A stratospheric precursor of East Asian summer droughts and floods.

East Asian floods and droughts in summer show a typical dipole pattern with a north-south oscillation centered near 30°N, called the southern drought-northern flood (SDNF) pattern, which has caused significant economic losses and casualties in the past three decades. However, effective explanations and predictions are still challenging, making suitable disaster prevention more difficult. Here, we find that a key predictor of this dipole pattern is the Quasi-Biennial Oscillation (QBO, tropical winds above 10 km). The QBO can modulate precipitation in East Asia, contributing the largest explained variation of this dipole pattern. A QBO-included statistical model can effectively predict summer floods and droughts at least three months in advance and explain at least 75.8% of precipitation variation. More than 30% of the SDNF pattern is attributed to the QBO in July-August 2020 and 2021. This result suggests a good prospect for using the tropical mid- to upper atmosphere in seasonal forecasts for summer.

A commentary of "Antarctic ozone layer is gradually recovering" in 10 remarkable discoveries from 2020 in Nature.

In the mid-1980s, scientists discovered a spring atmospheric ozone hole over Antarctica, revealing the threat of human-made ozone-depleting substances to the atmosphere. The Antarctic ozone hole located 10 to 20 km above sea level, also affects the circulation of the atmosphere in the southern hemisphere, which in turn affects the global climate. One of its most noticeable effects is that the westerly jet in summer begins to move to the poles. The westerly jet is a planetary-scale atmospheric circulation phenomenon; there are several jet zones on the Earth. The 1987 Montreal Protocol and its subsequent amendments banned the production and use of ozone-depleting substances. Therefore, the concentration of ozone-depleting substances in the atmosphere is declining, and the ozone layer has shown preliminary signs of recovery. The study by Banerjee et al. pointed out that ozone hole-related effects on circulation and climate have ceased since the ozone layer began to recover [1]. While others had noticed this trend of cessation of ozone hole effects before, Banerjee and others officially attributed it to the impact of the Montreal Protocol for the first time.

Responses of Arctic sea ice to stratospheric ozone depletion.

The Arctic has experienced several extreme springtime stratospheric ozone depletion events over the past four decades, particularly in 1997, 2011 and 2020. However, the impact of this stratospheric ozone depletion on the climate system remains poorly understood. Here we show that the stratospheric ozone depletion causes significant reductions in the sea ice concentration (SIC) and the sea ice thickness (SIT) over the Kara Sea, Laptev Sea and East Siberian Sea from spring to summer. This is partially caused by enhanced ice transport from Barents-Kara Sea and East Siberian Sea to the Fram Strait, which is induced by a strengthened and longer lived polar vortex associated with stratospheric ozone depletion. Additionally, cloud longwave radiation and surface albedo feedbacks enhance the melting of Arctic sea ice, particularly along the coast of the Eurasian continent. This study highlights the need for realistic representation of stratosphere-troposphere interactions in order to accurately predict Arctic sea ice loss.

Increasing trend of lightning activity in the South Asia region.

Lightning is an important natural source of wildfires and oxynitride, and hence significantly influences ecological systems and atmospheric chemistry. Here, we choose South Asia, an important region for global water reallocation and global climate changes, to examine lightning variations based on the longest existing lightning dataset from the OTD/LIS observations. We identify a clear increase in lightning density in the research region, increasing at a rate of 0.096 fl km-2 a-1 over the last two decades. Multiple linear regression analysis is adopted to identify the main influencing factors among ten potential thermodynamic or microphysical factors and the crucial areas contributing to the increases in lightning. The surface latent heat flux along the west coast of the Indian subcontinent is the largest contributor, explaining 52% of the lightning variance and contributing to a 0.025 fl km-2 a-1 increase. The sea surface temperature in the Arabian Sea, the convective available potential energy (CAPE) over the northwestern Indian subcontinent, and the wind shear along the northwestern coast also make important contributions to the lightning increase, indicating that the thermodynamic effects overwhelm the microphysical effects on lightning activity over the South Asia region.

Recent strengthening of the stratospheric Arctic vortex response to warming in the central North Pacific.

The stratospheric Arctic vortex (SAV) plays a critical role in forecasting cold winters in northern mid-latitudes. Its influence on the tropospheric mid- and high-latitudes has attracted growing attention in recent years. However, the trend in the SAV during the recent two decades is still unknown. Here, using three reanalysis datasets, we found that the SAV intensity during 1998-2016 has a strengthening trend, in contrast to the weakening trend before that period. Approximately 25% of this strengthening is contributed by the warming of sea-surface temperature (SST) over the central North Pacific (CNP). Observational analysis and model experiments show that the warmed CNP SST tends to weaken the Aleutian low, subsequently weakening the upward propagation of wavenumber-1 planetary wave flux, further strengthening the SAV. This strengthened SAV suggests important implications in understanding the Arctic warming amplification and in predicting the surface temperature changes over the northern continents.

Stratospheric ozone loss over the Eurasian continent induced by the polar vortex shift.

The Montreal Protocol has succeeded in limiting major ozone-depleting substance emissions, and consequently stratospheric ozone concentrations are expected to recover this century. However, there is a large uncertainty in the rate of regional ozone recovery in the Northern Hemisphere. Here we identify a Eurasia-North America dipole mode in the total column ozone over the Northern Hemisphere, showing negative and positive total column ozone anomaly centres over Eurasia and North America, respectively. The positive trend of this mode explains an enhanced total column ozone decline over the Eurasian continent in the past three decades, which is closely related to the polar vortex shift towards Eurasia. Multiple chemistry-climate-model simulations indicate that the positive Eurasia-North America dipole trend in late winter is likely to continue in the near future. Our findings suggest that the anticipated ozone recovery in late winter will be sensitive not only to the ozone-depleting substance decline but also to the polar vortex changes, and could be substantially delayed in some regions of the Northern Hemisphere extratropics.

Indo-Pacific warm pool area expansion, Modoki activity, and tropical cold-point tropopause temperature variations.

The tropical cold-point tropopause temperature (CPTT), a potentially important indicator of global climate change, is of particular importance for understanding changes in stratospheric water vapor levels. Since the 1980s, the tropical CPTT has shown not only interannual variations, but also a decreasing trend. However, the factors controlling the variations in the tropical CPTT since the 1980s remain elusive. The present study reveals that the continuous expansion of the area of the Indo-Pacific warm pool (IPWP) since the 1980s represents an increase in the total heat energy of the IPWP available to heat the tropospheric air, which is likely to expand as a result. This process lifts the tropical cold-point tropopause height (CPTH) and leads to the observed long-term cooling trend of the tropical CPTT. In addition, our analysis shows that Modoki activity is an important factor in modulating the interannual variations of the tropical CPTT through significant effects on overshooting convection.