Please take a look at the code "phys/module_physics_init.F", in wit you can find the piece of code:
Code:
1850 ! Determine season (summer=1, winter=2)
1851 ISN=1
1852 IF(JULDAY.LT.105.OR.JULDAY.GT.288)ISN=2
1853 IF(CEN_LAT.LT.0.0)ISN=3-ISN
1854
ISN is the parameter that determines winter/summer.