Observed strengthening of the zonal sea surface temperature gradient across the equatorial Pacific Ocean

Decadal variations of very small amplitude (~0.3°C in sea surface temperature) in the tropical Pacific Ocean, the genesis region of the interannual El Niño–Southern Oscillation (ENSO) phenomenon, have been shown to have powerful impacts on global climate. Future projections from different climate models do not agree on how this critical feature will change under the influence of anthropogenic forcing. A number of attempts have been made to resolve this issue by examining trends from the 1880s to the present, a period of rising atmospheric concentrations of greenhouse gases. The most recent concluded that the three major data sets disagreed on the trend in the critical equatorial gradient of sea surface temperature (SST). Using a corrected version of one of these data sets, and extending the analysis to the seasonal cycle, we show here that all agree that the equatorial SST gradient has strengthened from 1880–2005 during the boreal fall when this gradient is normally strongest. This result appears to favor a theory for future changes based on ocean dynamics over one based on atmospheric energy considerations. Both theories incorporate the expectation, based on ENSO theory, that the zonal sea level pressure (SLP) gradient in the tropical Pacific is coupled to SST and should therefore strengthen along with the SST gradient. While the SLP gradient has not strengthened, we find that it appears to have weakened only during boreal spring, consistent with the SST seasonal trends. Most of the coupled models included in the IPCC Fourth Assessment Report underestimate the strengthening SST gradient in boreal fall, and show almost no change in the SLP gradient in any season. Our observational analyses suggest that both theories are at work but with relative strengths that vary seasonally and that the two theories need not be inconsistent with each other.