NOAA/WDS Paleoclimatology - Wenman Island, Eastern Equatorial Pacific Coral Sr/Ca, Stable Isotope and SST Data 1964-1998 CE

Coral reconstructed surface water D14C at 91.8°W, 1.4°N from Wenman Island, in the northern transition zone of the eastern equatorial Pacific cold tongue, exhibits seasonal and interannual variability in the late 20th century. During moderate to strong El Niño events (=1° Niño3 sea surface temperature, SST, anomaly) the D14C seasonal cycle is absent or significantly dampened even though there is still a local SST seasonal cycle (e.g., 1969 and 1972). This observation implies shallower upwelling and entrainment of higher D14C off-equatorial subtropical water and possibly a compression of upwelling to closer to the equator.
A two-endmember D14C mixing model was used to elucidate seasonal mixing and entrainment, i.e., water mass transformation, between waters upwelled at the equator and off equatorial waters incorporated in the northern transition zone. Such transformation strongly factors in meridional heat transport and downwelling which maintains the equatorial cold tongue in the presence of a large surface heat flux. Dilution of upwelled waters via entrainment and diapycnal mixing was estimated using Urvina Bay (0.4°S, 91.25°W) upwelling season D14C as the upwelling endmember and Wenman’s non-upwelling season D14C as that which it is mixed into. We estimate the mixing of upwelled was 31±2% (weighted error of the mean, n=11), i.e., a dilution factor of upwelled waters of ~3/7 for the period 1970-1981. Although there is a visual correspondence with the Niño3 sea surface temperature anomaly, precision of the mixing model is insufficient for direct year to year comparisons.