Some climate variables do not show the same response to declining atmospheric CO2 concentrations as before the preceding increase. A comprehensive understanding of this hysteresis effect and its regional patterns is, however, lacking. Here we use an Earth system model with an idealized CO2 removal scenario to show that surface temperature and precipitation exhibit globally widespread irreversible changes over a timespan of centuries. To explore the climate hysteresis and reversibility on a regional scale, we develop a quantification method that visualizes their spatial patterns. Our experiments project that 89% and 58% of the global area experiences irreversible changes in surface temperature and precipitation, respectively. Strong irreversible response of surface temperature is found in the Southern Ocean, Arctic and North Atlantic Ocean and of precipitation in the tropical Pacific, global monsoon regions and the Himalayas. These global hotspots of irreversible changes can indicate elevated risks of negative impacts on developing countries.

Paleo-proxy records suggest that the Atlantic Meridional Overturning Circulation (AMOC) exhibits a threshold for an abrupt change, a so-called tipping point. A classical bifurcation theory, a basis of the tipping dynamics of AMOC implicitly assumes that the tipping point is fixed. However, when a system is subjected to time-varying forcing (e.g., AMOC exposed to ice meltwater) an actual tipping point can be overshot due to delayed tipping, referred to as the slow passage effect. Here, using an Earth system model of intermediate complexity and a low-order model with freshwater forcing, we show that the tipping point of AMOC is largely delayed by the slow passage effect. It causes a large tipping lag of up to 1300 years, and strongly relaxes the abruptness of tipping as well. We further demonstrate that the tipping modulation can actively occur in past, present, and future climates by quantifying the effect during Dansgaard-Oeschger events, meltwater pulse 1A (MWP-1A), and current Greenland ice sheet melting. The suggested slow passage effect may explain the observed lagged AMOC collapse to MWP-1A of about 1000 years and provides implications tipping risk in the future.

Accurate representation of the Atlantic Meridional Overturning Circulation (AMOC) in global climate models is crucial for reliable future climate predictions and projections. In this study, we used 42 coupled atmosphere–ocean global climate models to analyze low-frequency variability of the AMOC driven by the North Atlantic Oscillation (NAO). Our results showed that the influence of the simulated NAO on the AMOC differs significantly between the models. We showed that the large intermodel diversity originates from the diverse oceanic mean state, especially over the subpolar North Atlantic (SPNA), where deep water formation of the AMOC occurs. For some models, the climatological sea ice extent covers a wide area of the SPNA and restrains efficient air–sea interactions, making the AMOC less sensitive to the NAO. In the models without the sea-ice-covered SPNA, the upper-ocean mean stratification critically affects the relationship between the NAO and AMOC by regulating the AMOC sensitivity to surface buoyancy forcing. Our results pinpoint the oceanic mean state as an aspect of climate model simulations that must be improved for an accurate understanding of the AMOC.

It is known that winter Atlantic-Niño events can induce the El Niño–Southern oscillation (ENSO) in the following winter with a lag of 1-year during one period. On the other hand, summer Atlantic-Niño events can lead to the ENSO in the subsequent winter with a half-year lag during another period. In this study, we investigate the distinct interdecadal modulation of the effect of the Atlantic-Niños on ENSO by analyzing observational reanalysis datasets. During the mid-twentieth century, the winter Atlantic-Niño exhibited increased intensity and extended westward due to warmer conditions in the tropical western Atlantic. As a result, convection occurred from the Amazon to the Atlantic, triggering an atmospheric teleconnection that led to trade wind discharging and equatorial Kelvin waves, ultimately contributing to the development of ENSO. In contrast, during late twentieth century, summer Atlantic-Niño events were closely linked to the South America low-level jet in boreal spring. This connection led to the formation of widespread and intense convection over the Amazon to the Atlantic region. Then, the Walker circulation was effectively modulated, subsequently triggering ENSO events. Further analysis revealed that the interdecadal modulation of the Atlantic–South America–Pacific mean state plays a crucial role in shaping the impact of Atlantic-Niños on ENSO by modifying not only the characteristics of the Atlantic-Niños but also ocean–atmospheric feedback process. Therefore, improving our understanding of the interdecadal modulation of the climatological mean state over the Pacific to Atlantic regions enables better anticipation of the interaction between the Atlantic and Pacific Oceans.

Understanding the inter-basin interactions between the Atlantic and Pacific Oceans is of great concern due to their substantial global climatic implications. By analyzing observational reanalysis datasets (1948–2020), we found that there are two regimes in Atlantic–Pacific inter-basin interactions: (1) the Pacific-driven regime, and (2) the Atlantic-driven regime. In the Pacific-driven regime before the mid-1980s, the El Niño–Southern Oscillation (ENSO) in winter effectively drives the primary mode of sea surface temperature anomaly (SSTA) in the tropical Atlantic (i.e., North Tropical Atlantic (NTA) mode) in the following spring. The NTA mode has a meridional contrast of SSTA along the Atlantic Intertropical Convergence Zone, similar to the Atlantic Meridional Mode. Whereas, in the Atlantic-driven regime after the mid-1980s, the ENSO effect on the NTA becomes remarkably weaker, so that the NTA mode is featured with a SSTA monopole. Notably, the NTA mode without the meridional contrast of SSTA is capable of modulating an ENSO event with a lag. Our analyses of the latest climate models participating in the Coupled Model Intercomparison Project (CMIP) phases 6 support the hypothesis that the two regimes engendered by the Atlantic–Pacific inter-basin interactions are likely due to natural variability.