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.