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Abstract
The interdecadal Pacific oscillation (IPO) shifted to a negative phase around the late 1990s. Its impact on the atmospheric quasi-biweekly oscillation (QBWO) intensity over the western North Pacific (WNP) during late summer was investigated. Corresponding to the phase transition of the IPO, La Niña–like SST anomalies and an enhanced Walker circulation appeared in the tropical Pacific, which led to decreased precipitation over the equatorial central and eastern Pacific. The decreased precipitation induced a Gill response with an anomalous anticyclone (cyclone) in the lower (upper) troposphere over the WNP. This resulted in anomalous background westerly vertical shear over the tropical WNP. Furthermore, the anomalous anticyclone induced anomalous horizontal divergence and descent motion in the planetary boundary layer, which led to decreased background surface moisture over the tropical WNP. These changes in background atmospheric conditions suppressed the development of QBWO perturbations over the tropical WNP. Therefore, the QBWO intensity weakened over the WNP after the late 1990s. The composite evolution of QBWO events before and after the late 1990s confirm the interdecadal change of the QBWO intensity. A simple model was employed to understand the relative role of the background moisture and vertical shear changes in modulating the QBWO activity. The result shows that the moisture change plays a more important role than the vertical shear change in weakening the QBWO intensity.
Abstract
The interdecadal Pacific oscillation (IPO) shifted to a negative phase around the late 1990s. Its impact on the atmospheric quasi-biweekly oscillation (QBWO) intensity over the western North Pacific (WNP) during late summer was investigated. Corresponding to the phase transition of the IPO, La Niña–like SST anomalies and an enhanced Walker circulation appeared in the tropical Pacific, which led to decreased precipitation over the equatorial central and eastern Pacific. The decreased precipitation induced a Gill response with an anomalous anticyclone (cyclone) in the lower (upper) troposphere over the WNP. This resulted in anomalous background westerly vertical shear over the tropical WNP. Furthermore, the anomalous anticyclone induced anomalous horizontal divergence and descent motion in the planetary boundary layer, which led to decreased background surface moisture over the tropical WNP. These changes in background atmospheric conditions suppressed the development of QBWO perturbations over the tropical WNP. Therefore, the QBWO intensity weakened over the WNP after the late 1990s. The composite evolution of QBWO events before and after the late 1990s confirm the interdecadal change of the QBWO intensity. A simple model was employed to understand the relative role of the background moisture and vertical shear changes in modulating the QBWO activity. The result shows that the moisture change plays a more important role than the vertical shear change in weakening the QBWO intensity.
Abstract
East Asian summer monsoon (EASM) prediction is difficult because of the summer monsoon’s weak and unstable linkage with El Niño–Southern Oscillation (ENSO) interdecadal variability and its complicated association with high-latitude processes. Two statistical prediction schemes were developed to include the interannual increment approach to improve the seasonal prediction of the EASM’s strength. The schemes were applied to three models [i.e., the Centre National de Recherches Météorologiques (CNRM), the Met Office (UKMO), and the European Centre for Medium-Range Weather Forecasts (ECMWF)] and the Multimodel Ensemble (MME) from the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) results for 1961–2001. The inability of the three dynamical models to reproduce the weakened East Asian monsoon at the end of the 1970s leads to low prediction ability for the interannual variability of the EASM. Therefore, the interannual increment prediction approach was applied to overcome this issue. Scheme I contained the EASM in the form of year-to-year increments as a predictor that is derived from the direct outputs of the models. Scheme II contained two predictors: both the EASM and also the western North Pacific circulation in the form of year-to-year increments. Both the cross-validation test and the independent hindcast experiments showed that the two prediction schemes have a much better prediction ability for the EASM than does the original scheme. This study provides an efficient approach for predicting the EASM.
Abstract
East Asian summer monsoon (EASM) prediction is difficult because of the summer monsoon’s weak and unstable linkage with El Niño–Southern Oscillation (ENSO) interdecadal variability and its complicated association with high-latitude processes. Two statistical prediction schemes were developed to include the interannual increment approach to improve the seasonal prediction of the EASM’s strength. The schemes were applied to three models [i.e., the Centre National de Recherches Météorologiques (CNRM), the Met Office (UKMO), and the European Centre for Medium-Range Weather Forecasts (ECMWF)] and the Multimodel Ensemble (MME) from the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) results for 1961–2001. The inability of the three dynamical models to reproduce the weakened East Asian monsoon at the end of the 1970s leads to low prediction ability for the interannual variability of the EASM. Therefore, the interannual increment prediction approach was applied to overcome this issue. Scheme I contained the EASM in the form of year-to-year increments as a predictor that is derived from the direct outputs of the models. Scheme II contained two predictors: both the EASM and also the western North Pacific circulation in the form of year-to-year increments. Both the cross-validation test and the independent hindcast experiments showed that the two prediction schemes have a much better prediction ability for the EASM than does the original scheme. This study provides an efficient approach for predicting the EASM.
Abstract
In this study, the authors found that the summer precipitation over China experienced different decadal variation features from north to south after the late 1990s. In northeastern and North China and the lower–middle reaches of the Yangtze River, precipitation decreased after 1999, while precipitation experienced a significant reduction over South and southwestern China and a significant increase over the southern parts of Hetao region and Huaihe River valley after 2003. The authors next analyzed the associated decadal variation of the atmospheric circulation and attempted to identify the mechanisms causing the two decadal variations of precipitation. The wind anomalies for the former exhibit a barotropic meridional dipole pattern, with anticyclonic anomalies over Mongolia to northern China and cyclonic anomalies over the southeastern Chinese coast to the northwestern Pacific. For the latter, there is a southeast–northwest-oriented dipole pattern in the middle and lower troposphere, with cyclonic anomalies over the northern parts of the Tibetan Plateau and anticyclonic anomalies over the lower–middle reaches of the Yangtze River to southern Japan. An anomalous anticyclone dominates the upper troposphere over China south of 40°N. The authors further found that the summer sea surface temperature (SST) warming over the tropical Atlantic played an important role in the decadal variation around 2003 via inducing teleconnections over Eurasia. In contrast, the decadal variation around 1999 may be caused by the phase shift of the Pacific decadal oscillation (PDO), as has previously been indicated.
Abstract
In this study, the authors found that the summer precipitation over China experienced different decadal variation features from north to south after the late 1990s. In northeastern and North China and the lower–middle reaches of the Yangtze River, precipitation decreased after 1999, while precipitation experienced a significant reduction over South and southwestern China and a significant increase over the southern parts of Hetao region and Huaihe River valley after 2003. The authors next analyzed the associated decadal variation of the atmospheric circulation and attempted to identify the mechanisms causing the two decadal variations of precipitation. The wind anomalies for the former exhibit a barotropic meridional dipole pattern, with anticyclonic anomalies over Mongolia to northern China and cyclonic anomalies over the southeastern Chinese coast to the northwestern Pacific. For the latter, there is a southeast–northwest-oriented dipole pattern in the middle and lower troposphere, with cyclonic anomalies over the northern parts of the Tibetan Plateau and anticyclonic anomalies over the lower–middle reaches of the Yangtze River to southern Japan. An anomalous anticyclone dominates the upper troposphere over China south of 40°N. The authors further found that the summer sea surface temperature (SST) warming over the tropical Atlantic played an important role in the decadal variation around 2003 via inducing teleconnections over Eurasia. In contrast, the decadal variation around 1999 may be caused by the phase shift of the Pacific decadal oscillation (PDO), as has previously been indicated.
Abstract
The mei-yu withdrawal date (MWD) is a crucial indicator of flood/drought conditions over East Asia. It is characterized by a strong interannual variability, but its underlying mechanism remains unknown. We investigated the possible effects of the winter sea surface temperature (SST) in the North Pacific Ocean on the MWD on interannual to interdecadal time scales. Both our observations and model results suggest that the winter SST anomalies associated with the MWD are mainly contributed to by a combination of the first two leading modes of the winter SST in the North Pacific, which have a horseshoe shape (the NPSST). The statistical results indicate that the intimate linkage between the NPSST and the MWD has intensified since the early 1990s. During the time period 1990–2016, the NPSST-related SST anomalies persisted from winter to the following seasons and affected the SST over the tropical Pacific in July. Subsequently, the SST anomalies throughout the North Pacific strengthened the southward migration of the East Asian jet stream (EAJS) and the southward and westward displacement of the western North Pacific subtropical high (WPSH), leading to an increase in mei-yu rainfall from 1 to 20 July. More convincingly, the anomalous EAJS and WPSH induced by the SST anomalies can be reproduced well by numerical simulations. By contrast, the influence of the NPSST on the EASJ and WPSH were not clear between 1961 and 1985. This study further illustrates that the enhanced interannual variability of the NPSST may be attributed to the more persistent SST anomalies during the time period 1990–2016.
Abstract
The mei-yu withdrawal date (MWD) is a crucial indicator of flood/drought conditions over East Asia. It is characterized by a strong interannual variability, but its underlying mechanism remains unknown. We investigated the possible effects of the winter sea surface temperature (SST) in the North Pacific Ocean on the MWD on interannual to interdecadal time scales. Both our observations and model results suggest that the winter SST anomalies associated with the MWD are mainly contributed to by a combination of the first two leading modes of the winter SST in the North Pacific, which have a horseshoe shape (the NPSST). The statistical results indicate that the intimate linkage between the NPSST and the MWD has intensified since the early 1990s. During the time period 1990–2016, the NPSST-related SST anomalies persisted from winter to the following seasons and affected the SST over the tropical Pacific in July. Subsequently, the SST anomalies throughout the North Pacific strengthened the southward migration of the East Asian jet stream (EAJS) and the southward and westward displacement of the western North Pacific subtropical high (WPSH), leading to an increase in mei-yu rainfall from 1 to 20 July. More convincingly, the anomalous EAJS and WPSH induced by the SST anomalies can be reproduced well by numerical simulations. By contrast, the influence of the NPSST on the EASJ and WPSH were not clear between 1961 and 1985. This study further illustrates that the enhanced interannual variability of the NPSST may be attributed to the more persistent SST anomalies during the time period 1990–2016.
Abstract
The reversal of surface air temperature anomalies (SATA) in winter brings a great challenge for short-term climate prediction, and the mechanisms are not well understood. This study found that the reversal of SATA between December and January over China could be demonstrated by the second leading mode of multivariate empirical orthogonal function analysis on the December–January SATA. It further reveals that the central Pacific El Niño–Southern Oscillation (CP ENSO) has contributed more influence on such a reversal of SATA since 1997. CP ENSO shows positive but weak correlations with SATA over China in both December and January during the pre-1996 period, whereas it shows significant negative and positive correlations with the SATA in December and January, respectively, during the post-1997 period. The CP ENSO–related circulations suggest that the change of the Siberian high has played an essential role in the reversal of SATA since 1997. The pattern of sea surface temperature anomalies associated with the CP ENSO leads to a westward-replaced Walker circulation that alters the local meridional circulation and, further, has impacted the Siberian high and SATA over China since 1997. Moreover, the seasonal northward march of the convergence zone from December to January causes a northward-replaced west branch of the Walker circulation in January compared with that in December. The west branch of the Walker circulation in December and January directly modulates local Hadley and Ferrel circulations and then causes contrasting Siberian high anomalies by inducing opposite vertical motion anomalies over Siberia. The reversal of SATA between December and January, therefore, has been more frequently observed over China since 1997. The abovementioned mechanisms are validated by the analysis at pentad time scales and confirmed by numerical simulations.
Abstract
The reversal of surface air temperature anomalies (SATA) in winter brings a great challenge for short-term climate prediction, and the mechanisms are not well understood. This study found that the reversal of SATA between December and January over China could be demonstrated by the second leading mode of multivariate empirical orthogonal function analysis on the December–January SATA. It further reveals that the central Pacific El Niño–Southern Oscillation (CP ENSO) has contributed more influence on such a reversal of SATA since 1997. CP ENSO shows positive but weak correlations with SATA over China in both December and January during the pre-1996 period, whereas it shows significant negative and positive correlations with the SATA in December and January, respectively, during the post-1997 period. The CP ENSO–related circulations suggest that the change of the Siberian high has played an essential role in the reversal of SATA since 1997. The pattern of sea surface temperature anomalies associated with the CP ENSO leads to a westward-replaced Walker circulation that alters the local meridional circulation and, further, has impacted the Siberian high and SATA over China since 1997. Moreover, the seasonal northward march of the convergence zone from December to January causes a northward-replaced west branch of the Walker circulation in January compared with that in December. The west branch of the Walker circulation in December and January directly modulates local Hadley and Ferrel circulations and then causes contrasting Siberian high anomalies by inducing opposite vertical motion anomalies over Siberia. The reversal of SATA between December and January, therefore, has been more frequently observed over China since 1997. The abovementioned mechanisms are validated by the analysis at pentad time scales and confirmed by numerical simulations.
Abstract
Accurate sub-seasonal (2-8 weeks) prediction of monsoon precipitation is crucial for mitigating flood and heatwave disasters caused by intra-seasonal variability (ISV). However, current state-of-the-art sub-seasonal-to-seasonal (S2S) models have limited prediction skills beyond one week when predicting weekly precipitation. Our findings suggest that predictability primarily arises from strong ISV events, and the prediction skills for ISV events depend on the propagation stability of preceding signals, regardless of models. This allows us to identify opportunities and barriers (OBs) within S2S models, clarifying what the models can and cannot achieve in ISV event prediction. Focusing on the complex East Asian summer monsoon (EASM), we discover that stable propagation of Eurasian and tropical atmospheric wave trains towards East Asia serves as an opportunity. This opportunity offers a one-week leading prediction skill of up to 0.85 and skillful prediction up to 13 days ahead for 43% of all ISV events. However, the Tibetan Plateau barrier highlights the limitation of EASM predictability. Identifying these OBs will help us gain confidence in making more accurate sub-seasonal prediction.
Abstract
Accurate sub-seasonal (2-8 weeks) prediction of monsoon precipitation is crucial for mitigating flood and heatwave disasters caused by intra-seasonal variability (ISV). However, current state-of-the-art sub-seasonal-to-seasonal (S2S) models have limited prediction skills beyond one week when predicting weekly precipitation. Our findings suggest that predictability primarily arises from strong ISV events, and the prediction skills for ISV events depend on the propagation stability of preceding signals, regardless of models. This allows us to identify opportunities and barriers (OBs) within S2S models, clarifying what the models can and cannot achieve in ISV event prediction. Focusing on the complex East Asian summer monsoon (EASM), we discover that stable propagation of Eurasian and tropical atmospheric wave trains towards East Asia serves as an opportunity. This opportunity offers a one-week leading prediction skill of up to 0.85 and skillful prediction up to 13 days ahead for 43% of all ISV events. However, the Tibetan Plateau barrier highlights the limitation of EASM predictability. Identifying these OBs will help us gain confidence in making more accurate sub-seasonal prediction.
Abstract
The heterogeneous land surface spanning the Yellow River irrigated oasis and the adjacent Kubuqi and Ulan Buh Desert (Hetao area) in Inner Mongolia, China, has been noted to frequently generate planetary boundary layer convergence line (BLCL), providing an important source of low-level lifting for convection initiation (CI). As the first field experiment to collect comprehensive observations of vegetation-contrast-resulting thermal circulations that consistently generate BLCLs and lead to CI, the DEsert-oasis COnvergence line and Deep convection Experiment (DECODE) was conducted from 5 July to 9 August 2022, in the Hetao area. Two oasis and four desert observation sites were set up in the region that exhibits the highest frequency of BLCL and CI occurrences, equipped with a suite of advanced instruments probing land-atmosphere interactions, planetary boundary layer processes, and evolution of BLCLs and their associated CI, including Doppler LiDARs, microwave radiometers, soil temperature and moisture sensors, eddy correlation systems, portable radiosondes, C-band polarimetric Doppler radar, aircraft, and Geostationary High-speed Imager onboard FY-4B satellite. DECODE captured 29 BLCLs (13 with CI), 66 gust fronts, 12 horizontal convective rolls, and one tornado. The observations unveiled full thermal circulations spanning the desert-oasis boundary characterized by a horizontal width of ∼25 km, a convergence height of ∼1 km above ground level (AGL), and divergence from 2 to ∼3.5 km AGL, with vertical wind speeds up to 2 m s−1. Future publications stemming from DECODE will delve into a spectrum of scientific inquiries, including but not limited to land surface and boundary layer processes, BLCL dynamics, CI mechanisms, convective organization, predictability, and model evaluation, among others.
Abstract
The heterogeneous land surface spanning the Yellow River irrigated oasis and the adjacent Kubuqi and Ulan Buh Desert (Hetao area) in Inner Mongolia, China, has been noted to frequently generate planetary boundary layer convergence line (BLCL), providing an important source of low-level lifting for convection initiation (CI). As the first field experiment to collect comprehensive observations of vegetation-contrast-resulting thermal circulations that consistently generate BLCLs and lead to CI, the DEsert-oasis COnvergence line and Deep convection Experiment (DECODE) was conducted from 5 July to 9 August 2022, in the Hetao area. Two oasis and four desert observation sites were set up in the region that exhibits the highest frequency of BLCL and CI occurrences, equipped with a suite of advanced instruments probing land-atmosphere interactions, planetary boundary layer processes, and evolution of BLCLs and their associated CI, including Doppler LiDARs, microwave radiometers, soil temperature and moisture sensors, eddy correlation systems, portable radiosondes, C-band polarimetric Doppler radar, aircraft, and Geostationary High-speed Imager onboard FY-4B satellite. DECODE captured 29 BLCLs (13 with CI), 66 gust fronts, 12 horizontal convective rolls, and one tornado. The observations unveiled full thermal circulations spanning the desert-oasis boundary characterized by a horizontal width of ∼25 km, a convergence height of ∼1 km above ground level (AGL), and divergence from 2 to ∼3.5 km AGL, with vertical wind speeds up to 2 m s−1. Future publications stemming from DECODE will delve into a spectrum of scientific inquiries, including but not limited to land surface and boundary layer processes, BLCL dynamics, CI mechanisms, convective organization, predictability, and model evaluation, among others.
