Altimetry data from the first 7-yr TOPEX/Poseidon (T/P) mission (October 19
92-September 1999) are analyzed to investigate the interannual variability
in regions of the Kuroshio Extension and its southern recirculation gyre. L
arge-scale, interannual changes of the Kuroshio Extension system are charac
terized by the oscillation between an elongated state and a contracted stat
e. In the elongated state, the Kuroshio Extension has a larger eastward sur
face transport, a greater zonal penetration, and a more northerly zonal-mea
n path. All these characteristics are closely connected to the presence of
an intense, zonally elongated southern recirculation gyre. In its contracte
d state, the Kuroshio Extension has a smaller eastward surface transport, a
more southerly mean path, and is accompanied by a weaker southern recircul
ation gyre. During the T/P period, the Kuroshio Extension changed from the
elongated state in 1992-93 to the contracted state in 1996-97, and back to
the elongated state since late 1997.
In addition to these large-scale changes, the mesoscale eddy field also exh
ibited prominent interannual changes in the Kuroshio Extension region. In t
he upstream region between the Izu Ridge and the Shatsky Rise, the eddy kin
etic energy level was generally low while the Kuroshio Extension was in its
elongated state, and high while in the contracted state. Downstream of the
Shatsky Rise, on the other hand, the EKE level was high (low) when the Kur
oshio Extension was in the elongated (contracted) state. The large-scale, i
nterannual changes in the Kuroshio Extension system have a significant impa
ct on the regional wintertime SST anomaly field: the warm (cold) wintertime
SST anomalies tend to persist in years when the Kuroshio Extension is in i
ts elongated (contracted) state. A diagnostic analysis of the surface ocean
heat balance indicates that the nonseasonal geostrophic advection by the o
cean circulation works to reduce (increase) the wintertime SST anomalies wh
en the Kuroshio Extension changes from an elongated (contracted) state to a
contracted (elongated) state. The SST anomalies associated with the large-
scale changes of the Kuroshio Extension have an area-averaged, peak-to-peak
amplitude of >1 degrees C and appear independent of the interannual SST ch
anges in the tropical Pacific Ocean.