Following a significant increase in globally averaged surface temperatures during the last quarter of the 20th century, this warming trend decelerated between 1998 and 2013. Because the slowdown did not match the sustained increase in anthropogenic greenhouse gas emissions, this so-called global warming hiatus triggered intense scientific and public debate. Numerous scientists have argued the hiatus resulted from a redistribution of heat from the upper to the deep oceans that is associated with natural variations in Earth’s climate system such as the El Niño–Southern Oscillation and the Pacific Decadal Oscillation.
Now Cha et al. present evidence that since 2011, the tropical Pacific Ocean has been shifting toward more El Niño–like conditions that coincide with a resumption of global warming. Using hindcast simulations from the Regional Oceanic Modeling System combined with ensemble empirical mode decomposition statistical analyses, the team determined that the tropical Pacific is experiencing a slow, decadal-scale shift that is distinct from interannual, El Niño–like variability.
The results indicate the observed changes are strongly correlated with a shift in trade wind patterns related to an alteration in the phase of the Pacific Decadal Oscillation. Because these winds help control the speed of the Equatorial Undercurrent, the new pattern has altered the tropical Pacific’s upper ocean circulation and contributed to the regional redistribution of heat, resulting in sea surface warming in the central eastern tropical Pacific. The authors argue these changes have contributed to substantial increases in sea level in the central eastern tropical Pacific, as well as subsurface cooling and corresponding decreases in sea level in the western tropical Pacific.
By linking changes in trade wind patterns to ocean circulation and surface warming trends, the researchers offer convincing support that the Pacific Decadal Oscillation and other natural, longer-term variations in climate may contribute substantially to ocean warming. Because this proposed mechanism has important implications for predicting sea level and ocean warming on decadal timescales, the authors argue that ocean-atmosphere interactions—which were not included in this study—should be incorporated into future research to better understand climate-related processes in the tropical Pacific. (Geophysical Research Letters, https://doi.org/10.1029/2018GL080651, 2018)
—Terri Cook, Freelance Writer