The variable in your mat file is called NEWSST, not NEWSST_2015.
It is usually better though to load a mat file into a struct and then reference from that e.g.
loadedData = load( 'NEWSST_2015.mat' );
NEWSST_2015 = loadedData.NEWSST;
then you will have a variable of the name you use. I haven't checked if the rest of your code is fine, but the variable not existing is certainly the main problem.