Are El Niño events becoming more prevalent?

The hypothesis that as the earth warms, El Niño events are becoming more prevalent is tested. Particular emphasis is placed on the observed tendency toward higher sea-level pressure at Darwin, Australia pointed out by Trenberth and Hoar (1996). Pressure at Darwin is representative of the western pole of the Southern Oscillation, the atmospheric expression of ENSO. Results presented here support the notion of a sustained positive pressure tendency at Darwin, but not necessarily a tendency toward the warm polarity of the ENSO cycle. Sea-level pressure over Tahiti, the eastern pole of the Southern Oscillation, has not fallen significantly in recent decades, as would be expected if the tendency at Darwin were entirely in response to ENSO, and the warming of the equatorial Pacific during this period, relative to the global oceans as a whole, was quite modest in comparison to the large year-to-year varriability of sea-surface temperature in this region. Much of the tendency toward higher pressure at Darwin may be a reflection of subtle trends in tropical sea surface temperatures that are not ENSO-related.