Abstract: Based on the SODA_1.4.2 and NCAR/NCEP monthly reanalysis data from 1958 to 2001, the relationship between the seasonal and annual variations of the Northwestern Subtropical Mode Water (STMW) and the midhigh latitude atmospheric circulation over East Asiathe North Pacific is studied using the P-vector method in this paper. It is showed that: (1) The STMW is of significant seasonal variation: its volume increases in April, then gradually weakens in summer and autumn, and eventually shranks to the smallest in December. It is also investigated that the significant seasonal variation of the STMW is closely linked to the seasonal evolution of the sea surface wind stress and net heat flux over the Northwestern Pacific, reflecting the characteristics of the seasonal evolution of midhigh latitude atmospheric circulation anomalies over East Asia to some extent. (2) Meanwhile, on the interannual timescale, the 3-yearperiod is one of the most significant oscillation cycles of the STMW index averaged from May to July. Besides the ocean dynamic process, the annual variation of the STMW is also controlled by the wind stress curl and net heat flux anomalies, which is relevant to the abnormal configuration of the midhigh latitude atmospheric circulation anomalies over East Asia during the precious winter. When the previous winter monsoon is strong (weak) over East Asia, the wind stress mixing and the net sea surface flux over the formation area of the STMW are enhanced (reduced), leading to the strong (weak) oceanic mixing process. Eventually, more (less) water mass with low potential vorticity comes into the thermocline area through subduction, causing the strong (weak) STMW event as time goes. Therefore, the intensity of the STMW reflects the integrated features of midhigh latitude atmospheric and oceanic circulations over the East Asiathe North Pacific during the previous winter, which might-impose an effect on the East Asia climate anomaly.