CoAuthors

Bo Qiu (University of Hawaii at Manoa, USA); Shinichiro Kida (Application Laboratory, JAMSTEC, Japan)

Regional sea level trend and variability in the Pacific Ocean has often been
considered to be induced by the low-frequency changes in the surface wind
stress forcing. In this study, we demonstrate that significant sea level
trend and varaibility can also be generated by eddy momentum flux forcing due
to instability of the background upper ocean circulation. In the North Pacific
Ocean, the eddy momentum flux forcing is particularly intense in the Kuroshio
Extension region (30-40N, 140-165E) and the Subtropical Countercurrent (STCC)
region (18-28N, 130-165E). Using the two-layer Hallberg Isopycnal Model driven
by the ECMWF ERA reanalysis wind stresses and the eddy momentum fluxes inferred
from the AVISO SSH anomaly data, we quantified the relative importance of the
wind- and eddy-forced reginal sea level trends in the past two decades. It
is found that the increasing (decreasing) trend south (north) of the Kuroshio
Extension axis is largely due to the strengthening of the eddy forcing
associated with the decreasing PDO index. On the other hand, the decreasing
(increasing) sea level trend south (north) of the STCC is caused by the
local, decadally-weakened eddy and wind forcing. Both of these forcing
changes are again caused by the emhancement of the negative PDO phase in the
North Pacific Ocean.