Seasonal variability of the long term data

I have 12 years of daily data of the ozone, latitude and the potential temperature. I want to find the seasonal variablity of the datasets (for example: DJF, JJA). I have 4 days data in each month. In matlab, I really do not know how to find the seasonality and that is the reason I did not attach any script here. I have attached datasets for reference also I attached figure that I want in result.

Answer 1

[Chris Turnes]

The new trenddecomp function may give you what you're looking for.