Argo Reveals the Scales and Provenance of Equatorial Island Upwelling Systems. Issue 16 (27th August 2022)

• Record Type: Journal Article
• Authors: Karnauskas, Kristopher B.; Giglio, Donata
• Is Part Of: Geophysical research letters. Volume 49:Issue 16(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-27
• Subjects: upwelling -- equatorial -- Argo -- islands -- Galápagos; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL098744
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Equatorial islands have distinct oceanographic signatures, including cool sea surface temperature and high productivity immediately to their west. It has long been hypothesized that topographic upwelling is responsible for such characteristics—upward deflection by the islands of the eastward‐flowing equatorial undercurrent (EUC). Using 22 years of in situ measurements by Argo, we provide the first direct observations of this process occurring with consistency at two prominent archipelagos in the equatorial Pacific. Argo measurements resolve a clear subsurface thermal fingerprint of vertical divergence at the depth of the EUC, confined to within 100 km of both the Gilbert (∼175°E) and Galápagos Islands (∼90°W). This signal at the Galápagos is well‐reproduced by a high‐resolution ocean reanalysis, enabling the estimation of vertical velocities balancing the zonal convergence of the EUC upon the islands. This sharpened view of the physics underpinning such important tropical ecosystems has implications for strategies to model and predict them. Plain Language Summary: Upwelling fuels marine life by bringing nutrient‐rich water into the sunlit surface layer. Upwelling can also impact the atmosphere, because the water brought to the surface is colder, which affects the wind and clouds. The surprising abundance and diversity of life—from corals to fish to seabirds—around equatorial islands are thought to be caused by a unique form of upwelling. A global array of thousands of floats called Argo is used to reveal the structure of this unique form of upwelling. Unlike most upwelling zones (e.g., California), which are driven directly by the wind combined with Earth's rotation, upwelling at equatorial islands is shown here to be due to a massive underwater current that flows along the equator colliding with the islands. The results are corroborated with a high‐resolution model and recent data from underwater gliders near the Galápagos. These insights have immediate implications for predictions of how such important tropical ecosystems will change in the future. Key Points: Twenty‐two years of Argo profiles reveal the localized topographic upwelling signal west of the two equatorial archipelagos The observed thickening of the thermocline west of the Galápagos is associated with the localized mean upwelling velocity of 12 m per day The blockage of the equatorial undercurrent by islands contributes to the productivity of waters west of the equatorial archipelagos