The ionospheric Weddell Sea Anomaly (WSA) was first reported more than five decades ago based on ionosonde data near the Antarctica peninsula. The WSA is an ionospheric structure characterized by a larger nighttime electron density than daytime density. Recent satellite observations indicate that the WSA can extend from South America and Antarctica to the central Pacific. The major physical mechanisms that have been suggested for the WSA formation are an equatorward neutral wind, an electric field, the photoionization, and the downward diffusion from the plasmasphere. On the basis of the theoretical modeling performed in this study using the SAMI2 model, an equatorward neutral wind is identified as the major cause of the WSA, while the downward flux from the plasmasphere provides an additional plasma source to enhance or maintain the density of the anomalous structure.