Search

Based on long-term observational and reanalysis datasets from 1901 through 2014, this study investigates the characteristics and physical causes of the interdecadal variations in the summer precipitation over the East Asian monsoon boundary zone (EAMBZ), which is a peculiar domain defined from the perspective of the interplay between climatic systems (i.e., mid-latitude westerly and East Asian summer monsoon). Observational evidence reveals that, similarly to previous studies, the EAMBZ precipitation featured prominent interdecadal fluctuations, e.g., with dry summers during the periods preceding 1927, 1939–1945, 1968–1982, and 1998–2010 and wet summers during the periods of 1928–1938, 1946–1967, and 2011 onwards. Further analyses identify that, amongst the major interdecadal oceanic forcings (e.g., Atlantic multidecadal oscillation and Pacific decadal oscillation), the Indian Ocean basin mode (IOBM) is a significant oceanic forcing responsible for the interdecadal variations of the EAMBZ precipitation, playing an independent and critical modulation role. When the cold phase of the IOBM occurs, an anomalous cyclonic circulation is excited around the northeast corner of the tropical Indian Ocean, which further induces a north-low–south-high meridional seesaw pattern over the Northeast China–subtropical western Pacific (SWP) sector. Such seesaw pattern is conducive to the enhanced EAMBZ precipitation by linking favorable environments for the transportation of water vapor from the SWP and the convergence over the EAMBZ at interdecadal timescales. For this reason, a physical–empirical model for the EAMBZ precipitation is developed in terms of the IOBM cooling. Despite the fact that the extreme summer EAMBZ precipitation cannot be captured by this model, it can still well capture its interdecadal fluctuations and reflect their steady relationship. The key physical pathway connecting the IOBM cooling with the interdecadal variations of the summer EAMBZ precipitation is supported by the numerical results based on the large ensemble experiment and the Indian Ocean pacemaker experiment. Our findings may provide new insights into the understanding of the causes of the interdecadal variations in the summer EAMBZ precipitation, which may favor the long-term policy decision-making for the local hydrometeorological planning. [ABSTRACT FROM AUTHOR]

Based on long-term observational and reanalysis datasets from 1901 through 2014, this study investigates the characteristics and physical causes of the interdecadal variations in the summer precipitation over the East Asian monsoon boundary zone (EAMBZ). Observational evidence reveals that the EAMBZ precipitation featured prominent interdecadal fluctuations, e.g., with dry summers during the periods preceding 1927, 1968–1982, and 1998–2010, and wet summers during the periods of 1928–1938, 1946–1967, and 2011 onwards. Further analyses identify that the Indian Ocean basin mode (IOBM) is an important oceanic modulator responsible for the interdecadal variations of the EAMBZ precipitation. When the cold phase of the IOBM occurs, an anomalous cyclonic circulation is excited around the northeast corner of the tropical Indian Ocean, which further induces a "north-low–south-high" meridional seesaw pattern over the Northeast China–subtropical western Pacific (SWP) sector. Such seesaw pattern is conducive to the enhanced EAMBZ precipitation through linking favorable environments for the transportation of water vapor from the SWP and the convergence over EAMBZ at interdecadal timescales. For this reason, a physical-empirical model for the EAMBZ precipitation is developed in terms of the IOBM cooling, which can well capture its interdecadal fluctuations and reflect their steady relationship. The key physical pathway connecting the IOBM cooling with the interdecadal variations of the summer EAMBZ precipitation is supported by the numerical results based on the large ensemble experiment and the Indian Ocean pacemaker experiment. Our findings may provide new insights into the understanding of the causes of the interdecadal variations in the summer EAMBZ precipitation, which may favor the long-term policy decision making for the local hydrometeorological planning. [ABSTRACT FROM AUTHOR]

Based on multiple long‐term observational and reanalysis datasets, this study investigated the characteristics and physical mechanisms of the interdecadal variations in late spring (i.e., May) precipitation (LSP) over the southeastern extension of the Tibetan Plateau (SETP) since 1900. It was revealed that by and large, LSP over the SETP experienced interdecadal decrease during the period preceding 1927, 1962–1988, and 2004 onwards, but saw an increase during the periods of 1928–1961 and 1989–2003. The atmospheric circulations responsible for interdecadal variations in LSP over the SETP were also analysed. These analyses identified significant synergistic impacts of decreased mid‐latitude upstream westerlies and increased low‐latitude monsoonal southerlies over the Central North Bay of Bengal (CNBOB) on interdecadal variations in precipitation, suggesting striking interactions between extratropical eastward cold air and tropical northward warm/humid air. Further observational and modelling evidence suggested that Atlantic Multidecadal Oscillation (AMO) was likely to be a salient oceanic driver for the interdecadal synergy between upstream westerlies and CNBOB monsoonal southerlies. The elevated sea surface temperature anomalies associated with the warm phase of the AMO could spark favourable local atmospheric anomalies, forcing an upper‐tropospheric, planetary‐scale teleconnection emanating from the east of the North Atlantic sector, which may serve as an effective bridge linking the remote AMO signal and the synergy between westerlies and monsoonal southerlies around the SETP on interdecadal timescales. Our findings provided new insights into the understanding of the synergistic roles of westerlies and monsoons in the modulation of interdecadal LSP over the SETP, prior to the peak Asian summer monsoon season. [ABSTRACT FROM AUTHOR]

Hazy conditions have a significant impact on the environment and societal development. Their occurrence and persistence depend largely on climatological conditions, including the important role of climate change. Based on monthly data from 15 Coupled Model Intercomparison Project Phase 6 (CMIP6) models under three Tier 1 scenarios (SSP1–2.6, SSP2–4.5, and SSP5–8.5), the meteorological potential for winter haze pollution over Beijing was assessed during periods of peak carbon emissions (PCP; 2021–2030) and carbon neutrality (approximate net‐zero emissions) (PCN; 2051–2060) in China. The results show that a possible high‐emission strategy during PCP may not obviously enhance winter hazy conditions over Beijing in the near future. Rather, the high‐emission scenario in this period may decrease the meteorological potential that is favourable for the enhanced Beijing haze pollution by inducing a Northeast Asia cyclonic anomaly, compared to situations under SSP2–4.5, with medium emissions. Further analyses indicated that, under the continued sustainable pathway (SSP1–2.6 in PCN plus SSP1–2.6 in PCP; the better pathway simulating the effects of approximate net‐zero emissions during PCN through model results), atmospheric conditions such as the East Asian winter monsoon as well as decadal oceanic forcings of the mega‐El Niño and the warm Atlantic Multidecadal Oscillation, which are conducive to more frequent Beijing haze, are significantly suppressed in the farther future PCN. Under such circumstances, in situ haze pollution is highly repressed, suggesting a critical and positive role for the low‐carbon policy advocated by China during PCN in suppressing longer‐term hazy conditions over Beijing. In contrast, Beijing haze pollution during PCN is frequent under the discontinued sustainable pathway (SSP2–4.5/SSP5–8.5 in PCN plus SSP1–2.6 in PCP). Our findings demonstrate the possible insignificant impact of the high‐emission strategy of China during the near‐future PCP on Beijing haze exacerbation, and an important role for carbon neutrality advocated by China to suppress the haze potential in the farther future PCN, highlighting the significance of future pathway selections. [ABSTRACT FROM AUTHOR]

A recent study reported a notable westerly‐monsoon interplay (WMI) to the northeast of the Tibetan Plateau in boreal summer. However, this study introduces another striking and unique WMI over the southeastern TP (SETP) in late spring (May), emphasizing the critical roles of the upstream mid‐latitude westerly and the downstream Bay of Bengal (BOB) summer monsoon (BOBSM) in linking interannual variations of precipitation over SETP. In this study, the research SETP domain (22.5°–27.5°N, 87.5°–97.5°E) as well as a westerly index (WI) over the pivotal upstream westerly domain (22.5°–30°N, 65°–80°E) and a monsoon index (MI) over the pivotal downstream BOBSM domain (12.5°–22.5°N, 87.5°–100°E) are defined to investigate connections between the westerly and BOBSM and their combined effects on precipitation over SETP. The results indicate that the mid‐latitude westerly and BOBSM show opposite effects on precipitation over SETP. The weaker westerly and the stronger BOBSM correspond to enhanced SETP precipitation, and vice versa. The enhanced BOBSM acts as a predominant role on the strengthening of precipitation, while the decelerated westerly plays a secondary dynamical amplification role. To quantify their synergistic roles in linking SETP precipitation, a WMI index is defined. Statistical analysis suggests that this newly defined WMI index has a higher positive correlation with the precipitation over SETP. The correlation has a prominent increase compared to individual correlations between WI/MI and SETP precipitation, and dynamic conditions in high WMI index years are more favorable for the formation of SETP precipitation. The possible mechanism is that the BOBSM southerly strengthens and the southern branch of the upstream mid‐latitude westerly weakens in high WMI index years, resulting in an anomalous cyclone over the northwestern BOB and an anomalous anticyclone over the western TP. The anomalous cyclone resembles a Rossby wave response to the increased precipitation over SETP, and the precipitation in turn contributes to maintaining the BOB cyclonic anomaly and the above anticyclonic anomaly via the latent heating release. This positive feedback effect is conducive to enhanced SETP precipitation. [ABSTRACT FROM AUTHOR]

This paper provides a comprehensive assessment of Asian summer monsoon prediction skill as a function of lead time and its relationship to sea surface temperature prediction using the seasonal hindcasts of the Beijing Climate Center Climate System Model, BCC CSM1.1(m). For the South and Southeast Asian summer monsoon, reasonable skill is found in the model's forecasting of certain aspects of monsoon climatology and spatiotemporal variability. Nevertheless, deficiencies such as significant forecast errors over the tropical western North Pacific and the eastern equatorial Indian Ocean are also found. In particular, overestimation of the connections of some dynamical monsoon indices with large-scale circulation and precipitation patterns exists in most ensemble mean forecasts, even for short lead-time forecasts. Variations of SST, measured by the first mode over the tropical Pacific and Indian oceans, as well as the spatiotemporal features over the Ni˜no3.4 region, are overall well predicted. However, this does not necessarily translate into successful forecasts of the Asian summer monsoon by the model. Diagnostics of the relationships between monsoon and SST show that difficulties in predicting the South Asian monsoon can be mainly attributed to the limited regional response of monsoon in observations but the extensive and exaggerated response in predictions due partially to the application of ensemble average forecasting methods. In contrast, in spite of a similar deficiency, the Southeast Asian monsoon can still be forecasted reasonably, probably because of its closer relationship with large-scale circulation patterns and El Ni˜no-Southern Oscillation. [ABSTRACT FROM AUTHOR]

Using hindcasts of the Beijing Climate Center Climate System Model, the relationships between interannual variability (IAV) and intraseasonal variability (ISV) of the Asian-western Pacific summer monsoon are diagnosed. Predictions show reasonable skill with respect to some basic characteristics of the ISV and IAV of the western North Pacific summer monsoon (WNPSM) and the Indian summer monsoon (ISM). However, the links between the seasonally averaged ISV (SAISV) and seasonal mean of ISM are overestimated by the model. This deficiency may be partially attributable to the overestimated frequency of long breaks and underestimated frequency of long active spells of ISV in normal ISM years, although the model is capable of capturing the impact of ISV on the seasonal mean by its shift in the probability of phases. Furthermore, the interannual relationships of seasonal mean, SAISV, and seasonally averaged long-wave variability (SALWV; i.e., the part with periods longer than the intraseasonal scale) of the WNPSM and ISM with SST and low-level circulation are examined. The observed seasonal mean, SAISV, and SALWV show similar correlation patterns with SST and atmospheric circulation, but with different details. However, the model presents these correlation distributions with unrealistically small differences among different scales, and it somewhat overestimates the teleconnection between monsoon and tropical central-eastern Pacific SST for the ISM, but underestimates it for the WNPSM, the latter of which is partially related to the too-rapid decrease in the impact of El Niño-Southern Oscillation with forecast time in the model. [ABSTRACT FROM AUTHOR]

Subseasonal forecast skills and biases of global summer monsoons are diagnosed using daily data from the hindcasts of 45-day integrations by the NCEP Climate Forecast System version 2. Predictions for subseasonal variability of zonal wind and precipitation are generally more skillful over the Asian and Australian monsoon regions than other monsoon regions. Climatologically, forecasts for the variations of dynamical monsoon indices have high skills at leads of about 2 weeks. However, apparent interannual differences exist, with high skills up to 5 weeks in exceptional cases. Comparisons for the relationships of monsoon indices with atmospheric circulation and precipitation patterns between skillful and unskillful forecasts indicate that skills for subseasonal variability of a monsoon index depend partially on the degree to which the observed variability of the index attributes to the variation of large-scale circulation. Thus, predictions are often more skillful when the index is closely linked to atmospheric circulation over a broad region than over a regional and narrow range. It is also revealed that, the subseasonal variations of biases of winds, precipitation, and surface temperature over various monsoon regions are captured by a first mode with seasonally independent biases and a second mode with apparent phase transition of biases during summer. The first mode indicates the dominance of overall weaker-than-observed summer monsoons over major monsoon regions. However, at certain stages of monsoon evolution, these underestimations are regionally offset or intensified by the time evolving biases portrayed by the second mode. This feature may be partially related to factors such as the shifts of subtropical highs and intertropical convergence zones, the reversal of biases of surface temperature over some monsoon regions, and the transition of regional circulation system. The significant geographical differences in bias growth with increasing lead time reflect the distinctions of initial memory capability of the climate system over different monsoon regions. [ABSTRACT FROM AUTHOR]