The Auto Correlative Plot

The annual trend is very siciall right, but it's not smooth. That's jus sort of noise in the data that we can't explain alone. We'd like to take the auto correlative function, which is a measure of how much it correlates with itself. The reason you see that wavy pattern is because, as you identified earlier, very recent days are very highly correlated. But longer you go back in time, the less correlated it is.