To understand the possible reason, we checked model initial subsurface conditions in the winters before those pIOD events. One common feature is that strong cold subsurface signals appeared in the tropical southwestern IO. The structure is similar to Rossby wave characteristics. The cold signal will propagate westward and reflect as equatorial eastward-propagating Kelvin wave, providing favorable conditions for the pIOD development. This is consistent with IOD dynamics. So the strong cold subsurface signal in the southwestern IO may provide important preconditions for the long-lead forecasts of pIOD.