Abstract: The variability of subsurface ocean in the tropical Pacific was analyzed based on the simple ocean data assimilation (SODA) dataset. The results indicate that the path of anomalous temperature signal transmitting to the western Pacific in an ENSO cycle has close relationship with the depth distribution of thermocline. The climatological thermocline was very shallow along 10ºN. Therefore, the thermocline was like a ridge through the eastern to western Pacific under the sea surface. As a result, the anomalous ENSO signal coming from the equator could not get across this area to the north, so the signal in the eastern Pacific could only transmit to the western Pacific along this region. On the contrary, the thermocline in the southern Pacific does not have such a character and could not block the signal’s propagation from the equator to higher latitude. Further research indicates that the signal intensity of temperature anomaly changes in the ENSO cycle, especially when the signal is transmitted from the eastern Pacific to the western Pacific along 10ºN. Except for the temperature anomaly from 10ºN in the eastern Pacific, the temperature anomaly that ultimately approaches the western Pacific warm pool was also affected by the local surface wind in the North Pacific and the temperature anomaly from higher latitude.

Key words: ENSO, thermocline, primary period