The role of remote versus local climatic influences in shaping seasonal to interannual rainfall isotopic variations in northern Borneo

Abstract:

While interannual and intraseasonal variability are the dominant influences on modern rainfall water isotopes (δ¹⁸O and δD) in northern Borneo (Moerman et al., 2013), the strong resemblance between stalagmite δ¹⁸O and equatorial boreal fall insolation over the Holocene and late Pleistocene suggests that seasonal δ¹⁸O variability is an important control on Borneo stalagmite δ¹⁸O over glacial/interglacial timescales (Carolin et al., 2013). A weak, bimodal seasonal cycle of 2-3‰ exists in northern Borneo rainfall δ¹⁸O, with relative minima during winter/summer and relative maxima during spring/fall. The seasonal cycle in rainfall δ¹⁸O, however, is poorly correlated to seasonal variations in precipitation amount. As a result, the processes driving rainfall δ¹⁸O seasonality at Borneo remain unclear. To better constrain the controlling mechanisms, we compare a 7-yr-long timeseries of daily Borneo rainfall δ¹⁸O to overlapping satellite-based measurements of GOSAT and TES tropospheric water vapor δD. To investigate the role of moist processes such as evaporation, condensation, and convection, we explore the relationship between seasonal composites of water vapor δD and specific humidity in the Borneo region. We also use HYSPLIT air mass back-trajectories to differentiate local (e.g. moisture recycling, local convection/evaporation) versus regional (e.g. moisture source region, trajectory, and convective activity) controls on the seasonal isotopic composition of Borneo rainfall. Given the sensitivity of Borneo rainfall δ¹⁸O to interannual shifts in the zonal location of deep convection in the western Pacific – which drive rainfall δ¹⁸O variations of up to 6-8‰ – we perform similar investigations during the weak-to-moderate and moderate-to-strong ENSO cycles of 2006-2008 and 2009-2011 respectively. With this study, we identify the relative influence of local moist processes as well as meridional and zonal shifts in regional hydrology on past western Pacific hydroclimate variability as revealed by Borneo stalagmite δ¹⁸O. More generally, we outline a roadmap for how local high-frequency monitoring of water isotopes in conjunction with satellite-based products can improve interpretations of δ¹⁸O-based paleoclimate reconstructions from terrestrial archives.