Marine heatwave (MHW), a prolonged period of anomalously warm seawater, has a catastrophic repercussion on marine ecosystems. With global warming, MHWs have become increasingly frequent, intense, and prolonged. To avoid irreversible damages from such extreme events, net-zero carbon emissions by the 2050s, called carbon neutrality, were proposed. Here, we evaluate the impact of carbon neutrality on MHWs in the late 21st century using multi-model projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) Shared Socioeconomic Pathway (SSP)1-1.9 and SSP3-7.0 scenarios. It is found that if the current regional rivalry over carbon emissions continues (i.e., SSP3-7.0), the MHWs in the late 21st century will become stronger and longer than historical ones, especially in the western boundary current and equatorial current regions. Approximately 68% of the global ocean will be exposed to permanent MHWs, regionally 93% in the Indian Ocean, 76% in the Pacific Ocean, 68% in the Atlantic Ocean, 65% in the Coastal Ocean, and 48% in the Southern Ocean. Such MHWs can be significantly reduced by achieving carbon neutrality (i.e., SSP1-1.9). In particular, the spatial proportion of the ocean exposed to permanent MHWs can be reduced to as low as 0.02 to 0.07%, depending on the regions. This result underscores the critical importance of ongoing efforts to achieve net-zero carbon emissions to reduce the potential ecological risks induced by extreme MHWs.

Climate extremes, such as hot temperature and heavy precipitation events, have devastating effects on human societies. As the planet gets warmer, they have become more intense and more frequent. To avoid irreversible damages from climate extremes, many countries have committed to achieving net-zero anthropogenic carbon emissions, or carbon neutrality, by the 2050s. Here, we quantify the impact of carbon neutrality on population exposure to climate extremes using multi-model projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) Shared Socioeconomic Pathway (SSP)1-1.9 and SSP3-7.0 scenarios. It is found that the increasing population exposure to hot-temperature and heavy-precipitation extremes under SSP3-7.0 scenario can be substantially reduced by 87–98% in the late 21st century by achieving the carbon neutrality based on SSP1-1.9 scenario. The benefits of carbon neutrality are particularly pronounced in Africa and Asia. The potential benefits of carbon neutrality are also significant in North America, Europe, and Oceania, where a reduction in climate extremes is more than twice as important as population decline in reducing population exposure to climate extremes. These results provide important scientific support for ongoing efforts to achieve net-zero carbon emissions by the 2050s to reduce potential climate risk and its inequity across continents.

In most emissions scenarios consistent with the temperature targets of the Paris Agreement, carbon dioxide removal (CDR) would be required to achieve net negative emissions. The efficiency of CDR depends on the behavior of the natural carbon reservoirs, land and ocean, that regulate atmospheric CO2 concentrations, but their change in response to negative emissions is highly uncertain. Here, we investigate the response of the terrestrial and oceanic carbon cycle to negative emissions based on an idealized emission-driven simulation using a state-of-the-art Earth system model. The terrestrial and oceanic carbon sinks become carbon sources ~30 years after the onset of negative emissions. Thereafter, although the atmospheric CO2 concentration returns to its initial level, the terrestrial ecosystem and the ocean continue to release carbon. The ocean recovers as a carbon sink within a few decades, while the terrestrial ecosystem remains a carbon source until the end of the simulation. The prolonged carbon emissions from the land are due to the delayed response of respiration to the earlier increase in terrestrial carbon uptake. As a result, the total carbon stock on land gradually decreases but still remains higher than its initial state. The ocean carbon inventory shows an irreversible change within a few centuries due to the accumulation of anthropogenic CO2 in the deep ocean, which would take more than centuries to be removed naturally.

In the austral spring seasons of 2020-2022, the Antarctic stratosphere experienced three consecutive strong vortex events. In particular, the Antarctic vortex of October-December 2020 was the strongest on record in the satellite era for that season at 60°S in the mid- to lower stratosphere. However, it was poorly predicted by the Australian Bureau of Meteorology’s operational seasonal climate forecast system of that time, ACCESS-S1, even at a short lead time of a month. Using the current operational forecast system, ACCESS-S2, we have, therefore, tried to find a primary cause of the limited predictability of this event and conducted forecast sensitivity experiments to climatological versus observation-based ozone to understand the potential role of the ozone forcing in the strong vortex event and associated anomalies of the Southern Annular Mode (SAM) and south-eastern Australian rainfall. Here, we show that the 2020 strong vortex event did not follow the canonical dynamical evolution seen in previous strong vortex events in spring, whereas the ACCESS-S2 control forecasts with the climatological ozone did, which likely accounts for the inaccurate forecasts of ACCESS-S1/S2 at 1-month lead time. Forcing ACCESS-S2 with observed ozone significantly improved the skill in predicting the strong vortex in October-December 2020 and the subsequent positive SAM and related rainfall increase over south-eastern Australia in the summer of December 2020 to February 2021. These results highlight an important role of ozone variations in seasonal climate forecasting as a source of long-lead predictability, and therefore, a need for improved ozone forcing in future ACCESS-S development.

Spatial and temporal distributions of Clear-Air Turbulence (CAT) in the Northern Hemisphere were investigated using 41 years (1979 – 2019) of the European Centre for Medium-range Weather Forecast Reanalysis version 5 (ERA5) data. We used two groups of CAT diagnostics to determine occurrence frequencies: 1) commonly used empirical turbulence indices (TI1, TI2, and TI3) and their components [vertical wind shear (VWS), deformation, divergence, and divergence tendency], and 2) theoretical instability indicators Richardson number (Ri), potential vorticity (PV), and Brunt-Vӓisӓlӓ frequency. The empirical indices showed high frequencies of MOG-level CAT potential over the East Asian, Eastern Pacific, and Northwest Atlantic regions in winter. Over East Asia, the entrance region of the strong upper-level jets, showed the highest frequencies in TI1, TI2, and TI3 mainly due to strong VWS. The Eastern Pacific and Northwestern Atlantic areas near the exit region of the jet had relatively high frequencies of these and also Ri. PV frequency was high on the southern side of the jet primarily due to negative relative vorticity. Long-term increasing trends of MOG-level CAT potential also appeared in those three regions mainly due to the warming in lower latitudes. The most significant increasing trend was found over East Asia, due to the strengthening of the East Asian jet and VWS due to the strong meridional temperature gradients in the mid-troposphere induced by warming in the tropics and cooling in eastern Eurasia. These trends over East Asia are expected to be of importance to efficient aviation operations across the northwestern Pacific Ocean.

Downslope windstorms are responsible for wildfires, wind gusts, and turbulence in the lee side of the Taebaek Mountains, called Yeongdong region (YD) in Korea. We classified the synoptic conditions of the windstorms in the YD using a Self-Organizing Map (SOM). For the windstorm events from 1979 to 2019, sea level pressure anomalies were used to train the SOM. It was found that the synoptic patterns could be classified into three representative types: 1) the south high and north low pattern in spring, 2) the west high and east low pattern in winter, and 3) the strong low-pressure system passing the northern part of Korea. At the 850 hPa level, prevailing southwesterly (nortwesterly) flow with warm (cold) advection was dominant in Type 1 (2), and Type 3 presented a well-developed baroclinic system of cyclone. Adiabatic warming by downslope windstorm is the strongest in Type 1, which is likely to have a huge impact on the spread of wildfires. Three mesoscale generation mechanisms were examined under different synoptic patterns. Hydraulic jump theory was dominant for the windstorms in Type 2 due to upstream flows with moderate Froude numbers and inversion layers. The partial reflection of mountain waves was found in all types but more frequent in Type 1 than others. Downslope windstorms with wave breaking at critical levels mostly occurred in Type 1. This objective classification of weather patterns responsible for downslope windstorm in the YD is useful for better prediction and future projection of this event with climate change.

An intermediate complexity moist General Circulation Model is used to investigate the factor(s) controlling the magnitude of the surface impact from Southern Hemisphere springtime ozone depletion. In contrast to previous idealized studies, a model with full radiation is used, which allows focus on the full range of feedbacks between incoming ultraviolet radiation and temperature variations. In addition, the model can be run with a varied representation of the surface, from a zonally uniform aquaplanet to a highly realistic configuration. The model captures the positive Southern Annular Mode response to ozone depletion evident in observations and comprehensive models in December through February. It is shown that while synoptic waves dominate the long-term poleward jet shift, the initial response includes changes in planetary waves which simultaneously moderate the polar cap cooling (i.e., a negative feedback), but also constitute nearly half of the initial momentum flux response that shifts the jet polewards. Enhanced ultraviolet absorption at the surface due to the ozone hole drives an additional negative feedback on the poleward jet shift. The net effect is that stationary waves and surface radiative effects weaken the circulation response to ozone depletion, and also delay the response until summer rather than spring when ozone depletion peaks.