Future changes in the influence of the Southern Annular Mode on South Pacific chlorophyll-a in the CESM2 large ensemble

초록

The Southern Annular Mode (SAM) is the leading mode of atmospheric variability in the Southern Hemisphere, exerting a significant influence on ocean circulation and marine productivity. In the present climate, positive SAM phases enhance Ekman-driven northward nutrient transport, leading to delayed increases in chlorophyll-a (Chl-a) concentrations in the mid-latitude South Pacific. However, the extent to which this relationship will persist under future climate change remains uncertain. Here, we investigate future changes in the SAM-Chl-a relationship using Community Earth System Model version 2 Large Ensemble simulations under the shared socioeconomic pathway3-7.0 scenario. A subset of ensemble members that successfully reproduce observed present-day SAM-Chl-a lag correlations was selected for analysis. In the historical period (1970-2014), Chl-a anomalies in the mid-latitude eastern South Pacific show a distinct peak approximately three months after positive SAM events, associated with enhanced northward nutrient transport. In contrast, future projections (2070-2100) reveal a weakened or absent lagged response, coinciding with reduced westerly wind anomalies and diminished Ekman transport. These results suggest that the SAM's influence to regulate marine productivity in the mid-latitude South Pacific may decline under continued climate change. This has important implications for future biogeochemical dynamics and highlights the need for high-resolution modeling to better represent fine-scale ocean-atmosphere interactions.

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