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The Pacific Ocean plays a crucial role in the global climate system, with a strong east-west gradient in sea surface temperatures (SSTs) and thermocline depth in the tropical Pacific. The West Pacific Warm Pool (WPWP) is the warmest water body on Earth, contributing to the heating of the tropical atmosphere. Research suggests that during the Miocene Climate Optimum (MCO), around 15.5-13.3 million years ago, there was still a temperature gradient between the western and eastern Pacific, with a gradient of ~4°C. This gradient remained constant throughout an interval of ice sheet expansion, indicating that oceanographic processes producing the modern gradient were present and active. A 2°C sea surface cooling in the eastern equatorial Pacific was found to lag the benthic foraminiferal δ18O positive shift by 150 kyr, suggesting that tropical temperature did not decrease synchronously with the expansion of the Antarctic ice sheet. Reconstructed variations in the δ18O composition of seawater revealed a freshening in the eastern Pacific Ocean after 13.8 Ma, indicating changes in the hydrological cycle and tropical fronts in response to the new icehouse regime.