Interannual and Decadal Temperature Variability in the Southwest Pacific Ocean between 1955 and 1988

Abstract

The spatial and temporal variability of the southwest Pacific Ocean is examined with the aim of describing the physical processes operating on interannual and decadal timescales. The study takes advantage of a new temperature atlas of the upper 450 m of the southwest Pacific Ocean, obtained from 40?000 bathythermograph profiles between 1955 and 1988. Rotated principal components analysis was used to filter the important spatial and temporal scales of temperature variability in the data. Three different analyses are presented. They include two intraocean analyses and a joint analysis of subsurface ocean temperature, sea level pressure, and surface winds. The dominant El Niño mode describes the large vertical excursions of the thermocline in the western tropical Pacific in response to atmospheric forcing at a 3?6-month lag. More importantly, most of the retained modes, outside of the equatorial region, have time variations that correlate with El Niño. One ocean mode, with a spatial pattern representing sea surface temperature anomalies in the western Coral Sea (linked to the interannual migration of the South Pacific convergence zone), correlates significantly with (at the 99% level) and leads (by 3?6 months) the Southern Oscillation index (SOI), suggesting that sea surface temperature anomalies in this region may be a useful indicator for the onset of El Niño. A separate mode whose spatial pattern corresponds to the main oceanographic gyre also shows statistically significant temperature variations in phase with, or slightly leading, the SOI. The main decadal variations occur in the midlatitudes, in the subtropical gyre, and in another mode associated with sub-Antarctic mode water (SAMW). The subtropical gyre warmed to a maximum in the mid-1970s and has been cooling since. In the SAMW a long-term warming of the upper 100 m of the southwest Tasman Sea is identified between 1955 and 1988. The depth-integrated warming in this region is found to be about 0.015°C yr?1, representing a contribution to sea level rise, through thermal expansion, of about 0.3 mm yr?1.