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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 ascent mechanisms 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., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

At midnight on 27–28 June 2016, a Tibetan Plateau (TP) Vortex (TPV) generated over the western TP that subsequently caused a downstream record-breaking rainstorm and extremely severe natural disaster. Based on reanalysis data and satellite imagery, this study investigates the formation of this TPV from a potential vorticity (PV) perspective. Results show that, in late June 2016, a remarkable circulation anomaly occurred over the TP and its peripheral area, with easterly flow in the middle and lower troposphere developing in the subtropical zone, replacing the normal westerly flow there. Its forefront merged with the southwesterly flow from the west and penetrated and converged over the western TP where the surface was warmer than normal, forming a low-level jet and downward slantwise isentropic surfaces in-situ. When the air parcel slid down the slantwise isentropic surface, its vertical relative vorticity developed owing to slantwise vorticity development associated with PV restructuring. At the same time, the penetrating southwesterly flow brought abundant water vapor to the western TP and induced increasing sub-cloud entropy and air ascent there. Low-layer cloud formed and the cloud liquid water content increased. The strong latent heat that was released in association with the formation of cloud produced strong diabatic heating near 400 hPa at night and strong PV generation below. The normal diurnal variation was interrupted and the vortex was generated near the surface. These results demonstrate that, against a favorable circulation background, both adiabatic and diabatic PV processes are crucial for TPV genesis.

The relationship between the Indian Ocean dipole (IOD) and the South Asian summer monsoon (SASM), which remains a subject of controversy, was investigated using data analyses and numerical experiments. We categorized IOD events according to their sea surface temperature anomaly (SSTA) pattern: type W and type E are associated with stronger SSTA amplitudes in the western and eastern poles of the IOD, respectively, while type C has comparable SSTA amplitudes in both poles during boreal autumn. Type W is associated with a weak SASM from May to summer, which contributes to substantial warming of the western pole in autumn; the east–west SST gradient linked to the warming of the western pole causes weak southeasterly wind anomalies off Sumatra and feeble and cold SSTAs in the eastern pole during the mature phase. Type E is associated with a strong SASM and feeble warming of the western pole; interaction between the strong SASM and cold SSTAs in the eastern pole in summer results in strong southeasterly wind anomalies off Sumatra and substantial cooling of the eastern pole during the mature phase. For type C, warming of the western pole and cooling of the eastern pole develop synchronously without apparent SASM anomalies and reach comparable intensities during the mature phase. Observations and numerical simulation results both indicate the role of disparate SASM anomalies in modulating SSTA patterns during the development of positive IODs. Warming of the tropical Indian Ocean becomes established in the winter and spring following type W and type C IODs but not following type E events.

This study indicates a significant variation of humidity on extreme heat (EH) days over South Korea and southern–central Japan during the period 1979–2018. EH is therefore classified into three categories: type-A wet EH, type-B wet EH, and dry EH. Their statistical characteristics and formation mechanisms are investigated and compared. Our results suggest that the type-A wet EH is the most destructive, with the highest intensity, longest duration, and broadest spatial scale covering most of midlatitude East Asia. By contrast, type-B wet EH and dry EH are weaker, shorter, and mostly confined to northeast Asia. Despite these differences in characteristics, both types of wet EH are caused by the poleward advance of tropical warm and humid air masses as a result of the northward displacement of the Asian westerly jet. By contrast, dry EH is primarily induced by an increase in adiabatic heating and solar radiation resulting from anomalous subsidence. The three types of EH are associated with distinct large-scale teleconnections over Eurasia. A stable and persistent tripole wave pattern is responsible for type-A wet EH. The activity of atmospheric blocking over northern Europe, where the pattern originates, plays a crucial role in maintaining this pattern. By contrast, type-B wet EH and dry EH are related to a quadrupole pattern and a Silk Road pattern–like teleconnection, respectively, both lasting for a shorter time. These results highlight the diversity of EH, which suggests that multiple local and large-scale circulations should be considered to improve the forecast skills for EH.

The interannual variability of spring lightning activity over South China (SC) and its driving mechanism are examined using the Tropical Rainfall Measuring Mission (TRMM) satellite-estimated convective rain rate and merged lightning flash rate datasets together with atmospheric and oceanic reanalysis products over the TRMM period (1998–2014). SC is climatologically characterized by high lightning flash densities, strong convective rainfall, and large convective available potential energy (CAPE) during spring. The spring SC lightning activity (SCLA) has a strong year-to-year variability, and the standard deviation of flash rates shows a spatially coherent annular distribution centered over the Pearl River Delta. The interannual variations of spring SCLA are not only highly correlated with those of convective rainfall and CAPE, but also significantly correlated with El Nino–Southern Oscillation (ENSO). The anomalously frequent SCLA tends to follow El Nino events, while infrequent SCLA preferentially follows La Nina events. Composite analyses demonstrate that during spring for the more frequent SCLA cases, the upper tropospheric atmospheric response to El Nino-related sea surface temperature anomaly (SSTA) patterns leads to strong southwesterlies over SC in conjunction with tropical westerlies over the tropical Indian Ocean, forming a divergent environment over SC. The upper-level divergence is coupled with low-level moisture convergence, generating strong mid-tropospheric updrafts to lift more air parcels to reach their level of free convection, thereby producing stronger convection with frequent SCLA. In contrast, the anomalous circulation forced by La Nina-related SSTA patterns is conducive to suppressed convection over SC and infrequent SCLA.

Interannual variations of the Bay of Bengal summer monsoon (BOBSM) onset in association with El Nino-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 Nino (La Nina) 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 Nino (La Nina) 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 Nino-related (La Nina-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.

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).

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.

Clouds strongly modulate regional radiation balance and their evolution is profoundly influenced by circulations. This study uses 2001–16 satellite and reanalysis data together with regional model simulations to investigate the spring shortwave cloud radiative effect (SWCRE) and the associated circulations over southeastern China (SEC). Strong SWCRE, up to −110 W m−2, persists throughout springtime in this region and its spring mean is the largest among the same latitudes of the Northern Hemisphere. SWCRE exhibits pronounced subseasonal variation and is closely associated with persistent regional ascending motion and moisture convergence, which favor large amounts of cloud liquid water and resultant strong SWCRE. Around pentad 12 (late February), SWCRE abruptly increases and afterward remains stable between 22° and 32°N. The thermal and dynamic effects of Tibetan Plateau and westerly jet provide appropriate settings for the maintenance of ascending motion, while water vapor, as cloud water supply, stably comes from the southern flank of the Tibetan Plateau and South China Sea. During pentads 25–36 (early May to late June), SWCRE is further enhanced by the increased water vapor transport caused by the march of East Asian monsoon systems, particularly after the onset of the South China Sea monsoon. After pentad 36, these circulations quickly weaken and the SWCRE decreases accordingly. Individual years with spring strong and weak rainfall are chosen to highlight the importance of the strength of the ascending motion. The simulation broadly reproduced the observed results, although biases exist. Finally, the model biases in SWCRE–circulation associations are discussed.

The break of the western North Pacific (WNP) summer monsoon (WNPSM) occurs climatologically in early August and is accompanied by a remarkable suppression of convection over the ocean east of the Mariana Islands (10°–20°N, 140°–160°E). This suppression of convection is sandwiched between two convection peaks in late July and mid-August. Two types of monsoon break are identified in the interannual variation of the WNPSM break in the period 1979–2015, exhibiting a distinct subseasonal evolution of convection that is either in phase or out of phase with the climatological evolution. The preceding SST anomalies in the tropical WNP during early and mid-July are responsible for the interannual variation of the monsoon break. Warm (cold) SST anomalies induce an advanced (delayed) evolution of the WNPSM, with the establishment of strong convection in late July (early August) followed by a monsoon break in early August (mid-August). The subseasonal evolution of convection is therefore in phase (out of phase) with that of the climatological mean. The above SST anomalies mainly result from the local wind–evaporation–SST positive feedback during spring and summer. This local air–sea interaction is still robust after the linear regression components related to the variability of ENSO are excluded from the original fields, indicating that it is, to a large extent, independent of ENSO. The ENSO decaying phases have a secondary role in modulating the SST anomalies related to the WNPSM break.

The large-scale circulation anomalies associated with extreme heat (EH) in South Korea and southern–central Japan are examined using data during the time period 1979–2016. Statistical analysis indicates that EH days in these two regions are concentrated in July and August and tend to occur simultaneously. These EH days are therefore combined to explore the physical mechanisms leading to their occurrence. The composite results indicate that the anomalous atmospheric warming during EH days is dominantly caused by a significant subsidence anomaly, which is associated with a deep anomalous anticyclone over East Asia. Further investigation of the evolution of circulation anomalies suggests that the anomalous anticyclone over East Asia related to EH is primarily initiated by wave trains originating from upstream regions, which propagate eastward along the Asian westerly jet in the upper troposphere. These wave trains can be categorized into two types that are characterized by the precursor anticyclonic and cyclonic anomalies, respectively, over central Asia. The distinction between these two types of wave train can be explained by the wavenumbers of the Rossby waves, which are modulated by both the intensity and the shape of the Asian westerly jet as the background basic flow.

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 SST-forced 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.

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. 摘要 2014年夏季长江流域(YRV)发生的多次阶段性强降水事件显著地受10–20天准双周振荡所调控. 代表性振荡过程与合成分析表明, 准双周湿位相主要取决于对流层高层南侵的高位势涡度(PV)与低纬度向西南平流的高PV导致南亚高压形态改变而产生的YRV高空辐散. 高空向南的正PV平流与低层南风输送的负PV平流在YRV上空形成正的PV平流垂直梯度, 激发出等熵面位移的上升运动分量; 高空的高PV同时向中低层伸展, 导致等熵面坡度增大从而增强气块沿等熵面上滑的上升运动分量, 由此产生更强的非绝热有关的上升运动分量. 干位相情况则相反.

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.

The spatio-temporal variations of eastern China spring rainfall are identified via empirical orthogonal function (EOF) analysis of rain-gauge (gridded) precipitation datasets for the period 1958–2013 (1920–2013). The interannual variations of the first two leading EOF modes are linked with the El Nino–Southern Oscillation (ENSO), with this linkage being modulated by the Pacific Decadal Oscillation (PDO). The EOF1 mode, characterized by predominant rainfall anomalies from the Yangtze River to North China (YNC), is more likely associated with out-of-phase PDO–ENSO events [i.e., El Nino during cold PDO (EN_CPDO) and La Nina during warm PDO (LN_WPDO)]. The sea surface temperature anomaly (SSTA) distributions of EN_CPDO (LN_WPDO) events induce a significant anomalous anticyclone (cyclone) over the western North Pacific stretching northward to the Korean Peninsula and southern Japan, resulting in anomalous southwesterlies (northeasterlies) prevailing over eastern China and above-normal (below-normal) rainfall over YNC. In contrast, EOF2 exhibits a dipole pattern with predominantly positive rainfall anomalies over southern China along with negative anomalies over YNC, which is more likely connected to in-phase PDO–ENSO events [i.e., El Nino during warm PDO (EN_WPDO) and La Nina during cold PDO (LN_CPDO)]. EN_WPDO (LN_CPDO) events force a southwest–northeast oriented dipole-like circulation pattern leading to significant anomalous southwesterlies (northeasterlies) and above-normal (below-normal) rainfall over southern China. Numerical experiments with the CAM5 model forced by the SSTA patterns of EN_WPDO and EN_CPDO events reproduce reasonably well the corresponding anomalous atmospheric circulation patterns and spring rainfall modes over eastern China, validating the related mechanisms.

Austral summer (December–February) surface air temperature over southeast Australia (SEA) is found to be remotely influenced by sea surface temperature (SST) in the South Atlantic at decadal time scales. In austral summer, warm SST anomalies in the southwest South Atlantic induce concurrent above-normal surface air temperature over SEA. This decadal-scale teleconnection occurs through the eastward propagating South Atlantic–Australia (SAA) wave train triggered by SST anomalies in the southwest South Atlantic. The excitation of the SAA wave train is verified by forcing experiments based on both linear barotropic and baroclinic models, propagation pathway and spatial scale of the observed SAA wave train are further explained by the Rossby wave ray tracing analysis in non-uniform basic flow. The SAA wave train forced by southwest South Atlantic warming is characterized by an anomalous anticyclone off the eastern coast of the Australia. Temperature diagnostic analyses based on the thermodynamic equation suggest anomalous northerly flows on western flank of this anticyclone can induce low-level warm advection anomaly over SEA, which thus lead to the warming of surface air temperature there. Finally, SST-forced atmospheric general circulation model ensemble experiments also demonstrate that SST forcing in the South Atlantic is associated with the SAA teleconnection wave train in austral summer, this wave train then modulate surface air temperature over SEA on decadal timescales. Hence, observations combined with numerical simulations consistently demonstrate the decadal-scale teleconnection between South Atlantic SST and summertime surface air temperature over SEA.

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.

This study utilizes daily Asian Precipitation–Highly-Resolved Observational Data Integration Towards Evaluation (APHRODITE) gridded rainfall and the U.S. National Centers for Environmental Predicti...

Anomalous warming occurred in the equatorial central-eastern Pacific in early May 2014, attracting much attention to the possible occurrence of an extreme El Nino 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 Nino event. Based on NCEP (National Centers for Environmental Prediction) oceanic and atmospheric reanalysis data, the physical processes responsible for the strength of El Nino 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 Nino was suspended in summer 2014.

The El Nino event in 2014 starts with the substantial warm sea surface temperature anomaly (SSTA) in the equatorial eastern Pacific, following the westerly wind event (WWE) over equatorial western Pacific from February to March 2014. But it is suspended from June to August after the warm SSTA in equatorial eastern Pacific weakening in April when the WWE disappears. Results show that the 2014 El Nino event is triggered by the WWE, which is induced by the wet phase of MJO propagating eastward from the equatorial Indian Ocean in February. In early March the dry phase of MJO arrives at equatorial western Pacific to weaken the WWE. And the MJO is decayed evidently after mid March. Afterwards the anomalous easterly over equatorial western Pacific is determined by the zonal SSTA gradient in equatorial western-central Pacific. After the MJO disappearance, the anomalous surface easterly could warm the local SST via the wind-evaporation feedback to enhance the sea surface depression in situ . Thus the evident cold advection anomalies are formed in the mixed layer of equatorial central Pacific, where the continuous warming is prevented to increase the local sea surface pressure. As a result, a negative zonal gradient of sea surface pressure anomalies is enhanced over the equatorial western-central Pacific, developing the anomalous easterly over the equatorial western Pacific. Such positive feedback between the easterly anomalies over equatorial western Pacific and the zonal SSTA gradient in equatorial western–central Pacific maintains from June to August 2014, when the El Nino event is suspended with the absence of WWE.

Time-irreversible symmetry is a fundamental property of nonlinear time series. Time-irreversible behaviors of mean temperature measured on 182 stations over China from 1960 to 2012 are analyzed by directed horizontal visibility graph (DHVG for short), and significance of results has been estimated by Monte Carlo simulations. It is found that dominated time irreversibility emerges in nearly all daily temperature anomaly variations over China. Further studies indicate that these time-irreversible behaviors result from asymmetric distributions of persistent daily temperature increments and decrements, and this kind of symmetry can be quantified by distributions of consecutive daily mean temperature increasing or decreasing steps. At the same time, the findings above have been confirmed by artificially generated time series with given value of multiscale asymmetry.

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.

[1] This study investigates the modulation of the Pacific Decadal Oscillation (PDO) on the predictability of interannual early summer south China rainfall (SCR) using high-quality station rainfall data. Of particular interest is the difference in impact between negative and positive phases of the PDO on the predictability of interannual early summer SCR. A clear difference in the correlation between the interannual early summer SCR and the preceding sea surface temperature (SST) over the Pacific Ocean appears in negative and positive phases of the PDO. In the negative PDO phase, the correlation between interannual early summer SCR and SST is dominated by a pattern with significant negative correlations in the subtropical western North Pacific and southeast Pacific and significant positive correlations in the tropical central Pacific. However, in the positive PDO phase, significant positive correlations are observed in the tropical eastern Pacific. It is found that, for each PDO phase, the preceding SST anomalies in some regions in the Pacific may act as predictors of the interannual early summer SCR. As such, a two-regime regression model for the relationship between interannual early summer SCR and preceding SST anomalies is established based on the negative and positive PDO phases using respective multiple linear regression models. Results suggest that the interannual early SCR is more predictable in PDO positive phase than in negative phase. It offers a support for the argument that a segmented statistical forecasting approach associated with the decadal modulation effect of the coupled ocean atmospheric mode should be adopted to forecast the early summer SCR.

Based on a control experiment (CTL), we conduct three sensitivity experiments to focus on the influence of different cloud microphysical latent heat processes on the intensity of Typhoon Hagupit (2008). The results show the following: (1) Because of changes in the latent heat budget, different cloud microphysical processes have little influence on the typhoon track, but have significant influence on its intensity, propagation speed, cloud and precipitation, and thermal and dynamic structures. (2) The positive feedback effect between latent heat release and convection modulates typhoon intensity. In the absence of latent heat release of deposition and sublimation, it is difficult to produce deep convection, the cloud wall is weak and disorganized, the warm core structure is shallow, low-level radial inflow and high-level radial outflow weaken, the cyclonic circulation weakens, and the intensity decreases. In the absence of the latent heat release of evaporation and cooling of rain water, low levels are warm and humid, the development of convection in the cloud wall is strong, there is a deep warm core structure, low-level radial inflow and radial outflow at high level are strengthened, and the cyclonic circulation and intensity are strengthened. (3) The dominant latent heat processes that strengthen the typhoon are water vapor condensation into cloud water, and the deposition growth of snow and of cloud ice. The main latent heat processes weakening the typhoon are evaporation of rainwater, sublimation of snow, and melting of graupel.

The Advanced Research WRF (ARW) version of the Weather Research and Forecasting (WRF) model is used to conduct simulation experiments with a 3 km resolution for Typhoon Hagupit (2008). The primary results show the following: (1) the control experiment reproduces well the environmental field, track, storm propagation speed, intensity change, latent heating vertical profile of the typhoon inner core area, and wind and precipitation distribution of the typhoon. (2) The average total latent heating profile of the inner core reaches its peak value at a height of 7 km. At heights below 500 m, there is mainly an evaporative cooling effect. From 1 to 4 km, mainly condensation heating is present. Above 10 km, there is mainly a deposition effect. (3) Condensation heating exists principally within the inner flank of the cloud wall. Its maximum height is 2–5 km, and can reach 11 km. Melt cooling takes place in the outer flank of the cloud wall, whereas freeze heating is in the cloud wall, above the zero degree isotherm. (4) The large magnitude of microphysical process conversion rates is attributable to rainwater collected by graupel, melting of graupel, rainwater collected by snow, and water vapor condensation into cloud water. The dominant microphysical processes for releasing latent heat are water vapor condensation into cloud water and depositional growth of snow and cloud ice. The dominant microphysical processes for absorbing latent heat are evaporation of rainwater, sublimation of snow, and melting of graupel.

Multi-fractal behaviors of diurnal temperature range (DTR for short) from 100 stations over China during 1956–2010 are analyzed by means of multi-fractal detrended fluctuation analysis. By making a Monte-Carlo simulation, we obtain two criterions which can be used to decide whether a DTR series is significantly multi-fractal or not. With these criterions, different multi-fractal behaviors are found over the north and the south of China, and Yangtze River is roughly the dividing line. Over the north region, nearly all the considered DTR series do not show multi-fractal behaviors, while the results are completely the opposite over the south. The findings are confirmed by the scaling behaviors of the corresponding DTR magnitude series and indicate that more scale-dependent structure differences may be hidden in DTR series over the north and the south of China. Therefore, an extensive analysis of the multi-fractal behaviors are essential for a better understanding of the complex structures of the climate changes.

Effects of extreme value loss on long-term correlated time series are analyzed by means of detrended fluctuation analysis (DFA) and power spectral density analysis. Weaker memory can be detected after removing of extreme values for the artificial long-term correlated data, indicating the emergence of extreme events may be closely related to long-term memory. For observational temperature records, similar results are obtained, but not in all stations. For example, in some stations, only extending of scaling range to smaller time scales occurs, which may be due to the asymmetric distribution of values in the record. By comparing our findings with previous works, clustered positions of the extreme events are recognized as an important property in long-term correlated records. Through a simple numerical test, close relations between extreme events and long-term memory are discovered, which is helpful for our understanding of the effects of extreme value loss on long-term correlated records.

This study reveals the barotropic dynamics associated with the formation and growth of tropical cyclone Nargis in 2008,during its formation stage.Strong equatorial westerlies occurred over the southern Bay of Bengal in association with the arrival of an intraseasonal westerly event during the period 22-24 April 2008. The westerlies,together with strong tropical-subtropical easterlies,constituted a large-scale horizontal shear flow,creating cyclonic vorticity and thereby promoting the incipient disturbance that eventually evolved into Nargis.This basic zonal flow in the lower troposphere was barotropically unstable,with the amplified disturbance gaining more kinetic energy from the easterly jet than from the westerly jet during 25-26 April. This finding suggests that more attention should be paid to the unstable easterly jet when monitoring and predicting the development of tropical cyclones.Energetics analyses reveal that barotropic energy conversion by the meridional gradient of the basic zonal flow was indeed an important energy source for the growth of Nargis.

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.

Based on the aqua-planet experiments, the wavenumber-frequency characteristics of tropical waves and their influencing factors in SST distribution and the convective parameterization scheme are investigated using the spectral atmospheric general circulation model (SAMIL). Space-time spectral analysis is used to obtain the variance of convectively coupled tropical waves. In the Control experiment with maximum SST located at the equator the simulated tropical-wave behaviors are in agreement with those in observations and theoretical solutions. When the maximum SST is located at 5°N, the symmetric and antisymmetric waves are much weaker than those in the control experiment, suggesting that tropical wave activities are very sensitive to the SST distributions. Importantly, the variance maximum of Madden-Julian oscillation (MJO) is found to occur around 5°N, which suggests that the development of the MJO depends largely on the latitude of maximum SST. Furthermore, the seasonal variations of MJO may be mainly caused by the seasonal variations of the maximum SST.

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

The US National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data are employed to study the relationship between the variation in the structure of subtropical anticyclone and the summer monsoon onset over the South China Sea (SCS). The ridge surface of the subtropical anticyclone is defined by the boundary between westerly to the north and easterly to the south [or westerly-easterly boundary (WEB) in brief]. The WEB well represents the three-dimensional structure of the subtropical anticyclone. Under the thermal wind constraint, the WEB usually tilts toward the warmer zone in the vertical. In the Asian monsoon area, the ridgelines of the subtropical anticyclone at various levels in the troposphere in winter are continuous and exhibit a relatively zonal structure. The WEB tilts southward with increasing height and the mean position of the WEB is closer to the equator. In summer, the ridgelines of the subtropical anticyclone in the middle and lower (upper) troposphere are discontinuous (continuous) and the WEB either tilts northward or is vertical, with the mean position of the WEB being located at 25-30°N. May is the period when the structure of the subtropical anticyclone varies most significantly. During the seasonal transition, the tilt of the WEB changes from southward to northward. When the WEB becomes perpendicular to the earth's surface or tilts northward, the meridional temperature gradient vanishes or becomes positive, implying the replacement of the winter monsoon by the summer monsoon. Defining the “seasonal transition axis” (STA) as the vertical ridge axis that results from the switching of the tilt of the WEB from southward to northward, it is found that the STA first establishes over the eastern Bay of Bengal and Indochina Peninsula during the first pentad of May. The onset of the Asian summer monsoon circulation is therefore closely associated with the location of the STA. When the STA reaches the northeastern SCS in the fourth pentad of May, the SCS summer monsoon occurs. The south Asian summer monsoon onset corresponds to the establishment of the STA over central India in the first two pentads of June. Based on the close relationship between the STA and monsoon onset, the area-averaged meridional mean upper tropospheric (200-500 hPa) temperature gradient in the vicinity of the WEB is suggested to be an index to define the SCS summer monsoon onset. In comparison with the onset dates as determined by the area-averaged zonal wind at 850 hPa and OLR, the dates determined by these three indices are consistent in a large number of individual years, which indicates that the establishment of the STA, or the reversal of the meridional temperature gradient, captures the essential feature associated with the SCS summer monsoon onset.

[1] This study reveals some unique features associated with the impact of Typhoon Chanchu on the South China Sea (SCS) summer monsoon (SCSSM) onset in 2006. With Typhoon Chanchu entering the SCS in mid-May, southwesterlies were induced over the SCS and its upstream region due to a thermally forced Rossby wave response, leading to a reversal of low-level winds and a positive meridional temperature gradient (MTG) in the upper troposphere over the SCS. Typhoon Chanchu thus acted as an immediate trigger for the SCSSM onset, and the onset date was suggested to occur in the third pentad of May. After the typhoon landed in the coast of southern China, it still contributed to the establishment of the SCSSM due to warming the air column over the northern SCS, resulting in a persistent positive MTG with the ridge surface tilting northward.