Pronounced northward shift of the westerlies during MIS 17 leading to the strong 100-kyr ice age cycles

•Strongest expansion of the Mediterranean forest of the last 800,000 yr.•Maximum summer warmth in SW Iberia during the peak of the cool MIS 17.•Pronounced two-step northward shift of the westerlies centered at ∼693 ka.•Increasing arrival of moisture at northern high latitudes at the end of MIS 17.•Important contribution of the westerlies in explaining the strong 100-kyr ice cycles.