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Most El Niño events decay after a peak in boreal winter, but some persist and strengthen again in the following year. Several mechanisms for regulating its decay pace have been proposed; however, their relative contributions have not been thoroughly examined yet. By analyzing the fast-decaying and persistent types of the events in a 1200-year coupled simulation, we quantify the key dynamic and thermodynamic processes in the decaying spring that are critical to determining the decay pace of El Niño. The zonal advection due to upwelling Kelvin wave accounts for twice as much the cooling difference as evaporation or meridional advection does. The upwelling Kelvin wave is much stronger in the fast-decaying events than the others, and its strength is equally attributed to the reflected equatorial Rossby wave and the equatorial easterly wind forcing over the western Pacific in the preceding 2-3 months. Relative to the fast-decaying events, the evaporative cooling is weaker but the meridional warm advection is stronger in the persistent events. The former is due to more meridionally asymmetric wind and sea surface temperature anomalies (SSTA) signaling positive Pacific Meridional Mode (PMM). The latter results from the advection of equatorial warm SSTA by climatological divergent flow, and the warmer SSTA persists from the mature stage subject to weaker cloud-radiative cooling in response to the Central-Pacific-type SSTA distribution in the persistent events relative to the fast-decaying events. Our result consolidates the existing knowledge and provides a more comprehensive and physical pathway for the causality of El Niño’s diverse duration.

Eastern China (EC) suffered an extreme drought with long-lasting duration and record-breaking intensity in late summer–autumn 2019. Our diagnosed results show that the central Pacific (CP) El Niño, in tandem with warm sea surface temperature anomalies (SSTAs) over the Kuroshio Extension (KE) region, induces the meridionally elongated cyclonic circulation anomalies stretching from the western North Pacific (WNP) to the Yellow Sea. Its western flank corresponds to overwhelming low-level northerly wind anomalies over EC, which result in deficient moisture and anomalous descent over EC and hence cause the extreme drought in 2019. To investigate the relative contributions of SSTAs over different regions, we performed sensitivity experiments, and analyzed the relationship between extreme drought like what occurred in 2019 (a 2019Drought-like event) and the SSTAs in CMIP6 historical simulations. Modeling evidences reveal that both warm SSTAs over the central equatorial Pacific and the KE region are indispensable for shaping the meridionally elongated cyclone anomaly. Specifically, the cyclone anomaly over the WNP induced by CP El Niño aligns with the cyclone anomaly over the Yellow Sea induced by the warm SSTAs over the KE region, merging into a meridionally stretched cyclone anomaly to the east of EC. Consequently, the northerly anomalies stretch across EC, leading to unfavorable atmospheric conditions and the rainfall deficit there. Projection results show that the occurrence probability of a 2019Drought-like event will increase by 20% (decrease by 40%–50%) under a high (medium-low) emission scenario compared to present-day climate, indicating the nonlinear response of extreme drought to different emission scenarios and the urgency of carbon emission reduction. Significance Statement An extreme drought hit the Eastern China (EC) region in 2019 and caused tremendous losses. This study proposed that both the 2019 CP El Niño and the warm SST anomalies over the Kuroshio Extension (KE) region induce the meridionally elongated circulation anomalies and the resultant extreme drought. We showed that the typical circulation anomalies induced by central Pacific (CP) El Niño cannot totally explain the meridionally elongated circulation anomalies in 2019. Our modeling evidences confirmed the indispensable role of warm SST anomalies over KE region in the 2019 extreme drought’s formation. The projection results show that extreme drought like that in 2019 will occur more (less) frequently under a high (medium-low) emission scenario compared to modern-day level, indicating the urgency of carbon emission reduction.

Differing from classic midlatitude dry baroclinic instability theory, atmospheric motions over the subtropical mei-yu front in boreal summer are dominated by synoptic-scale disturbances coupled with precipitation and moisture under a weaker background vertical shear. This moisture–precipitation–circulation interactive feature, along with a preferred zonal wavelength of about 3400 km and eastward phase propagation, is explained by a moist baroclinic instability theoretical framework. The new framework is an extension of a traditional two-level baroclinic model by considering a prognostic moisture equation, the moisture–precipitation–circulation feedback, and an interactive planetary boundary layer. An eigenvalue analysis of the model shows that the most unstable mode has a preferred zonal wavelength of 3400 km, a westward-tilted vertical structure, and a phase leading of maximum moisture and precipitation anomalies relative to a lower-tropospheric trough, all of which are in good agreement with observations. Both anomalous horizontal and vertical advection processes contribute to the moisture increase. Further sensitivity tests show that the instability and the zonal-scale selection primarily arise from the moisture–convection–circulation feedback, while the vertical shear provides an additional energy source for the perturbation growth. This moist baroclinic instability theory explains well the observed characteristics of the development of synoptic-scale disturbances along the mei-yu front.

As the most dominant tropical climate mode on the interannual time scale, El Niño–Southern Oscillation (ENSO) is suggested to significantly influence the interannual variation of East Asian summer monsoon rainfall (IEASMR). However, the leading mode of IEASMR remains almost untouched when the impacts of preceding ENSO events are linearly removed, suggesting the existence of alternative impact factors and predictability sources of IEASMR. After removing the impact of ENSO, the sea surface temperature anomalies (SSTAs) over both the tropical Atlantic and extratropical North Atlantic are found to be related to IEASMR through atmospheric teleconnections. Positive SSTA over the tropical Atlantic could induce tropical diabatic heating, which triggers an equivalent barotropic Rossby wave train emanating from the Atlantic, going across the Eurasian continent, and ending with a cyclonic anomaly over northeast Asia. The tropical diabatic heating could also induce western North Pacific anomalous anticyclone via tropical routes. The dipole SSTA pattern with cooling in the west and warming in the east over the extratropical North Atlantic induces local circulation anomalies through heat flux exchange, which could further perturb a Rossby wave train with a cyclonic anomaly over northeast Asia, thus modulating IEASMR. Numerical experiments with prescribed atmospheric heating associated with Atlantic SSTAs could realistically reproduce these teleconnections toward IEASMR. By adding the predictability sources of Atlantic SSTAs, the seasonal hindcast skills of IEASMR could be significantly improved over both the tropical western North Pacific and subtropical land regions such as central China and Japan. Significance Statement The purpose of this article is to identify the alternative impact factors of the interannual variation of East Asian summer monsoon rainfall (IEASMR), after removing the impact of ENSO, considering the limited contribution of ENSO to the variances of IEASMR. Here, we find that the Atlantic sea surface temperature anomalies (SSTAs) play a considerable role in driving IEASMR. The impact of ENSO on IEASMR is mainly confined to the tropical western North Pacific, while the Atlantic SSTAs influence rainfall over subtropical East Asia and the tropical western North Pacific through both tropical and extratropical routes. The results unravel the important roles of Atlantic SSTAs in driving ENSO-independent IEASMR, which will have a large implication for the seasonal prediction of East Asian summer climate.

Despite an insignificant linear relation between the boreal summer equatorial Atlantic sea surface temperature anomaly (EA SSTA) and preceding winter El Niño–Southern Oscillation (ENSO), an EA warming event was found to be mostly preceded by either phase of ENSO. Physical mechanisms of this asymmetric impact of El Niño and La Niña on EA warming were investigated through observational and modeling analyses. For the El Niño–induced EA warming group, the El Niño–related SSTA induces a warm SSTA in the South Atlantic during developing fall and winter via the Pacific–South American (PSA) pattern. The induced extratropical SSTA extends to the equator through local air–sea interaction processes and favors EA warming development. In contrast, for the La Niña–induced EA warming group, there is no significant SSTA in the South Atlantic from developing fall to winter. It is not until La Niña decaying spring that a significant westerly anomaly is generated and thus an EA warming through the remote Gill-type response. The asymmetric South Atlantic response is attributed to the stronger (weaker) heating strength over the central Pacific and Maritime Continent for El Niño (La Niña). The distinctive evolutions of El Niño (fast transition) and La Niña (long persistence) also contribute to the asymmetric EA response. The persistent La Niña makes a stronger equatorial Atlantic response during ENSO decaying spring. Our results suggest that the South Atlantic SSTA and ENSO temporal evolution are important in explaining the asymmetric impacts of the El Niño and La Niña on EA warming. Furthermore, the Indian Ocean SSTA is also suggested to contribute to the asymmetric impacts.

While enhanced rainbands progressed northward in East Asia from June to August during the regular El Niño decaying summer, strengthened rainbands were only observed in the earlier summer and disappeared in August in the super El Niño composite. The cause of this distinctive feature is investigated through a combined observational and modeling study. The relative roles of the mean state and anomalous heating in causing the northward progression in the regular El Niño group are assessed through idealized numerical experiments. The result shows that the monthly evolving mean state is more important, while the anomalous forcing also plays a role. The distinctive rainfall feature in the super El Niño composite was primarily contributed by the 1982/83 and 2015/16 events, whereas the rainband evolution in 1998 resembled the regular El Niño composite. The cause of the different rainfall pattern in August among the super El Niño events is further investigated. A marked difference exists in the tropical sea surface temperature anomaly (SSTA) and associated anomalous precipitation patterns. A low-level cyclonic (anticyclonic) anomaly appeared south of Japan in August 1983 and 2016 (1998), inducing northerly (southerly) anomalies and thus suppressed (enhanced) rainfall in eastern China. Whereas an anomalous anticyclone in the western North Pacific (WNP) is a typical response to an El Niño during its mature and decaying phases, the formation of a cyclonic anomaly in the WNP resulted from anomalous enthalpy advection associated with the eastward retreat of an anomalous anticyclone triggered by a local cold SSTA belt in August 1983 and from a Pacific meridional mode (PMM)-like positive SSTA pattern in August 2016.

The Advanced version of the Weather Research and Forecasting (WRF-ARW) Model is used to investigate the influence of an easterly wave (EW) on the genesis of Typhoon Hagupit (2008) in the western North Pacific. Observational analysis indicates that the precursor disturbance of Typhoon Hagupit (2008) is an easterly wave (EW) in the western North Pacific, which can be detected at least 7 days prior to the typhoon genesis. In the control experiment, the genesis of the typhoon is well captured. A sensitivity experiment is conducted by filtering out the synoptic-scale (3–8-day) signals associated with the EW. The absence of the EW eliminates the typhoon genesis. Two mechanisms are proposed regarding the effect of the EW on the genesis of Hagupit. First, the background cyclonic vorticity of the EW could induce the small-scale cyclonic vorticities to merge and develop into a system-scale vortex. Second, the EW provides a favorable environment in situ for the rapid development of the typhoon disturbance through a positive moisture–convection feedback.

The diversity of the Madden–Julian oscillation (MJO) initiation (i.e., initial onset of active convection before moving eastward) regions was explored using a clustering method. Regions favorable for MJO initiations are grouped into four longitude zones: the Atlantic and Africa (AA), the Indian Ocean (IO), the Maritime Continent (MC), and the western Pacific (WP). The region-dependent dominant initiation mechanisms are explored using a composite procedure. The AA initiation is attributed to a circumnavigating process associated with a preceding MJO. As upper-tropospheric westerly anomalies move into the AA region, the associated descending motion leads to suppressed convection over the IO, which further triggers convection onset to its west through anomalous westward moisture advection. The IO initiation arises from the downstream forcing of a preceding suppressed phase of MJO. A delayed air–sea interaction process also plays a role. The MC initiation is triggered by a westward-propagating dry equatorial Rossby wave in the Pacific. The low-level poleward flows associated with the anticyclonic Rossby wave gyres advect high mean moisture, promoting the convection onset over the MC. The WP initiation is triggered by a preceding suppressed phase of MJO that moves eastward, in a way similar to the downstream scenario in the IO. The AA initiation is usually associated with a La Niña–like background sea surface temperature pattern, which favors the decoupling of upper-tropospheric westerly anomalies from the preceding MJO. The MC and WP initiations are more frequent during El Niño, as the relevant meridional moisture gradient is sharper and the eastern Pacific is moister.

The annual cycle of sea surface temperature (SST) in the equatorial eastern Pacific (EEP) shows a distinctive change between the warm and cold phases of the El Niño-Southern Oscillation (ENSO), with a much-enhanced cold tongue in boreal fall during La Niña than during El Niño. The specific dynamic and thermodynamic processes through which ENSO modulates the annual cycle during the various phase of the annual cycle were investigated through an analytical heat budget model. It is found that the modulation of ENSO is primarily through ocean dynamic heating processes, including anomalous temperature advection by mean vertical and meridional currents and mean temperature advection by anomalous zonal and vertical currents. While El Niño induced increase of column water vapor causes the increase of downward longwave radiation and thus the weakening of the cold tongue, an enhanced surface latent heat flux due to the increase of surface wind speed tends to strengthen the cold tongue and thus oppose the ocean dynamic effect. Such ENSO impacts persist from boreal fall to boreal winter and weaken substantially during the subsequent spring.

This study examined the contribution of the Pacific decadal oscillation (PDO) to the record-breaking 2013–17 drought in the Korean Peninsula. The meteorological drought signal, measured by the Standardized Precipitation Index (SPI), in 2013 and 2016 co-occurred with a heat wave. The positive phase of the PDO during the mid-2010s was responsible for the precipitation deficit, particularly in 2014, 2015, and 2017, resulting in 5 years of meteorological drought. The enhanced atmospheric heating anomalies over the subtropical central Pacific, induced by the in situ PDO-related sea surface temperature (SST) warming, led to a low-atmospheric cyclonic flow centered over the midlatitude Pacific. The northerly wind anomalies at the western edge of this low-level cyclonic flow were responsible for the horizontal negative advection of moist energy, which contributed to the decreased precipitation and the resultant negative SPI over the Korean Peninsula in 2014, 2015, and 2017. The large-ensemble simulation supported the observational findings that the composited SST anomalies during the 5 years of persistent drought exhibited prominent and persistent SST warming over the subtropical central Pacific, along with large-scale cyclonic flow over the North Pacific. The findings of this study imply that the SST anomalies over the North Pacific and subtropical central Pacific can be a predictable source to potentially increase the ability to forecast multiyear droughts over the Korean Peninsula.

The unusual movement of Typhoon Lionrock (2016) that posed great challenges for operational numerical predictions was investigated. Analysis of the steering flow at different levels shows that Lionrock’s southwestward motion before 25 August was mainly controlled by the upper-level steering, and the dominant steering shifted to lower levels as the storm turned northeastward abruptly afterward. To examine the influence of the environmental flow on this major turning of Lionrock, three numerical simulations are conducted using the Weather Research and Forecasting (WRF) Model with different starting times. The study indicates that the initial southwestward movement of Lionrock is attributed to the westward extension of the mid- to upper-level subtropical high, and the later turning to northeast is caused by the low-level southwesterly flow associated with the monsoon gyre northeast of Lionrock. In an experiment in which the monsoon gyre is removed from the initial and boundary fields, Lionrock continues its southwestward movement without turning northeastward. This result suggests that the transition of the steering from high to low levels plays a crucial role in the major turning of Lionrock. More sensitivity experiments with modifications of the initial and/or the boundary conditions indicate a low predictability of Lionrock’s major turning.

The Asian summer monsoon (ASM) is the most energetic circulation system. Projecting its future change is critical for the mitigation and adaptation of billions of people living in the region. There are two important components within the ASM: South Asian summer monsoon (SASM) and East Asian summer monsoon (EASM). Although current state-of-the-art climate models projected increased precipitation in both SASM and EASM due to the increase of atmospheric moisture, their circulation changes differ markedly-A robust strengthening (weakening) of EASM (SASM) circulation was projected. By separating fast and slow processes in response to increased CO

The global carbon cycle is strongly influenced by terrestrial productivity and ocean plankton concentration in the upper ocean1-4, with some carbon being transported from land to the ocean by chemical processes 5-9 and possibly returned to the atmosphere. High-latitude Ocean plankton may contribute to rapid surface warming, which could remotely affect terrestrial ecosystems by modifying adjacent continental climate conditions. However, this possible impact of ocean chlorophyll concentration on terrestrial activities and carbon sinks remains unclear. Here, we found that high-latitude North Atlantic chlorophyll blooming could have a positive impact on terrestrial productivity and agriculture over Europe, potentially contributing to increased carbon sinks. A stable supply of nutrients in high latitudes allows for chlorophyll blooming, which traps more incoming shortwave radiation and contributes to surface warming. This, in turn, leads to increased precipitation over Europe through atmospheric teleconnection, accelerating terrestrial productivity (15-20% of the gross primary productivity), crop yields, and carbon sink during the growing season. Model results suggest that chlorophyll concentrations over the North Atlantic ocean significantly impact terrestrial activities in Europe, and an increase in future chlorophyll blooming events could enhance the terrestrial ecosystem as a carbon sink, contributing to negative carbon-climate feedback. Our results highlight the positive interaction between the marine and terrestrial ecosystems and the importance of considering these interactions in future assessments.

Precipitation was claimed to increase over Asian and North African monsoon (AAM) regions during past interglacial epochs and also under future global warming scenarios. Using CMIP6 model experiments, this study compares the changes of AAM in interglacial epochs to global warming. Moisture budget analysis shows that the increased monsoon rainfall during interglacial epochs primarily results from the dynamic process associated with strengthened monsoon circulation, but is caused by thermodynamic process under global warming associated with increased mean moisture. To disentangle the mechanism for the distinct changes in vertical and horizontal monsoon circulation, we further decompose the response of AAM to global warming into the direct effect from CO2 radiative forcing and the indirect effect due to increased sea surface temperature (SST), based on idealized CMIP6 experiments. The results show that the effect of direct CO2 radiative forcing on the AAM is an analogue to that in interglacial epochs driven by enhanced land-ocean equivalent potential temperature contrast, both of which are characterized by strengthened vertical and horizontal monsoon circulation despite regional difference. However, the above effect is overwhelmed by the substantially increased SST under global warming, which is absent during interglacial epochs. The substantial SST warming acts to weaken the monsoon circulation by decreasing the land-ocean equivalent potential temperature contrast and enhancing the atmospheric static stability. Our results demonstrate that the interglacial epoch is not an analogue of the AAM response to global warming, and the lack of global SST warming is responsible for their difference.

The longitudinal location of precipitation anomalies over the equatorial Pacific shows a distinctive feature with the westernmost location for La Niña, the easternmost location for eastern Pacific (EP) El Niño, and somewhere between for central Pacific (CP) El Niño, even though the center of the sea surface temperature anomaly (SSTA) for La Niña is located slightly east of that of CP El Niño. The mechanisms for such a precipitation diversity were investigated through idealized model simulations and moisture and moist static energy budget analyses. It is revealed that the boundary layer convergence anomalies associated with the precipitation diversity are mainly induced by underlying SSTA through the Lindzen–Nigam mechanism, that is, their longitudinal locations are mainly controlled by the meridional and zonal distributions of the ENSO SSTA. The westward shift of the precipitation anomaly center during La Niña relative to that during CP El Niño is primarily caused by the combined effects of nonlinear zonal moist enthalpy advection anomalies and the Lindzen–Nigam mechanism mentioned above. Such a zonal diversity is further enhanced by the “convection–cloud–longwave radiation” feedback, the SST-induced latent heat flux anomalies, and the advection of mean moist enthalpy by anomalous winds. This diversity in the longitudinal location of precipitation anomalies has contributions to the diversities in the longitudinal locations of anomalous Walker circulation and western North Pacific anomalous anticyclone/cyclone among the three types of ENSO.

The changes of the MJO behavior and its impact on global precipitation, precipitation extremes, and high-frequency variability during the past century (1901–2010) were investigated using the NOAA-20CR dataset. It was found that the MJO amplitude was significantly strengthened, while its eastward phase speed hardly changed. The impacts of the MJO on precipitation in South China (SC), northern Australia (AU), and California (CA) were investigated. The anomalous rainfall in the regions was strengthened from the early to late twentieth century, with the percentage increase ranging from 67% in AU to 14% in CA. A moisture budget analysis indicated that the enhanced precipitation was primarily attributed to the effect of anomalous wind while the effect of the mean moisture change was small. The impact of the local meridional wind anomaly was critical in SC, while in AU and CA the zonal wind component dominated. The precipitation extremes had a significant increase from the early to late twentieth century. The cumulative extreme precipitation amount increased by 140%–150% in SC and AU and by 100% in CA, and the number of the extreme days increased by 110% in SC and AU and 70% in CA. Such increases are consistent with the strengthening of the high-frequency (HF; with a period less than 10 days) variability across the globe. The pattern of percentage increase of the HF variability resembles the pattern of percentage increase of MJO-scale precipitation. This implies that the MJO teleconnection pattern could exert a large-scale control on the HF variability and weather extremes. Significance Statement The Madden–Julian oscillation (MJO), a dominant mode of tropical intraseasonal oscillations, plays an important role in affecting global weather and climate. To understand to what extent the MJO and associated teleconnections change with global warming, this study adopts an alternative approach by analyzing a century-long reanalysis dataset rather than the outputs of climate models that consist of large uncertainties due to poor MJO simulations. We found that MJO-induced tropical and midlatitude precipitation and associated extremes and high-frequency variabilities were strengthened during the past century while the global mean surface temperature increases approximately by 1°C. The results derived from the present study provide a basis for assessing the future MJO behavior and associated climate impacts and for improving the extended-range prediction of severe weather and climate extremes.

A conventional method to define short-term climate anomalies for atmospheric and oceanic variables, recommended by the World Meteorological Organization (WMO), is the departure from a 30-yr climatological mean in the preceding three decades. Such a method, however, introduces spurious errors such as sudden jumps and artificial trends. A new method, named a trend correctional method, is introduced to eliminate the errors. To demonstrate the capability of this new method, we examine a set of idealized cases first by superposing a “true” interannual or interdecadal signal onto a linear or a nonlinear trend. Comparing to the conventional method, the trend correctional method is able to retain, to a large extent, the “true” anomaly signals. Next, we examined real-time indices. The anomaly time series derived based on the trend correctional method show a better agreement with the observed anomaly series. The root-mean-square error is greatly improved, comparing to that calculated based on the conventional method. Therefore, the results from both the idealized and real cases demonstrate that the new method has a clear advantage to the conventional method in deriving true climate anomalies, in particular under the ongoing global warming circumstance.

Summer precipitation over the Yangtze River basin (YRB) in 2020 experienced a strong subseasonal and synoptic fluctuation in addition to contributing to an exceptionally large seasonal mean precipitation. The cause of this higher-frequency fluctuation is examined based on observational analyses. Apart from the continuous northward movement of the climatological mei-yu rainband, the mei-yu rainbelt in the summer of 2020 experienced multiple northward and southward swings. The cause of the swings was attributed to the subseasonal variability of southerly winds to the south and northeasterly winds to the north of the YRB. In addition, synoptic-scale variability, characterized by the eastward propagation of low-level cyclonic vorticity and precipitation anomalies, was also commonplace in the summer of 2020. While the strengthening of both the subseasonal and synoptic variabilities in the summer of 2020 was attributed to the increase of the background mean moisture, the synoptic variability was greatly affected by the subseasonal rainfall variability. As a result, both the synoptic-scale and subseasonal variabilities contributed to the north-south swings of the rainbelt. The large-scale modulations by both the seasonal mean and subseasonal anomalies provide insight regarding the optimization of issuing accurate, extended-range forecasts of extreme weather events.

Features of the dominant modes of surface air temperature (SAT) on the intraseasonal timescale over the mid–high-latitude Eurasia (MHE) during boreal summer (June–September) are investigated based on the ERA5 reanalysis data from 1979 to 2016. The intraseasonal variability (ISV) of SAT over MHE is primarily characterized by an eastward propagation along 60°N, which is found to impact the regional weather in China, including summertime extreme hot and cool events. The forecast skill and potential predictability of the ISV of SAT over MHE are assessed for 5 dynamical models that have participated in the subseasonal-to-seasonal (S2S) prediction project, by analyzing 12 years’ (1999–2010) model reforecast/hindcast data. By using the principal component (PC) index of the leading intraseasonal SAT modes as a predictand, we found that the forecast skill for ISV of SAT can reach out to 11–17 days, and the ECMWF model exhibits the best score. All the S2S models tend to show 1) a relatively higher skill for strong intraseasonal oscillation (ISO) cases, 2) a systematic underestimate of the amplitude of the SAT ISV signal, and 3) different skills during different phases of ISO cases. Analysis of potential predictability based on the perfect-model assumption reveals a 4–6-day skill gap for most models, and the skill gap also varies among different phases of ISO events. The results imply the need for continued development of operational forecasting systems to improve the actual prediction skills for the ISV of SAT over MHE.