Deep-reaching submesoscale fronts generate ageostrophic secondary circulations that transport heat vertically through the ocean interior, yet their seasonal variability remains largely unknown. Here, we combine more than 75,000 sub-kilometer hydrographic profiles collected by 13 instrumented southern elephant seals over six years with satellite derived mesoscale strain to diagnose submesoscale vertical velocities and vertical heat transport in the Indian sector of the Southern Ocean. We show that both quantities are highly intermittent, extend to at least 500 m depth, and exhibit a pronounced seasonal cycle, with root-mean-square amplitudes nearly doubling between fall and spring. In contrast, the mean vertical heat transport is persistently upward throughout the year and reaches a maximum during winter, exceeding 150 W/m 2 below 200 m. These results reveal that the influence of deep-reaching submesoscale secondary circulations on upper ocean heat transport and restratification is strongly seasonally dependent.