The present study examines the relationship of ENSO Modoki and canonical ENSO, respectively, with the tropical cyclone (TC) frequency over the western North Pacific (WNP) for the period 1960–2008. ...The TC frequency is significantly positively correlated with ENSO Modoki index. The Niño‐3 index has a markedly negative (positive) correlation with the TC frequency in the northern (southeastern) portion of the WNP. In response to heating source related to El Niño Modoki, a large‐scale cyclonic anomaly forms over the WNP. In contrast, during the canonical El Niño years, zonally‐elongated heating source and sink exhibit a meridional dipole pattern, which induces an anticyclonic anomaly in the subtropics and a cyclonic anomaly near the equatorial central Pacific. Numerical experiments under the realistic mean state and heating profiles validate that the anomalous circulation responses to heating play essential roles in different modulations of two kinds of Pacific Ocean warming on the TC frequency.
Dynamical downscaling is an important approach to obtaining fine-scale weather and climate information. However, dynamical downscaling simulations are often degraded by biases in the large-scale ...forcing itself. We constructed a bias-corrected global dataset based on 18 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the European Centre for Medium-Range Weather Forecasts Reanalysis 5 (ERA5) dataset. The bias-corrected data have an ERA5-based mean climate and interannual variance, but with a non-linear trend from the ensemble mean of the 18 CMIP6 models. The dataset spans the historical time period 1979-2014 and future scenarios (SSP245 and SSP585) for 2015-2100 with a horizontal grid spacing of (1.25° × 1.25°) at six-hourly intervals. Our evaluation suggests that the bias-corrected data are of better quality than the individual CMIP6 models in terms of the climatological mean, interannual variance and extreme events. This dataset will be useful for dynamical downscaling projections of the Earth's future climate, atmospheric environment, hydrology, agriculture, wind power, etc.
The Regional Climate Model system version 4 (RegCM4) has been used to dynamically downscale outputs from four different general circulation models (GCM) participating in the Coupled Model ...Intercomparison Project Phase 5 (CMIP5) to the horizontal resolution of 25 km × 25 km, in order to study changes in the Southern China hydrological cycle according to the Representative Concentration Pathway (RCP) 8.5 between 2050‐to‐2099 and 1979‐to‐2003. Accompanying wetter boreal spring and summer, the interannual rainfall variability for these seasons is also enhanced. A novel moisture budget analysis shows that changes in mean background humidity (anomalous wind convergence) dominate the increase in the interannual variability in spring (summer). Extreme daily precipitation in these seasons (based on the 95th percentile) is projected to become more intense, roughly following the Clausius–Clapeyron (CC) relation for the aforementioned seasons. On the other hand, the annual number of maximum consecutive dry days (CDD) is found to increase by about 3 to 5 days over locations south of 32° N, where autumn mean rainfall rate is projected to reduce (although this might be subjected to models' ability in capturing tropical cyclone activities). Analyses of the GCM raw outputs indicate that strengthened northerlies over coastal East Asia, which is likely associated with the so‐called tropical expansion, are responsible for the drier autumn.
Southern China precipitation interannual variability is projected to increase under a warmer climate, which can be attributed to changes in mean background humidity and anomalous wind convergence for spring and summer, respectively. Extreme daily rainfall is projected to intensify by ~5% per temperature rise in low‐level troposphere. On the other hand, there will be less autumn rainfall and longer maximum consecutive dry days because of the enhanced northerly in relation to tropical expansion.
The Mid‐to‐Lower Reaches of the Yangtze River (MLRYR) suffered an extreme drought in the post‐monsoon season of 2019, contemporaneous with a strong Central Pacific (CP) El Niño and a super positive ...Indian Ocean Dipole (pIOD) event. The present work shows that CP El Niño‐related Pacific Sea Surface Temperature Anomalies (SSTAs) weakened the western North Pacific anticyclone to prevent moisture transport to the MLRYR and explained 60% of the drought intensity. The super pIOD with an extremely cold SSTA in the tropical Southeastern Indian Ocean contributed 40% of the drought amplitude via an atmospheric teleconnection. The Indian Ocean cold SSTAs first strengthened the post‐monsoon rainfall and enhanced diabatic heating over South Asia, leading to baroclinic circulation anomalies with induced descending motion over the MLRYR. The aggravated dry conditions there ultimately broke the historical drought record for the period since 1979.
Plain Language Summary
In the post‐monsoon season of 2019, the Mid‐to‐Lower Reaches of the Yangtze River (MLRYR) experienced a record‐breaking drought, which severely disrupted water supplies and affected the planting of crops. At the same time, a super positive Indian Ocean Dipole (pIOD) event occurred, along with a central Pacific (CP) El Niño in the tropical Pacific. The present study indicates that in addition to the CP El Niño, the extremely cold SSTAs in the tropical Southeastern Indian Ocean associated with the super pIOD event was also an important factor in the record‐breaking drought event. This factor first shifted the intertropical convergence zone northward to intensify the post‐monsoon rainfall and its released condensation heating over South Asia. Then, a vertically baroclinic circulation was stimulated to strengthen a descending motion over the MLRYR via an atmospheric teleconnection. On the other hand, the tropical Pacific warm SSTAs related to the strong CP El Niño weakened the western North Pacific anticyclone, which reduced the moisture supply to the MLRYR. In this way, both the pIOD and CP El Niño events jointly resulted in the record‐breaking MLRYR drought in 2019 and explained approximately 40% and 60% of this extreme drought, respectively.
Key Points
Drought over the mid‐to‐lower reaches of the Yangtze River (MLRYR) broke historical records in 2019 during the post‐monsoon season
A strong central Pacific El Niño contributed ∼60% of the extreme MLRYR drought intensity due to the tropical Pacific air‐sea interaction
Another 40% of this MLRYR drought can be attributed to a super positive Indian Ocean Dipole via a teleconnection effect over South Asia
The impacts of the eastern-Pacific (EP) and central-Pacific (CP) El Niño-Southern Oscillation (ENSO) on the southern China wintertime rainfall (SCWR) have been investigated. Results show that ...wintertime rainfall over most stations in southern China is enhanced (suppressed) during the EP (CP) El Niño, which are attributed to different atmospheric responses in the western North Pacific (WNP) and South China Sea (SCS) during two types of ENSO. When EP El Niño occurs, an anomalous low-level anticyclone is present over WNP/the Philippines region, resulting in stronger-than-normal southwesterlies over SCS. Such a wind branch acts to suppress East Asian winter monsoon (EAWM) and enhance moisture supply, implying surplus SCWR. During CP El Niño, however, anomalous sinking and low-level anticyclonic flow are found to cover a broad region in SCS. These circulation features are associated with moisture divergence over the northern part of SCS and suppressed SCWR. General circulation model experiments have also been conducted to study influence of various tropical sea surface temperature (SST) patterns on the EAWM atmospheric circulation. For EP El Niño, formation of anomalous low-level WNP anticyclone is jointly attributed to positive/negative SST anomalies (SSTA) over the central-to-eastern/ western equatorial Pacific. However, both positive and negative CP Niño-related-SSTA, located respectively over the central Pacific and WNP/SCS, offset each other and contribute a weak but broad-scale anticyclone centered at SCS. These results suggest that, besides the vital role of SST warming, SST cooling over SCS/WNP during two types of El Niño should be considered carefully for understanding the El Niño-EAWM relationship.
Future projections of the eastern-Pacific (EP) and central-Pacific (CP) types of El Niño in the twenty-first century, as well as their associated tropical circulation and precipitation variability, ...are investigated using historical runs and representative concentration pathway 8.5 (RCP8.5) simulations from 31 coupled models in phase 5 of the Coupled Model Intercomparison Project (CMIP5). As inferred from CMIP5 models that best capture both El Niño flavors, EP El Niño sea surface temperature (SST) variability will become weaker in the future climate, while no robust change of CPEl Niño SST is found. Models also reach no consensus on the future change of relative frequency from CP to EP El Niño. However, there are robust changes in the tropical overturning circulation and precipitation associated with both types of El Niño. Under a warmer climate, magnitudes of precipitation anomalies during EP El Niño are projected to increase, presenting significant enhancement of the dry (wet) signal over the western (central–eastern) Pacific. This is consistent with an accelerated hydrological cycle in the deep tropics; hence, a “wet get wetter” picture appears under global warming, accompanied by a weakened anomalous Walker circulation. For CP El Niño, drier-than-normal conditions will be intensified over the tropical central–eastern Pacific in the future climate, with stronger anomalous sinking related to the strengthened North Pacific local Hadley cell. These results suggest that, besides the enhanced basic-state hydrological cycle over the tropics, other elements, such as the anomalous overturning circulation, might also play a role in determining the ENSO precipitation response to a warmer background climate.
Changes in the tropical atmospheric overturning circulation can strongly influence the global weather pattern via affecting the location, extent, and strength of tropical convective heating. However, ...due to limitations in obtaining reliable global data before the satellite era, there are still uncertainties on long‐term changes in the tropical overturning features, such as the Walker Circulation. By analysing the 20th century reanalysis products, ship‐based observations and gauge‐based land precipitation data, robust intensification of convection is found over the western equatorial Pacific and Maritime Continent (WEP‐MC; 90°E–150°E/15°S–15°N), where rainfall and total cloud cover increased by 15–20% in boreal spring during 1901–2010. The signal is noticeably seasonally dependent, and both rainfall and cloud cover show consistent and significant increasing trends only in the boreal spring season. General circulation model (GCM) experiments were conducted using two different reconstructed SST data sets as forcing; results from both runs indicate springtime intensification of WEP‐MC convection, even with relatively uniform SST warming. Further, numerical experiments using a simplified model show that the Pacific trade wind can be accelerated by intensifying cumulus heating; however, this can be sensitive to the location of convection, and only occurs if the latter is within ±5° in latitude along the equator. Our findings suggest that the secular change in the Walker Circulation is potentially discovered from the current reanalysis and observational data sets if “regional‐up” approaches are applied.
(a) Rainfall (shading; units: mm⋅day–1) and 10‐m height wind (arrows; see scale arrow at upper right) difference between 1981–2010 and 1901–1930 March–April–May (MAM) climatology. Signals failed to pass the 90% confidence level Student's t test are masked out. (b) Raster fillings showing 1981–2010 and 1901–1930 2.5° × 2.5° upscaled MAM precipitation differences, based on 0.5° × 0.5° PRCC land surface precipitation data. Green hatching denotes where the signals pass 90% confidence level Student's t test. (c) Total cloud cover percentage change (shading) and surface wind (vectors; see scale arrow at upper right) difference between 1981–2010 and 1901–1930 MAM average, based on ICOADS data set. Signals failed to pass the 90% confidence level Student's t test are masked out.
We have investigated changes of western North Pacific land-falling tropical cyclone (TC) characteristics due to warmer climate conditions, using the pseudo-global-warming (PGW) technique. Historical ...simulations of three intense TCs making landfall in Pearl River Delta (PRD) were first conducted using the Weather Research and Forecasting (WRF) model. The same cases were then re-simulated by superimposing near- (2015-2039) and far- (2075-2099) future temperature and humidity changes onto the background climate; these changes were derived from the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model projections according to the Representative Concentration Pathway (RCP) 8.5 scenario. Peak intensities of TCs (maximum surface wind in their lifetimes) are expected to increase by ~ (3) 10% in the (near) far future. Further experiments indicate that surface warming alone acts to intensify TCs by enhancing sea surface heat flux, while warmer atmosphere acts in the opposite way by increasing the stability. In the far future, associated storm surges are also estimated to increase by about 8.5%, computed by the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model. Combined with sea level rise and estimated land vertical displacement, TC-induced storm tide affecting PRD will increase by ~1 m in the future 2075-2099 period.
In this study, we assess the performance of state-of-the-art coupled models in reproducing El Niño diversity and its impacts on the East Asian Summer Monsoon (EASM), based on simulations taken from ...the Coupled Model Intercomparison Project phase 5 (CMIP5). All of the 17 models used in this study are identified to give reasonable eastern-Pacific (EP) El Niño-related sea surface temperature anomalies (SSTA) and anomalous Walker circulation compared with observations, whereas one-third of the models fail to reproduce the Walker circulation anomalies for central-Pacific (CP) El Niño. In these models, negative SSTA over the eastern-to-central Pacific are found to be too strong, while positive SSTA shift to the western Pacific during CP El Niño. There is thus sinking (rising) motion over the central (western) part of the Pacific basin, implying westward displaced anomalous Walker cells, relative to the observed. This results in simulated low-level cyclonic flow and positive rainfall anomalies over the far western north Pacific/South China Sea during and after the peak phase of CP El Niño, which are opposite to the observed anomalous climate state. The other two-thirds of models, with realistic CP El Niño-related anomalous Walker cell, can better reproduce the corresponding wind circulation and rainfall anomalies over East Asian (EA) region. The contrasting CP El Niño-related teleconnection among models mainly arises from the different simulated CP El Niño SSTA, which in turn determine the circulation anomalies through a Gill-type response. Further analyses reveal that, in the mean state of models with erroneous teleconnection, there exists a large cold SST bias over the eastern Pacific. The stronger Walker circulation, and possibly the stronger Bjerknes feedback, might be responsible for the overly negative SSTA associated with CP El Niño in the vicinity. Our results highlight the sensitivity of El Niño–EA climate linkage to the oceanic state in models, especially for CP El Niño events.
This study examines the impacts of climate change on precipitation extremes in the Asian monsoon region during boreal summer, based on simulations from the 20-km Meteorological Research Institute ...atmospheric general circulation model. The model can capture the summertime monsoon rainfall, with characteristics similar to those from Tropical Rainfall Measuring Mission and Asian Precipitation-Highly-Resolved Observational Data Integration Towards Evaluation. By comparing the 2075–2099 with the present-day climate simulations, there is a robust increase of the mean rainfall in many locations due to a warmer climate. Over southeastern China, the Baiu rainband, Bay of Bengal and central India, extreme precipitation rates are also enhanced in the future, which can be inferred from increases of the 95th percentile of daily precipitation, the maximum accumulated precipitation in 5 consecutive days, the simple daily precipitation intensity index, and the scale parameter of the fitted gamma distribution. In these regions, with the exception of the Baiu rainband, most of these metrics give a fractional change of extreme rainfall per degree increase of the lower-tropospheric temperature of ~ 5 to 8.5% K
−1
, roughly consistent with the Clausius–Clapeyron relation. However, over the Baiu area extreme precipitation change scales as ~ 3.5% K
−1
only. We have also stratified the rainfall data into those associated with tropical cyclones (TC) and those with other weather systems. The AGCM gives an increase of the accumulated TC rainfall over southeastern China, and a decrease in southern Japan in the future climate. The latter can be attributed to suppressed TC occurrence in southern Japan, whereas increased accumulated rainfall over southeastern China is due to more intense TC rain rate under global warming. Overall, non-TC weather systems are the main contributor to enhanced precipitation extremes in various locations. In the future, TC activities over southeastern China tend to further exacerbate the precipitation extremes, whereas those in the Baiu region lead to weaker changes of these extremes.
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