The Equatorial Pacific and ENSO have climatologically important seasonal cycles, with maximum Western Pacific SSTs occurring during boreal autumn and ENSO events peaking during boreal winter. In this work, we use the concept of a monsoonal mode to show that the presence of a large landmass in the northern hemisphere leads to this seasonal cycle. Specifically, warm air moving east from the Asian summer monsoon suppresses surface fluxes in the West Pacific, leading to increased heat content there during the following months. This, in turn, enhances ENSO growth rates during boreal autumn and causes ENSO events to peak in boreal winter.