Long-Term Monitoring Records Generate a New Hypothesis: Active-Layer Variation as a Markovian Process

Abstract:

The hypothesis is examined that near-surface components of the permafrost system, including the seasonally frozen active layer and the ice-rich transient layer, respond to extreme events in a manner resembling a Markovian process over multi-decadal timescales. Anthropogenic disturbance or an abnormally warm summer can cause thaw penetration into the transient layer, resetting active-layer thickness (ALT) to a tightly constrained range about a new mean value over consecutive ensuing years. Conversely, extreme cool summers can reset ALT to lower values for subsequent years. The data sets used to test this hypothesis have been expanded substantially relative to those used in previous studies through use of long-term Circumpolar Active Layer Monitoring (CALM) program records. ALT measurements extend as far back as the 1960s at one site and annual ALT and meteorological measurements began in the early to mid-1990s at all sites. Unlike earlier studies, observations after 2001 also include subsidence measurements for the ground surface. As records from CALM sites have lengthened to multi-decadal series, departures from the standard bivariate Stefan relation between ALT and annual degree days of thaw have become more apparent. The frequency of ALT “resets” may increase with climate variability. The CALM sites used in this study are all in natural environments and therefore only climate-induced disturbances, including particularly warm/cool and wet/dry summers, were considered in the search for Markovian-like behavior in the active layer/transient layer/permafrost system. Recognizing potential Markovian behavior in the near-surface components of the system has significance for a variety of regional modeling efforts, but particularly for regional thaw subsidence.