Cross-equatorial heat transports and the monsoons

Monsoons, the lifeline for billions of people, result from the cross-equatorial Hadley circulation (HC), which imports moisture into monsoon regions and exports heat. The inter-annual relationships between cross-equatorial heat transports and monsoons are largely unexplored, which is vital for water resources and climate stability. Using reanalysis data, it is revealed that increased cross-equatorial atmospheric heat transport (CE-AHT) during JJA weakens the Indian, West African, and North American monsoons by contracting the HC, shifting the subtropical jets and Inter-tropical Convergence Zone (ITCZ) equatorward, weakening the Walker circulation, and inducing El Nino- ˜ like conditions. During DJF, the Australian, South African, and South American monsoons strengthen as the HC expands, the ITCZ and subtropical jets move poleward, strengthening the Walker circulation, and promoting La Nina-like conditions. Global monsoon precipi- ˜ tation increases in both JJA and DJF. Conversely, increased cross-equatorial oceanic heat transport (CE-OHT) in JJA and DJF elicits effects opposite to those of CE-AHT. A few CMIP6 models simulate the historical trends observed in reanalysis for CE-AHT and CE-OHT. However, the relationship between CE-AHT, CE-OHT, and monsoons in most models mirrors reanalysis patterns. Under the shared socioeconomic pathways 5-8.5 scenario, most CMIP6 models exhibit upward CE-AHT and downward CE-OHT trends in JJA, but these trends are downward and insignificant for DJF. In JJA, all models indicate increases in Indian monsoon precipitation for 2080–2100 compared to 1995–2014, while disagreement arises regarding the West African and North American monsoons. In multi-model space, greater CE-AHT weakens Indian monsoon rainfall, while greater CE-OHT weakens global monsoon rainfall. In DJF, models differ on Australian, South African, and South American monsoon precipitation changes. This research suggests that changes in CE-AHT and CE-OHT are deeply intertwined with monsoons, and future warming could alter monsoon dynamics and heighten uncertainty in climate modelling and predictions.