Low-frequency ocean-atmosphere variability across the tropical western Pacific

Oceanic and atmospheric data from the tropical western Pacific are analyzed to describe decadal-scale trends during the last 20 years, and these low-frequency trends are compared with shorter-term Southern Oscillation-related variations. Regional indices of western and central equatorial Pacific SST exhibit significant upward trends in recent decades. The decadal variability in the tropical Pacific is large enough relative to interannual variability to significantly affect the interpretation of standardized SST anomaly indices used to monitor Southern Oscillation phenomena. Specific humidity in the tropical western Pacific boundary layer exhibits a statistically significant upward trend consistent with previously published results based on a shorter data record. The convective instability of the tropical troposphere is increasing, but two indices related to precipitation show no evidence of a trend. These trends cannot be explained as an aggregate of the effects of more frequent El Nino warm events in recent years because the tropical western Pacific response to El Nino includes negative (i.e., dry) boundary-layer humidity anomalies and decreased convective instability. On interannual timescales there seems to be a distinct separation between the processes affecting tropospheric temperature within and above the tropical western Pacific boundary layer.