Potential Reemergence of Seasonal Soil Moisture Anomalies in North America

Soil moisture is an important predictor of regional climate variations and water resources. Most previous research on soil moisture and associated climate predictability has focused on sub-seasonal to seasonal time scales. We produce a new analysis using long-term in situ soil moisture observations, found a rebound in root zone soil moisture predictability on inter-seasonal to inter-annual (ISI) time scales. We call this rebound potential soil moisture “re-emergence” because it is related to more persistent soil moisture memory, on time scales of several months to a year or longer, within the sub-surface soil layer. An analogous climate process in which a reservoir of long-term memory is tapped on a seasonally varying basis, sea surface temperature anomaly re-emergence, enhances decadal scale climate variability in the North Pacific, e.g. Pacific Decadal Oscillation (PDO). Soil moisture re-emergence could similarly enhance hydroclimate predictability at ISI time scales, including by reddening remotely forced climate variability signals over land.

Long-term soil moisture observations are limited. Hence, we evaluated soil moisture re-emergence in various land data assimilation products, e.g. North American Data Assimilation System version 2 (NLDAS2) and climate model simulations. We found pronounced spatial and seasonal dependence of soil moisture re-emergence, which was sometimes but not always robust across these datasets despite the fact that in all cases their corresponding land surface/hydrology models were forced with observed climate forcing. An analysis of the CESM-Large Ensemble showed similar re-emergence characteristics, but varying across the 42 ensemble members, suggesting a potential but undetermined role of internal variability. In a separate land surface modeling experiment, we found that soil moisture re-emergence in some regions intensified when we muted inter-annual precipitation variability compared to a corresponding control experiment. Finally, in a new small ensemble climate modeling experiment (the GLACE-Hydrology experiment) using CESM, we found an important but spatially variable role of land-atmosphere coupling upon re-emergence. Overall, we documented for the first time a soil moisture re-emergence process that can potentially contribute to ISI hydroclimate predictability.