Revitalization of Namias' Climatological Isentropic Analysis

This study revisits, updates, and evaluates the climatological isentropic analysis pioneered by Jerome Namias in the 1930s. As Wexler and Namias (1938) noted, climatological isentropic analysis is particularly useful in that it gives “a much better picture of flow pattern than fixed-level [and constant pressure] charts and show(s) the dry and moist tongues. This form of analysis allows a much better correlation of rainfall-patterns with upper-air phenomena than was possible [with isobaric charts]” (p. 164). Namias (1940) subsequently created a summertime 315K isentropic analysis that was one of the original analyses used to define the North American Monsoon. Our updated (1957-2008) examination of the summertime 315K isentropic analysis displays many of the climatological features identified by Namias. Given this verification of the technique, we extend his analyses by creating average wintertime (295K), autumn (305K) and springtime (300K) isentropic analyses for 1957-2008. Importantly, we identify a prominent “isentropic inverted wave” over the Great Plains in the spring 300K isentropic surface corresponding to prevalent severe storm and tornadic activity in that region during the spring season.

We also update and evaluate one of Namias' original applications of climatological isentropic analyses, drought identification, using the modern Standardized Precipitation Index, or SPI, as our drought indicator. Adapting from the concept of modern climatological anomaly patterns for isobaric (e.g., 500 hPa) analyses, we create “isentropic anomaly” charts showing the difference between the pressures of the isentropic surface for drought (and contrasting wet episodes) and the long-term average isentropic surface. Modern drought (and wet episodes) occurrences are clearly and distinctly displayed on climatological isentropic analyses with anomalous dry tongue/ridge isentropic intrusions unmistakably linked to drought episodes and anomalous wet tongue/trough isentropic intrusions noticeably prominent over regions experiencing wet periods. Fundamentally, we demonstrate that climatological isentropic analysis as pioneered over seven decades ago by Namias is still notably applicable to modern climate forecasting concerns.