The sea ice cover of the Southern Ocean undergoes a rapid decrease from mid-November to mid-January. The atmosphere-to-ocean heat flux is insufficient to account for this melting, even in the presence of the high percentage of open water characteristic of the Southern Ocean sea ice. It is estimated that sea-air heat exchange in the 60¿ to 70 ¿S zone cna account for roughly 50% of the required spring heating. The remainder must be supplied by the relatively warm deep water, residing below the Southern Ocean pycnocline. Deep-to-surface water heat flux is accomplished by upwelling of the pycnocline due to the regional Ekman divergence of the surface layer and by cross-pycnocline mixing. It is estimated that the required vertical mixing coefficient for the pycnocline is 1.5 cm2/s. This value is considered as realistic in view of the relatively weak pycnocline which is a consequence of low levels of fresh water input to the surface layer. It is suggested that the magnitude of fresh water input is a major factor in determining the degree of seasonality of Southern Ocean sea ice.