# How to calculate the interannual variability and Seasonal Amplitude?

I have a time series of remote sensing observables. For example, the observed brightness from 2007 to 2019. And we have 2 datapoints per day. How can I calculate the interannual variability of this variable? and the seasonal amplitude? and how to define them? I want to study the relationship between interannual variability and seasonal amplitude of this variable. It's better to do a scatter plot with X-axis as interannual variability and Y-axis as seasonal amplitude. But I have no idea about the definition and the calculation of inter annual variability and seasonal amplitude (and also seasonal variability). I am a beginner, is there anyone who can help me? thanks a lot!

In what context are these terms? Surely they must be defined somewhere if needed. If you thought about them, then their definition is up to you.

"Inter-annual variability" doesn't tell much. It could be the variance between yearly averages, i.e. the average of the variance within year 1, variance within year 2 etc. but that would give you only one number. Or it could be the year-over-year variance, i.e. the variance of: variable at day 1 year 1, day 1 year 2 etc., like stock returns (though I don't see this making much sense here). It may make more sense to get the "intra-annual variance" here, i.e. the variance of the variable within a year.

"Seasonal amplitude" is just as vague. Could mean the absolute distance of the most extreme value observed (negative or positive) from the mean of that year/season. Or it could mean fitting a Fourier series (a sine or cosine curve may do) for each year/season and calculating the magnitude of the amplitude of the wave.