Topographic Setting Drives the Imprint of the Madden–Julian Oscillation ( <scp>MJO</scp> ) on Tree Growth in the Northern Sierra Nevada

ABSTRACT Understanding how large‐scale tropical climate patterns, such as the Madden–Julian Oscillation (MJO), influence tree growth is crucial for improving regional climate projections and water resource management. We investigate whether tree growth from conifer forests in the northern Sierra Nevada, United States can serve as a proxy for MJO variability. Increment cores were collected from Pinus jeffreyi (site = JSJ; n = 21) and Abies magnifica (site = JSR; n = 44) trees growing on opposite slopes, resulting in chronologies that spanned 1690–2022 and 1673–2022, respectively. We used these records to assess climate drivers of tree growth from local (e.g., temperature, precipitation, drought) to broad scales (e.g., ocean–atmosphere conditions over the Indo‐Pacific, MJO indices). Both records exhibit significant correlations ( p &lt; 0.05) with local climate variables, particularly moisture availability, though with key differences in sensitivity most likely related to species‐specific traits and topographic setting. As such, JSJ, positioned on a south‐facing slope, has markedly stronger connections with large‐scale climate drivers, including zonal ( u ) and meriodional ( v ) winds, sea‐level pressure (SLP) and sea surface temperature (SST), particularly across the tropical Pacific, while the JSR (from a north‐east facing slope) shows more localised responses. Notably, JSJ demonstrated significant correlations with multiple MJO indices ( p &lt; 0.01), marking the first documented link between MJO dynamics and tree‐ring data, whereas no significant MJO correlations were found for JSR. We show that relationships between JSJ and early MJO phases (1–3) have strengthened in recent decades, while those with later phases (6–8) peaked in the early 2000s but have since weakened. These findings suggest that the MJO influences tree growth through its modulation of atmospheric rivers and precipitation patterns, with key shifts aligning with California's ongoing drought. This study provides the first published evidence of an MJO signal in tree rings, offering the potential to develop a novel proxy for past MJO variability. We underscore the importance of understanding tropical–extratropical climate linkages and their impact on regional ecosystems, with implications for improving climate projections and managing water resources under future climate scenarios. PACS: 0000, 1111 MSC2000 Classification: 0000, 1111.