Interdecadal variability in sea level pressure (SLP) and sea surface temperature (SST) anomalies in the Pacific Ocean was "quasiperiodic" from 1900-91. The coherent variability of this phenomenon is investigated using gridded observational data from the turn of the century (SST and SLP) and of upper ocean heat content (HS) from the recent two and a half decades. The nominal cycle in atmosphere-ocean variables is roughly two decades long, but growth and decay can happen on a shorter timescale (e.g., half a cycle or so). The authors divide the full cycle into four phases: An onset phase, during which a weak SLP anomaly pattern off Japan takes approximately 2-4 yr to expand eastward, leads to large SLP anomalies in the region of the Aleutian low. A quasistationary growth phase, with the midlatitude SLP anomaly pattern in the eastern ocean, intensifies over a 2-4 yr period. The persistent SLP anomalies evolve in concert with large SST (and HS anomalies) of the same polarity located to the south-southwest along the subarctic frontal zone (SAFZ). During the growth phase, SST anomalies with opposite polarity develop to the east, associated with anomalous atmospheric circulation along the North American coastline. Near the end of the growth phase a narrow tongue of enhanced SST anomalies is found along the subtropical front near Hawaii, slightly to the west of the subduction region associated with the subtropical gyre. Following is a decay phase during which the midocean SST and SLP anomalies weaken, while the HS anomalies persist in the southern part of the subtropical gyre. Concomitantly, a weak anomalous east-west SLP gradient is established in the tropical Pacific and could contribute to the interdecadal variability of the southern oscillation index. Finally, a return phase occurs (identical to the onset phase but with opposite polarity), during which SST anomalies move from the Alaskan gyre and from the center of the subtropical gyre to merge onto the SAFZ. During the evolution of the interdecadal phenomenon, the overall structure of IIS highlights the notion that the subtropical gyre integrates the thermal and dynamical forcing induced by the persistent surface circulation anomaly. Since only the oceanic anomalies retain their character throughout the seasonal cycle, the interdecadal phenomenon may owe its existence to coupled ocean-atmosphere interaction in which ocean surface anomalies feed back on the atmosphere providing the necessary links between consecutive winter seasons. It is suggested that this can occur through interactions in either midlatitudes (delayed-negative feedback) and/or the Tropics.
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