Decadal and seasonal dependence of North Pacific sea surface temperature persistence

Decadal and seasonal dependence of the persistence characteristics of area-averaged sea surface temperature (SST) anomalies in the North Pacific (150 degrees E similar to 140 degrees W, 20 degrees N similar to 60 degrees N) are investigated using two different SST data sets for the period 1948-2005. It is found that a persistence barrier exists around July-September (especially in September). This July-September persistence barrier is accompanied by a summer decline in the wind stress. The results confirm the existence of the July-September persistence barrier in the North Pacific SST discovered by Namias and Born (1970). Besides the seasonal change, North Pacific SST persistence also exhibits a pronounced decadal change. Taking all calendar months into account, North Pacific SST persistence is relatively strong from the mid-1950s to the mid-1960s but then weak from the mid-1960s to the mid-1980s, and becomes stronger again from the mid-1980s until the mid-1990s, after which it tends to become weak again. The recurrence of SST anomalies from one winter to the next is obvious from the mid-1950s to mid-1960s, but no obvious recurrence occurs after the mid-1960s. Decadal changes of the Pacific-North America (PNA) pattern, the SST-clouds feedback, and the Southern Oscillation Index (SOI) are found to be related to those of North Pacific SST persistence. The PNA index shows a significant upward trend after the 1980s. Besides, the PNA pattern also exhibits a high persistence in winter from the mid-1980s to the mid-1990s. These changes of PNA pattern are favorable to the occurrence of strong SST persistence in winter from the mid-1980s to the mid-1990s. In summer, the positive feedback between the marine boundary clouds and SST enhances the SST persistence in the North Pacific. It is found that the positive feedback between the SST and clouds in the North Pacific during summer becomes stronger from the mid-1980s to the mid-1990s, which would contribute to the longer SST persistence in summer from the mid-1980s to the mid-1990s. The SOI shows negative correlation with the North Pacific SST persistence and the PNA index, indicating the remote forcing of ENSO on the North Pacific climate change. In addition, the high north Pacific SST persistence from the mid-1980s to the mid-1990s coincides with the warm phase of the Pacific Decadal Oscillation (PDO). We concluded that the changes in the tropical SST or the PDO phase might explain the origin of decadal changes of North Pacific SST persistence.