Why Has the Land Memory Changed?

Abstract

The ?land memory? refers to an interseasonal predictability of the summer monsoon rainfall in the southwestern United States, describing a relationship of the summer monsoon rainfall anomaly with anomalies in the antecedent winter season snow and land surface conditions in the western United States. This relationship has varied, however, showing a peculiar on-and-off feature in the last century. It is important to understand this variation so that the relationship can be used to assist making predictions of the monsoon rainfall for that region. This note offers the evidence and shows that the change of the land memory may have been a reflection of an irregular variation in the persistence of the sea surface temperature anomaly (SSTA) in the North Pacific Ocean; in epochs when the SSTA persisted from winter through summer, the SSTA and related anomalies in atmospheric circulation could have dominated the summer monsoon variation, whereas in epochs when the persistence collapsed the SSTA effect weakened and the effect of the land processes on the summer monsoon rainfall became prominent.