How Fast is the Mean Upwelling in the Equatorial Pacific Ocean?
Journal Article
Overview
abstract
Abstract; Theory for the steady, wind-driven ocean circulation predicts upwelling at the equator under easterly winds, easily confirmed by observations of temperature and chlorophyll. The time-mean upwelling in the eastern equatorial Pacific is critical to the surface heat balance and the global carbon cycle and has been proposed to play a key role in shaping the response of the tropical Pacific to anthropogenic radiative forcing. Estimating the time mean of any variable in a noisy system from direct measurements, not to mention one at the limit of measurement uncertainty like vertical velocity, is fraught with challenges. Since the 1960s, oceanographers have attempted to circumvent the latter problem by applying the continuity equation to measured horizontal divergence. A systematic review of published estimates reveals a remarkable range explained by differences in sampling strategy, leaving one unable to reject the highest of the range. After accounting for sampling, even Wyrtki’s famously estimated 1 m day−1 is consistent with both the low and high extrema. Careful examination of modern observations and techniques indicates a time-mean upwelling in the equatorial Pacific of 13.1 ± 6.9 m day−1—an order of magnitude faster than previously assumed and with a very sharp equatorial peak. The latest generation of climate models simulates time-mean upwelling velocities of ∼3.1 m day−1, the spread of which is well explained by resolution, and the remaining offset is probably related to parameterizations. It is proposed that the relatively slow simulated upwelling contributes to the mismatch between simulated and observed trends in the equatorial Pacific with important implications for future climate projections.; ; Significance Statement; The equatorial Pacific Ocean drives a vital engine of Earth’s climate, cooling the surface and cycling heat and carbon between the ocean and atmosphere. This study reveals that upwelling—the slow upward movement of deep, cold water to the surface—is much faster than previously thought and tightly focused near the equator. These findings help explain why climate models struggle to capture key trends in the tropical Pacific, offering a clearer view of how this region responds to global warming and its role in shaping the planet’s future climate.;
