Bad data design forces you to write slow, complex, obfuscated, buggy code which is hard to debug:
Your code would be simpler and more efficient if you just used a structure, e.g.:
S.Jan = [...]
S.Feb = [...]
...
S.Dec = [...]
Then you could use dynamic fieldnames to access any of the arrays:
fld = 'Jan';
S.(fld)
Or use a table, which would be similarly simple and efficient to access:
Note that you should fix your badly-designed data at the point where those separate arrays are generated in the workspace, e.g. by load-ing into an output variable (which is a scalar structure) instead of directly into the workspace:
S = load(...)
