Rei Chemke

The winter storm track in the Southern Hemisphere has large weather and climate impacts, as it drives daily to multi-decadal variations in extratropical winds, precipitation, and temperature. In the lower mid-latitudes, where the southern parts of the continents reside, the storm track is projected to initially increase with greenhouse gas emissions but to weaken as emissions continue. Given the extensive efforts to mitigate climate change, it is critical to assess the reversibility of the storm track under a negative emissions scenario. Here, we show that while changes in the storm track are reversible under a doubling of CO₂ concentrations, beyond that point, the storm track weakening becomes irreversible for centuries. The persistent storm track weakening stems from a prolonged polar amplification, driven by ocean heat transport changes, and is associated with intensity changes of poleward heat flux and extreme warm and cold temperatures. Our results suggest that implementing mitigation pathways prior of reaching a doubling of CO₂ concentrations would allow avoiding the persistent impacts of the weakening storm track.

Accurately assessing future precipitation changes presents one of the greatest challenges of climate change. In the tropics, changes in the Hadley circulation are expected to considerably affect precipitation in dry subtropical and wet equatorial regions. However, while climate models project a robust weakening of the Northern Hemisphere circulation in the coming decades, currently, there is low confidence in the magnitude of such weakening and its impact on regional precipitation patterns. Here we use emergent constraint analyses and observation-based Hadley circulation strength changes to show that the projected circulation weakening will probably be larger than in current predictions. The more pronounced weakening of the flow results in a doubling of the subtropical precipitation increase compared with current forecasts, specifically over Asia, Africa and the Pacific Ocean. Our findings provide more accurate tropical circulation and precipitation projections and have considerable societal impacts, given the scarcity of water in subtropical regions.

The Hadley circulation plays a central role in determining the location and intensity of the hydrological cycle in tropical and subtropical latitudes. Thus, the human-induced historical and projected weakening of the Northern Hemisphere Hadley circulation has considerable societal impacts. Yet, it is currently unknown how unparalleled this weakening is relative to the response of the circulation to natural forcings in past centuries. Here, using state-of-the-art climate models, we show that in contrast to the recent and future human-induced Hadley circulation weakening, natural forcings acted to intensify the circulation by cooling the climate over the last millennium. The reversal of a naturally-forced multi-centennial trend by human emissions highlights their unprecedented impacts on the atmospheric circulation. Given the amplifying effect of natural forcings on the Hadley circulation, our analysis stresses the importance of adequately incorporating natural forcings in climate model projections to better constrain future tropical climate changes.

By accounting for most of the poleward atmospheric heat and moisture transport in the tropics, the Hadley circulation largely affects the latitudinal patterns of precipitation and temperature at low latitudes. To increase our preparednesses for human-induced climate change, it is thus critical to accurately assess the response of the Hadley circulation to anthropogenic emissions(1-3). However, at present, there is a large uncertainty in recent Northern Hemisphere Hadley circulation strength changes(4). Not only do climate models simulate a weakening of the circulation(5), whereas atmospheric reanalyses mostly show an intensification of the circulation(4-8), but atmospheric reanalyses were found to have artificial biases in the strength of the circulation(5), resulting in unknown impacts of human emissions on recent Hadley circulation changes. Here we constrain the recent changes in the Hadley circulation using sea-level pressure measurements and show that, in agreement with the latest suite of climate models, the circulation has considerably weakened over recent decades. We further show that the weakening of the circulation is attributable to anthropogenic emissions, which increases our confidence in human-induced tropical climate change projections. Given the large climate impacts of the circulation at low latitudes, the recent human-induced weakening of the flow suggests wider consequences for the regional tropical-subtropical climate.

The strength of mid-latitude storm tracks shapes weather and climate phenomena in the extra-tropics, as these storm tracks control the daily to multi-decadal variability of precipitation, temperature and winds. By the end of this century, winter mid-latitude storms are projected to intensify in the Southern Hemisphere, with large consequences over the entire extra-tropics. Therefore, it is critical to be able to accurately assess the impacts of anthropogenic emissions on these storms to improve societal preparedness for future changes. Here we show that current climate models severely underestimate the intensification in mid-latitude storm tracks in recent decades. Specifically, the intensification obtained from reanalyses has already reached the model-projected end-of-the-century intensification. The biased intensification is found to be linked to biases in the zonal flow. These results question the ability of climate models to accurately predict the future impacts of anthropogenic emissions in the Southern Hemisphere mid-latitudes.

The latitudinal position of mid-latitude storm tracks has large climate impacts affecting the distribution of precipitation, temperature, humidity, and winds over the extratropics. By the end of this century, climate models project a poleward shift of summer mid-latitude storm tracks in the Southern Hemisphere. Most previous mechanisms for the poleward shift of the storm tracks focused on the role of atmospheric temperature changes. However, the relative roles of other climate system components in the projected storm tracks shift have not been examined to date. Here it is shown that thermodynamic ocean coupling is responsible for the future poleward shift of the storm tracks as it overcomes the effect of dynamic ocean coupling to shift the storm tracks equatorward. These results stress the importance of using full-physics ocean models to investigate the future shift of the storm tracks, and of better monitoring ocean coupling processes to improve our preparedness for future climate changes.

Atmospheric waves control the weather and climate variability, by affecting winds, temperature, and precipitation. It is thus critical to assess their future response to anthropogenic emissions. Most previous studies investigated the projected regional changes in the intensity of atmospheric waves, by pooling across waves with different scales. However, the waves' projected changes might vary with their scale, and thus, their future climate impacts might also be scale dependent. Here we show that both in the tropics and midlatitudes while large waves will get stronger, small waves will get weaker by the end of this century. Thus, investigating the response of atmospheric waves to human activity by pooling across all wave scales masks the future climate impacts of large waves. We further reveal that the opposite response of large and small waves stems from the opposite effect of static stability and zonal wind on the growth rate of the different waves.

North Atlantic sea surface temperatures have large climate impacts affecting the weather of the Northern Hemisphere. In addition to a substantial warming over much of the North Atlantic, caused by increasing greenhouse gases over the 21st century, climate projections show a surprising region of considerable future cooling at midlatitudes, referred to as the North Atlantic warming hole. A similar pattern of surface temperature trends has been observed in recent decades, but it remains unclear whether this pattern is of anthropogenic origin or a simple manifestation of internal climate variability. Here, analyzing state-of-the-art climate models and observations, we show that the recent North Atlantic warming hole is of anthropogenic origin. Our analysis reveals that the anthropogenic signal has only recently emerged from the internal climate variability, and can be attributed to greenhouse gas emissions. We further show that a declining northward oceanic heat flux in recent decades, which is linked to this surface temperature pattern, is also of anthropogenic origin.

The Hadley circulation has large climate impacts at low latitudes by transferring heat and moisture between the tropics and subtropics. Climate projections show a robust weakening of the Northern Hemisphere Hadley circulation by the end of the twenty-first century. Over the past several decades, however, atmospheric reanalyses indicate a strengthening of the Hadley circulation. Here we show that the strengthening of the circulation in the Northern Hemisphere is not seen in climate models; instead, these models simulate a weakening of the circulation in the past 40 years. Using observations and a large ensemble of model simulations we elucidate this discrepancy between climate models and reanalyses, and show that it does not stem from internal climate variability or biases in climate models, but appears related to artefacts in the representation of latent heating in the reanalyses. Our results highlight the role of anthropogenic emissions in the recent slowdown of the atmospheric circulation, which is projected to continue in coming decades, and question the reliability of reanalyses for estimating trends in the Hadley circulation.