Concurrent Decadal Mesoscale Eddy Modulations in the Western North Pacific Subtropical Gyre

Satellite altimeter data of the past two decades are used to investigate the low-frequency mesoscale eddy variability inside the western North Pacific subtropical gyre. Eddy activity modulations with a decadal time scale are detected concurrently within the 18 degrees-28 degrees N band, including the three branches of the Subtropical Countercurrent (STCC) and the Hawaiian Lee Countercurrent (HLCC). Lagging behind the Pacific decadal oscillation (PDO) index by six months, enhanced eddy activities were detected in 1995-98 and 2003-06, whereas the eddy activities were below the average in 1999-2002 and 2009-11. Analysis of the temperature and salinity data that became available after 2001 via the International Argo Program reveals that the modulating eddy activities are due to the decadal change in the upper-ocean eastward shear in the broad-scale STCC HLCC band. By conducting an upper-ocean temperature budget analysis, the authors found that this observed eastward shear change can be effectively accounted for by the decadal-varying surface heat flux forcing. Using the Argo-based temperature and salinity data, it is further found that the decadal subsurface potential vorticity (PV) signals to the north and beneath the STCC HLCC were vertically coherent and not confined to the mode water isopycnals. Adjusting to the PDO-related surface forcing, these subsurface PV anomalies lagged behind the upper-ocean eastward shear signals and likely made minor contributions to generate the decadal-varying eddy signals observed in the western North Pacific subtropical gyre.