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Song Yang
and
Xingwen Jiang

Abstract

The eastern Pacific (EP) El Niño–Southern Oscillation (ENSO) and the central Pacific (CP) ENSO exert different influences on climate. In this study, the authors analyze the hindcasts of the NCEP Climate Forecast System, version 2 (CFSv2), and assess the skills of predicting the two types of ENSO and their impacts on East Asian climate. The possible causes of different prediction skills for different types of ENSO are also discussed.

CFSv2 captures the spatial patterns of sea surface temperature (SST) related to the two types of ENSO and their different climate impacts several months in advance. The dynamical prediction of the two types of ENSO by the model, whose skill is season dependent, is better than the prediction based on the persistency of observed ENSO-related SST, especially for summer and fall. CFSv2 performs well in predicting EP ENSO and its impacts on the East Asian winter monsoon and on the Southeast Asian monsoon during its decaying summer. However, for both EP ENSO and CP ENSO, the model overestimates the extent of the anomalous anticyclone over the western North Pacific Ocean from the developing autumn to the next spring but underestimates the magnitude of climate anomalies in general. It fails to simulate the SST pattern and climate impact of CP ENSO during its developing summer. The model’s deficiency in predicting CP ENSO may be linked to a warm bias in the eastern Pacific. However, errors in simulating the climate impacts of the two types of ENSO should not be solely ascribed to the bias in SST simulation.

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Xingwen Jiang
and
Mingfang Ting

Abstract

Intraseasonal variability of rainfall over the Indian subcontinent (IS) and the Tibetan Plateau (TP) has been discussed widely but often separately. In this study, we investigate the covariability of rainfall across the IS and the TP on intraseasonal time scales and its impact on interannual variability of regional rainfall. The most dominant mode of rainfall intraseasonal variability across the region features a dipole pattern with significant out-of-phase rainfall anomalies between the southeastern TP and the central and northern IS. This dipole rainfall pattern is associated with intraseasonal oscillations (ISOs) of 10–20 days and 30–60 days, especially the latter. An active spell of rainfall in the central and northern IS (southeastern TP) is associated with the strengthening (northward shift) of water vapor transport of the Indian summer monsoon, resulting in more water vapor entering into the central and northern IS (southeastern TP) and thus more rainfall. The 10–20-day ISO of the dipole rainfall pattern is caused by the 10–20-day atmospheric ISO in both the tropics and the extratropics, whereas the 30–60-day ISO of the dipole rainfall pattern is only associated with atmospheric ISO in the tropics. The dipole rainfall pattern resembles the most dominant mode of interannual variability of July–August mean rainfall. The 30–60-day ISO of the dipole rainfall pattern has an important contribution to the dipole pattern of July–August mean rainfall anomalies on an interannual time scale due to the different frequencies of occurrence of the active and break phases.

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Xingwen Jiang
and
Mingfang Ting

Abstract

The Tibetan Plateau (TP) has long been regarded as a key driver for the formation and variations of the Indian summer monsoon (ISM). Recent studies, however, have indicated that the ISM also exerts a considerable impact on rainfall variations in the TP, suggesting that the ISM and the TP should be considered as an interactive system. From this perspective, the covariability of the July–August mean rainfall across the Indian subcontinent (IS) and the TP is investigated. It is found that the interannual variation of IS and TP rainfall exhibits a dipole pattern in which rainfall in the central and northern IS tends to be out of phase with that in the southeastern TP. This dipole pattern is associated with significant anomalies in rainfall, atmospheric circulation, and water vapor transport over the Asian continent and nearby oceans. Rainfall anomalies and the associated latent heating in the central and northern IS tend to induce changes in regional circulation that suppress rainfall in the southeastern TP and vice versa. Furthermore, the sea surface temperature anomalies in the tropical southeastern Indian Ocean can trigger the dipole rainfall pattern by suppressing convection over the central IS and the northern Bay of Bengal, which further induces anomalous anticyclonic circulation to the south of TP that favors more rainfall in the southeastern TP by transporting more water vapor to the region. The dipole pattern is also linked to the Silk Road wave train via its link to rainfall over the northwestern IS.

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Zongjian Ke
,
Xingwen Jiang
,
Jinming Feng
, and
Zunya Wang

Abstract

In the last two decades, southwestern China (SWC) has experienced severe droughts, which are always accompanied by severe deficiencies in precipitation. In this study, we found that the interannual variability in boreal winter precipitation in SWC is modulated by the Philippine Sea anomalous anticyclone (PSAC). The interannual relationship between the PSAC and SWC precipitation experienced an interdecadal change around the early 1980s. The correlation between them was enhanced in the period from 1981 to 2001 (P2) compared to the period from 1961 to 1980 (P1). In P1, the moisture transported by the PSAC mainly affected eastern China, as the PSAC was located over the northern Philippine Sea, and the moisture budget of SWC was dominated by moisture transport at the western boundary. The PSAC, however, strengthened and shifted southwestward in P2, accompanied by a deepened India–Burma trough. As such, the PSAC transported moist air from the western North Pacific and the Indian Ocean into SWC through its southern boundary. Meanwhile, the stronger PSAC in P2 was accompanied by an upper-level convergence from the western North Pacific to the Bay of Bengal, which induced an upper-level divergence and ascending motion over SWC. Thus, the PSAC caused a significant increase in precipitation in P2. Stronger air–sea interactions in the western North Pacific induced by El Niño–Southern Oscillation may be responsible for the enhancement and southwestward shift of the PSAC in P2 compared to that in P1.

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Zunya Wang
,
Song Yang
,
Zongjian Ke
, and
Xingwen Jiang

Abstract

Based on the observational datasets of rime and glaze from 743 stations in China and the atmospheric circulation data from the NCEP–NCAR reanalysis during 1954–2009, large-scale atmospheric and oceanic conditions for extensive and persistent rime and glaze events were examined with a composite analysis. Results show that rime events mostly occur in northern China while glaze events are mainly observed in southern China. The icing events are accompanied by low temperature and high humidity but not necessarily by above-normal precipitation. The Asian low, blocking highs, strong moisture transport, and an inversion layer related to major abnormal circulation systems contribute to the occurrence and persistence of icing events in China. The Ural blocking high plays a major role in the glaze events, and the Okhotsk blocking high is closely related to the rime events. For glaze events, extratropical circulation anomalies and the southward outbreak of cold air play a dominant role. In contrast, the strong northward transport of warm and moist airflows plays a leading role and the blocking high and the southward outbreak of extratropical cold air take a supporting role for rime events. There is nearly an equal chance for occurrences of rime events under La Niña and El Niño backgrounds. However, glaze events more likely occur under the background of La Niña. Additionally, the sea surface temperatures from the tropical Indian Ocean to the tropical northwestern Pacific Ocean also contribute to the occurrence and maintenance of icing events in China.

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Tuantuan Zhang
,
Song Yang
,
Xingwen Jiang
, and
Ping Zhao

Abstract

The authors analyze the seasonal–interannual variations of rainfall over the Maritime Continent (MC) and their relationships with El Niño–Southern Oscillation (ENSO) and large-scale monsoon circulation. They also investigate the predictability of MC rainfall using the hindcast of the U.S. National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2).

The seasonal evolution of MC rainfall is characterized by a wet season from December to March and a dry season from July to October. The increased (decreased) rainfall in the wet season is related to the peak-decaying phase of La Niña (El Niño), whereas the increased (decreased) rainfall in the dry season is related to the developing phase of La Niña (El Niño), with an apparent spatial incoherency of the SST–rainfall relationship in the wet season. For extremely wet cases of the wet season, local warm SST also contributes to the above-normal rainfall over the MC except for the western area of the MC due to the effect of the strong East Asian winter monsoon.

The CFSv2 shows high skill in predicting the main features of MC rainfall variations and their relationships with ENSO and anomalies of the large-scale monsoon circulation, especially for strong ENSO years. It predicts the rainfall and its related circulation patterns skillfully in advance by several months, especially for the dry season. The relatively lower skill of predicting MC rainfall for the wet season is partly due to the low prediction skill of rainfall over Sumatra, Malay, and Borneo (SMB), as well as the unrealistically predicted relationship between SMB rainfall and ENSO.

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Nan Xing
,
Jianping Li
,
Xingwen Jiang
, and
Lanning Wang

Abstract

Local sea surface temperature (SST) plays an important role in the onset of the Bay of Bengal (BoB) summer monsoon (BoBSM). Previous study indicated that the meridionally warmest SST axis (WSSTA) appears in mid-April in the central BoB, which may be a precursor for the BoBSM onset. In this study, it is found that a warm but not the meridionally warmest center, which is defined as the secondary WSSTA (SWSSTA), occurs in early April in the central BoB, leading the BoBSM onset by five pentads. Dates of the SWSSTA occurrence are significantly positively correlated with dates of the WSSTA occurrence in the central BoB and the BoBSM onset on an interannual time scale. The SWSSTA is an earlier precursor for the BoBSM onset. The formation of the oceanic precursor and its impact on the BoBSM onset are as follows. Before the BoBSM onset, resulting from more surface heat input and shallower mixed layer affected by the low-level anticyclone and subtropical high in the central BoB, local SST shows the most rapid increase. Meanwhile, the situation is adverse to the rapid increase of SST in the equatorial BoB. For this reason, the SWSSTA occurs, and the WSSTA subsequently appears in the central BoB. The WSSTA in turn enhances local convection, eliminates the low-level anticyclone, and moves the subtropical high outward away from the BoB by inducing atmospheric instability, thus developing a heating center. Convectional heating further strengthens southwesterlies in the BoB by exciting mixed planetary–gravity waves, resulting in the BoBSM onset.

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Tuantuan Zhang
,
Song Yang
,
Xingwen Jiang
, and
Bohua Huang

Abstract

Seasonal prediction of extratropical climate (e.g., the East Asian climate) is partly dependent upon the prediction skill for rainfall over the Maritime Continent (MC). A previous study by the authors found that the NCEP Climate Forecast System, version 2 (CFSv2), had difference in skill between predicting rainfall over the western MC (WMC) and the eastern MC (EMC), especially in the wet season. In this study, the potential mechanisms for this phenomenon are examined. It is shown that observationally in the wet season (from boreal winter to early spring) the EMC rainfall is closely linked to both ENSO and local sea surface temperature (SST) anomalies, whereas the WMC rainfall is only moderately correlated with ENSO. The model hindcast unrealistically predicts the relationship of the WMC rainfall with local SST and ENSO (even opposite to the observed feature), which contributes to lower prediction skill for the WMC rainfall. In the dry season (from boreal late summer to fall), the rainfall over the entire MC is significantly influenced by both ENSO and local SST in observations and this feature is well captured by the CFSv2. Therefore, the hindcasts do not show apparently different skill in rainfall prediction for EMC and WMC in the dry season. The possible roles of atmospheric internal processes are also discussed.

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Juan Li
,
Haoming Chen
,
Puxi Li
, and
Xingwen Jiang

Abstract

Based on the hourly merged precipitation product, the performance of the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) in simulating the diurnal variations of precipitation during warm season over the western periphery of the Sichuan basin (SCB) has been evaluated, and the underlying physical causes associated with the wet biases have also been investigated. The results show that the IFS well reproduces the spatial distributions of precipitation amount, frequency, and intensity over the SCB, as well as their diurnal variations, but the simulated precipitation peaks earlier than the observation with notable wet biases over the western periphery of the SCB. In addition, the strong wet biases exhibit notable regional differences over the western periphery of the SCB. The simulated wet biases over the southwestern periphery of the SCB expand westward to higher altitudes along the windward slope, with the maximum wet biases occurring at night. The westward expansion of the simulated stronger upward motions results in a westward shift of precipitation. However, the simulated precipitation over the northwestern periphery of the SCB has small difference in terms of the location; hence, the overestimated precipitation is associated with the stronger atmospheric instability, resulting from the higher potential temperature and the larger specific humidity near the surface. The findings revealed in this study indicate that the ECMWF forecast shows distinct uncertainties over the different complex terrain, and thus offers a promising way forward for improvements of model physical processes.

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Zunya Wang
,
Xingwen Jiang
,
Zongjian Ke
, and
Yafang Song

Abstract

The related atmospheric and oceanic factors are investigated in this analysis to understand the natural attributes responsible for the significant increase of the high temperature extremes (HTEs) on the Tibetan Plateau (TP) in summer. It is found that stronger-than-normal South Asia high (SAH) and corresponding weaker-than-normal East Asia jet (EAJ), an anomalous anti-cyclone and intensified mid-level westerly wind over the TP, and the more extensive, stronger, further westward and northward stretching western Pacific subtropical high (WPSH) motivate more occurrence of HTEs over the TP on the interannual timescale. From 1961 to 2021, these crucial circulation patterns show a significant changing trend favorable for the occurrence of HTE, and thus contribute to its great increase. Further, the significant warming of sea surface temperature (SST) in the tropical western Indian, northern North Pacific and western North Atlantic make great contributions through different air-sea interactive processes as the Matsuno-Gill response, zonal vertical circulation cell and mid-high latitude teleconnection wave train, respectively. Meanwhile, the inter-decadal variability plays an import role. A breakpoint at the early 21st century is detected in the occurrence of summer HTEs on the TP. Both the crucial circulation patterns and the SST anomalies (SSTA) in the key oceanic regions occurred significant inter-decadal transition to favor the occurrence of HTEs. In particular, Atlantic Multi-decadal Oscillation (AMO) is significantly and positively correlated to the inter-decadal variation of summer HTEs on the TP. The zonal teleconnection wave train triggered by AMO forms a stronger-than-normal SAH and strengthened mid-level westerly airflow over the TP, inductive to the increase of summer HTEs on the TP.

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