Modal structure of variations in the tropical climate system. Part II: Origins of the LF mode

Simulations with ocean and atmospheric general circulation models and a hybrid coupled model reproduce well the observed features of variability in the low frequency (LF) mode described in Part I. The model results show the origins of the LF to be in the ocean and suggest this phe- nomenon is a natural mode of the tropical Pacific Basin. Air-sea interactions amplify the ocean mode by a factor of 5-6 so it obtains climatologicalximportance. .These same inter- actions introduce the LF to the atmosphere. The physical processes responsible for these results are presented. The LF mode of interannual variability is not directly driven by the annual cycle. But it does depend importantly on the fact that the ocean-atmosphere coupling strength vary with respect to the annual cycle. The LF mode appears to be rather sharply peaked in wave number space but broadbanded in frequency space