A Role of the Soil Enthalpy in Land Memory*

Recent studies have identified a connection between the summer monsoon rainfall in the southwest United States and anomalies of the antecedent winter precipitation and snowpack in the northwest United States. This connection shows a seasonal-scale predictability of the precipitation and indicates a seasonal predictability of the land–atmosphere system (the “land memory”) in the western United States. Although some efforts have been devoted to understanding this predictability, the physical processes constituting it remain unexplained. In this empirical study, a potential source, the soil enthalpy, and its role in land memory are examined for the recent epoch of a strong land memory (1961–90). The rationale is that the soil enthalpy variation has magnitudes comparable to the atmospheric enthalpy changes at various time scales, and the soil enthalpy anomaly in the top 20–50-cm soil column can persist for 2–3 months. As shown by the major results of this study, a persistent negative anomaly of the soil enthalpy in the northwest United States is related to negative anomalies of the surface and the lower-troposphere temperature in that region. Subsequently, the lower-troposphere temperature and related higher-atmospheric pressure anomalies in the northwest United States during late spring and the early summer months encourage a northward position of the lower-troposphere monsoon ridge in the western United States and, therefore, create a circulation that favors an above-average monsoon rainfall in the southwest United States. A weaker summer monsoon occurs when a sequence of opposite anomalies develops after a warm and dry winter in the northwest United States. In this regard, the soil enthalpy variations may serve to “record” the winter precipitation and temperature anomalies and “release” their effects on summer monsoon rainfall through interactions of soil enthalpy with the surface and lower-troposphere temperatures.