The Growth and Decay of Low-Frequency Anomalies in a GCM

The temporal evolution of a regional-scale persistent low-frequency anomaly is examined with data from a 2100-day perpetual January general circulation model. The persistent episodes are determined with an objective analysis of the low-pass (.10 day) 350-mb streamfunction field that uses both pattern correlations and empirical orthogonal function (EOF) analysis. The composite evolution of each term in the streamfunction tendency equation is calculated relative to the onset day (the first day of the persistent episode). By projecting each term in the streamfunction tendency equation onto an EOF (the EOF is associated with a particular low-frequency anomaly), the contribution of these terms toward the tendency of the corresponding principal component can be obtained. It is found that the sum of the linear terms dominates during most of the growth and the decay of the low-frequency anomaly. The linear term that accounts for the growth and maintenance of the low-frequency anomaly is the interaction between the anomaly and the time-mean zonally asymmetric flow. After the anomaly attains sufficient amplitude, its decay is accomplished through the divergence term. For one phase of the EOF, it is found that the high-frequency transients prolong the anomaly, whereas in the other phase they do not. Implications of this study for examining monthly averaged anomalies are also discussed.