Middle to late Miocene climate and Atlantic-Mediterranean gateway changes. (a) Approximate time trend of the gateway width and depth throughout the late Miocene including the combined dimensions of existing gateways for a given time (reconstructed linearly from palaeogeographic constraints, Fig. 2). (b) Benthic δ18O composite3; PDB, PeeDee Belemnite. (c) Sea surface temperatures for Northern Hemisphere high- and mid-latitudes, Southern Hemisphere mid-latitudes, and Tropics1. Grey shaded area represents the duration of late Miocene surface water cooling1. (d) Velocity of Mediterranean Overflow Water (MOW) with varying dimensions of the Atlantic-Mediterranean gateways (a) computed with rotational- and hydraulic-control theories53. Bold lines indicate the preferred theory to compute velocity for a given state of the gateway: rotational control theory is more accurate for larger gateways, whereas hydraulic control becomes preferable once the two lines intersect. (e) Proportion of the water from the upper layers of the North Atlantic gyre (NACW) that is entrained into the MOW-generated plume, based on the calculations and references shown in Supplementary Table 1 (Supplementary Material). Evidence of strengthening arctic overflows are based on refs60,61. Pockets of glacial activity in Greenland are from refs62,63.