Potential Reemergence of Seasonal Soil Moisture Anomalies in North America

Abstract

Soil moisture anomalies within the root zone (roughly, soil depths down to ~0.4 m) typically persist only a few months. Consequently, land surface–related climate predictability research has often focused on subseasonal to seasonal time scales. However, in this study of multidecadal in situ datasets and land data assimilation products, we find that root zone soil moisture anomalies can recur several or more seasons after they were initiated, indicating potential interannual predictability. Lead–lag correlations show that this recurrence often happens during one fixed season and also seems related to the greater memory of soil moisture anomalies within the layer beneath the root zone, with memory on the order of several months to over a year. That is, in some seasons, notably spring and summer when the vertical soil water potential gradient reverses sign throughout much of North America, deeper soil moisture anomalies appear to return to the surface, thereby restoring an earlier root zone anomaly that had decayed. We call this process “reemergence,” in analogy with a similar seasonally varying process (with different underlying physics) providing winter-to-winter memory to the extratropical ocean surface layer. Pronounced spatial and seasonal dependence of soil moisture reemergence is found that is frequently, but not always, robust across datasets. Also, some of its aspects appear sensitive to spatial and temporal sampling, especially within the shorter available in situ datasets, and to precipitation variability. Like its namesake, soil moisture reemergence may enhance interannual-to-decadal variability, notably of droughts. Its detailed physics and role within the climate system, however, remain to be understood.

Persistence and Plasticity in Conifer Water‐Use Strategies

“…However, changes in seasonal origins of plant water do not necessarily imply that trees are changing the depth they are drawing water from. Rather, the changes in the seasonal origins of the water could be a reflection of changes in the downward mobility of summer precipitation through the root zone or upward mobility of snowmelt from deeper soil layers into the root zone (Kumar et al, ). During periods of increased snowfall, anomalously wet soil conditions can persist at depth for multiple years.…”

“…This observation could reflect multiple interacting processes including: the ubiquitous reliance of trees on deeper water pools; deeper water from a previous season migrating upward into the root zone; winter precipitation being retained in micropores; or that summer precipitation evaporated or passed through the root zone in macropores. Nonetheless, the common use of winter precipitation by trees strongly supports the role of soil hydrology in adding a seasonal or even interannual legacy component to ecosystems (Kumar et al, ).…”

“…One interpretation of this low frequency behavior is that it reflects biotic processes such as the use of a previous year's nonstructural carbon pool or changes in leaf area that affect photosynthesis for subsequent seasons (Bréda et al, 2006;Franke et al, 2013;Sala et al, 2012). However, year-to-year persistence in aboveground productivity may also emerge abiotically through the dynamics of subsurface hydrology such as the lagged and sustained response of water table depths or deep soil moisture content to precipitation variability (Amenu et al, 2005;Bierkens & Van den Hurk, 2007;Kumar et al, 2019;Maxwell & Kollet, 2008). It follows, if plants are relying on deeper soil moisture or water pools held in low matric potential pores with long residence times, then multi-annual persistence in ecosystem productivity could emerge through the delayed and sustained response of soil hydrology to surface climate (Ghannam et al, 2016;Rempe & Dietrich, 2018).…”

Advances in understanding large‐scale responses of the water cycle to climate change

“…75 The control of soil moisture on evapotranspiration determines feedbacks onto surface climate, which vary across simulations 110 and can cause delayed responses over multiple seasons. 111 The response of vegetation to climate change and increased atmospheric CO 2 concentrations also determines regional P-E, as well as aridity. Depending on their responses, plants may either amplify 112 or ameliorate 113 warming impacts on drought at the surface.…”

Seasonal to multi-year soil moisture drought forecasting

“…For example, the NOAA's Seasonal Climate Outlook did not predict the 2012 agricultural drought (also commonly referred to as soil moisture drought) in the central US which resulted in $30 billion economic losses 15,16 . In this study, we assess the initialized 16 and uninitialized 5 large ensemble experiments of the Community Earth System Model (CESM) for North America and show that recent advances in earth system modeling 17 combined with an improved understanding of long-memory land surface processes 18,19 can enable skillful predictions of soil moisture drought several months in advance ( Fig. 1) despite limited skills in the precipitation forecast.…”