Nonlinear Dynamic Aspects of Generalized Frosts in the Pampa Húmeda of Argentina

Abstract

Generalized frosts have a significant impact on the Pampa Húmeda of Argentina, particularlythose without persistence (0DP), defined as events that do not last more than oneday, and are the most frequent generalized frosts. This study investigates the dynamicaland physical mechanisms that sustain these events, emphasizing the nonlinear interactionsrepresented by the RossbyWave Source (RWS) equation. Composite analysis of pressure,temperature, wind and geopotential height fields were performed, showing that 0DP eventsare related to abrupt cold air intrusion linked to the enhancement of upper levels troughsover the eastern Pacific Ocean and transient surface anticyclones over South America.This linear analysis only showed a lack of persistent upper-level maintenance and didnot explain the dynamics of the rapid weakening of the circulation. For this reason, anonlinear analysis based on the decomposition of the RWS equation into its advective anddivergent terms is performed. The advective term only acts as an initial trigger, deepeningtroughs and favoring meridional cold air advection, while the divergent term dominatesthe events, representing 63–67% of the affected area. This term reinforces ridges, promotessubsidence and favors clear sky conditions that enhance nocturnal radiative cooling andfrost formation. Positive anomalies of the divergent RWS term strengthen the ridge andadvect cold air over the Pampa Húmeda, whereas subsequent negative anomalies overthe southwestern Atlantic act as sinks of wave activity, leading to the rapid dissipation ofthe synoptic configuration. Consequently, the same mechanism that generates favorableconditions for frost development also determines their lack of persistence. These findingsdemonstrate that the short-lived nature of 0DP frosts is not due to the absence of dynamicalforcing, but rather to nonlinear processes that both enable and constrain frost occurrence.This highlights the importance of incorporating nonlinear diagnostics, such as the RWS, toimprove the understanding of short-lived atmospheric extremes.