Marine Isotope Stage (MIS) 13, ~533e478 ka, has received particular attention due to the unexpected
enhancement of monsoon systems under a cool climate characterized by lower atmospheric CO2 and
larger ice volume than many other interglacials. Key questions remain about its regional expression
(intensity, climate variability, length), and underlying forcing factors, in particular at the mid-latitudes.
Here we examine the SW Iberian vegetation, terrestrial climate and sea surface temperature (SST)
variability during MIS 13 by combining pollen and biomarker data from IODP Site U1385 with climatemodel
experiments. We show, for the first time, that despite strong precessional forcing, MIS 13 stands
out for its large forest expansions with a reduced Mediterranean character alternating with muted forest
contractions, indicating that this stage is marked by a cool-temperate climate regime with high levels of
humidity. Results of our data-model comparison reveal that MIS 13 orbitally driven SW Iberian climate
and vegetation changes are modulated by the relatively strong ice-sheet forcing. We find that the
Northern Hemisphere ice-sheets prescribed at the MIS 13 climate optimum reinforce the insolation effect
by increasing the tree fraction and both winter and summer precipitation. We propose that the interactions
between ice-sheets and major atmospheric circulation systems may have resulted in the
persistent influence of the mid-latitude cells over the SW Iberian region, which led to intensified
moisture availability and reduced seasonality, and, in turn, to a pronounced expansion of the temperate
forest