Large-Scale Variability in the Midlatitude Subtropical and Subpolar North Pacific Ocean: Observations and Causes

Altimetric data from the 8-yr TOPEX/Poseidon (T/P) mission (Oct 1992-Jul 2000) are used to investigate large-scale circulation changes in the three current systems of the midlatitude North Pacific Ocean: the North Pacific Current (NPC), the Alaska gyre, and the western subarctic gyre (WSG). To facilitate the understanding of the observed changes, a two-layer ocean model was adopted that includes first-mode baroclinic Rossby wave dynamics and barotropic Sverdrup dynamics. The NPC intensified steadily over the T/P period from 1992 to 1998. Much of this intensification is due to the persistent sea surface height (SSH) drop on the northern side of the NPC. A similar SSH trend is also found in the interior of the Alaska gyre. Both of these SSH changes are shown to be the result of surface wind stress curl forcing accumulated along the baroclinic Rossby wave characteristics initiated from the eastern boundary. In addition to the interior SSH signals, the intensity of the Alaska gyre is shown to depend also on the SSH anomalies along the Canada/Alaska coast, and these anomalies are shown to be jointly determined by the signals propagating from lower latitudes and those forced locally by the alongshore surface winds. The WSG changed interannually from a zonally elongated gyre in 1993-95 to a zonally more contracted gyre in 1997-99. This structural change is due to the interannual SSH anomalies within the WSG as a result of the baroclinic Rossby wave adjustment attenuated by eddy dissipation. Along the western boundary of the subpolar North Pacific, variability of the East Kamchatka Current (EKC) and Oyashio is in balance with that of the interior Sverdrup flow on the annual and year-to-year timescales. On the multiyear timescales, the EKC/Oyashio variability is shown to be determined by the baroclinic SSH signals.