Seasonal to decadal modulation of the impact of El Niño-Southern Oscillation on New Caledonia (SW Pacific) rainfall (1950-2010)

Abstract

[1] New Caledonia (NC; ∼166°E, 22°S) rainfall anomalies are more sensitive to central Pacific (CP) El Nino and La Nina events than to those exhibiting highest sea surface temperature (SST) anomalies in the eastern Pacific (EP). The linear relationship between NC rainfall anomalies and CP SST indices peaks from September to March (S–M). The seasonal S–M atmospheric anomalies observed in the South West (SW) Pacific during the warm CP events are highly dissimilar to the EP ones, while there are more similarities during the cold events with a higher amplitude during the CP ones. The warm CP events strengthen the southern Hadley cell around NC longitudes, with positive rainfall anomalies in the equatorial Pacific leading to an anomalous release of latent heat in the upper troposphere and an increased subsidence in the SW Pacific. Atmospheric anomalies are strongest in September–November because of a combination of a rather strong zonal SST gradient with the warmest SST in the equatorial Pacific just west of the dateline. The cold CP and EP events are associated with a southwestward shift of the South Pacific Convergence Zone with strongest atmospheric anomalies during the CP events. Squared wavelet coherence between NC rainfall and Nino 4 SST index shows that their negative correlations are mostly carried by two distinct timescales: the classical El Nino–Southern Oscillation (i.e., 3–6 years) variability and a quasi-decadal one (i.e., 10–12 years). The high-frequency (>1/8 cycle per year) correlations peak around Christmas and are quasi-stationary since 1950, whereas the low-frequency ones (<1/8 cycle per year) peak from the austral autumn to the austral spring and have strengthened from ∼1975 to 1980 onward with a subtle warming trend in the equatorial Pacific near the dateline.