Auto correlation refers to the relationship between different values in a set of data. It helps us understand if there is a pattern or similarity between the values at different points in time or space. If the values are positively correlated, it means that as one value increases, the other tends to increase as well. Conversely, if they are negatively correlated, as one value increases, the other tends to decrease.
Full definition
The tests that you did in the cases, changing time periods, looking
at auto correlation etc. to you seem non trivial, but that is because you found them relevant.
We have the mean and variance but the is
auto correlation, that in some years is going to be highly positive, like at start of El Niño, and right after El Niño peak we have a negative auto correlation and then positive auto correlation right after that.