Radiocarbon as a thermocline proxy for the eastern equatorial Pacific

An ocean model is used to test the idea that sea surface Δ 14 C behaves as a thermocline proxy in the eastern equatorial Pacific. The ORCA2 model, which includes Δ 14 C as a passive tracer, has been forced with reanalysis fluxes over 1948–1999, and the output is compared with a previously reported Galapagos Δ 14 C record. The model reproduces the abrupt increase in the seasonally minimum Δ 14 C in 1976/77 found in the data. This increase is associated with neither a shift of thermocline depth over the NINO3 region, nor a change in the relative proportion of Northern/Southern source waters. Rather, it is due to a decrease in the Sub‐Antarctic Mode Water (SAMW) component of the upwelling water, thereby representing a decrease in entrainment of water from below the base of the directly ventilated thermocline.