A possible mechanism for the North Pacific regime shift in winter of 1998/1999

This study examines a possible mechanism for the North Pacific regime shift in the winter of 1998/1999. Since the 1976/1977 climate regime shift, the sea surface temperature (SST) in the North Pacific has exhibited a long-term warming trend in the western and central regions, which is mainly due to the two regime shifts that occurred in the winter of 1988/1989 and 1998/1999, respectively. In particular, the 1998/1999 regime shift is characterized by a dipole-like structure along 40 degrees N where a significant warming is prominent in the southwestern and central North Pacific. The slow dynamic adjustments of SST and zonal wind to the meridional heat exchange through thermal advection may be responsible for the 1998/1999 regime shift. We also assert that an intrinsic multidecadal SST oscillation in the North Pacific contributes to the 1998/1999 regime shift. Furthermore, another possibility, which is associated with the oceanic teleconnection from the tropics to the midlatitude, is also briefly discussed.