Wing-Le Chan

The Atlantic Meridional Overturning Circulation (AMOC) transports heat globally, and its potential weakening would be crucial for future climate projections. Valuable insights into climatic impacts of AMOC slowdowns can be gained from the last glacial period, when millennial-scale AMOC reductions recurrently triggered large-scale reorganizations of the atmosphere-ocean system. However, hydroclimatic responses within the Southern Hemisphere Westerly Wind (SWW) belt—a key region for the global carbon cycle—remain poorly constrained. Here we present a marine sediment record from ~150 km offshore Chile in the eastern South Pacific that documents millennial-scale increases in detrital discharge from the western Patagonian Ice Sheet, temporally associated with AMOC slowdowns. These discharge events likely resulted from ice-sheet expansion and the associated marginal melt linked to gradually enhanced orographic precipitation under SWW and temperature rise. Our simulations with a coupled atmosphere-ocean general circulation model support enhanced precipitation along the Chilean margin south of 45°S and warming-induced melting due to millennial-scale AMOC slowdowns. The precipitation increase reflects the progressive southward intensification of the SWW and the enhanced sea-surface evaporation resulting from Southern Ocean warming through the thermal bipolar seesaw. Together, proxy and modeling results demonstrate that abrupt AMOC slowdowns drove coupled atmospheric-oceanic responses in the South Pacific that promoted Patagonian hydroclimate and ice-sheet changes and may have amplified wind-driven ventilation of the Southern Ocean. These findings highlight the tight coupling between interhemispheric atmospheric and oceanic circulation, cryosphere dynamics, and the global carbon cycle, and raise the possibility of future abrupt shifts in far-field hydroclimate and atmospheric CO ₂ .

Orbital forcing is a primary driver of paleoclimate change, but its influence on deep time climates, particularly in the polar regions of the Late Cretaceous, is poorly understood. Here, we conduct climate simulations with an atmosphere‐ocean‐vegetation coupled model, MIROC4mV, to investigate the response of northern high latitude temperature using Maastrichtian geography. By accounting for the full range of orbital parameters, the model successfully reproduces proxy‐based estimates of polar temperatures. Our experiments show that the sensitivity of northern high latitude surface temperature to orbital forcing is higher with Maastrichtian geography than with Modern. In particular, Maastrichtian geography greatly amplifies obliquity‐driven fluctuations due to the large continental landmass at northern high latitudes and the absence of ice sheets. These findings highlight the importance of considering the full range of orbital sensitivity when interpreting geological records from deep‐time periods.

The Arctic during the Last Interglacial period (LIG) was considered warmer than it is today. The previous study points to a large difference in the degree of simulated annual-mean Arctic warming among models. While recent reconstructions suggest the disappearance of summer sea ice in the Arctic at the LIG, many climate models fail to capture this feature. It is thus essential to investigate sources of uncertainty in climate models. The current study examines the impact of the temperature-cloud phase relationship. Sensitivity studies are conducted for the first time to explore the potential importance of this relationship in simulating the LIG climate. Two different cloud parameter sets are used for an atmosphere-ocean general circulation model with and without the dynamic vegetation feedback. The model with cloud parametrization that permits liquid water at lower temperatures and a larger fraction of supercooled liquid water at the same temperature simulates a warmer preindustrial (PI) climate, greater annual-mean Arctic warming at the LIG, and substantially reduced summer sea ice cover at the LIG. It is demonstrated that the low-level clouds play a crucial role in controlling the Arctic response via the greenhouse effect. The result indicates the importance of the temperature-cloud phase relationship in simulating the Arctic climate at the LIG. It also highlights the importance of accurately simulating modern sea ice thickness and representing the processes that affect the fraction of supercooled liquid water in clouds.

Tides are the main energy source for diapycnal mixing in the ocean interior. However, energy-constrained tidal mixing parameterizations are not routinely included in ocean models applied to the deep-time past of the Earth. Instead, diapycnal mixing is usually parameterised by a constant vertical diffusivity or a prescribed vertical profile of vertical diffusivity.Here, by leveraging outputs from the DeepMIP project, we compute the power effectively consumed by parameterized diapycnal mixing in each DeepMIP model and for different CO2 concentrations. We show that this power slightly increases with increasing CO2 in simulations integrated to quasi-equilibrium but skyrockets in warming, out-of-equilibrium, simulations. This reflects the increased stratification in a warming ocean, even though in principle the same amount of tidal energy is available for mixing. We find no evident relationships between the intensity of the overturning circulation and the power consumed by diapycnal mixing across the DeepMIP models. Finally, we use coupled climate-biogeochemistry simulations performed with the IPSL-CM5A2 model to show that the marine biogeochemistry is largely impacted by the vertical mixing scheme employed, even if the total power consumed by diapycnal mixing remains similar.

The Pliocene is thought to have been warmer than present-day and has been the focus of many studies as a possible analogue of near-future warming. In the Pliocene, there were less tundra and more forests in the Arctic, the Sahara Desert was smaller than in present-day, and there were more grassland and forests elsewhere. Studies with multiple climate models in PlioMIP2 have indicated that, factors other than atmospheric CO2, such as paleogeography, vegetation, and ice sheets, also play an important role in shaping the Pliocene climate. In many modelling studies, vegetation, which can change in response to the other forcings, is fixed. In this study, in order to investigate the role of CO2 and paleogeography including the effect of vegetation feedback, we conducted experiments using an atmosphere-ocean-vegetation coupled model, MIROC4m-AOV. We set CO2 concentration and paleogeography to those of pre-industrial and of the Pliocene as prescribed by the PlioMIP3 protocol. We also conducted experiments in which conditions at northern and southern high latitudes are set to Pliocene separately. While Pliocene CO2 concentration contributes to a globally warmer climate than pre-industrial, paleogeography has a large effect, both seasonally and locally. With Pliocene paleogeography, the continents at northern high latitudes tend to be warmer in summer and colder in winter. The summer warming in these regions causes a reduction in tundra and an increase in forests, further enhancing the warming. The Pliocene paleogeography of the northern high latitudes also enhances the precipitation in North Africa via the summer monsoon.

The early Eocene represents one of the warmest periods in Earth’s history, with atmospheric CO₂ concentrations and global temperatures far higher than today. Studying this period offers a useful way to explore how monsoon systems behave under extreme greenhouse conditions. However, the markedly different paleogeography, including altered land–sea distributions and the absence of the Himalayas, makes direct comparison with the modern monsoon challenging. Here, we examine the behavior of low-level monsoonal circulation over the Indian Ocean during the early Eocene using five climate model simulations from the Deep-time Model Intercomparison Project (DeepMIP). All simulations show a coherent monsoon-like circulation, indicating that a proto-monsoon system existed during this warm climate state. We further identify low-level jet structures aligned with paleotopographic features over the Eastern African Rift and the Deccan Plateau, which we refer to as the Proto-LLJ. Despite enhanced land–sea temperature contrasts under elevated CO₂, the strength of the Proto-LLJ weakens across the simulations. This contrasts with present-day behavior, where a stronger land–sea contrast is often linked to intensified or poleward-shifted monsoon jets. Our results indicate that CO₂-driven warming leads to increased tropical atmospheric stability, reduced vertical temperature gradients, and weaker convective overturning. As a result, the vertical motion needed to sustain strong low-level monsoon winds is suppressed. These findings suggest that in very warm climates, increased atmospheric stability can outweigh thermal forcing and lead to weaker monsoonal circulation, highlighting a key control on paleo-monsoon dynamics under extreme greenhouse conditions.

Simulations of the Eocene climate using state‐of‐the‐art Earth system models provide a reference state for the future climate, as the Eocene was one of the warmest geological epoch with high atmospheric carbon dioxide (CO ₂ ) concentration and global temperatures comparable to projections for the coming centuries. However, the paleogeographic configuration of the Eocene impact's distinct climate features. Here, we decompose the response of low‐level monsoon dynamics over the Indian Ocean to the early Eocene hothouse using five climate model simulations from the Deep‐time Model Intercomparison Project (DeepMIP). We see a circulation pattern resembling the paleo‐monsoon across all models over the Indian Ocean. Surprisingly, we find low‐level jet (LLJ) forming along the topographic barriers of the Eastern African Rift and the Deccan Plateau, which we refer to as the “Proto‐LLJ.” Based on the analysis of the DeepMIP results, we find a reduction in the Proto‐LLJ strength with elevated CO ₂ . Under present‐day conditions, the northward shift of monsoonal LLJ is attributable to the increased land‐sea contrast under global warming. Even though land‐ocean temperature contrasts increased during the Eocene hothouse, Proto‐LLJ weakened due to tropical atmospheric stabilization. This stabilization reduced vertical temperature gradients, suppressed convection, and weakened atmospheric overturning, limiting the upward motion needed to drive strong monsoonal winds under CO ₂ ‐induced warming.

The Arctic during the last interglacial period (LIG) was considered warmer than today. While a recent proxy-based study suggests the disappearance of summer sea ice in the Arctic at the LIG, many climate models fail to capture this feature. It is thus essential to investigate sources of uncertainty in numerical models. The current study examines the impact of the temperature-cloud phase relationship. Sensitivity studies are conducted for the first time to explore the potential importance of this relationship in simulating the LIG climate. Two different cloud parameter sets are used for an atmosphere-ocean general circulation model with and without the dynamic vegetation feedback. The model with cloud parametrization permitting liquid water at a lower temperature and a larger fraction of supercooled liquid water at the same temperature simulates a warmer preindustrial (PI) climate, larger annual mean Arctic warming at the LIG, and substantially reduced sea ice cover during summer at the LIG. It is demonstrated that the low-level clouds play a crucial role in controlling the Arctic response via the greenhouse effect. The result indicates the importance of the temperature-cloud phase relationship in simulating the Arctic climate at the LIG. It also highlights the importance of accurately simulating modern sea ice thickness and representing the processes that affect the fraction of supercooled liquid water in clouds.

The Arctic during the last interglacial period (LIG) was considered warmer than today. While a recent proxy-based study suggests the disappearance of summer sea ice in the Arctic at the LIG, many climate models fail to capture this feature. It is thus essential to investigate sources of uncertainty in numerical models. The current study examines the impact of the temperature-cloud phase relationship. Sensitivity studies are conducted for the first time to explore the potential importance of this relationship in simulating the LIG climate. Two different cloud parameter sets are used for an atmosphere-ocean general circulation model with and without the dynamic vegetation feedback. The model with cloud parametrization permitting liquid water at a lower temperature and a larger fraction of supercooled liquid water at the same temperature simulates a warmer preindustrial (PI) climate, larger annual mean Arctic warming at the LIG, and substantially reduced sea ice cover during summer at the LIG. It is demonstrated that the low-level clouds play a crucial role in controlling the Arctic response via the greenhouse effect. The result indicates the importance of the temperature-cloud phase relationship in simulating the Arctic climate at the LIG. It also highlights the importance of accurately simulating modern sea ice thickness and representing the processes that affect the fraction of supercooled liquid water in clouds.