Recent increase in surface-water stability during warming off California as recorded in marine sediments

Warming of surface waters in the California Current since the 1950s has coi ncided with a significant decline in zooplankton volume. This has been attr ibuted to reduced upwelling of nutrient-rich waters caused by increased the rmal stratification across the thermocline. Proxy microfossil evidence pres erved 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 up-welled radiolarians in the basin has steadily d eclined since 1900, while oxygen isotopes in surface-dwelling planktonic fo raminifera reflect increasing surface temperatures. Comparison of the delta (18)O records between surface and thermocline-dwelling planktonic foraminif era 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 r esults. This evidence suggests that relaxation of North Pacific anticycloni c gyre circulation deepened isopycnics, causing onshore movement of warmer, less saline waters and reduced upwelling of cool, nutrient-rich waters.