Initial Conditions for Optimal Growth in a Coupled Ocean–Atmosphere Model of ENSO*

Abstract

Abstract Several studies have examined the conditions in the equatorial Pacific basin that lead to the maximum growth over a fixed time period, τ. These studies have the purpose of finding the characteristic precursor to an ENSO warm event, or more generally to explore error growth and predictability of the coupled ocean–atmosphere system. This paper develops a linearized version of the Battisti model (similar to the Zebiak–Cane model) with a time-invariant background state. The optimal initial conditions for time period τ (τ-optimals) were computed for a range of τ and for a selection of background states. A number of interesting characteristics of the τ-optimals emerged: 1) The τ-optimals grow more quickly than even the most unstable mode (the ENSO mode) of the system. 2) The τ-optimals develop quickly into the ENSO mode—in around 90 days. 3) The ENSO mode produced by a given τ-optimal does not in general peak at time τ. For τ less than 360 days the ENSO modes peak after time τ, and for τ greater than 3...