Observed El Nino conditions in the eastern tropical Pacific in October 2015

A strong El Nino developed in early 2015. Measurements from a research cruise on the R/V Sonne in October 2015 near the Equator east of the Galapagos Islands and off the shelf of Peru are used to investigate changes related to El Nino in the upper ocean in comparison with earlier cruises in this region. At the Equator at 85 degrees 30' W, a clear temperature increase leading to lower densities in the upper 350 m had developed in October 2015, despite a concurrent salinity increase from 40 to 350 m. Lower nutrient concentrations were also present in the upper 200 m, and higher oxygen concentrations were observed between 40 and 130 m. In the equatorial current field, the Equatorial Undercurrent (EUC) east of the Galapagos Islands almost disappeared in October 2015, with a transport of only 0.02 Sv in the equatorial channel between 1 degrees S and 1 degrees N, and a weak current band of 0.78 Sv located between 1 and 2 degrees 30' S. Such near-disappearances of the EUC in the eastern Pacific seem to occur only during strong El Nino events. Off the Peruvian shelf at similar to 9 degrees S, characteristics of upwelling were different as warm, saline, and oxygen-rich water was upwelled. At similar to 12, similar to 14, and similar to 16 degrees S, the upwelling of cold, low-salinity, and oxygen-poor water was still active at the easternmost stations of these three sections, while further west on these sections a transition to El Nino conditions appeared. Although from early 2015 the El Nino was strong, the October measurements in the eastern tropical Pacific only showed developing El Nino water mass distributions. In particular, the oxygen distribution indicated the ongoing transition from typical to El Nino conditions progressing southward along the Peruvian shelf.