Search

The North Atlantic storm track plays a critical role in setting the regional weather and climate over Western Europe. In response to anthropogenic emissions, the summer North Atlantic storm track has weakened in recent decades and is projected to continue weakening by the end of this century. In light of growing efforts to mitigate climate change, it is crucial to assess how reversible the CO2‐induced storm track weakening is. Here, I show that under CO2 removal scenarios, the recovery timescale of the storm track is more than double its weakening period. It is found that due to a prolonged high‐latitude warming, postponing the execution of mitigation policies beyond doubling of CO2 concentrations would lead to a centennial‐scale recovery of the storms. Given the impacts of weakening summer storms on weather and extreme events, the delayed recovery of the storms might have broader regional consequences for Western Europe. Plain Language Summary: The summer North Atlantic storm track sets the regional weather and climate over Europe, by transferring heat, moisture and momentum across the North Atlantic basin. Anthropogenic emissions were found to weaken the storm track, which would greatly affect precipitation and hot‐dry extreme events over Europe. It is thus critical to assess the reversibility of the summer North Atlantic storm track weakening under a reduction in greenhouse gases. Here, using targeted numerical simulations, I show that the reversibility time‐scale of the storm track is more than double its weakening time‐scale. Thus, postponing the execution of mitigation strategies increases the reversibility time‐scales of the storm track. Specifically, avoiding centennial‐scale impacts of the weakening storm track can be achieved by executing mitigation strategies prior of reaching doubling of CO2 concentrations. Our analysis reveals that the prolonged recovery of the storm track stems from a prolonged warming of the high latitudes relative to the lower latitudes. Key Points: The reversibility time‐scale of the North Atlantic summer storm track is more than double its weakening time‐scalePostponing mitigation policies beyond a doubling of CO2 levels yields centennial‐scale recovery of the storm trackThe prolonged recovery of the storm track stems from a prolonged warming of the high latitudes relative to the lower latitudes [ABSTRACT FROM AUTHOR]

The projected weakening of the Northern Hemisphere Hadley cell will have large climatic impacts at low latitudes. Several mechanisms have been proposed to explain this weakening. In order to isolate and assess their relative importance, we here use the abrupt 4 × CO2 experiment of the Coupled Model Intercomparison Project phase 5, as this forcing separates the different mechanisms which respond on different time scales. We find that the Hadley circulation responds relatively quickly to quadrupling CO2 concentrations, reaching its steady‐state value after less than a decade. This fast response demonstrates that the weakening could not be solely due to the much slower increase in surface temperature. In addition, we show that the Hadley cell's weakening results from a combination of an increase in tropical static stability, partially offset by an increase in the latitudinal gradient of latent heating. Plain Language Summary: The Hadley circulation has large effects on the variability of temperature and precipitation at low latitudes. By the end of the 21st century, climate models project a weakening of the circulation, which will have great societal impacts. It is thus critical to elucidate the underlying mechanisms of the projected weakening. Here, we assess different mechanisms for the projected weakening and show that it stems from an increase in the vertical temperature gradient in the tropics, which is partially offset by an increase in latent heating. Key Points: Surface warming alone is unable to explain Hadley cell weakening under 4 × CO2 forcing, as the weakening exhibits a much faster responseEach of the previously proposed mechanisms analyzed in this study cannot fully explain the weakening of the Hadley cellThe weakening stems from a combination of the opposing effects of increases in stratification and in meridional gradient of latent heating [ABSTRACT FROM AUTHOR]

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., (© 2024. The Author(s).)

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., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

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., (© 2022. The Author(s).)

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.