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The cross-equatorial southwesterly winds from the eastern equatorial Pacific direct moisture toward the Pacific coast of northwestern South America, where subsequent orographic lifting creates the wettest regions in the world. The Choco low-level jet is emblematic of broader westerly winds in this region and is projected to weaken by the end of the 21st century, but climate models show considerable disagreement about the extent of weakening. Using contemporary observations, we demonstrate that the configuration of westerly winds in the eastern equatorial Pacific is reflected by hydrogen isotopes in precipitation (δDp) in western Ecuador. As westerly winds strengthen, δDp increases from greater transport of δDvapor enriched in deuterium from the Eastern Pacific Warm Pool. We apply this framework to a new record of reconstructed δDp using leaf waxes in ocean sediments off the coast of Ecuador (ODP1239, 0◦40.32′ S, 82◦4.86′ W) that span the Plio-Pleistocene. Low δDp in the early Pliocene indicates weak westerly water vapor transport in a warmer climate state, which is attributed to a low sea surface temperature gradient between the cold tongue and off-equatorial regions in the eastern equatorial Pacific. Near 3 Ma, westerly water vapor transport weakens, possibly as a result of shifts in the Intertropical Convergence Zone forced by high latitude Northern Hemisphere cooling. In complementary isotope-enabled climate simulations, a weak Choco jet and westerly water vapor transport in the early Pliocene are matched by a decrease in δDp and hydroclimate changes in western Ecuador. Precipitation from the Choco jet can cause deadly landslides and weakened westerly winds in the early Pliocene implies a southward shift of these hazards along the Pacific coast of northwestern South America in the future.