  The adjoint of an ocean general circulation model is used as a tool for
investigating the causes of changes in ENSO SST indices. We identify adjoint
Kelvin and Rossby waves in the sensitivities to sea level and wind stress at
earlier times, which can be traced back for more than a year through western
and weak eastern boundary reflections. Depending on the thermocline depth the
first and second baroclinic modes are excited. The sensitivities to the heat
flux and SST are local and decay in about a month. The sensitivities to the
fluxes are converted into the influence of SST using the adjoint of a
statistical atmosphere model. Focusing on SST perturbations in the index region
itself, we recover, up to a scale factor, the delayed oscillator concept.
