Regional climate variability and ocean heat transport in the southwest Pacific Ocean

The winter of 1992 was the coldest on record in New Zealand since the 1940s. Temperatures in New Zealand were as much as 3øC below average, with heavy rain and unusual snow. The oceanic surface layer in the southwest Pacific was also anomalously cold over the same period. A World Ocean Circulation Experiment (WOCE) hydrographic section (P14C) between Auckland and Fiji during September 1992 found cold sea surface temperatures and deep mixed layers near New Zealand when compared to 8 years of high-resolution expendable bathythermograph (XBT) temperature measurements collected along the same transect. High nutrient and low dissolved oxygen concentrations in the surface layer indicated recent entrainment of thermocline waters. The Auckland to Fiji XBT section is one of three WOCE high-resolution XBT survey lines in the "Tasman Box" region, whose boundaries are Auckland-Fiji, Fiji-Brisbane and Wellington-Sydney transects plus the Australian coast. Geostrophic shear and transport were estimated from 10 realizations of the Tasman Box during the period 1991-1993. The time series of geostrophic transport shows that following a convergence in late 1991, early in 1992 there was a substantial divergence of mass in the upper waters, equivalent to a thinning of the warm water layer. The phase of this anomalous divergence is matched to an observed amplification of the seasonal oceanic heat storage cycle in 1991-1992. The top 200-m average temperature was warmer in the 1992 summer than in 1991 or 1993, but the winter of 1992 was the coldest of the 8-year record along the Auckland-Fiji line. The divergence (thinning) of the warm water layer appears to have preconditioned the region for the exceptionally cold 1992 winter. The alternative, cool conditions from anomalous air-sea heat exchange caused by variability in the wind field, is considered unlikely as large fluctuations in heat loss are not observed in the air-sea flux data during this period. The severe weather conditions and anomalous ocean heat transport are most likely related to the prolonged E1 Nifio-Southern Oscillation episode that began in early 1991.