rainfall. Significant 25–60‐day ISOs of rainfall over the MLYR are a major driver of persistently severe floodThe 25–60‐day ISOs of rainfall originate from the synergistic effect of tropical and midlatitude atmospheric ISOsThe existence of 25–60‐day midlatitude ISO is associated with the stronger barotropic energy transfer over the northeastern Atlantic Significant 25–60‐day ISOs of rainfall over the MLYR are a major driver of persistently severe flood The 25–60‐day ISOs of rainfall originate from the synergistic effect of tropical and midlatitude atmospheric ISOs The existence of 25–60‐day midlatitude ISO is associated with the stronger barotropic energy transfer over the northeastern Atlantic

Abstract: Climatological characteristics of diurnal variations in summer precipitation over the Asian monsoon region are comprehensively investigated based on the Tropical Rainfall Measuring Mission (TRMM) satellite data during 1998–2008. The topographic influence on the diurnal variations and phase propagations of maximum precipitation are identified according to spatiotemporal distributions of the amplitude and peak time of the diurnal precipitation. The amplitude and phase of diurnal precipitation show a distinct geographical pattern. Significant diurnal variations occur over most of continental and coastal areas including the Maritime Continent, with the relative amplitude exceeding 40%, indicating that the precipitation peak is 1.4 times the 24-h mean. Over the landside coasts such as southeastern China and Indochina Peninsula, the relative amplitude is even greater than 100%. Although the diurnal variations of summer precipitation over the continental areas are characterized by an afternoon peak (1500–1800 Local Solar Time (LST)), over the central Indochina Peninsula and central and southern Indian Peninsula the diurnal phase is delayed to after 2100 LST, suggesting the diurnal behaviors over these areas different from the general continental areas. The weak diurnal variations with relative amplitudes less than 40% exist mainly over oceanic areas in the western Pacific and most of Indian Ocean, with the rainfall peak mainly occurring from midnight to early morning (0000–0600 LST), indicating a typical oceanic regime characterized by an early morning peak. However, apparent exceptions occur over the South China Sea (SCS), Bay of Bengal (BOB), and eastern Arabian Sea, with the rainfall peak occurring in daytime (0900–1500 LST). Prominent meridional propagations of the diurnal phase exist in South Asia and East Asia. Along the eastern Indian Peninsula, there is not only the southward phase propagation with the peak occurring around 25°N but also the northward phase propagation with the peak beginning with the southernmost continent, and both reach the central Indian continent to finish. Along the same longitudes where southern China and Kalimantan are located, the diurnal phase of the former propagates from the oceanic area (northern SCS) toward the inland continent, while the phase of the latter propagates from the land area toward the outside sea, showing a landward or seaward coastal diurnal regime. A distinct zonal propagation of the diurnal phase is observed over the BOB oceanic area. The maximum precipitation zone originates from the land-sea boundary of the eastern coast of the Indian peninsula at around 0300 LST, and then propagates eastward with increasing time to reach the eastern coast of the BOB on 1800 LST, finally migrates into the Indochina continent on about 2100 LST.

Abstract: Physical processes associated with onset of the 1998 Asian summer monsoon were examined in detail using multi-source datasets. We demonstrated that strong ocean-atmosphere-land interaction in the northern Indian Ocean and tropical Asian area during spring is a fundamental factor that induces the genesis and development of a monsoon onset vortex over the Bay of Bengal (BOB), with the vortex in turn triggering onset of the Asian summer monsoon. In spring, strong surface sensible heating over India and the Indochina Peninsula is transferred to the atmosphere, forming prominent in situ cyclonic circulation, with anticyclonic circulations over the Arabian Sea and northern BOB where the ocean receives abundant solar radiation. The corresponding surface winds along the North Indian Ocean coastal areas cause the ocean to produce the in situ offshore currents and upwelling, resulting in sea surface temperature (SST) cooling. With precipitation on the Indochina Peninsula increasing from late April to early May, the offshore current disappears in the eastern BOB or develops into an onshore current, leading to SST increasing. A southwest-northeast oriented spring BOB warm pool with SST >31°C forms in a band from the southeastern Arabian Sea to the eastern BOB. In early May, the Somali cross-equatorial flow forms due to the meridional SST gradient between the two hemispheres, and surface sensible heat over the African land surface. The Somali flow overlaps in phase with the anticyclone over the northern Arabian Sea in the course of its inertial fluctuation along the equator. The convergent cold northerlies on the eastern side of the anticyclone cause the westerly in the inertial trough to increase rapidly, so that enhanced sensible heat is released from the sea surface into the atmosphere. The cyclonic vorticity forced by such sensible heating is superimposed on the inertial trough, leading to its further increase in vorticity strength. Since atmospheric inertial motion is destroyed, the flow deviates from the inertial track in an intensified cyclonic curvature, and then turns northward toward the warm pool in the northern BOB. It therefore converges with the easterly flow on the south side of the anticyclone over the northern BOB, forming a cyclonic circulation center east of Sri Lanka. Co-located with the cyclonic circulation is a generation of atmospheric potential energy, due to lower tropospheric heating by the warm ocean. Eventually the BOB monsoon onset vortex (MOV) is generated east of Sri Lanka. As the MOV migrates northward to the warm pool it develops quickly such that the zonal oriented subtropical high is split over the eastern BOB. Thus, the tropical southwesterly on the southern and eastern sides of the MOV merges into the subtropical westerly in the north, leading to active convection over the eastern BOB and western Indochina Peninsula and onset of the Asian summer monsoon.

A series of heavy rainfall events occurred over the Yangtze River Valley (YRV) in summer 2014, which were modulated by the 10–20-day quasi-biweekly oscillation (QBWO). Thus, the strongest QBWO cycle for the period 10–24 July was used as a representative case to reveal the dynamical mechanism for the QBWO of the YRV rainfall from the potential vorticity (PV) perspective and based on MERRA-2 reanalysis data. The quasi-biweekly YRV rainfall was found to depend closely on the QBWO of the upper-tropospheric South Asian high (SAH), with the SAH configuration modified by the southward-intruding midlatitude high PV stream along with southwestward-advected high PV, altering the divergent condition over the YRV. Quantitative diagnoses for the anomalous vertical motion demonstrated that, in the wet phase of the QBWO cycle, the upper-tropospheric southward-intruding high PV stream acted as a positive PV advection, while negative PV advection was generated due to the lower-tropospheric southerlies, thereby forming a positive vertical gradient of horizontal PV advection to induce evident isentropic-displacement ascending motion. On the other hand, the southward-intruding high PV stream extended downward to the middle troposphere, causing the isentropic surfaces to become more sloping, thus producing a strong isentropic-gliding ascending component. Subsequently, the stronger diabatic heating-related ascending motion was induced to generate positive rainfall anomalies over the YRV. The opposite situation arose in the dry phase, with weak descending motion in magnitude.

This paper reviews progress in the study of Tibetan Plateau (TP) climate dynamics over the past decade. Several theoretical frameworks, including thermal adaptation and the TP sensible heat (SH) driving air-pump, have been developed to identify the mechanisms responsible for the circulation anomaly produced by thermal forcing of the TP. Numerical simulations demonstrate that the thermal effects of large-scale orography, including the Tibetan and Iranian Plateaus (TIP), are crucial for the formation of the East Asian and South Asian summer monsoons (SASM) because the surface SH of the TIP is the major driver of the water vapor transport required for the genesis of the north branch of the SASM. The large-scale orography of the TP affects the Asian climate through thermal forcing in spring and summer, and mechanical forcing in winter. The TP forcing can also influence the Asian summer monsoon (ASM) onset over the Bay of Bengal (BOB) by enhancing the BOB warm pool at the surface and by modulating the South Asian High (SAH) in the upper troposphere. On intra-seasonal timescales, the TP thermal forcing significantly modulates spring rainfall in southern China and generates the biweekly oscillation of the SAH in summer. Despite climate warming, the atmospheric heat source over the TP, particularly the spring SH, exhibits a clear weakening trend from the 1980s to 2000s. This weakening of the spring SH contributed to the anomalous ‘dry in the north’ and ‘wet in the south’ rainfall pattern observed over East China. Also discussed are challenges to further understanding the mechanism of TP forcing on the multi-scale variability of the ASM.