Strong seasonal temperature variation below the thermocline is detected by Argo profiling floats. The variation is associated with a long baroclinic Rossby wave. The results of this study show that seasonal signals can be captured at 1000–2000 dbar not only in basin boundary regions or deep convection regions but also in the interior ocean. A clear, westward propagation signal is observed. The phase speed is of the same order as a theoretically derived speed. Below the thermocline, the first mode of empirical orthogonal function analysis of potential density variations in the strong signal region shows a clear seasonal variation. Even in the middle layer, the amplitude of the temperature anomaly is 0.05–0.12° C at 1200 dbar, which is equivalent to the annual standard deviation from the WOA01. These results are obtained using the dense spatial and temporal distributions provided by the Argo observations.