The Medieval Climatic Anomaly

Distinctive in this case means, does not match well with the first abut is explained by the later. This is not the greatest sentence format, but distinct means stands out from. Generally when you refer to something as distinct with respect to X, then it means X is the odd part about it or it stands out from X. In this cased becasue they are talking about a predictive model and the thing being predicted, then it means it is odd because it does not fit the predictions of the prevailing climate model. BUT it is also distinct becasue it is better explained by local factors, for a given value of "local", which is not normally the case for an event so large. It also might be a reference to more recent research showing the anomaly is likely not global. Combining these two slightly different usages is not great sentence composition but is grammatically correct.

In this particular case the regional forcing mechanism is believed to be a large volcano erupting.