Abstract
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.