Qinghai Meteorological Bureau

Changes in indices of climate extremes are studied on the basis of daily series of temperature and precipitation observations from 116 meteorological stations in central and south Asia. Averaged over all stations, the indices of temperature extremes indicate warming of both the cold tail and the warm tail of the distributions of daily minimum and maximum temperature between 1961 and 2000. For precipitation, most regional indices of wet extremes show little change in this period as a result of low spatial trend coherence with mixed positive and negative station trends. Relative to the changes in the total amounts, there is a slight indication of disproportionate changes in the precipitation extremes. Stations with near‐complete data for the longer period of 1901–2000 suggest that the recent trends in extremes of minimum temperature are consistent with long‐term trends, whereas the recent trends in extremes of maximum temperature are part of multidecadal climate variability.

Abstract. A Nationwide Nitrogen Deposition Monitoring Network (NNDMN) containing 43 monitoring sites was established in China to measure gaseous NH3, NO2, and HNO3 and particulate NH4+ and NO3− in air and/or precipitation from 2010 to 2014. Wet/bulk deposition fluxes of Nr species were collected by precipitation gauge method and measured by continuous-flow analyzer; dry deposition fluxes were estimated using airborne concentration measurements and inferential models. Our observations reveal large spatial variations of atmospheric Nr concentrations and dry and wet/bulk Nr deposition. On a national basis, the annual average concentrations (1.3–47.0 μg N m−3) and dry plus wet/bulk deposition fluxes (2.9–83.3 kg N ha−1 yr−1) of inorganic Nr species are ranked by land use as urban > rural > background sites and by regions as north China > southeast China > southwest China > northeast China > northwest China > Tibetan Plateau, reflecting the impact of anthropogenic Nr emission. Average dry and wet/bulk N deposition fluxes were 20.6 ± 11.2 (mean ± standard deviation) and 19.3 ± 9.2 kg N ha−1 yr−1 across China, with reduced N deposition dominating both dry and wet/bulk deposition. Our results suggest atmospheric dry N deposition is equally important to wet/bulk N deposition at the national scale. Therefore, both deposition forms should be included when considering the impacts of N deposition on environment and ecosystem health.

Daily and monthly maximum and minimum surface air temperatures at 66 weather stations over the eastern and central Tibetan Plateau with elevations above 2000 m were analyzed for temporal trends and spatial variation patterns during the period 1961–2003. Statistically significant warming trends were identified in various measures of the temperature regime, such as temperatures of extreme events and diurnal temperature range. The warming trends in winter nighttime temperatures were among the highest when compared with other regions. We also confirmed the asymmetric pattern of greater warming trends in minimum or nighttime temperatures as compared to the daytime temperatures. The warming in regional climate caused the number of frost days to decrease significantly and the number of warm days to increase. The length of the growing season increased by approximately 17 days during the 43‐year study period. Most of the record‐setting months for cold events were found in the earlier part of the study period, while that of the warm events occurred mostly in the later half, especially since the 1990s. The changes in the temperature regime in this region may have brought regional‐specific impacts on the ecosystems. It was found that grain production in Qinghai Province, located in the area of prominent warming trends, exhibited strong correlations with the temperatures, although such relationships were obscured by the influence of precipitation in this arid/semiarid environment in juniper tree ring records. In western Sichuan Province under a more humid environment, the tree growth (spruces) was more closely related to the changing temperatures.

Abstract In this study, we apply temperature, precipitation, and other data from 66 Chinese meteorological stations including Xining and Lhasa to analyze the extreme climate events and their impacting factors over the Qinghai-Tibet Plateau during the period 1961–2007. We focus on the spatial and temporal features of extreme climate events and their long-term changes over five climate zones of alpine grassland, meadow, and desert areas. Results show that, during the past decades, the changes in climate over the Qinghai-Tibet Plateau present trends towards warm and wet conditions. These changes in temperature and precipitation are evident in both seasonal means and extreme events, and the changes in precipitation are apparent in both precipitation amount and number of precipitation days. Clearly, warm and wet events increase, but cold and dry events decrease over the plateau region. Features of the warming climate are relatively consistent in spatial and seasonal distributions, with the most significant changes in winter and autumn and at nighttime. Northern Qinghai exhibits the greatest and most significant decrease in the frequency of extremely low-temperature events. However, the wetting trend shows more distinctive spatial features and is more seasonally dependent. While the trends in both precipitation amount and the number of precipitation days are positive in all climate zones for winter and spring, both positive and insignificant negative trends appear in summer and autumn. The largest decrease in the frequency of severely dry events is found over southeastern Tibet and western Sichuan.

Field investigations show that the surface wave magnitude (Ms) 8.1 Central Kunlun earthquake (Tibetan plateau) of 14 November 2001 produced a nearly 400-kilometer-long surface rupture zone, with as much as 16.3 meters of left-lateral strike-slip along the active Kunlun fault in northern Tibet. The rupture length and maximum displacement are the largest among the co-seismic surface rupture zones reported on so far. The strike-slip motion and the large rupture length generated by the earthquake indicate that the Kunlun fault partitions its deformation into an eastward extrusion of Tibet to accommodate the continuing penetration of the Indian plate into the Eurasian plate.

Abstract Frozen soil was simulated at six seasonally frozen and seven permafrost stations over the northern Tibetan Plateau using the Variable Infiltration Capacity (VIC) model for the period of 1962–2009. The VIC model resolved the seasonal cycle and temporal evolution of the observed soil temperatures and liquid soil moisture well. The simulated long‐term changes during 1962–2009 indicated mostly positive trends for both soil temperature and soil moisture, and negative trends for soil ice content at annual and monthly time scales, although differences existed among the stations, soil layers, and seasons. Increases in soil temperature were due mainly to increases in daily air temperature maxima and internal soil heat conduction, while decreases in soil ice content were related to the warming of frozen soil. For liquid soil moisture, increases in the cold months can be attributed to increases in soil temperature and enhanced soil ice melt while changes in the warm months were the results of competition between positive precipitation and negative soil temperature effects. Precipitation and liquid soil moisture were strongly correlated with evapotranspiration and runoff but had various degrees of correlations with base flow during May–September. Seasonally frozen stations displayed longer and more active hydrological processes than permafrost stations. Slight enhancement of the surface hydrological processes at the study stations was indicated, due to the combined effects of precipitation changes, which were dominant, and frozen soil degradation.

Abstract. Reliable precipitation data are highly necessary for geoscience research in the Third Pole (TP) region but still lacking, due to the complex terrain and high spatial variability of precipitation here. Accordingly, this study produces a long-term (1979–2020) high-resolution (1/30∘, daily) precipitation dataset (TPHiPr) for the TP by merging the atmospheric simulation-based ERA5_CNN with gauge observations from more than 9000 rain gauges, using the climatologically aided interpolation and random forest methods. Validation shows that TPHiPr is generally unbiased and has a root mean square error of 5.0 mm d−1, a correlation of 0.76 and a critical success index of 0.61 with respect to 197 independent rain gauges in the TP, demonstrating that this dataset is remarkably better than the widely used datasets, including the latest generation of reanalysis (ERA5-Land), the state-of-the-art satellite-based dataset (IMERG) and the multi-source merging datasets (MSWEP v2 and AERA5-Asia). Moreover, TPHiPr can better detect precipitation extremes compared with these widely used datasets. Overall, this study provides a new precipitation dataset with high accuracy for the TP, which may have broad applications in meteorological, hydrological and ecological studies. The produced dataset can be accessed via https://doi.org/10.11888/Atmos.tpdc.272763 (Yang and Jiang, 2022).

PURPOSE: To study the prevalence of dry eye in a hospital-based population and to evaluate the various risk factors attributable to dry eye. MATERIALS AND METHODS: In this cross-sectional study, 500 patients above 20 years of age were screened randomly for dry eye. A 13-point questionnaire, Lissamine Green test, Tear film break-up time (TBUT), Schirmer's test and presence of strands/filaments were used to diagnose dry eye. The diagnosis was made when at least three of the tests were positive. The role of air pollution, sunlight, excessive winds, smoking, drugs and refractive status as dry eye risk factors was assessed. RESULTS: Ninety-two (18.4%) patients had dry eye. Dry eye prevalence was maximum in those above 70 years of age (36.1%) followed by the age group 31-40 years (20%). It was significantly higher (P = 0.024) in females (22.8%) than in males (14.9%), more common in rural residents (19.6%) than in urban (17.5%) and highest among farmers/labourers (25.3%). A 2.15 fold increase was found in the odds for dry eye in those exposed to excessive wind, 1.91 fold to sunlight exposure, 1.42 to smoking, 1.38 to air pollution and 2.04 for persons on drugs. Dry eye prevalence was 14% in emmetropes, 16.8% in myopes and 22.9% in hypermetropes. It was 15.6% in those with corrected and 25.3% in those with uncorrected refractive errors. CONCLUSION: Dry eye is an under-diagnosed ocular disorder. Reduction in the modifiable risk factors of dry eye is essential to reduce its prevalence.

Abstract. Measurements of speciated atmospheric mercury were conducted at a remote mountain-top station (Waliguan (WLG) Baseline Observatory) at the edge of northeastern part of the Qinghai-Xizang Plateau, Western China. Mean concentrations of total gaseous mercury (TGM), particulate bound mercury (PBM), and gaseous oxidized mercury (GOM) during the whole sampling campaign were 1.98±0.98 ng m−3, 19.4±18.1 pg m−3, and 7.4±4.8 pg m−3, respectively. Levels of speciated Hg at WLG were slightly higher than those reported from remote areas of North America and Europe. Both regional sources and long-rang transport played important roles in the distribution of atmospheric TGM and PBM at WLG, whereas GOM showed major links to the regional sources, likely as well as the in-situ productions via photochemical processes. Regional sources for speciated Hg were mostly located to the east of WLG, which is the most industrial and urbanized areas of Qinghai province. Potential source contribution function (PSCF) results showed that eastern Gansu, western Ningxia and Shanxi Province were likely the potential source regions of WLG, with good accordance with locations of urban areas and industrial centers. Moreover, we found that Northern India may be a significant source region for WLG during the sampling campaign, and this is the first published evidence suggesting long-range transport of atmospheric Hg from India to the Northeastern Tibetan Plateau. Seasonal and diurnal variations of TGM were in contrast to most of the previous studies in China, with relatively higher levels in warm seasons and night, respectively. The temporal trend of TGM also highlighted the impact of long-range transport on the distribution of TGM in ambient air at WLG.

Abstract. Groundwater origin, flow and geochemical evolution in the Golmud River watershed of the Qaidam Basin was assessed using hydrogeochemical, isotopic and numerical approaches. The stable isotopic results show groundwater in the basin originates from precipitation and meltwater in the mountainous areas of the Tibetan Plateau. Modern water was found in the alluvial fan and shallow aquifers of the loess plain. Deep confined groundwater was recharged by paleowater during the late Pleistocene and Holocene under a cold climate. Groundwater in the low-lying depression of the central basin is composed of paleobrines migrated from the western part of the basin due to tectonic uplift in the geological past. Groundwater chemistry is controlled by mineral dissolution (halite, gypsum, anhydrite, mirabilite), silicate weathering, cation exchange, evaporation and mineral precipitation (halite, gypsum, anhydrite, aragonite, calcite, dolomite) and varies from fresh to brine with the water types evolving from HCO3 ⋅ Cl-Ca ⋅ Mg ⋅ Na to Cl-Na, Cl-K-Na and Cl-Mg type along the flow path. Groundwater flow patterns are closely related to stratigraphic control and lithological distribution. Three hierarchical groundwater flow systems, namely local, intermediate and regional, were identified using numerical modeling. The quantity of water discharge from these three systems accounts for approximately 83 %, 14 % and 3 %, respectively, of the total groundwater quantity of the watershed. This study can enhance the understanding of groundwater origin, circulation and evolution in the Qaidam Basin as well as other arid endorheic watersheds in northwestern China and elsewhere worldwide.

Abstract. Knowledge of aerosol size and composition is important for determining radiative forcing effects of aerosols, identifying aerosol sources and improving aerosol satellite retrieval algorithms. The ability to extrapolate aerosol size and composition, or type, from intensive aerosol optical properties can help expand the current knowledge of spatiotemporal variability in aerosol type globally, particularly where chemical composition measurements do not exist concurrently with optical property measurements. This study uses medians of the scattering Ångström exponent (SAE), absorption Ångström exponent (AAE) and single scattering albedo (SSA) from 24 stations within the NOAA/ESRL Federated Aerosol Monitoring Network to infer aerosol type using previously published aerosol classification schemes.Three methods are implemented to obtain a best estimate of dominant aerosol type at each station using aerosol optical properties. The first method plots station medians into an AAE vs. SAE plot space, so that a unique combination of intensive properties corresponds with an aerosol type. The second typing method expands on the first by introducing a multivariate cluster analysis, which aims to group stations with similar optical characteristics and thus similar dominant aerosol type. The third and final classification method pairs 3-day backward air mass trajectories with median aerosol optical properties to explore the relationship between trajectory origin (proxy for likely aerosol type) and aerosol intensive parameters, while allowing for multiple dominant aerosol types at each station.The three aerosol classification methods have some common, and thus robust, results. In general, estimating dominant aerosol type using optical properties is best suited for site locations with a stable and homogenous aerosol population, particularly continental polluted (carbonaceous aerosol), marine polluted (carbonaceous aerosol mixed with sea salt) and continental dust/biomass sites (dust and carbonaceous aerosol); however, current classification schemes perform poorly when predicting dominant aerosol type at remote marine and Arctic sites and at stations with more complex locations and topography where variable aerosol populations are not well represented by median optical properties. Although the aerosol classification methods presented here provide new ways to reduce ambiguity in typing schemes, there is more work needed to find aerosol typing methods that are useful for a larger range of geographic locations and aerosol populations.

Abstract. Tropospheric ozone is an important atmospheric oxidant, greenhouse gas and atmospheric pollutant at the same time. The oxidation capacity of the atmosphere, climate, human and vegetation health can be impacted by the increase of the ozone level. Therefore, long-term determination of trends of baseline ozone is highly needed information for environmental and climate change assessment. So far, studies on the long-term trends of ozone at representative sites are mainly available for European and North American sites. Similar studies are lacking for China and many other developing countries. Measurements of surface ozone were carried out at a baseline Global Atmospheric Watch (GAW) station in the north-eastern Tibetan Plateau region (Mt Waliguan, 36°17′ N, 100°54′ E, 3816 m a.s.l.) for the period of 1994 to 2013. To uncover the variation characteristics, long-term trends and influencing factors of surface ozone at this remote site in western China, a two-part study has been carried out, with this part focusing on the overall characteristics of diurnal, seasonal and long-term variations and the trends of surface ozone. To obtain reliable ozone trends, we performed the Mann–Kendall trend test and the Hilbert–Huang transform (HHT) analysis on the ozone data. Our results confirm that the mountain-valley breeze plays an important role in the diurnal cycle of surface ozone at Waliguan, resulting in higher ozone values during the night and lower ones during the day, as was previously reported. Systematic diurnal and seasonal variations were found in mountain-valley breezes at the site, which were used in defining season-dependent daytime and nighttime periods for trend calculations. Significant positive trends in surface ozone were detected for both daytime (0.24 ± 0.16 ppbv year−1) and nighttime (0.28 ± 0.17 ppbv year−1). The largest nighttime increasing rate occurred in autumn (0.29 ± 0.11 ppbv year−1), followed by spring (0.24 ± 0.12 ppbv year−1), summer (0.22 ± 0.20 ppbv year−1) and winter (0.13 ± 0.10 ppbv year−1), respectively. The HHT spectral analysis identified four different stages with different positive trends, with the largest increase occurring around May 2000 and October 2010. The HHT results suggest that there were 2–4a, 7a and 11a periodicities in the time series of surface ozone at Waliguan. The results of this study can be used for assessments of climate and environment change and in the validation of chemistry–climate models.

Abstract Sea fog is frequently observed over the Yellow Sea, with an average of 50 fog days on the Chinese coast during April–July. The Yellow Sea fog season is characterized by an abrupt onset in April in the southern coast of Shandong Peninsula and an abrupt, basin-wide termination in August. This study investigates the mechanisms for such steplike evolution that is inexplicable from the gradual change in solar radiation. From March to April over the northwestern Yellow Sea, a temperature inversion forms in a layer 100–350 m above the sea surface, and the prevailing surface winds switch from northwesterly to southerly, both changes that are favorable for advection fog. The land–sea contrast is the key to these changes. In April, the land warms up much faster than the ocean. The prevailing west-southwesterlies at 925 hPa advect warm continental air to form an inversion over the western Yellow Sea. The land–sea differential warming also leads to the formation of a shallow anticyclone over the cool Yellow and northern East China Seas in April. The southerlies on the west flank of this anticyclone advect warm and humid air from the south, causing the abrupt fog onset on the Chinese coast. The lack of such warm/moist advection on the east flank of the anticyclone leads to a gradual increase in fog occurrence on the Korean coast. The retreat of Yellow Sea fog is associated with a shift in the prevailing winds from southerly to easterly from July to August. The August wind shift over the Yellow Sea is part of a large-scale change in the East Asian–western Pacific monsoons, characterized by enhanced convection over the subtropical northwest Pacific and the resultant teleconnection into the midlatitudes, the latter known as the western Pacific–Japan pattern. Back trajectories for foggy and fog-free air masses support the results from the climatological analysis.

Abstract Gridded daily precipitation, temperature minima and maxima, and wind speed are generated for the northern Tibetan Plateau (NTP) for 1957–2009 using observations from 81 surface stations. Evaluation reveals reasonable quality and suitability of the gridded data for climate and hydrology analysis. The Mann–Kendall trends of various climate elements of the gridded data show that NTP has in general experienced annually increasing temperature and decreasing wind speed but spatially varied precipitation changes. The northwest (northeast) NTP became dryer (wetter), while there were insignificant changes in precipitation in the south. Snowfall has decreased along high mountain ranges during the wet and warm season. Averaged over the entire NTP, snowfall, temperature minima and maxima, and wind speed experienced statistically significant linear trends at rates of −0.52 mm yr−1 (water equivalent), +0.04°C yr−1, +0.03°C yr−1, and −0.01 m s−1 yr−1, respectively. Correlation between precipitation/wind speed and climate indices characterizing large-scale weather systems for four subregions in NTP reveals that changes in precipitation and wind speed in winter can be attributed to changes in the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), the East Asian westerly jet (WJ), and the El Niño–Southern Oscillation (ENSO) (wind speed only). In summer, the changes in precipitation and wind are only weakly related to these indices. It is speculated that in addition to the NAO, AO, ENSO, WJ, and the East and South Asian summer monsoons, local weather systems also play important roles.

The first Tropospheric Ozone Assessment Report (TOAR) provides information on present-day distributions and long-term trends of ozone metrics relevant for climate change, human health, and vegetation. However, only few results are available in TOAR for China due to limited long-term ozone observations. Here, we present an integrated analysis of long-term measurements of surface ozone from eight sites distributed in the North China Plain (NCP) and Yangtze River Delta (YRD), the relatively underdeveloped region Northeast China, and the remote regions in Northwest and Southwest China. Trends and present-day values for seven annual and five seasonal ozone metrics were calculated following the TOAR methodologies. We compare the seasonal and diurnal cycles of ozone concentrations as well as the present-day values of ozone among sites and discuss the long-term trends in the ozone metrics. Large and significant increases of ozone are detected at the background site in the NCP, moderate increases at the global baseline site in western China, significant decreases at the northwestern edge of China, and nearly no trend at other sites. Extremely high values of ozone occurred in the NCP and YRD, particularly in warmer seasons. The present-day levels of summer ozone metrics in the NCP are much higher than the thresholds set in TOAR for the highest value groups of ozone metrics. The summer ozone metrics at the Shangdianzi background site in the NCP indicate increases at rates of more than 2%/yr during 2004–2016. In contrast, ozone at the Lin’an background site in the YRD was constant over the period 2006–2016. Our results fill some knowledge gaps in spatiotemporal changes of ozone in China and may be of useful in the assessment of ozone impacts on human health and vegetation.

[1] Surface ozone (O3), carbon monoxide (CO), and total and speciated reactive nitrogen compounds (NOy, NO, NO2, PAN, HNO3, and particulate were measured at Mount Waliguan (WLG; 36.28°N, 100.90°E, 3816 m above sea level (asl)) in the summer of 2006 to further understand the sources of ozone and reactive nitrogen and to investigate the partitioning of reactive nitrogen over the remote Qinghai-Tibetan Plateau. The mean mixing ratios of O3, CO, NOy, and daytime NO were 59 ppbv, 149 ppbv, 1.44 ppbv, and 71 pptv, respectively, which (except for NOy) were higher than those measured from a previous campaign in summer 2003, which is consistent with more frequent transport of anthropogenic pollution from central and eastern China in the measurement period of 2006 (55%) than that of 2003 (25%). The abnormally high values of NOy observed in 2003 were suspected to be due to the positive interference from ammonia (NH3) to the particular catalytic converter used in that study. Varied diurnal patterns were observed for the various NOy components. The ozone production efficiencies (ΔO3/ΔNOz), which were estimated from the slope of the O3-NOz scatterplot, were 7.7–11.3 for the polluted plumes from central and eastern China. The speciation of reactive nitrogen was investigated for the first time in the remote free troposphere in western China. PAN and particulate were the most abundant reactive nitrogen species at WLG, with average proportions of 32% and 31%, followed by NOx (24%) and HNO3 (20%). The relatively large contribution of particulate to NOy was due to the presence of high concentrations of NH3 and crustal particles, which favor the formation of particulate nitrate. An analysis of backward trajectories for the recent 10 years revealed that air masses from central and eastern China dominated the airflow at WLG in summer, suggesting strong impact of anthropogenic forcing on the surface ozone and other trace constituents on the Plateau.

Abstract. Particle number size distributions in size range 12–570 nm were measured continuously at Mount Waliguan, a remote mountain-top station in inland China. The station is located at the altitude of 3816 m a.s.l., and some 600–1200 m above the surrounding area. The measurement period lasted from September 2005 to May 2007. The measurements were verified with independent CPC measurements at the same site. The average particle concentration in ambient conditions was 2030 cm−3, which is higher than the values measured at similar altitude in other regions of the world. On average, the Aitken mode contributed to roughly half of the particle number concentration. The concentrations were found to be higher during the summer than during the winter. The diurnal variation was also investigated and a clear pattern was found for the nucleation mode during all seasons, so that the nucleation mode particle concentration increased in the afternoon. The same pattern was visible in the Aitken mode during the summer, whereas the accumulation mode did not show any level of diurnal pattern during any season. Excluding the nucleation mode, the average day-time particle concentrations were not significantly higher than those measured at night-time, indicating no systematic pattern of change between planetary boundary layer conditions and free troposphere conditions. In air masses coming from east, the number concentration of particles was higher than in other air masses, which indicates that the air mass might be affected anthropogenic pollution east of the station. Also other factors, such as active new-particle formation, keep aerosol number concentrations high in the area.

Using a large amount of aircraft measurements of cloud droplet size distributions, the relationship between cloud spectral relative dispersion (ɛ) and cloud droplet number concentration ( N c ) is studied. The results indicate that the value of ɛ varies between 0.2 to 0.8 when the cloud droplet number concentration is low (about 50 cm −3 ), and converges toward a narrow range of 0.4 to 0.5 when the cloud number concentration is higher. Because the distribution of the cloud droplet size is an important parameter in estimating the first indirect radiative effect of aerosols on the climate system, the uncertainty in the corresponding radiative forcing can be reduced by 10–40% (depending on cloud droplet number density) under high aerosol loading. This finding is important for improving climate change projections, especially for the regions where aerosol loading is high and continues to increase.

Abstract Using observed daily precipitation and National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis data, what induced winter large spatial persistent heavy rainfall (PHR) events in south China was examined, based on composite analyses of 30 large spatial PHR events during 1951–2015. The results showed that wave trains within North Africa‐Asia (NAA) westerly jet existed in upper troposphere during these PHR processes. The wave trains shared the characteristic of a Rossby wave. The Rossby wave originated from northwest Europe, entered into the NAA jet through strong cold air advection to form convergence over the Mediterranean, and then propagated eastward along subtropical NAA jet. The Rossby wave propagated toward Southeast Asia and caused strong divergence in the upper troposphere. The strong divergence in the upper troposphere induced vertical convection and favored large spatial PHR events in south China. In addition, the enhanced India‐Burma trough and subtropical high in the northwestern Pacific supplied enough water vapor transportation. This mechanism would be useful to the medium‐range forecast of such winter rainfall processes over south China.

Abstract This study identifies several modes of coevolution of various types of El Niño–Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) by performing rotated season-reliant empirical orthogonal function (S-EOF) analysis with consideration of ENSO asymmetry. The first two modes reveal that early-onset ENSO is associated with subsequent strong IOD development, whereas late-onset ENSO forces an obscure IOD pattern with marginal SST anomalies in the western Indian Ocean. Further studies show that El Niño starting before early summer can more easily force an IOD event than that starting in late summer or fall, even when they are of equivalent magnitudes. This is because the atmospheric responses over the Indian Ocean to the eastern Pacific warming are in sharp contrast between early and late summer. Early-onset (late onset) El Niño can (cannot) cause favorable atmospheric circulation conditions over the Indian Ocean for inducing the western Indian Ocean warming, which facilitates the subsequent IOD development. In addition, the different propagations of ocean dynamic Rossby waves during the early- or late-onset types of ENSO are also accountable for the different IOD development. For the higher-order modes, the rotated S-EOF of “Niño only” cases shows a coevolution between a negative IOD mode and a date line Pacific El Niño, with warm sea surface temperature anomalies originating from the northern Pacific meridional mode.