Temperature anomaly reemergence in seasonally frozen soils

[1] In cold regions where soils experience seasonally freezing and thawing, past soil temperature anomalies are stored as variations in the amount of ground ice and can reemerge at the surface after frozen soils thaw. Warmer soils in autumn result in shallower freeze depths in winter, requiring less energy to thaw in spring, and resulting in warmer soils the following summer. We identified reemergence from in situ soil temperature data across the former Soviet Union and simulated reemergence using a soil heat transfer model with phase change. Reemergence is triggered by a sudden drop in the apparent soil specific heat associated with the latent heat of fusion of water. Past soil temperature anomalies persisting just below the maximum freeze depth, which varies from less than one to three meters, amplify the reemergence signal. Reemergence strength increases with soil water content and does not occur if the soil never freezes. To simulate the reemergence, models need a soil column at least 7 m deep with enough vertical resolution to accurately capture variability in the frozen layer. Reemergence of past soil temperature anomalies is a new class of time delayed, land-atmosphere feedbacks that can potentially help to explain observed variability in climate and improve seasonal climate prediction in regions with seasonally frozen soils.