Winter Insolation Modulates Boreal Tropical Monsoonal Temperatures in the Late Pleistocene

During past glacial-interglacial cycles, the boreal summer insolation is the most crucial external forcing for climate change. However, the question of whether summer insolation is a key forcing on temperatures in the boreal tropics remains under debate, hampering our understanding of climate change at low latitudes. To shed further light on this issue, we performed a series of equilibrium simulations with the NorESM-L model over the past 425 ka. Our simulations show that in the boreal tropical monsoon region, the simulated annual temperature is anti-phased towards the boreal summer insolation. This antiphase relation is also supported by some available geological data. Additional diagnostic analyses reveal that the tropical warmth throughout the year is more reliant on winter temperatures than on summer temperatures. This stands in contrast to the situation in middle to high latitudes, particularly in the Mediterranean region. Further correlation analysis and spectrum analysis suggest that the annual temperature in boreal tropics is highly linked to local winter insolation. Our results highlight complex hydrothermal configurations in the boreal tropics, suggesting a decoupling of temperature and precipitation. Specifically, variations in annual temperature and precipitation in the boreal tropics are driven by distinct patterns of seasonal insolation. We deduce that the unique hydrothermal configurations in North Africa may have influenced the dispersal of early humans out of Africa. On the orbital timescale, whether the temperature in boreal tropics is driven by boreal summer insolation still remains unclear. In this study, we conduct a comprehensive analysis of simulated results spanning the 425 ka. Our findings indicate that the annual temperature in boreal tropics is anti-phased to the local summer insolation. Specifically, when local summer insolation is high (low), overall cooling (warming) occurs in the North African and South Asian monsoon regions. Some available paleo-temperature records from the tropics also provide partial support for our simulation results. The temperature pattern in the boreal tropics is linked to the strong internal feedback in the summer from the unique climate background of the tropics, which results in relatively slight summer cooling at high boreal summer insolation, while the annual temperature is largely dependent on winter temperatures driven by local winter insolation. Our simulations reveal highly complex hydrothermal configurations in the boreal tropics on the orbital timescale, suggesting a decoupling of temperature and precipitation, with the former influenced by winter insolation and the latter forced by summer insolation. The temperature in boreal tropical monsoon regions is anti-phased to the boreal summer insolation The annual temperature in boreal tropics is more dependent on the winter than summer temperatures The key role of local winter insolation in regulating the boreal tropical temperature is closely linked to its unique climatic background