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Pacific decadal variability has strong impacts on the statistics of weather, atmosphere extremes, droughts, hurricanes, marine heatwaves, and marine ecosystems. Sea surface temperature (SST) observations show that the variance of the El Nino-like decadal variability has increased by similar to 30% (1920-2015) with a stronger coupling between the major Pacific climate modes. Although we cannot attribute these trends to global climate change, the examination of 30 members of the Community Earth System Model Large Ensemble (LENS) forced with the RCP8.5 radiative forcing scenario (1920-2100) suggests that significant anthropogenic trends in Pacific decadal variance will emerge by 2020 in response to a more energetic North Pacific Meridional Mode (PMM)-a well-known El Nino precursor. The PMM is a key mechanism for energizing and coupling tropical and extratropical decadal variability. In the LENS, the increase in PMM variance is consistent with an intensification of the winds-evaporation-SST thermodynamic feedback that results from a warmer mean climate.

Plain Language Summary Decadal variability modulates weather, droughts, hurricanes, and marine heatwaves in the Pacific Ocean with dramatic societal and ecological impacts. Understanding how decadal variability may change in a warming climate remains difficult to assess because of the limited observational record and poor reproducibility of decadal dynamics in climate projection models. We combine theory with available reanalysis products and a large climate model ensemble, to show that the Pacific decadal variance increases under anthropogenic forcing as a result of stronger thermodynamic coupling between ocean and atmosphere. Given that thermodynamic coupling is also increasing in other ocean basins, this study provides a mechanistic framework to understand the amplification of climate variability on global scales under anthropogenic forcing.