Fig. 6: Mechanism favoring a La Niña-like condition from a general circulation perspective.

Left: On average, the strongest convection/rainfall occurs over Indonesia/Maritime Continent. The divergent circulation, resulting from the convection, advects absolute vorticity poleward. This leads to the generation of planetary-scale Rossby waves in the subtropics²⁰⁰ that can propagate poleward and then upward once they reach mid- and high-latitudes. As these waves propagate upward, they transport heat poleward, which warms the Arctic. Right: Paleo-climate data suggests that when the global climate was warmer than today’s climate, the Arctic was considerably warmer while the tropics were about the same temperature as today’s climate. The higher Arctic temperature implies that longwave cooling to space must have been greater as well. To maintain the warmth against the increased longwave cooling, more heat needs to be transported from lower latitudes. Amongst the various atmospheric processes, the most probable candidate that can generate a stronger poleward heat transport in the face of a weakened meridional temperature gradient is a more enhanced tropical heating/convection around the Maritime continent, which is suggestive of a more La Niña-like pattern in convective heating and SST.