Warming of surface waters in the California Current since the 1950s has coincided with a significant decline in zooplankton volume. This has been attributed to reduced upwelling of nutrient-rich waters caused by increased thermal stratification across the thermocline. Proxy microfossil evidence preserved in the Santa Barbara Basin suggests that stability increased early in the 1900s, intensified after the early 1940s, and became well established by 1960. Accumulation of upwelled radiolarians in the basin has steadily declined since 1900, while oxygen isotopes in surface-dwelling planktonic foraminifera reflect increasing surface temperatures. Comparison of the δ18O records between surface and thermocline-dwelling planktonic foraminifera reveals that the temperature difference between surface and thermocline water has increased during the twentieth century. Instrumental records of surface and thermocline temperatures, monitored since 1950, support these results. This evidence suggests that relaxation of North Pacific anticyclonic gyre circulation deepened isopycnics, causing onshore movement of warmer, less saline waters and reduced upwelling of cool, nutrient-rich waters.