Precipitation and Seawater Isotopic Variability from Hawaii to the equator: the 2014-2015 ENSO cycle

Abstract:

An increasing number of paleoclimate reconstructions rely on the isotopic variability of precipitation or seawater as a proxy for past hydrological variability, even though modern-day water isotope variability is poorly constrained by observations. Nowhere is this more apparent than in the tropical Pacific, where paleo-water isotope reconstructions imply that anthropogenic climate change has driven dramatic shifts in the isotopic composition of surface waters (Nurhati et al., 2009), yet water isotope observations in this region are virtually non-existent. Here we present a new set of weekly seawater and daily precipitation isotope observations along a meridional gradient in the tropical Pacific, spanning from Hawaii (21N, 158W) to Palmyra Island (6N, 162W) to Christmas Island (2N, 157W), that spans the development and growth of the current ENSO cycle that began in 2014. We use a suite of high-quality in situ observations of ocean conditions (salinity, temperature) as well as surface meteorological measurements (relative humidity, precipitation amount, wind speed and direction) to provide an interpretive framework for the observed isotopic variations, with a focus on the expression of seasonal to interannual features in the dataset. A complementary dataset of precipitation and seawater isotopes from across the equator in the tropical Pacific basin provides additional diagnostic context. We also compare our observed isotopic variations to output from numerical simulations of precipitation and seawater isotopes in the tropical Pacific. We discuss the implications of our findings for the design of long-term monitoring programs in the tropical Pacific, as well as the interpretation of proxy-based reconstructions of seawater and precipitation water isotopes.