Local and Regional Modes of Hydroclimatic Change Expressed in Modern Multidecadal Precipitation Oxygen Isotope Trends.

Stable isotope ratios of precipitation trace mechanisms of hydroclimatic change in the modern and paleoclimate record. Patterns and drivers of isotopic change at multidecadal timescales have remained unclear, however, due to limitations in the observational record. Here, we use a 65‐year global data compilation to estimate solstial season δ18Op trends. Spatially organized regions of change suggest divergent controls, and we propose that changes in atmospheric water balance dominate trends in moisture‐limited areas, whereas changes in upwind source region conditions drive trends where atmospheric water flux is large relative to precipitation. Positive trends on windward coasts suggest the latter effect, whereas we attribute the dipole patterns in trends over North America and Europe and decreasing trends in southern Australia during boreal winter to the former. Simulations from the isotope‐enabled Community Atmospheric Model match predictions for water balance, implying that the model may underrepresent the effects of changing vapor source conditions. Plain Language Summary: Impacts of global climate change extend beyond rising temperatures and include shifts in rain and snowfall. The changes to precipitation occur both because warmer ocean and atmospheric temperatures cause increased evaporation and also because changing atmospheric circulation alters patterns of moisture transport. It's difficult to figure out what aspects of the water cycle have changed due to climate change by simply using measurements of rain and snow amounts. However, stable isotopes of oxygen in precipitation can reveal process changes. We compiled a global data set of precipitation oxygen isotope data for the past half century. We identified trends in the data set and found a complex response, characterized by regions of both positive and negative change. In some places this differs from the patterns expected if only the local balance between vapor and precipitation is considered. In the places where the patterns differ, our results suggest that the observed pattern must arise due to changes in evaporation location, conditions or condensation in the air mass prior to reaching the collection site. This occurs most often on the coasts, where precipitation comes from relatively unaltered ocean vapor. Within some continental interiors, changes to vapor flux become more important in governing the oxygen isotope response. However, oxygen isotope trends simulated by an atmospheric circulation model suggest that most of the model's response comes from changes to water balance, including water vapor flux, meaning that the model may miss some water cycle changes. Key Points: Spatial analysis of a multidecadal data set shows strong regional increases and decreases in precipitation δ18Op valuesThe sensitivity of δ18O to local water balance is not ubiquitous and may be greater in vapor limited regionsAtmospheric circulation model δ18O indicates mostly positive trends that match local water balance estimates [ABSTRACT FROM AUTHOR]