A 170‐year Sr/Ca and Ba/Ca coral record from the western Pacific warm pool: 2. A window into variability of the New Ireland Coastal Undercurrent

A Porites coral from New Ireland (2.5°S, 150.5°E), at the heart of the Pacific warm pool, records variations of Ba/Ca back to the early 1820s. The New Ireland Coastal Undercurrent, which flows along the north coast and transports high‐nutrient thermocline waters, is thought to be the main source of the Ba enrichment observed in the coral during El Niño. Between the 1850s and the 1960s, frequent large Ba peaks indicate that nutrients were available during both phases of El Niño–Southern Oscillation (ENSO). During La Niña, Ba could be advected along the South Equatorial Current. Also during this period, clusters of Ba/Ca peaks at nearly decadal timescale generally coincide with the return time of a strong El Niño, in accord with the high decadal variance of NINO3 sea surface temperatures. Ba enrichment in this coral primarily reflects the stratification in the thermocline that controls vertical mixing. The coral records long‐term changes in those properties affecting nutrients in surface waters, with reduced Ba after the 1960s, and even less between 1823 and 1850. These reductions are tentatively attributed to the high rate of warming at the end of the Dalton Minimum and since the mid‐20th century. An ensuing weakening of the trade winds may have produced a more stratified equatorial thermocline, hindering the transport of Ba and nutrients from the undercurrent to the shallow coastal waters north of New Ireland.