Significant climate anomalies persist through the summer (June-August) after El Nino dissipates in spring over the equatorial Pacific. They include the tropical Indian Ocean (TIO) sea surface temperature (SST) warming, increased tropical tropospheric temperature, an anomalous anticyclone over the subtropical northwest Pacific, and increased mei-yu-baiu rainfall over East Asia. The cause of these lingering El Nino effects during summer is investigated using observations and an atmospheric general circulation model (GCM). The results herein indicate that the TIO warming acts like a capacitor anchoring atmospheric anomalies over the Indo-western Pacific Oceans. It causes tropospheric temperature to increase by a moist-adiabatic adjustment in deep convection, emanating a baroclinic Kelvin wave into the Pacific. In the northwest Pacific, this equatorial Kelvin wave induces northeasterly surface wind anomalies, and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone. The GCM results support this Kelvin wave-induced Ekman divergence mechanism. In response to a prescribed SST increase over the TIO, the model simulates the Kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest Pacific. An additional experiment further indicates that the north Indian Ocean warming is most important for the Kelvin wave and northwest Pacific anticyclone, a result corroborated by observations. These results have important implications for the predictability of Indo-western Pacific summer climate: the spatial distribution and magnitude of the TIO warming, rather than simply whether there is an El Nino in the preceding winter, affect summer climate anomalies over the Indo-western Pacific and East Asia.

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Indian Ocean Capacitor Effect on Indo–Western Pacific Climate during the Summer following El Niño

In recent years, the Indian Ocean (IO) has been discovered to have a much larger impact on climate variability than previously thought This paper reviews climate phenomena and processes in which the IO is, or appears to be, actively involved We begin with an update of the IO mean circulation and monsoon system It is followed by reviews of ocean/atmosphere phenomenon at intraseasonal, interannual, and longer time scales Much of our review addresses the two important types of interannual variability in the IO, El Nino–Southern Oscillation (ENSO) and the recently identified Indian Ocean Dipole (IOD) IOD events are often triggered by ENSO but can also occur independently, subject to eastern tropical preconditioning Over the past decades, IO sea surface temperatures and heat content have been increasing, and model studies suggest significant roles of decadal trends in both the Walker circulation and the Southern Annular Mode Prediction of IO climate variability is still at the experimental stage, with varied success Essential requirements for better predictions are improved models and enhanced observations

Indian Ocean circulation and climate variability

From 2013 to 2017, with the implementation of the toughest-ever clean air policy in China, significant declines in fine particle (PM2.5) concentrations occurred nationwide. Here we estimate the drivers of the improved PM2.5 air quality and the associated health benefits in China from 2013 to 2017 based on a measure-specific integrated evaluation approach, which combines a bottom-up emission inventory, a chemical transport model, and epidemiological exposure-response functions. The estimated national population-weighted annual mean PM2.5 concentrations decreased from 61.8 (95%CI: 53.3-70.0) to 42.0 µg/m3 (95% CI: 35.7-48.6) in 5 y, with dominant contributions from anthropogenic emission abatements. Although interannual meteorological variations could significantly alter PM2.5 concentrations, the corresponding effects on the 5-y trends were relatively small. The measure-by-measure evaluation indicated that strengthening industrial emission standards (power plants and emission-intensive industrial sectors), upgrades on industrial boilers, phasing out outdated industrial capacities, and promoting clean fuels in the residential sector were major effective measures in reducing PM2.5 pollution and health burdens. These measures were estimated to contribute to 6.6- (95% CI: 5.9-7.1), 4.4- (95% CI: 3.8-4.9), 2.8- (95% CI: 2.5-3.0), and 2.2- (95% CI: 2.0-2.5) µg/m3 declines in the national PM2.5 concentration in 2017, respectively, and further reduced PM2.5-attributable excess deaths by 0.37 million (95% CI: 0.35-0.39), or 92% of the total avoided deaths. Our study confirms the effectiveness of China's recent clean air actions, and the measure-by-measure evaluation provides insights into future clean air policy making in China and in other developing and polluting countries.

https://www.pnas.org/content/pnas/116/49/24463.full.pdf

Drivers of improved PM2.5 air quality in China from 2013 to 2017.

Fine-particle pollution associated with winter haze threatens the health of more than 400 million people in the North China Plain. Sulfate is a major component of fine haze particles. Record sulfate concentrations of up to ~300 μg m −3 were observed during the January 2013 winter haze event in Beijing. State-of-the-art air quality models that rely on sulfate production mechanisms requiring photochemical oxidants cannot predict these high levels because of the weak photochemistry activity during haze events. We find that the missing source of sulfate and particulate matter can be explained by reactive nitrogen chemistry in aerosol water. The aerosol water serves as a reactor, where the alkaline aerosol components trap SO 2 , which is oxidized by NO 2 to form sulfate, whereby high reaction rates are sustained by the high neutralizing capacity of the atmosphere in northern China. This mechanism is self-amplifying because higher aerosol mass concentration corresponds to higher aerosol water content, leading to faster sulfate production and more severe haze pollution.

Reactive nitrogen chemistry in aerosol water as a source of sulfate during haze events in China

Recent climate changes over the Tibetan Plateau and their impacts on energy and water cycle: A review

In January 2013, a severe fog and haze event (FHE) of strong intensity, long duration, and extensive coverage occurred in eastern China. The present study investigates meteorological conditions for this FHE by diagnosing both its atmospheric background fields and daily evolution in January 2013. The results show that a weak East Asian winter monsoon existed in January 2013. Over eastern China, the anomalous southerly winds in the middle and lower troposphere are favorable for more water vapor transported to eastern China. An anomalous high at 500 hPa suppresses convection. The weakened surface winds are favorable for the fog and haze concentrating in eastern China. The reduction of the vertical shear of horizontal winds weakens the synoptic disturbances and vertical mixing of atmosphere. The anomalous inversion in near-surface increases the stability of surface air. All these meteorological background fields in January 2013 were conducive to the maintenance and development of fog and haze over eastern China. The diagnosis of the daily evolution of the FHE shows that the surface wind velocity and the vertical shear of horizontal winds in the middle and lower troposphere can exert dynamic effects on fog and haze. The larger (smaller) they are, the weaker (stronger) the fog and haze are. The thermodynamic effects include stratification instability in middle and lower troposphere and the inversion and dew-point deficit in near-surface. The larger (smaller) the stratification instability and the inversion are, the stronger (weaker) the fog and haze are. Meanwhile, the smaller (larger) the dewpoint deficit is, the stronger (weaker) the fog and haze are. Based on the meteorological factors, a multi-variate linear regression model is set up. The model results show that the dynamic and thermodynamic effects on the variance of the fog and haze evolution are almost the same. The contribution of the meteorological factors to the variance of the daily fog and haze evolution reaches 0.68, which explains more than 2/3 of the variance.

Meteorological conditions for the persistent severe fog and haze event over eastern China in January 2013

Recent advances in studies on the interaction between the East Asian monsoon and the ENSO cycle are reviewed in this paper. Through the recent studies, not only have the responding features and processes of the East Asian winter and summer monsoon circulation anomalies and summer rainfall anomalies in East Asia to the ENSO cycle during its different stages been understood further, but also have the thermal and dynamic effects of the tropical western Pacific on the ENSO cycle been deeply analyzed from the observational facts and dynamic theories. The results of observational and theoretical studies showed that the dynamical effect of the atmospheric circulation and zonal wind anomalies in the lower troposphere over the tropical western Pacific on the ENSO cycle may be through the excitation of the equatorial oceanic Kelvin wave and Rossby waves in the equatorial Pacific. These studies demonstrated further that the ENSO cycle originates from the tropical western Pacific. Moreover, these recent studies also showed that the atmospheric circulation and zonal wind anomalies over the tropical western Pacific not only result from the air-sea interaction over the tropical western Pacific, but are also greatly influenced by the East Asian winter and summer monsoons. Additionally, the scientific problems in the interaction between the Asian monsoon and the ENSO cycle which should be studied further in the near future are also pointed out in this paper.

Recent advances in studies of the interaction between the East Asian winter and summer monsoons and ENSO cycle

A diagnostic study is made to investigate the relationship between water vapor transport from Indian 
monsoon and that over East Asia in Northern summer. It is found that water vapor transport from Indian 
monsoon is inverse to that over East Asia. More (less) Indian monsoon water vapor transport corresponds 
to less (more) water vapor transport over East Asia and less (more) rainfall in the middle and lower reaches 
of the Yangtze River valley. The Indian summer monsoon water vapor transport is closely related to the in
tensity of the western Pacific subtropical high in its southwestern part. The stronger (weaker) the Indian sum
mer monsoon water vapor transport, the weaker (stronger) the western Pacific subtropical high in its 
southwestern part, which leads to less (more) water vapor transport to East Asia, and thus less (more) rain
fall in the middle and lower reaches of the Yangtze River valley. Analysis of the out-going longwave radia
tion anomalies suggests that the convective heating anomalies over the Indian Ocean may have significant 
impact not only on the Indian monsoon, but also on the East Asian monsoon.

Relations of Water Vapor Transport from Indian Monsoon with That over East Asia and the Summer Rainfall in China

The circulation and zonal wind anomalies in the lower troposphere over the equatorial western Pacific and their roles in the developing and decaying processes of the 1982—1983, 1986—1987, 1991—1992 and 1997—1998 El Nino events and the occurrence of La Nina events are analyzed by using the observed data in this paper. The results show that before the developing stage of these El Nino events, there were cyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies brought the westerly anomalies over the Indonesia and the tropical western Pacific. However, when the El Nino events developed to their mature phase, there were anticyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies made the easterly anomalies appear over the tropical western Pacific. A simple, dynamical model of tropical ocean is used to calculate the response of the equatorial oceanic waves to the observed anomalies of wind stress near the sea surface of the equatorial Pacific during the 1997/98 ENSO cycle, which was the strongest one in the 20th century. It is shown that the zonal wind stress anomalies have an important dynamical effect on the development and decay of this El Nino event and the occurrence of the following La Nina event.

Dynamical effect of the zonal wind anomalies over the tropical western Pacific on ENSO cycles

The observed data of the sea surface temperature (SST) anomalies and the sea temperature (ST) in the sub-layer of the equatorial Pacific, the NCEP/ NCAR reanalysis data and the data set of daily precipitation in China are used to analyze the characteristics of the 1997 / 98 ENSO cycle and its impact on summer climate anomalies in East Asia. The results show that the 1997/98 ENSO cycle, the strongest one in the 20th century, might be characterized by rapid development and decay and eastward propagation from the West Pacific warm pool. Influenced by the ENSO cycle, in 1997, the serious drought and hot summer occurred in North China, and in the summer of 1998, the severe floods occurred in the Yangtze River valley, especially in the Dongting Lake and Boyang Lake valleys, South Korea and Japan. The analysis also shows that: influenced by the 1997/98 ENSO cycle, the water vapor transportation by the Asian monsoon in the summer of 1997 was very different from that in the summer of 1998. In the summer of 1997, the water vapor transportation by the Asian summer monsoon was weak in North China and the northern part of the Korea Peninsula. Thus, it caused the drought and hot summer in North China. However, in the summer of 1998, the sea temperature in the sub-layer of the West Pacific warm pool dropped, the western Pacific subtropical high shifted southward. Thus, a large amount of water vapor was transported from the Bay of Bengal, the South China Sea and the tropical western Pacific into the Yangtze River valley of China, South Korea and Japan, and the severe flood occurred there.

Zhang Renhe

Papers

Meteorological conditions for the persistent severe fog and haze event over eastern China in January 2013

Recent advances in studies of the interaction between the East Asian winter and summer monsoons and ENSO cycle

Relations of Water Vapor Transport from Indian Monsoon with That over East Asia and the Summer Rainfall in China

Dynamical effect of the zonal wind anomalies over the tropical western Pacific on ENSO cycles

The 1997/ 98 enso cycle and its impact on summer climate anomalies in east asia