Testing Latitudinally Dependent Explanations of the Circulation Response to Increased CO2 Using Aquaplanet Models

The atmospheric circulation exhibits robust responses to increased CO₂ that emerge across the climate model hierarchy. Existing theoretical explanations of the circulation response can be grouped according to latitude. Here we test latitudinally dependent explanations of the circulation response to increased CO₂ using slab ocean aquaplanet models with latitudinally dependent CO₂ concentration. Quadrupling CO₂ in the tropics (0–20°) accounts for the strengthening and upward shift of the subtropical jet but does not account for the poleward shift of the Hadley cell edge or extratropical circulation. The tropical response is dominated by regions of descent. When CO₂ is quadrupled in high latitudes (60–90°), there is a negligible circulation response. The response to latitudinally dependent increased CO₂ is mostly linear and increased CO₂ in the midlatitudes (20–60°) dominates. Within the midlatitudes, the subtropics (20–40°) dominate. Thus, story lines explaining the circulation shift in response to increased CO₂ should focus on the thermodynamic response in the subtropics.