The phrase "multidecadal time" refers to a period of time spanning several decades. It refers to a long-term timeframe that looks at changes and events happening over many years, rather than just a few. Full definition
Testing the hypotheses must be accomplished by using «hindcast» simulations that attempt to reproduce past climate behavior over multidecadal time scales. (physicstoday.scitation.org)
Steve the challenge will be not only finding «snapshots» of those variables, but finding repeated measures over multidecadal time spans. (climateaudit.org)
Is there a reason to believe that internal mechanisms will generate same - sign anomalies in both hemispheres on multidecadal time scales? (gfdl.noaa.gov)