Decadal Atmosphere-Ocean Variability

Unforced climate variability on decadal and longer time scales is of interest both because of its potential predictability and because it is a source of noise that makes it difficult to determine the regional impacts of anthropogenic climate change. Our research focuses on identifying patterns of sea-surface temperature (SST) variability that are associated with low-frequency climate variability in the world’s ocean and understanding their mechanisms and impacts. Our work has provided insight into the role of ocean circulation in maintaining persistent SST anomalies, the relative role of air-sea fluxes of heat and momentum in ocean circulation dynamics, and the feedback of persistent SST anomalies onto the atmospheric circulation.

Our work on Pacific climate variability (Download Wills et al. 2018; Download Wills et al. 2019b) has shown that the decadal variability of Pacific SSTs is more independent of El Niño than previously thought, has improved our ability to separate the signals of global warming, the Pacific Decadal Oscillation (PDO), and El Niño in observations, and has established the role of the North Pacific subpolar gyre variability in the PDO. Our work on Atlantic climate variability (Download Wills et al. 2019a; Download Oldenburg et al. 2021; Download Årthun et al. 2021) has improved the mechanistic understanding of the Atlantic Multidecadal Oscillation (AMO), which results from the two-way interactions between the Atlantic Meridional Overturning Circulation (AMOC) and the overlying atmosphere. Our work has also helped to quantify the impacts of different modes of variability on Earth's energy budget and global-mean surface temperature (Download Wills et al. 2021).

One major outstanding question about decadal atmosphere-ocean variability is about the extent to which midlatitude SST anomalies impact the atmospheric circulation. This impact of ocean anomalies on the atmospheric circulation is key to determining how predictable regional climate anomalies are on decadal timescales. Our work has shown that the impact of midlatitude SST anomalies on the atmospheric circulation is larger at higher resolution (Download Wills et al. 2024), and the Ph.D. project of Joas Müller focuses on understanding the mechanisms of this larger response at higher resolution and the consequences of the larger response for decadal climate prediction.