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To demonstrate the challenge of summer rainfall prediction and simulation in the eastern China, in this work, we examine the skill of the state-of-the-art climate models, evaluate the impact of sea surface temperature (SST) on forecast skill and estimate the predictability by using perfect model approach. The challenge is further demonstrated by assessing the ability of various reanalyses in capturing the observed summer rainfall variability in the eastern China and by examining the biases in reanalyses and in a climate model. Summer rainfall forecasts (hindcasts) initiated in May from eight seasonal forecast systems have low forecast skill with linear correlation of − 0.3 to 0.5 with observations. The low forecast skill is consistent with the low perfect model score (~ 0.1–0.3) of atmospheric model forced by observed SST, due to the fact that external forcing (SST) may play a secondary role in the summer rainfall variation in the eastern China. This is a common feature for the climate variation over the middle and high latitude lands, where the internal dynamical processes dominate the rainfall variation in the eastern China and lead to low predictability, and external forcing (such as SST) plays a secondary role and is associated with predictable fraction. Even the reanalysis rainfall has some remarkable disagreements with the observation. Statistically, more than 20% of the observed variance is not captured by the mean of six reanalyses. Among the reanalyses, JRA55 stands out as the most reliable one. In addition, the reanalyses and climate model have pronounced biases in simulating the mean rainfall. These defaults mean an additional challenge in predicting the summer rainfall variability in the eastern China that has low predictability in nature.

The evolution of Sahel summer rainfall in the context of global warming is a severe socio-economic concern because of its widespread influences on local agriculture, water resource management, food security, infrastructure planning, and ecosystems. Based on the mid-Pliocene simulations from the Pliocene Model Intercomparison Project Phase 2 and the historical simulations and shared socio-economic pathway 5–8.5 experiments from the Coupled Model Intercomparison Project phase 6, the present study contrasts the Sahel summer rainfall changes between the past mid-Pliocene and near future global warming climates. The results show that the Western African summer monsoon (WASM) circulation, closely linked with the Sahel summer rainfall change, tends to strengthen in both the past and future global warming climates, but the monsoonal circulation strengthening is much more intense in the past warm period than in the projected warm future. This causes that the multi-model ensemble (MME) mean increase ratio of Sahel summer rainfall in the past warming climate is about twice to three times larger than that in the future warming climate for the same increase of global mean surface temperature (the regional rainfall increase ratio in the MME mean: about 19.6% per one degree Celsius of global warming in the mid-Pliocene simulations versus about 7.7% per one degree Celsius of global warming in the SSP5-8.5 future projections). Such a striking discrepancy in the regional circulation and hydrological cycle changes is mainly attributed to a dramatically stronger warming over the Canadian Archipelago and Greenland during the mid-Pliocene warm period relative to the projected near future. The more significant northern high-latitude warming during the mid-Pliocene enhances the meridional temperature gradient between the extratropical and tropical regions, which could induce an excessive northward shift of the Intertropical Convergence Zone and a stronger WASM, and thus result in a more intense hydrological cycle around the Sahel region. Our results highlight that besides the global mean temperature increase, meridional warming patterns are also essential for the changes of WASM and regional hydrological cycle in a warmer world. Implications for projecting the regional monsoon and hydrological cycle changes at longer time scales than in the near future are discussed.

The strength of the dominant variability of contiguous United States (CONUS) summer rainfall during 1960–2013 experiences an interdecadal change in the early 1990s. Before the early 1990s, the variation in CONUS summer rainfall is relatively small in amplitude (standard deviation: 0.64 mm day−1), whereas after it amplifies remarkably, with its standard deviation (1.31 mm day−1) roughly doubling. Observational diagnoses and simulation results show that enhanced East Asian subtropical monsoon variability plays a direct role in strengthening the CONUS summer rainfall dipole variability. Besides, a northward shift of the East Asian summer monsoon is also responsible for the amplification of the CONUS summer rainfall variability. This northward shift of the East Asian rain belt pushes the rainfall perturbation farther to the north, much closer to the subtropical East Asian upper-level westerly jet stream. As a result, the East Asian subtropical monsoon heating induces the upper-level Asia–North America teleconnection pattern more effectively, leading to the larger amplitude of CONUS summer rainfall variability.

The present study examines subseasonal prediction skills and biases of the summer rainfall over eastern China in the Beijing Climate Center (BCC) Atmospheric General Circulation Model (BCC_AGCM2.2) and assesses the predictability of eastern China summer rainfall based on the multi-member forecasts. The BCC_AGCM2.2 model shows some skill in predicting summer rainfall over eastern China within the lead-times of 0–9 days. However, the subseasonal prediction skill is low on average, which is linked to the ability of the model in predicting the western Pacific subtropical high (WPSH). The low prediction skills may partially be attributed to biases in the model, including a wider meridional span and a weaker intensity of rainbelt in Yangtze River valley, earlier meridional movement, and a further northward shift of WPSH compared to that in the observations. These biases result in an obvious dry bias along the monsoon rainbelt, and a wet bias to both the north and south sides. Moreover, as a major contributor to the predictability, the Pacific-Japan pattern is not well predicted for both its spatial pattern and subseasonal evolution. The low forecast skill of summer rainfall in eastern China seems due to a dominant role of the atmospheric internal variability and a minor influence of the sea surface temperature in extratropical climate variability. Nevertheless, enhanced prediction skills under the assumption of a perfect model imply the potential to improve the prediction skill of the summer rainfall over eastern China through reducing model biases.

In this study, based on the ECMWF 20-year (1997 ~ 2016) hindcasts, the subseasonal prediction of the weekly summer rainfall anomaly over the middle and lower reaches of the Yangtze River (YR) were studied. The skill at 2-week lead time exhibits prominent interannual variation, which is significantly correlated with the preceding winter ENSO. That is, rainfall anomaly can be better predicted in El Niño decaying summer than in La Niña decaying summer. Observation analyses show that in El Niño decaying summer the intraseasonal variation of YR summer rainfall is featured by strong low-frequency (> 30 days) intraseasonal oscillation (ISO) partly associated with the first boreal summer ISO mode (BSISO1) activity, in comparison with La Niña decaying summer. This is possibly because El Niño-induced mean state western North Pacific (WNP) anti-cyclone blocks the northward propagation of convection over the WNP, resulting in BSISO1 stagnation in phases 3–4. The phase stagnation could force stable atmospheric teleconnection, which is favorable to sufficient moisture transportation to the YR and persistent rainfall formation. Finally, prediction verification showed that more accurate prediction for the middle-low-level circulation contribute to the better prediction of rainfall anomaly in El Niño decaying summer than in La Niña decaying summer.

This paper reveals a change in the relationship between the Yangtze River valley (YRV) summer rainfall (YRVSR) and the spring Southern Hemisphere annular mode (SAM) during the period of 1958–2012. It is found that the positive correlation between the YRVSR and the previous May SAM is insignificant during the period of 1958–1987 (P1), while it becomes significant during the period of 1988–2012 (P2). Such interdecadal change is relevant to the interdecadal change of the relationship between the May SAM and the Indian Ocean sea surface temperature anomaly (SSTA). During P2, the strong exchange of the air-sea heat flux related to the May SAM can excite a prominent meridional Indian Ocean tri-pole (IOT) SSTA pattern, whereas during P1, the May SAM associated air-sea heat exchange is weak and the IOT SSTA is not evident. Further observational and simulated evidences show that the May SAM associated IOT SSTA can persist through the following boreal summer and modulate the rainfall anomalies over the Maritime Continent (MC) by changing the zonal circulation over the tropical Indian Ocean-MC region. Subsequently, wave energies generated by the latent heat release of the MC rainfall (MCR) anomalies can propagate northward to the YRV along the great circle route. Responding to the positive (or negative) MCR heating forcing, anticyclonic (or cyclonic) circulation anomalies are excited over the western North Pacific, transporting more (or less) moisture to the YRV. Correspondingly, abnormal ascending (or sinking) motions are induced over the YRV, favoring sufficient (or insufficient) YRVSR. Thus, the IOT SSTA and MCR act as the “ocean” and “atmosphere” bridge role in connecting the May SAM and the YRVSR, respectively. Those models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) which well capture the May SAM-YRVSR relation can reproduce the IOT SSTA pattern and MCR anomalies associated with the May SAM. This further affirms that both the IOT SSTA and MCR are responsible for the strengthened relationship between the SAM and the YRVSR.

A meridional dipolar atmospheric teleconnection between the Tasman Sea and the Southern Ocean, which describes a hybrid between the El Niño–Southern Oscillation, Indian Ocean Dipole and Southern Annular Mode, was referred to as the Hybrid Teleconnection (HT) pattern previously. We investigate its connection with East Asian summer rainfall and find the preceding May–June HT can be the precursor of the following July–August East Asian rainfall. The mechanism is examined based on analyses of observational datasets and historical runs of coupled models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). The results suggest that a positive HT with positive 500 hPa geopotential height anomalies over the Tasman Sea but negative over the Southern Ocean in the preceding boreal early summer contributes to generating negative sea surface temperature (SST) anomalies in the western tropical Pacific (WP) by influencing surface heat exchange. The SST anomalies over WP persist into the mid-summer via the thermal memory of seawater, inducing interhemispheric meridional circulation and then exciting a Pacific-Japan-Pattern-like atmospheric circulation anomaly, which manifests as an anticyclone over the subtropical northwestern Pacific along with a cyclone near the Sea of Japan. This pattern induces intensified convergence and vertical ascend motion, and subsequently intensifies rainfall in the Yangtze River Basin.

The impacts of internal variability on East Asia–Northwest Pacific (EA–NWP) summer rainfall trends on the multidecadal time scale are invested based on three large ensemble simulations, which have ensemble member of 30, 40 and 100. In all the three simulations, the summer rainfall trends during 1970–2005 are remarkably diverse across the individual ensemble members over the EA–NWP, and the signal-to-noise ratio is lower than 1 over the EA–NWP, suggesting a strong impact of internal variability on EA–NWP summer rainfall trends at this interval. Moreover, we found that the diversity of EA–NWP summer rainfall trends across individual members has a similar leading spatial pattern in all the three ensembles, featuring reverse trends between in Mei-yu region and in the tropical NWP. The leading pattern is likely caused by a gradient between the sea surface temperature (SST) trends in the North Indian Ocean (NIO) and in the tropical western Pacific (WP). When there is a warming trend in the NIO and a cooling trend in the tropical WP, a low-level anomalous anticyclone strengthens over the subtropical NWP, causing a dipole rainfall trend over the EA–NWP. The impact of the east–west SST gradient pattern is confirmed by numerical experiments. Our findings highlight that the internally-generated gradient of NIO–WP SST trends is an important source of the uncertainty in EA–NWP summer rainfall decadal changes in simulations.

This study characterizes the intraseasonal variability (ISV) in the Southeastern United States (SE US) rainfall in boreal summer and delineates the associated dynamical processes featuring three-way interactions among the SE US rainfall, the central US low-level jet (LLJ), and the North Atlantic subtropical high (NASH). The analysis reveals that the ISV of the SE summer rainfall peaks at the 10‒20-day timescales. The physical mechanisms for the three-way interactions on the 10‒20-day timescales are proposed. When the NASH attains a minimum strength, the reduced size of the NASH is accompanied with an eastward retreat of the western ridge of the NASH, leading to a decrease in the zonal pressure gradient and consequently a weakening of the LLJ 1 day after. The weakened LLJ and the eastward-shifted NASH western ridge induces anomalous cyclonic circulation over the SE US, moves preferred regions of moisture convergence from central US to the SE US, and 3 days later the SE US rainfall attains its maximum strength. The excessive latent heating associated with the enhanced SE US rainfall excites an anomalous anticyclone northeast of the rainfall region, resulting in an increase in the NASH intensity that peaks 2 days after the maximum SE US rainfall. The NASH subsequently expands with its western ridge moving westward, zonal pressure gradient restored, and LLJ strength recovered. An anomalous anticyclone then emerges over the SE US and suppresses rainfall, marking the shift from an intraseasonal wet phase to dry phase in this region. A more rigorous proof of these causalities demand carefully designed numerical experiments and further statistical analysis in future. Our results suggest that improved prediction of SE US summer rainfall across intraseasonal scales depends critically on the model representation of the three-way coupling among the NASH, the central US LLJ, and the SE US rainfall.

Based on observational and reanalysis data for 1979–2015, the possible impacts of spring sea surface temperature anomalies (SSTA) over the South Indian Ocean on the inter-annual variations of summer rainfall in Guangdong and Guangxi Provinces (i.e., the Guangdong-Guangxi area, GG) were analysed in this study. The physical mechanism behind these impacts was explored. Two geographic regions over [65°E–95°E, 35°S–25°S] and [90°E–110°E, 20°S–5°S] were defined as the western pole region and the eastern pole region, respectively, for the GG summer precipitation (P GG )-related South Indian Ocean dipole SSTA pattern (R-SIODP). The difference between springtime SST anomalies averaged over the western pole region and that averaged over the eastern pole region was defined as the R-SIODP index. The correlation between the spring R-SIODP index and GG summer precipitation can reach up to 0.52. In the spring of positive R-SIODP anomaly, southerly winds over the western pole of the R-SIODP weaken, whereas the southeast trade winds over the eastern pole strengthen. By means of the wind-evaporation-SST feedback mechanism, the enhanced southeast trade winds can weaken the evaporation over the western pole of the R-SIODP and enhance the evaporation over the eastern pole. This results in a sustained positive SSTA in the western pole of the R-SIODP and a sustained negative SSTA in the eastern pole, whereby the distribution of the SSTAs maintains until summer. The SST dipole abnormally enhances the cross-equatorial airflow near 105°E, which intensifies the anomalous anti-cyclonic circulation over South China Sea at 850 hPa and simultaneously results in abnormal enhancement of water vapour transport to GG. Additionally, the SST dipole promotes abnormal divergence in the lower troposphere and abnormal convergence in the upper troposphere over the maritime continent (MC) region. Moreover, the low-level convergence in GG is enhanced, which results in abnormal enhancement of ascending motion in the GG that is conducive to positive summer rainfall anomaly in this region. In this study, the spring R-SIODP index, the SST to the east of Australia and to the east of southern Africa, and the North Atlantic oscillation (NAO) index were used to construct a statistical prediction model for the inter-annual variability of the GG summer rainfall anomaly. This model can well predict the accuracy of the inter-annual variation of GG summer rainfall.