So, first of all, the style for Biogeosciences is a dependent style of copernicus-publications.csl.
So you will need to change that style and save it with a new name and also change the file ID and self-link in lines 5+6 of the code.

Then for the sorting, you will find this in the <citation part of the code (lines 146...). You can see it has a section called sort and another one called layout. The latter is just all the different elements of how the in-text citation will look (author, year etc.).
The sort part sets, hierarchically, for which variable/macro the style should sort first. You can see it first sorts for the author macro and THEN for the year-date macro. Switch those two and that's all you need.