"Internal oscillations" refers to repetitive movements or fluctuations that happen within something or someone's own system or structure. It implies a regular back-and-forth motion or rhythm occurring within an individual or an object.
Full definition
There may be self sustaining
internal oscillations which feed the changes in SST / air circulation / cloud cover / radiation balance, back and forth (AMO?).
Climate variability best describes the shorter multi-centennial trends which are merely the overprinting
internal oscillations on longer - term climate change.
Like El Niño, La Niña, and the Southern Oscillation, it is considered as
free internal oscillation of the climate system not subjected to external forcing.
Internal oscillations such as PDO, AMO, ENSO, and the like do not affect climate change / global warming unless they have a quantifiable impact on radiative forcing.
To be added, other greenhouse gas forcings with their feedbacks and
internal oscillations which may be — or not — enhanced by the primary forcings.