Freshening of the upper thermocline in the North Pacific subtropical gyre associated with decadal changes of rainfall

Repeated observations in the North Pacific subtropical gyre near Hawaii rev eal pronounced freshening (similar to0.15 psu) and cooling (similar to0.5 d egreesC) of the upper thermocline from 1991 through 1997. The freshening ap pears progressively later on deeper isopycnals consistent with subduction o f surface salinity anomalies at higher latitudes and subsequent southward a dvection. Winter rainfall anomalies in the central North Pacific are domina ted by the El Nino/Southern Oscillation (ENSO) phenomenon and by the Pacifi c Decadal Oscillation (PDO), with the wet phase of the PDO leading the obse rved freshening. The reversal of the freshening trend in the upper thermocl ine is related both to extreme drought near Hawaii during the 1997-98 ENSO, and to protracted drought associated with the PDO. The density compensatio n of decadal thermal anomalies by salinity anomalies means that they do not disperse dynamically, and that the hydrological cycle is an active compone nt of decadal climate variability in the North Pacific.