Impacts of Tibetan Plateau sensible heat and El Niño–Southern Oscillation on precipitation over South China under the background of the PDO

This study aims to investigate the impacts of the spring sensible heat (SH) over the Tibetan Plateau (TP) and the El Niño–Southern Oscillation (ENSO) in the preceding wintertime on midsummer (July–August) precipitation over South China under the different Pacific decadal oscillation (PDO) phases. More specifically, eight classifications are adopted at the demarcation point around 1996 when the spring SH over the TP and the midsummer precipitation in South China occurred as well as the PDO phase transition, including positive and negative SHs and ENSOs under a positive PDO phase (1979–1996) and a negative PDO phase (1997–2019), respectively, based on the Niño-3 index and the spring SH calculated from 48 stations over the central and eastern parts of the TP. The results show that both the spring SH and the ENSO in preceding wintertime have a significant impact on the midsummer precipitation over South China; that is, when the two factors are in their respective positive (negative) phase, the midsummer precipitation in South China is generally less (more). Importantly, the phase change of background field PDO can significantly enhance the effect of the SH and the ENSO on summer precipitation over South China. Moreover, compared with the preceding wintertime ENSO, the spring SH over the TP contributes more to the midsummer precipitation in South China based on analyses of their independent and synergistic effects. The main mechanism responsible for the anomalous midsummer precipitation over South China are the combined effects of the South Asian high (SAH) and the western Pacific subtropical high (WPSH), which are controlled by the spring SH anomaly over the TP and the ENSO, respectively. Deep understanding of the dominant factors of the midsummer precipitation over South China will help understand the local climate change and reduce the losses caused by drought and flood disasters

Warming effect of dust aerosols modulated by overlapping clouds below

Due to the substantial warming effect of dust aerosols overlying clouds and its poor representation in climate models, it is imperative to accurately quantify the direct radiative forcing (DRF) of above-cloud dust aerosols. When absorbing aerosol layers are located above clouds, the warming effect of aerosols strongly depends on the cloud macro- and micro-physical properties underneath, such as cloud optical depth and cloud fraction at visible wavelength. A larger aerosol-cloud overlap is believed to cause a larger warming effect of absorbing aerosols, but the influence of overlapping cloud fraction and cloud optical depth remains to be explored. In this study, the impact of overlapping cloud properties on the shortwave all-sky DRF due to springtime above-cloud dust aerosols is quantified over northern Pacific Ocean based on 10-year satellite measurements. On average, the DRF is roughly 0.62 Wm^(−2). Furthermore, the warming effect of dust aerosols linearly increases with both overlapping cloud fraction and cloud optical depth. An increase of 1% in overlapping cloud fraction will amplify this warming effect by 1.11 Wm^(−2)τ^(−1). For the springtime northern Pacific Ocean, top-of-atmosphere cooling by dust aerosols turns into warming when overlapping cloud fraction is beyond 0.20. The variation of critical cloud optical depth beyond which dust aerosols switch from exerting a net cooling to a net warming effect depends on the concurrent overlapping cloud fraction. When the overlapping cloud coverage range increases from 0.2 to –0.4 to 0.6–0.8, the corresponding critical cloud optical depth reduces from 6.92 to 1.16. Our results demonstrate the importance of overlapping cloud properties for determining the springtime warming effect of dust aerosols

Effects of Atmospheric Heat Source on the Tibetan Plateau Vortex in Different Stages: A Case Study in June 2016

The Tibetan Plateau (TP) vortex (TPV), one of the crucial weather systems triggering rainfall, plays a key role in modulating precipitation over TP and downstream regions. The role of atmospheric heat source in TPV development is explored by a case study in June 2016, using high-resolution ERA5 reanalysis, black-body temperature (TBB) obtained from the Fengyun-2E (FY-2E) satellite, and precipitation amount from the Tropical Rainfall Measurement Mission (TRMM). The evolutions of TPV can be split into three stages, i.e., generation, development, and pre-moving-off stage. The intensity of TPV increases with fluctuations, with weaker and shallower TPV in the generation stage, strongest in the development stage and deepest in the pre-moving-off stage. Importantly, the genesis of TPV is related to the surface warming center driven by surface sensible heating while its development is primarily dependent on the latent heat of condensation. The main contributor of the latent heat of condensation is further analyzed as a vertical transport of the water vapor that promotes TPV development

A Synergistic Effect of Blockings on a Persistent Strong Cold Surge in East Asia in January 2018

A persistent strong cold surge occurred in East Asia in late January 2018, causing mean near-surface air temperature in China to hit the second lowest since 1984. Moreover, the daily mean air temperature remained persistently negative for more than 20 days. Here, we find that a synergistic effect of double blockings in Western Europe and North America plays an important accelerating role in the rapid phase transition of Arctic Oscillation and an amplifying role in the strength of cold air preceding to the cold surge outbreaks by the use of an isentropic potential vorticity analysis. In mid-January, an Atlantic mid-latitude anticyclone merged with Western Europe blocking, which led to a strengthening of the blocking. Simultaneously, the Pacific-North American blocking was also significantly strengthened. The two blockings synchronously deeply stretched towards the Arctic, which resulted in, on the one hand, warm and moist air of the Pacific and the Atlantic being excessively transported into the Arctic, and on the other hand, the polar vortex being split and cold air being squeezed southwards and accumulating extensively on the West Siberian Plain. After the breakdown of the double blocking pattern, which lasted for about 10 days, the record-breaking cold surge broke out in East Asia. It was discovered that the synergistic effect of double blockings extending into the Arctic, which is conducive to extreme cold events, has been rapidly increasing in recent years

The Variation Characteristics of Stratospheric Circulation under the Interdecadal Variability of Antarctic Total Column Ozone in Early Austral Spring

Antarctic Total Column Ozone (TCO) gradually began to recover around 2000, and a large number of studies have pointed out that the recovery of the Antarctic TCO is most significant in the austral early spring (September). Based on the Bodeker Scientific Filled Total Column Ozone and ERA5 reanalysis dataset covering 1979–2019, the variation characteristics of the Antarctic TCO and stratospheric circulation for the TCO ‘depletion’ period (1979–1999) and the ‘recovery’ period (2000–2019) are analyzed in September. Results show that: (1) Stratospheric elements significantly related to the TCO have corresponding changes during the two eras. (2) The interannual variability of the TCO and the above-mentioned stratospheric circulation elements in the recovery period are stronger than those in the depletion period. (3) Compared with the depletion period, due to the stronger amplitude of the planetary wave 1, stronger Eliassen–Palm (EP) flux corresponds to EP flux convergence, larger negative eddy heat flux, and positive eddy momentum flux in the stratosphere during the recovery period. The polar temperature rises in the lower and middle stratosphere and the polar vortex weakens in the middle and upper stratosphere, accompanied by the diminished area of PSC. This contributes to the understanding of Antarctic ozone recovery

Impact of Ozone Valley over the Tibetan Plateau on the South Asian High in CAM5

Local climate effects of Tibetan Plateau Ozone Valley (OVTP) were investigated by numerical simulations using Community Atmosphere Model version 5.1.1 (CAM5). After a 20-year spin-up period, two additional 10-year experiments were conducted. CAM5 was driven by monthly mean climatological ozone in control experiment (CE) and OVTP in the sensitivity experiment (SE) was removed from May to September. After the removal of OVTP, South Asian High (SAH) becomes more robust and colder from June to August, especially in June. The reason for enhancement of SAH is that removal of OVTP increasing ozone in 200–30 hPa leads to significant enhancement of longwave and shortwave radiative heating rate in SAH region in June, and then enhancement of horizontal divergence resulting from the radiative warming leads to strengthening of SAH influenced by the Coriolis force, while the colder SAH is primarily caused by dynamic processes. Adiabatic expansion and ascending movement mainly bring about temperature decrease in SAH after OVTP removal, but the thermodynamic process related to radiative heating offsets part of the cooling response