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The Mediterranean region is located over the entrance of the Asian upper level jet. The anomalous atmospheric circulation over the region can excite wave train pattern propagating eastward and impacting the climate over the Eurasian continent. In this study, the summer (June–September) sea level pressure (SLP) prediction skill over the Mediterranean region is investigated, based on the May-start hindcasts of five state-of-the-art coupled models from the ENSEMBLES over the period of 1960–2005. The result shows that there are three models that exhibit an increased prediction skill for the Mediterranean SLP after the late 1970s. Tropical sea surface temperature (SST) is generally recognized as the major source for dynamical seasonal climate prediction. The analysis of the three models shows that they can capture well the observed impact of tropical Atlantic and Pacific SSTs on the Mediterranean atmospheric circulation after the late 1970s. Consequently, the three models have an increased prediction skill for the Mediterranean SLP over the time. In contrast, the other two models cannot reproduce the observed relationship of the Mediterranean SLP with the tropical Atlantic and Pacific SSTs, thereby exhibiting low prediction skill for the Mediterranean SLP. Further analysis indicates that the increased prediction of the Mediterranean SLP can improve dynamical prediction of surface air temperature (SAT) over the eastern Mediterranean and central East Asia, which is valuable for current regional dynamical seasonal climate prediction. [ABSTRACT FROM AUTHOR]

The duration of cross-equatorial flow (CEF) events over the Maritime Continent (MC) varies greatly, from 3 days to more than 1 month. In this study, we classify CEF events into short- and long-lived ones using a threshold of 7 days, and conduct separate investigations on their statistical and evolution characteristics, as well as the associated physical processes from the perspective of two dominant modes of the intraseasonal oscillation. Results indicate that short-lived events, characterized by westward-propagation southerly anomalies, are largely dependent on the 10–25-day oscillation, while the contribution of the 30–60-day component is negligible. The associated enhanced convection is primarily confined to the western North Pacific (WNP) and moves northwestward, accompanied by an anomalous cyclone. In contrast, long-lived events show consistent changes among different CEF branches, characterized by southerly anomalies dominating the MC, which benefit from the favorable background of the 30–60-day oscillation. Associated convection anomalies show a dipole pattern, with enhanced convection over WNP and suppressed convection over the Indian Ocean. The enhanced WNP convection gradually migrates northward, inducing CEF anomalies via a continuous anomalous cyclone, while the suppressed convection can be found to slightly expand to the MC. Meanwhile, the 10–25-day oscillation shows similar magnitude and evolution with that in short-lived events, but is no longer crucial to the establishment of long-lived events. [ABSTRACT FROM AUTHOR]

It is challenging to predict extreme precipitation over regions with complex terrain, such as Southwest China (SWC). This study develops a statistical seasonal prediction scheme for summer extreme precipitation frequencies (EPFs) over SWC. In this scheme, the year-to-year increment method is first applied to the SWC summer EPFs and boundary forcings in preceding seasons. The first six leading modes of SWC summer EPFs are extracted by empirical orthogonal function (EOF) decomposition. The relationships between the six modes and preceding boundary forcings are investigated. According to these relationships, physics-based predictors for the principal components (PCs) of the modes are selected. The first two modes are taken as examples to analyze the mechanisms by which the predictors influence SWC summer EPFs. Two machine learning (ML) models, random forest and extreme gradient boosting, are trained using the data during 1980–2011. The ML models that perform best for each PC on the training data are used to be independently tested on data during 2012–2021. The predicted EPF patterns can be reconstructed by predicted PCs and observed EOFs. Prediction results indicate that the optimal ML models can well capture the variations in the six modes and spatial patterns of SWC EPFs and outperform the traditional stepwise regression models. The prediction skills for SWC summer EPF anomaly patterns show year-to-year variations which depend on the prediction results of the PCs. This scheme may help the seasonal predictions of SWC summer EPFs. • A new seasonal prediction scheme for summer extreme precipitation frequencies over Southwest China is proposed • The physics-based predictors and machine learning are used to build statistical models • The optimal machine learning models outperform the traditional stepwise regression models [ABSTRACT FROM AUTHOR]

The predictability of spring (March–May) precipitation over East China is investigated, based on the February-start hindcasts of eight coupled models from DEMETER and ENSEMBLES during 1960–2001. Five out of the eight models exhibit significantly increased predictability of central East China spring precipitation (CECSP) after the late 1970s. The mechanism analysis indicates that CECSP variability is closely related to a meridional dipole vorticity pattern at 200 hPa and southerly wind at 850 hPa over East Asia, whose prediction skill increased significantly around the late 1970s, consistent with the changes in CECSP predictability. Observational analysis indicates that the sea surface temperature (SST) over the tropical Pacific and Indian Oceans experienced a notable decadal change around the late 1970s. After the decadal change, the tropical SST has an enhanced impact on the CECSP-related East Asian dipole vorticity pattern at the upper level and on the western North Pacific anticyclone at the lower level. The five models can adequately reproduce the observed enhanced connection between the tropical SST and East Asian atmospheric circulation after the late 1970s, consequently showing higher predictability of East Asian atmospheric circulation and CECSP. However, the other three models cannot reproduce the relationship between the tropical SST and East Asian atmospheric circulation; therefore, CECSP predictability in these models remains low during the entire period. The increased predictability is valuable for current dynamical seasonal prediction for central East China. [ABSTRACT FROM AUTHOR]

Multi-model ensemble prediction is an effective approach for improving the prediction skill short-term climate prediction and evaluating related uncertainties. Based on a combination of localized operation outputs of Chinese climate models and imported forecast data of some international operational models, the National Climate Center of the China Meteorological Administration has established the China multi-model ensemble prediction system version 1.0 (CMMEv1.0) for monthly-seasonal prediction of primary climate variability modes and climate elements. We verified the real-time forecasts of CMMEv1.0 for the 2018 flood season (June-August) starting from March 2018 and evaluated the 1991–2016 hindcasts of CMMEv1.0. The results show that CMMEv1.0 has a significantly high prediction skill for global sea surface temperature (SST) anomalies, especially for the El Niño-Southern Oscillation (EN-SO) in the tropical central-eastern Pacific. Additionally, its prediction skill for the North Atlantic SST triple (NAST) mode is high, but is relatively low for the Indian Ocean Dipole (IOD) mode. Moreover, CMMEv1.0 has high skills in predicting the western Pacific subtropical high (WPSH) and East Asian summer monsoon (EASM) in the June–July–August (JJA) season. The JJA air temperature in the CMMEv1.0 is predicted with a fairly high skill in most regions of China, while the JJA precipitation exhibits some skills only in northwestern and eastern China. For real-time forecasts in March–August 2018, CMMEv1.0 has accurately predicted the ENSO phase transition from cold to neutral in the tropical central-eastern Pacific and captures evolutions of the NAST and IOD indices in general. The system has also captured the main features of the summer WPSH and EASM indices in 2018, except that the predicted EASM is slightly weaker than the observed. Furthermore, CMMEv1.0 has also successfully predicted warmer air temperatures in northern China and captured the primary rainbelt over northern China, except that it predicted much more precipitation in the middle and lower reaches of the Yangtze River than observation. [ABSTRACT FROM AUTHOR]

The Indian and East Asian summer monsoons are two active components of the global monsoon system and are closely related. However, their relationship is weakened after the late 1970s. This study revisited the possible mechanism for the weakening relationship between Indian and central East Asian summer rainfall, focusing on the influence from the middle-to-high-latitude climate system. The results show that variability in the summer North Atlantic Oscillation since the late 1970s induces a zonal atmospheric teleconnection pattern, resulting in anomalous upper level atmospheric circulation south of the Baikal Lake, which is further closely related to central East Asian summer rainfall over the period after the late 1970s. When the signal of the atmospheric circulation variability south of the Baikal Lake is removed, the relationship between the Indian and central East summer rainfall becomes significant again over the period after the late 1970s. The summer North Atlantic Oscillation thus opposes the influence of the Indian summer rainfall on central East Asian summer rainfall, consequently contributing to the breakdown of the Indian–central East Asian summer rainfall relationship after the late 1970s. [ABSTRACT FROM AUTHOR]

Lake ice phenology, as an indicator for climate variability and change, exerts a great influence on regional climate and hydrometeorology. In this study, the changing characteristics of lake ice phenology at Lake Qinghai (LQH) are investigated using retrieved historical datasets during 1979–2016. The results show that the variation of the lake freeze-up date over LQH is characterized by a strong interannual variability. Further analysis has revealed that November sea ice concentration (SIC) variation in the Kara Sea can exert a great impact on the freeze-up date at LQH. During the low sea ice years, the open sea serves as a strong diabatic heating source, largely contributing to the enhanced Arctic Eliassen–Palmer flux, which then results in the deceleration of zonal wind in the middle and high latitudes. In addition to this, accompanied with the decreasing Kara SIC, the enhanced stationary Rossby wave flux propagating along the high-latitude regions may further exert remarkable influences in deepening the East Asian trough, which provides a favorable atmospheric circulation pattern for cold air intrusion from the Arctic and Siberian regions to mainland China. The decreased surface air temperature would thus advance the freezing date over LQH. Furthermore, the close relationship between atmospheric circulation anomalies and Kara SIC variations is validated by a large ensemble of simulations from the Community Earth System Model, and the atmospheric circulation patterns induced by the SIC anomalies are reproduced to some extent. Therefore, the November Kara Sea ice anomaly might be an important predictor for the variation in the freeze-up date at LQH. [ABSTRACT FROM AUTHOR]

The relationship between the boreal summer (June–August, JJA) precipitation over eastern China and Australia is investigated in this study. The relationship exhibits a decadal enhancement with correlation coefficients increasing from 0.07 before the early 1980s to 0.60 thereafter. A mechanistic analysis suggests that the variability in sea surface temperature (SST) over the central tropical Pacific could play an important role in enhancing the precipitation relationship between the two regions. Over the past half century, the central Pacific SST has shown a warming trend with a significant decadal change that occurred around the early 1980s. Such changes in the central Pacific SST have resulted in stronger convective activity over the tropics, as well as eastern China and Australia since the early 1980s. In addition, since the early 1980s, the central Pacific SST stimulated anomalous twin anticyclones located on opposite sides of the equator, which changed the transportation of water vapour to eastern China and Australia. These changes in the atmospheric circulation related to central tropical Pacific SST have enhanced the impact on the precipitation variability over eastern China and Australia. Consequently, this has contributed to the covariability in the boreal summer precipitation over the two regions since the early 1980s. The result indicates that concurrent climate anomalies can provide simultaneously occurring natural disasters over Asia and Australia, which creates a challenge for international disaster prevention and mitigation. The relationship between the boreal summer (June–August, JJA) precipitation over eastern China and Australia is investigated in this study. The relationship exhibits a decadal enhancement with correlation coefficients increasing from 0.07 before the early 1980s to 0.60 thereafter. A mechanistic analysis suggests that the variability in sea surface temperature (SST) over the central tropical Pacific could play an important role in enhancing the precipitation relationship between the two regions. [ABSTRACT FROM AUTHOR]

Observations show that the East Asian winter monsoon (EAWM) experienced an interdecadal weakening in the mid-1980s. This is evident for all members of the EAWM system (i.e., East Asian trough, upper-tropospheric jet stream, and lower-tropospheric monsoon circulation). Here, we investigate the relative contributions of natural (volcanic aerosols and solar variability) and anthropogenic [greenhouse gases (GHGs) and anthropogenic aerosols] forcings to this interdecadal weakening using multiple coupled models within phase 5 of the Coupled Model Intercomparison Project (CMIP5). The results indicate that in the midtroposphere, the increased GHG concentrations play an important role in weakening the East Asian trough (EAT) by increasing the sea surface temperatures (SSTs) over the North Pacific. In the upper troposphere, natural external forcings contribute to the observed weakening of the meridional shear of the East Asian jet stream (EAJS) by regulating the meridional temperature gradient (MTG) over the East Asian region. In the lower troposphere, both anthropogenic and natural forcings can weaken the Siberian high during this period. Overall, based on the present analysis of the CMIP5 output, GHGs and natural forcings play key roles in shaping the observed interdecadal weakening of the EAWM during the mid-1980s. Additionally, contributions from internal variability cannot be neglected and require further investigation. [ABSTRACT FROM AUTHOR]

Using the high-resolution regional climate model–Weather Research and Forecasting model(WRF)–nested within the National Center for Atmospheric Research Community Atmosphere Model(CAM4), a set of sensitivity experiments were conducted to investigate the Asian climate response to the regional uplift of the Tibetan Plateau (TP). The results indicated that the presence of Asian orography, including the TP, strengthened both the Indian and East Asian summer monsoon (ISM and EASM), respectively. The uplift of the central-southern TP and western TP caused an asynchronous response of monsoon precipitation over India and East Asia, while the presence of the Himalayan mountains, Gobi Altai mountains, and Mongolian Plateau increased both ISM and EASM precipitation. The enhanced summer monsoon precipitation over southeastern China due to the TP uplift was caused by strong water vapor transportation, vapor convergence and ascending movement. The Indian and East Asian winter monsoons (IWM and EAWM), however, showed asynchronous change in response to the TP uplift; the EAWM was strengthened, but the opposite holds for the IWM. This study provides further understanding of the response of Asian monsoons to the regional uplift of the TP and emphasizes the importance of high-resolution regional climate model simulation nested within global climate model. [ABSTRACT FROM AUTHOR]

The boreal spring relationship between variabilities of East China precipitation (ECP) and tropical Ocean sea surface temperature (SST) during the period 1951-2014 is investigated in this study. The results show that the leading mode of the ECP variability exhibits an enhanced response to the anomalous El Niño-Southern Oscillation (ENSO)-like SST after the late 1970s, when the SST underwent a decadal change, with two positive centers over the eastern tropical Pacific (ETP) and tropical Indian Ocean (TIO). To further understand the relative roles of the ETP and TIO SST anomalies (SSTAs) in the variability of ECP after the late 1970s, partial regression and correlation methods are used. It is found that, without the contribution of the TIO, ETP SSTA plays a limited role in the variability of ECP after the late 1970s; comparatively, a significant correlation between TIO SST and ECP is identified during the same period, when the ETP signal is linearly removed. Physical analyses show that, after the late 1970s, the TIO SSTA affects East Asian atmospheric circulation in two ways: by exciting a zonal wave-train pattern over the mid-latitude Eurasian Continent and by inducing anomalous convection over the Maritime Continent. Via these two mechanisms, the TIO SST variability results in an anomalous East Asian trough and vertical motion over East China and consequently leads to anomalous precipitation over the region. The physical processes linking the ECP and TIO SST are confirmed by an atmospheric general circulation model experiment forced with idealized TIO warming. [ABSTRACT FROM AUTHOR]