The 1970s' Weddell Polynya is simulated in the framework of a coupled ocean-sea ice model forced by reanalysis surface flux fields. A rapid emergence of strongly negative wind stress curl over the Weddell Sea intensifies the cyclonic Weddell gyre and thus causes the relatively warm and salty Weddell Deep Water (WDW) to upwell, generating an open-ocean polynya by melting sea ice or hindering its formation. Once the polynya occurs in the austral winter, the underlying water column is destabilized due to the combined effect of the high-salinity WDW, a massive cooling at the air-sea interface, and the ensuing brine rejection from newly forming ice, thus inducing open-ocean deep convection. Further analysis shows that the buildup of a large heat reservoir at depth by the mid-1970s was a necessary condition to establish the Weddell Polynya of the 1970s.