Earth’s hydrological cycle is expected to intensify in response to global warming, with a ‘wet-gets-wetter, dry-gets-drier’ response anticipated. The subtropics (~15-30°N/S) are predicted to become drier, yet proxy evidence from past warm climates suggests these regions may be characterised by wetter conditions. Here we use an integrated data-modelling approach to reconstruct global- and regional-scale rainfall patterns during the early Eocene (~48-56 million years ago), with an emphasis on the subtropics. Model-derived precipitation–evaporation (P–E) estimates in the tropics (0-15° N/S) and high latitudes (>60° N/S) are positive and increase in response to higher temperatures, whereas model-derived P–E estimates in the subtropics (15-30° N/S) are negative and decrease in response to higher temperatures. This is consistent with a ‘wet-gets-wetter, dry-gets-drier’ response. However, some DeepMIP model simulations predict increasing – rather than decreasing – subtropical precipitation at higher temperatures (e.g., CESM, GFDL). Using moisture budget diagnostics we find that the models with higher subtropical precipitation are characterised by a reduction in the strength of subtropical moisture circulation due to weaker meridional temperature gradients. These model simulations (e.g., CESM, GFDL) agree more closely with various proxy-derived climate metrics and imply a reduction in the strength of subtropical moisture circulation during the early Eocene. Although this was insufficient to induce subtropical wetting, if the meridional temperature was weaker than suggested by the DeepMIP models, this may have led to wetter subtropics. This highlights the important role of the meridional temperature gradient when predicting past (and future) rainfall patterns.

Subtropical anticyclones and midlatitude storm tracks are key components of the large-scale atmospheric circulation. Focusing on the southern hemisphere, the seasonality of the three dominant subtropical anticyclones, situated over the South Pacific, South Atlantic and South Indian Ocean basins, has a large influence on local weather and climate within South America, Southern Africa and Australasia, respectively. Generally speaking, sea level pressure within the southern hemisphere subtropics reaches its seasonal maximum during the winter season when the southern hemisphere Hadley Cell is at its strongest. One exception to this is the seasonal evolution of the South Pacific subtropical anticyclone. While winter maxima are seen in the South Atlantic and South Indian subtropical anticyclones, the South Pacific subtropical anticyclone reaches its seasonal maximum during local spring with elevated values extending into summer. In this study we investigate the hypothesis that strength of the austral summer South Pacific subtropical anticyclone is largely due to heating over the South Pacific Convergence Zone. Using reanalysis data, and AGCM added cooling and heating experiments to artificially change the strength of diabatic heating over the South Pacific Convergence Zone, our results show that increased heating triggers a Rossby wave train over the Southern Hemisphere mid-latitudes by increasing upper-level divergence. The propagating Rossby wave train creates a high-low sea level pressure pattern that projects onto the center of the South Pacific Subtropical Anticyclone to intensify its area and strength. The southern hemisphere storm tracks also shift poleward due to increased heating over the South Pacific Convergence Zone.