PMEL Publications Abstract
FY 2026
Observation-based estimates of vertical velocity in the tropical upper oceans
Tuchen, F.P., G.R. Foltz, R.C. Perez, and M.J. McPhaden
J. Geophys. Res., 131(4), e2025JC023435, doi: 10.1029/2025JC023435, View open access article at AGU/Wiley (external link) (2026)
Equatorial upwelling plays a crucial role in regulating regional climate variability and biological productivity from seasonal to interannual time scales. Typically, sea surface temperature or surface chlorophyll anomalies are used as proxies for upwelling because of the challenges in directly measuring vertical velocities. Here, we show that using a synthesis product of surface drifter observations, reanalysis winds, and satellite altimetry to infer vertical velocity in the equatorial band (±3°; weq) improves the representation of the spatiotemporal characteristics of weq compared to estimates from other commonly used products. We focus on the annual mean, seasonal cycle, interannual variations, and long-term trends of vertical velocity estimates, derived by applying the continuity equation to horizontal current divergence in the upper 30 m across the tropical Pacific, Atlantic, and Indian Oceans. Mean weq values of 0.6 ± 0.2, 0.3 ± 0.2 and 0.1 ± 0.2 m day−1 are observed in the Pacific (150°W–90°W), Atlantic (20°W–0°), and Indian Ocean (60°E–90°E), respectively. Vertical transport across 30 m is 46 ± 14 Sv in the Pacific and 8 ± 4 Sv in the Atlantic. Interannual variations in weq are primarily driven by climate modes specific to each basin and are up to ±0.4 m day−1 in the Pacific, ±0.1 m day−1 in the Atlantic, and up to ±0.6 m day−1 in the Indian Ocean. From 1993 to 2022, Pacific weq significantly increased by 24 ± 13% (0.14 ± 0.07 m day−1). This increase in Pacific equatorial upwelling aligns with stronger trade winds and intensified upper-ocean currents.
Plain Language Summary. Upwelling along the equator plays a key role in shaping regional climate variations and supporting marine ecosystems by bringing cold, nutrient-rich water from the deep ocean to the surface. This process affects sea surface temperatures, interaction between the atmosphere and the ocean, and biological productivity. However, because vertical water movement is 1,000–10,000 times weaker than horizontal ocean currents, directly measuring its speed is extremely challenging. A combination of ocean surface drifters, reanalysis winds, and satellite altimetry is used to infer estimates of vertical velocity and transport in the tropical upper oceans. This approach likely offers an improved view of the spatial and temporal characteristics of equatorial upwelling. We find that average upwelling speeds near the equator (±3°) are about 0.6 ± 0.2 m day−1 in the Pacific (150°W–90°W), 0.3 ± 0.2 m day−1 in the Atlantic (20°W–0°), and 0.1 ± 0.2 m day−1 in the Indian Ocean (60°E–90°E). Year-to-year changes closely align with well-known climate patterns such as El Niño in the Pacific and regional equivalents in the other oceans. From 1993 to 2022, equatorial upwelling in the Pacific increased by 24 ± 13%. This increase in Pacific equatorial upwelling aligns with stronger winds and intensified upper-ocean currents.