IIRC, Mars's global atmospheric pressure is about 25% higher in the southern summer than in the southern winter, due to differential sublimation and condensatio

IIRC, Mars's global atmospheric pressure is about 25% higher in the southern summer than in the southern winter, due to differential sublimation and condensation of carbon dioxide, from the southern ice cap as well as from the soil at lower latitudes, and southern hemisphere summer temperatures can range all the way above zero celsius -- so there's quite a range of weather conditions to which any briny water would be subjected, especially in the southern hemisphere. Edit to add, the Martian southern hemisphere seasons are more extreme because the effects of its significant eccentricity and its axial tilt are additive there, while they counteract each other to moderate the seasons in the north.