The Role of Bjerknes and Shortwave Feedbacks in the Tropical Pacific SST Response to Global Warming

The evolution of tropical sea surface temperatures (SSTs) in response to greenhouse warming is of great societal and scientific interest. Most state-of-the-art climate models predict a mean "El Nino-like" warming pattern by century-end, characterized by greater warming over the Pacific cold tongue compared to the western warm pool. However, it is unclear which proposed mechanism dominates in this response. Here, we present partially coupled abrupt CO2 doubling experiments in which surface wind stress and shortwave heating are overridden to values from a control simulation. Contrary to previous studies, we find that experiments with overriding of surface wind stress exhibit only 58% of the full reduction in east-west SST contrast. When both surface wind stress and shortwave flux are overridden, only 34% of the full reduction remains, controlled by spatially-varying evaporative cooling. These results underscore the importance of Bjerknes and shortwave feedbacks in the tropical Pacific SST response to global warming. Sea surface temperature patterns over the tropical Pacific have an outsized impact on regional and global climates, as exemplified by the impact of present day El Nino events. As such, how tropical SST patterns will evolve under global warming has been a topic of keen scientific interest and scrutiny. While most state-of-the-art climate models show a transition to a more "El Nino-like" mean state by the end of the century, the precise reason why remains debated. Here, we present a method that estimates the contributions of changing surface winds and solar radiation to the tropical ocean warming pattern simulated by a state-of-the-art climate model. While some previous studies have argued that changes in surface winds are unimportant, here we show that the weakening of surface easterly winds that maintain a colder eastern Pacific is a major factor leading to a reduced east-west temperature contrast. Climate models predict an eastern equatorial Pacific warming pattern in the tropical Pacific by the end of the 21st centuryWe use partially coupled climate simulations to assess contributions of wind stress, shortwave, and evaporative feedbacks to this patternWeakening of surface wind stress is found to be a major factor leading to a reduced zonal SST gradient over the tropical Pacific