Non-Stationary Interdecadal Variability in the North Atlantic Oscillation Revealed in a 185-Year Coral-Based Sea Surface Temperature Record from the Western Tropical North Atlantic

The North Atlantic Oscillation (NAO) is primarily a boreal winter climate phenomenon defined by fluctuations in atmospheric pressure between the Bermuda-Azores High and the Icelandic Low. The NAO is often characterized by a distinct tripole pattern of sea surface temperature (SST) anomalies across the North Atlantic, and significantly influences temperature and precipitation patterns, with far-reaching environmental and socioeconomic impacts. Previous studies on the NAO from paleo-archives and instrumental records reveal that the long-term variance of the NAO has been increasing since the 1800s, suggesting changes in its long-term behavior. However, our understanding of the NAO remains limited by short instrumental records. Moreover, most marine-based NAO reconstructions are in high-latitude or subtropical regions, or outside the Atlantic basin. Thus, highlighting the need for additional marine-based records from the tropical Atlantic. Here, we present a high-resolution 185-year Sr/Ca-based boreal winter (DJFM) SST reconstruction (1831-2015) in the western tropical North Atlantic from a massive Colpophyllia natans coral collected at Kelleston Drain, Tobago. We found that the coral-derived DJFM SST record significantly captures regional SST trends (p<0.05). A 20-year smoothed reconstruction of DJFM SST reveals significant interdecadal co-variability with the NAO since the mid-1800s, with two notable exceptions: (1) around the mid-1920s, and (2) after the mid-1970s. While multiple factors may have contributed to the observed decoupling between the NAO and tropical Atlantic SST, both events were found to have coincided with an eastward displacement and expansion of the Icelandic Low-pressure center of action, leading to a breakdown in the NAO-SST tripole relationship. Our findings suggest that boreal winter tropical North Atlantic SSTs are closely linked to interdecadal NAO variability and are sensitive to the non-stationary behavior of the NAO, which is likely driven by changes in the spatial orientation and configuration of its atmospheric centers. Our work contributes to filling a critical gap in marine-based records of the NAO in the tropical North Atlantic and underscores the importance of understanding long-term decadal-scale climate variability in this region.