Interannual variability in the midlatitude subtropical and subpolar North Pacific: Observations and causes

Altimetry data from the first 8-yr TOPEX/Poseidon mission (1992-2000) are analyzed to investigate large-scale, low-frequency changes in the midlatitude subtropical and subpolar North Pacific Ocean. Focuses are placed on the three current systems: the North Pacific Current (NPC), the Alaska Gyre, and the Western Subarctic Gyre (WSG). The NPC intensified steadily over the period from 1992 to 1998 and the intensifying trend leveled off following 1998. Much of this change of the NPC is caused by sea surface height (SSH) variations on the northern side of the NPC from 40N to 45N. A similar SSH trend is also detected in the interior of the Alaska Gyre. While the barotropic Sverdrup-balance response is found to be important for the annual and intra-annual SSH signals, the interannual SSH changes are the result of surface wind forcing through the first-mode baroclinic Rossby wave dynamics. The intensity of the Alaska Gyre depends not only on the interior SSH signals, but also on those along the Canada/Alaska coast. The coastal SSH anomalies are a combination of the signals that propagate from lower latitudes and those forced locally by along-shore surface winds. In contrast to the NPC and the Alaska Gyre, the WSG was relatively strong before 1996 and relatively weak after 1996. This change in the WSG is largely due to the SSH changes across the the eastward flowing subarctic current. For the interannual SSH signals across the subarctic current, first-mode baroclinic Rossby wave responses are again found to be important. To adequately explain the magnitude of the observed SSH signals, we find that eddy dissipation, with a linear e-folding dissipation rate of several years, is required.