def get_prefix4WOA18(input):
    """
    Convert WOA18 variable name string into the prefix used on files
    """
    d = {
    'temperature': 't',
    'salinity': 's',
    'phosphate': 'p',
    'nitrate':'n',
    'oxygen':'o',
    'silicate':'i',
    }
    return d[input]