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The Antarctic vortex of October–December 2020 was the strongest on record in the satellite era for the season 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 understand the potential role of ozone in the event and its associated anomalies of the Southern Annular Mode (SAM) and rainfall over south–eastern Australia and western Patagonia. Here, we show that the 2020 strong vortex event did not follow the canonical dynamical evolution seen in previous strong vortex events in spring but suddenly appeared as a result of the record‐low upward propagating wave activity in September 2020. The ACCESS‐S2 forecasts significantly underestimated the negative wave forcing in September even at zero lead time, irrespective of the ozone configuration, therefore falling short in predicting the record strength of the polar vortex in late spring 2020. Nevertheless, ACCESS‐S2 with prescribed realistic ozone that had large anomalies in the Antarctic stratosphere significantly better predicted the strong vortex and the subsequent positive SAM and related rainfall anomalies over south–eastern Australia and western Patagonia in the austral summer of 2020–21. This highlights the potentially important role of ozone variations for seasonal climate forecasting as a source of long‐lead predictability. Plain Language Summary: The Antarctic vortex of October–December 2020 was the strongest on record in the satellite observation era for that season when monitored at 60°S in the mid‐ to lower stratosphere (altitudes of 15–30 km). However, this super vortex event was poorly predicted by the Australian Bureau of Meteorology (BoM)'s seasonal climate forecast system even at 1‐month lead time. We argue that the 2020 strong vortex was likely caused by an abrupt reduction in the upward propagating wave activity from the troposphere in September 2020, which left the stratospheric vortex undisturbed and strong. The BoM seasonal forecast system substantially underpredicted the negative wave forcing in September 2020, resulting in a poor forecast performance for the vortex strength in the following season. Forecast experiments prescribed with observed versus climatological ozone concentrations further show that using the observed ozone, characterized by a significant loss over Antarctica in spring 2020, improved the ensemble mean forecasts for the 2020 vortex strength by ∼10%–20% at different levels of the stratosphere and the associated surface climate features such as the poleward shift of the Southern Hemisphere midlatitude jet and anomalously high rainfall over south–eastern Australia and low rainfall over western Patagonia in the following summer. Key Points: The Antarctic vortex of 2020 was the strongest event in the satellite era for the October–December mean in the mid‐ to lower stratosphereSignificant lack of tropospheric wave forcing in September 2020 was responsible for the sudden appearance of the record strong polar vortexImposing realistic ozone concentrations significantly improved forecasts of the 2020 polar vortex strength and its downward impact [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) are known to produce both beneficial and extreme rainfall, leading to natural hazards in Chile. Motivated to understand moisture transport during AR events, this study performs a moisture budget analysis along 50 zonally elongated ARs reaching the western coast of South America. We identify the convergence of moist air masses of tropical/subtropical origin along the AR as the primary source of vertically integrated water vapor (IWV). Over the open ocean, moisture convergence is nearly balanced by precipitation. The advection of moisture along the AR, although smaller compared to mass convergence, significantly increases toward the landfalling region. The near conservation of IWV over the open ocean, observed by tracking a Lagrangian atmospheric column along the ARs, is the explanation behind the seemingly tropical origin of ARs in time‐lapse visualizations of IWV. Plain Language Summary: Imagine atmospheric rivers (ARs) as massive, flowing rivers in the sky, but instead of water, they carry vapor from the ocean. When these "sky rivers" travel and hit the Andes Mountains in South America, they can cause a lot of rain and snow to fall. This precipitation is often good because it helps fill reservoirs and water crops. However, sometimes there's too much rain, leading to floods and landslides, which can be dangerous. Over the ocean, the amount of water vapor these atmospheric rivers pick up is almost exactly balanced by the rain that falls from them. As these atmospheric rivers get closer to South America, the movement of moisture along the river, though generally less significant than the gathering of moist air, becomes more pronounced. This means that as the atmospheric river approaches the land, it starts carrying more moisture toward its destination. We were able to see this process in action by following a moving "slice" of the atmosphere (a Lagrangian atmospheric column) as it travels along the path of the atmospheric river. This helped us understand how atmospheric rivers maintain their water content as they move. It also shows why atmospheric rivers seem to originate from tropical areas when we look at them in time‐lapse images of water vapor. Key Points: We calculate the moisture transport budget of 50 events of zonal atmospheric rivers over the Pacific that reach South AmericaHorizontal convergence of tropical and subtropical air masses act to maintain atmospheric rivers over the ocean while advection dominates near the coastFollowing a Lagrangian column, precipitable water is roughly conserved along atmospheric rivers, except near landfalling [ABSTRACT FROM AUTHOR]

The spatial distribution of surface air temperatures is essential for understanding and modelling high‐relief environments. Good estimations of the surface temperature lapse rate (STLR) and the 0°C isotherm height (H0) are fundamental for hydrological modelling in mountainous basins. Although STLR changes in space and time, it is typically assumed to be constant leading to errors in the estimation of direct‐runoff volumes and flash‐floods risk assessment. This paper characterizes daily and seasonal temporal variations of the in‐situ STLR and H0 over the western slope of the subtropical Andes (central Chile). We use temperature data collected during 2 years every 10 min by a 16 sensors network in a small catchment with elevations ranging between 700 and 3,250 m. The catchment drains directly into Santiago, the Chilean capital with more than seven million inhabitants. Resulting values are compared against those obtained using off‐site, operational data sets. Significant intra‐ and inter‐day variations of the in‐situ STLR were found, likely reflecting changes in the low‐level temperature inversion during dry conditions. The annual average in‐situ STLR is −5.9°C/km during wet‐weather conditions. Furthermore, STLR and H0 estimations using off‐site gauges are extremely sensitive to the existence of gauging stations at high elevations. [ABSTRACT FROM AUTHOR]

Central Chile, home to more than 10 million inhabitants, has experienced an uninterrupted sequence of dry years since 2010 with mean rainfall deficits of 20–40%. The so‐called Mega Drought (MD) is the longest event on record and with few analogues in the last millennia. It encompasses a broad area, with detrimental effects on water availability, vegetation and forest fires that have scaled into social and economical impacts. Observations and reanalysis data reveal that the exceptional length of the MD results from the prevalence of a circulation dipole‐hindering the passage of extratropical storms over central Chile—characterized by deep tropospheric anticyclonic anomalies over the subtropical Pacific and cyclonic anomalies over the Amundsen–Bellingshausen Sea. El Niño Southern Oscillation (ENSO) is a major modulator of such dipole, but the MD has occurred mostly under ENSO‐neutral conditions, except for the winters of 2010 (La Niña) and 2015 (strong El Niño). Climate model simulations driven both with historical forcing (natural and anthropogenic) and observed global SST replicate the south Pacific dipole and capture part of the rainfall anomalies. Idealized numerical experiments suggest that most of the atmospheric anomalies emanate from the subtropical southwest Pacific, a region that has experienced a marked surface warming over the last decade. Such warming may excite atmospheric Rossby waves whose propagation intensifies the circulation pattern leading to dry conditions in central Chile. On the other hand, anthropogenic forcing (greenhouse gases concentration increase and stratospheric ozone depletion) and the associated positive trend of the Southern Annular Mode also contribute to the strength of the south Pacific dipole and hence to the intensity and longevity of the MD. Given the concomitance of the seemingly natural (ocean sourced) and anthropogenic forcing, we anticipate only a partial recovery of central Chile precipitation in the decades to come. [ABSTRACT FROM AUTHOR]