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Abstract The behavior of the El Niño–Southern Oscillation (ENSO) has undergone significant changes since the year 2000. Meanwhile, a notable westward shift and strengthening in the atmosphere‐to‐ocean Bjerknes feedback were observed. We find that this shift can be primarily attributed to a weakened relationship between the zonal gradient of precipitation anomaly and that of sea surface temperature (SST) anomaly since 2000.This weakened relationship is a comprehensive manifestation of reduced El Niño‐related precipitation anomalies in the central‐eastern tropical Pacific and increased anomalies in the western tropical Pacific. These changes are connected to the mean state change in the tropical Pacific after 2000, where the cooler background SSTs in the central‐eastern tropical Pacific suppress upward motion, and the warmer background SSTs in the western tropical Pacific promote upward motion in the overlying atmosphere. Our findings offer a potential explanation for the westward shift and strengthening in the atmosphere‐to‐ocean Bjerknes feedback since 2000.

Abstract Many meteorological centers have operationally implemented global model‐based ensemble prediction systems (GEPSs), making tropical cyclone (TC) forecasts from these systems available. The relatively low resolution of these GEPSs means that limits previous studies primarily focused on TC track forecasting. However, recent GEPS upgrades mean that TC intensity predictions from GEPSs are now also becoming of interest. This study focuses on the verification and comparison of the latest generation of GEPSs for TC intensity forecasts, particularly during the rapid intensification (RI) period over the western North Pacific (WP), eastern North Pacific (EP), and North Atlantic (NA) basins in 2021–2022. On average, the National Centers for Environmental Prediction (NCEP) GEPS performed best in predicting both TC intensity and RI across all three basins. Nevertheless, the exact timing of RI remains highly uncertain for these GEPS, indicating significant limitations in using GEPSs to forecast RI.

In summer, the middle and lower reaches of the Yangtze River (MLYR) often suffers from severe floods as a result of a dominant intraseasonal oscillation (ISO) of rainfall with the quasi-period of 25–60 days. 75% of flooding events during summers of 1961–2022 are found to occur following wet phases of the 25−60-day ISO in rainfall. This study objectively extracts the optimal configuration (OC) of 25−60-day atmospheric ISOs for the wet phase of MLYR rainfall. The OC is characterized by the quadruple pattern of tropical convection configuring with the 25−60-day Silk Road Pattern (SRP) in the mid−high latitudes. The 25−60-day SRP in cooperation with tropical ISO over the western Pacific induces dipolar vertical cells over eastern China, with ascending branch between the vertical cells providing the vitally dynamical condition for MLYR rainfall. With topographic blocking of the Tibetan Plateau, the northward-propagating tropical ISO over the Indian Ocean enhances maintenance of the 25−60-day SRP via upper-tropospheric divergent flows, thus anchoring anomalous ascents to the MLYR. Numerical experiments further verify the remarkable tropical-extratropical interaction around the Tibetan Plateau in this OC. Under the global warming background, the tropical 25−60-day ISO tends to strengthen, leading to an increase in the occurrence frequency of strong OC through tropical-extratropical interaction, and thus the severe flood events doubled in summers of 1991–2020.

Abstract 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.

Eastern China experienced persistent regional extreme heatwaves in the summer of 2022, with disparate spatial features and formation mechanisms in different months. We quantitatively assessed the relative contributions of three oceans, i.e. tropical Indian Ocean and Pacific and North Atlantic, and the local soil moisture–temperature feedback using linear regression. The results showed that the monthly mean atmospheric circulation anomalies failed to explain the extreme heatwave in June 2022. The combined contribution of the tropical Indo-Pacific and North Atlantic sea surface temperature anomalies (SSTAs), together with the local soil moisture–temperature feedback, explaining approximately 10% of the temperature anomalies. In July, the tropical Indo-Pacific SSTAs promoted anomalous atmospheric circulation and extreme heat via meridional circulation originating in the Maritime Continent, accounting for approximately 10% of the temperature anomalies, with North Atlantic SSTAs contributing the same percentage by a mid-latitude steady Rossby wave. Local soil moisture–temperature feedback accounted for 42% of the anomalies. The tropical Indo-Pacific SSTAs produced a strong western North Pacific anticyclone in August, but their direct contribution to the temperature anomalies was negligible. The North Atlantic SSTAs contributed 9% of the total via the mid-latitude steady Rossby wave. Local soil moisture–temperature feedback contributed 66%, suggesting that the July heatwave and drought exerted a significant impact on the subsequent August extreme heatwave. Global warming has greatly facilitated extreme heatwaves, accounting for about 30%–40% of these events in summer 2022. These results also suggest that the climatic effects of tropical Indo-Pacific and North Atlantic SSTAs on Eastern China are evident in the month-to-month variation in summer. Our results thus contribute to the understanding and prediction of extreme heatwaves in Eastern China.

This 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.

A 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).

The sea surface temperature inter-hemispheric dipole (SSTID) is an important variability mode of global SST anomalies, characterized by an anti-phase variation of SST between the two hemispheres. In this study, the decadal variation of the northern hemisphere summer monsoon (NHSM) is found to be strongly regulated by the SSTID, with positive (negative) phases of the SSTID corresponding to the strengthening (weakening) of NHSM. Both observation and SST-forced atmospheric model simulations suggest that the SSTID related thermal forcing modulates the NHSM by causing planetary-scale atmospheric circulation adjustments. Positive SSTID events lead to coherent increase (decrease) of surface air temperature over the entire northern (southern) hemisphere, increasing the inter-hemispheric thermal contrast (ITC). As sea level pressure changes are just opposite to air temperature, the increase of ITC enhances the inter-hemispheric pressure gradient (southern hemisphere minus northern hemisphere), leading to the strengthening of summer monsoonal circulation and the increase of monsoon rainfall in the northern hemisphere.

Asian large-scale orography profoundly influences circulation in the North Hemisphere. Considerable spring top-of-the-atmosphere (TOA) radiative cooling over Southeast China (SEC) is very likely related to upstream orography forcing. Here we investigate the mechanical and thermal forcings of Asian large-scale orography, particularly the Tibetan Plateau (TP), on downstream East Asian cloud amount and atmospheric radiation budget during March–April using the Global Monsoons Model Intercomparison Project simulations. The thermal forcing drives significant surface heating and a low-level cyclone over the TP, pumping low-level air to the middle troposphere. Ascent and water vapor convergence triggered by the thermal forcing favor air condensation, low–middle clouds, and resultant strong spring cloud radiative cooling over SEC. Moreover, the thermal forcing moves the position of cloud radiative cooling westward toward the TP. The TP’s blocking role weakens low-level westerlies over SEC, but its deflecting role increases downstream highlevel westerlies, dynamically favoring cloud formation over SEC and the eastward ocean. In addition, the TP can force ascent and increase cloud amounts over the western and central TP. The thermal forcing contributes to 57.1% of total cloud amount and 47.6% of TOA cloud radiative cooling induced by the combined orography forcing over SEC while the mechanical one accounts for 79.4% and 95.8% of the counterparts over the ocean to the east of SEC. Our results indicate that Asian large-scale orography shapes the contemporary geographical distribution of spring East Asian cloud amount and atmospheric radiation budget to a large extent. [ABSTRACT FROM AUTHOR]

Excessive precipitation was observed throughout the Yangtze River Valley during the record-breaking Meiyu season in 2020. However, the mechanism of extreme precipitation over the upper reaches of the Yangtze River remains unclear. Our results show that the activities of high potential vorticity (PV) systems during their eastward propagation over the eastern Tibetan Plateau (TP) should be responsible for the above-normal rainfall not only over western China but also over eastern China. The activity of high-PV systems is characterized by a prominent diurnal cycle, and their formation is closely related to the thermal contrast between the near-surface and lower atmosphere. In the morning, surface sensible heating increases sharply after sunrise, leading to a decrease in diabatic heating with height. Accordingly, PV weakens in the lower atmosphere, and the formation of high-PV systems reaches a minimum. An increase in turbulence increases near-surface evaporation, which in turn reduces surface diabatic heating. At the same time, cloud formation increases diabatic heating at approximately 400 hPa. Consequently, the thermal contrast below 400 hPa leads to an increase in diabatic heating with height, favoring the generation of high-PV systems. Compared with the climatology, an excessive water vapor supply from the anomalous anticyclone over the northern Bay of Bengal, forced by the Indian Ocean warming in 2020, contributes to a stronger thermal contrast and enhanced activity of high-PV systems over the TP. The arrival timing of high-PV systems at the eastern flank of the TP plays an important role in the subsequent development of these systems. Early arrival in the afternoon or evening is generally accompanied by air convergence and sufficient water vapor supply downstream of the TP, which favors the systems moving off the TP and influencing precipitation downstream. [ABSTRACT FROM AUTHOR]

Global sea surface temperature (SST) evolution exhibits an antiphase variation between the two hemispheres that is referred to as the SST interhemispheric dipole (SSTID) mode. The impacts of the SSTID on extratropical atmospheric circulation in boreal winter are explored by both regression analysis and SSTforced numerical simulations. The responses of extratropical circulation to SSTID thermal forcing bear an equivalent barotropic structure. For the Southern Hemisphere (SH), positive SSTID events lead to a meridional dipolar perturbation in sea level pressure (SLP), similar in pattern to the positive southern annular mode (SAM). Although SSTID-forced SLP anomalies over the Northern Hemisphere (NH) do not exhibit a zonally symmetric pattern as is the case over the SH, they still show signs of a meridional dipole opposite to the SH over the oceans. Divergent circulation responses to SSTID forcing between the two hemispheres are suggested to be associated with contrasting storm-track variations. Positive SSTID events weaken oceanic fronts in both the North Atlantic and North Pacific, and thus lead to the decline of NH storm-track activity by decreasing atmospheric baroclinicity. In the SH, positive SSTID events correspond to the enhancement of SH transients by intensifying the Antarctic polar-frontal zone. Additionally, local baroclinic energy conversions are diagnosed to explain the SSTID-related storm-track variations over both hemispheres. Finally, an investigation of transient eddy feedback indicates that the SSTID mode modulates extratropical atmospheric circulation, primarily by regulating storm tracks and changing the corresponding eddy feedback. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This article provides a comprehensive review of the global monsoon that encompasses findings from studies of both modern monsoons and paleomonsoons. We introduce a definition for the global monsoon that incorporates its three-dimensional distribution and ultimate causes, emphasizing the direct drive of seasonal pressure system changes on monsoon circulation and depicting the intensity in terms of both circulation and precipitation. We explore the global monsoon climate changes across a wide range of timescales from tectonic to intraseasonal. Common features of the global monsoon are global homogeneity, regional diversity, seasonality, quasi-periodicity, irregularity, instability, and asynchroneity. We emphasize the importance of solar insolation, Earth orbital parameters, underlying surface properties, and land-air-sea interactions for global monsoon dynamics. We discuss the primary driving force of monsoon variability on each timescale and the relationships among dynamics on multiple timescales. Natural processes and anthropogenic impacts are of great significance to the understanding of future global monsoon behavior. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The interannual variations of the early summer South China monsoon rainfall (SCMR) in relation to different Pacific Decadal Oscillation (PDO) backgrounds are examined using station rainfall data and the European Centre for Medium-range Weather Forecasts (ECMWF) 40-year reanalysis (ERA-40) data. The objective of this study is to investigate the atmospheric circulation patterns responsible for such variations, thereby understanding the interdecadal modulation and the cause for the extreme wet and dry SCMR. The interannual SCMR variance shows an interdecadal change before and after the late 1970s, with significant differences in sea-surface temperature (SST) and atmospheric circulation anomalies between the negative (1958-1976) and positive (1980-1998) PDO epochs. The dominant atmospheric teleconnection patterns associated with extreme wet and dry SCMR are remarkably different in these two epochs. During 1958-1976, an anomalous wet (dry) SCMR is characterized by a significantly anomalous anticyclone (cyclone) over the South China Sea (SCS) and western Pacific in the lower troposphere in the form of a meridional wave-like coupling along the coast of western North Pacific in the early summer, and preceded by a monopole circulation anomaly over the Ural Mountain in the preceding winter. During 1980-1998, however, an anomalous wet (dry) SCMR features a relatively weak anomalous anticyclone (cyclone) over the SCS in the low level without a significantly meridional juxtaposition of anomalous circulations in the early summer, and it follows a remarkable coherent long wave train pattern in middle-high latitudes in the preceding winter. Such interdecadal differences in anomalous circulation structures suggest that the relative roles of internal atmospheric variability and external forcing may be different from different epochs. But for both epochs, the circulation anomaly over the Ural Mountain shows a highly negative correlation with anomalous SCMR, suggesting that the winter circulation anomaly around the Ural Mountain is of great significance for improving the seasonal prediction of SCMR. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The temporal clustering of the western North Pacific tropical cyclogenesis and its modulation by the Madden–Julian oscillation (MJO) during the 1991 summer were examined based on the tropical cyclone best track, outgoing longwave radiation, and NCEP/NCAR reanalysis datasets. The wavelet analysis shows that convective activities around the monsoon trough in the western North Pacific possessed a distinct MJO with a period of 20–60 days. Two or more tropical cyclones were observed to form successively during each active phase of the MJO, and tropical cyclones tended to generate around the southeastern part of the maximum vorticity of the low-frequency cyclonic circulation during the developing and peak stages of the active MJO phase. But tropical cyclogenesis scarcely occurred during inactive MJO phases. Thus the MJO was a major agent in modulating repeated development of tropical cyclones in the western North Pacific during the 1991 summer. The MJO in circulation was characterized by a huge anomalous cyclone (anticyclone) in the lower troposphere existing alternately over the western North Pacific, leading to an enhanced (weakened) monsoon trough. An examination of the meridional gradient of absolute vorticity associated with the zonal flow indicates that the zonal flow in the monsoon trough region satisfied the necessary conditions for barotropic instability, with both zonal flow and the meridional gradient of absolute vorticity varying on the similar MJO timescale. The intraseasonal oscillation of such an unstable zonal flow might thus be an important mechanism for temporal clustering of tropical cyclogenesis in the western North Pacific. The barotropic conversion could provide a major energy source for the formation and growth of tropical cyclones in the western North Pacific during active MJO phases, with the eddy kinetic energy generation being dominated by both terms of eddies interacting with zonal and meridional gradients of the basic zonal flow. [ABSTRACT FROM AUTHOR]

The spatio-temporal variability in summer rainfall within eastern China is identified based on empirical orthogonal function (EOF) analysis of daily rain-gauge precipitation data for the period 1979–2003. Spatial coherence of rainfall is found in the Yangtze Basin, and a wavelet transform is applied to the corresponding principal component to capture the intraseasonal oscillation (ISO) of Yangtze rainfall. The ensemble mean wavelet spectrum, representing statistically significant intraseasonal variability, shows a predominant oscillation in summer Yangtze rainfall with a period of 20–50 days; a 10–20-day oscillation is pronounced during June and July. This finding suggests that the 20–50-day oscillation is a major agent in regulating summer Yangtze rainfall. Composite analyses reveal that the 20–50-day oscillation of summer Yangtze rainfall arises in response to intraseasonal variations in the western North Pacific subtropical high (WNPSH), which in turn is modulated by a Rossby wave-like coupled circulation–convection system that propagates northward and northwestward from the equatorial western Pacific. When an anomalous cyclone associated with this Rossby wave-like system reaches the South China Sea (SCS) and Philippine Sea, the WNPSH retreats northeastward due to a reduction in local pressure. Under these conditions, strong monsoonal southwesterlies blow mainly toward the SCS–Philippine Sea, while dry conditions form in the Yangtze Basin, with a pronounced divergent flow pattern. In contrast, the movement of an anomalous anticyclone over the SCS–Philippine Sea results in the southwestward extension of the WNPSH; consequently, the tropical monsoonal southwesterlies veer to the northeast over the SCS and then converge toward the Yangtze Basin, producing wet conditions. Therefore, the 20–50-day oscillation of Yangtze rainfall is also manifest as a seesaw pattern in convective anomalies between the Yangtze Basin and the SCS–Philippine Sea. A considerable zonal shift in the WNPSH is associated with extreme dry (wet) episodes in the Yangtze Basin, with an abrupt eastward (westward) shift in the WNPSH generally leading the extreme negative (positive) Yangtze rainfall anomaly by a 3/8-period of the 20–50-day oscillation. This finding may have implications for improving extended-range weather forecasting in the Yangtze Basin. [ABSTRACT FROM AUTHOR]

The objective of this study is to explore, based on the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data, the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM) in terms of its structure and propagation, as well as interannual variations. A possible mechanism that is responsible for the origin of the 10–20-day oscillation of the SCS SM is also proposed. The 30–60-day (hereafter the 3/6 mode) and 10–20-day (hereafter the 1/2 mode) oscillations are found to be the two intraseasonal modes that control the behavior of the SCSSM activities for most of the years. Both the 3/6 and 1/2 modes are distinct, but may not always exist simultaneously in a particular year, and their contributions to the overall variations differ among different years. Thus, the interannual variability in the intraseasonal oscillation activity of the SCS SM may be categorized as follows: the 3/6 category, in which the 3/6 mode is more significant (in terms of the percentage of variance explained) than the 1/2 mode; the 1/2 category, in which the 1/2 mode is dominant; and the dual category, in which both the 3/6 and 1/2 modes are pronounced. Composite analyses of the 3/6 category cases indicate that the 30–60-day oscillation of the SCS SM exhibits a trough–ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the SCS, with anomalous cyclones (anticyclones), along with enhanced (suppressed) convection, migrating northward from the equator to the midlatitudes. The northward-migrating 3/6-mode monsoon trough–ridge in the lower troposphere is coupled with the eastward-propagating 3/6-mode divergence–convergence in the upper troposphere. It is also found that, for the years in the dual category, the SCS SM activities are basically controlled by the 3/6 mode, but modified by the 1/2 mode. Composite results of the 1/2-mode category cases show that the 10–20-day oscillation is manifest as an anticyclone–cyclone system over the western tropical Pacific, propagating northwestward into the SCS. A close coupling also exists between the upper-level convergence (divergence) and the low-level anticyclone (cyclone). It is found that the 1/2 mode of the SCS SM mainly originates from the equatorial central Pacific, although a disturbance from the northeast of the SCS also contributes to this mode. The flow patterns from an inactive to an active period resemble those associated with a mixed Rossby–gravity wave observed in previous studies. [ABSTRACT FROM AUTHOR]