Impact of Remote Reemergence of the Subtropical Mode Water on Winter SST Variation in the Central North Pacific

Abstract

Using long-term datasets of sea surface temperature (SST), core-layer temperature (CLT) of the North Pacific subtropical mode water (NPSTMW), and the North Pacific index, an impact of remote reemergence of NPSTMW on winter SST variation in the central North Pacific is quantitatively investigated. A running correlation analysis between CLT and winter SST in the remote reemergence area clearly shows that an occurrence of remote reemergence of NPSTMW strongly depends on the specific time period: occurrence period and nonoccurrence period. It is found that background conditions, such as formation rate of NPSTMW, winter mixed layer depth, ocean heat content, and buoyancy flux, play a crucial role in the period-dependent remote reemergence. In the occurrence (nonoccurrence) periods, since a large positive (negative) upper-ocean heat content anomaly is located around the central North Pacific, a deeper (shallower) winter mixed layer is formed in both the formation area and the reemergence area of NPSTMW. Therefore, a large (small) amount of NPSTMW is formed, and consequently the advective part of NPSTMW is preserved (dissipated) from (because of) a vigorous mixing due to salt-finger-type convection. In addition, larger (less) oceanic buoyancy loss contributes to an occurrence of reemergence. These are favorable (unfavorable) conditions for persistence of thermal anomalies and occurrence of reemergence in the central North Pacific. Using a multiple regression analysis, it is shown that remote reemergence gives a significant impact to an equivalent degree to the surface thermal forcing related with the Aleutian low activity on winter SST variation during the occurrence periods, while there is no significant contribution to SST variation during the nonoccurrence periods. It is also shown that the period-dependent reemergence closely connects with the Aleutian low activity with a lag of 6 to 8 yr, that is, the spinup/spindown of the subtropical gyre.