Eddies Drive Meridionally Asymmetric Upwelling in the Equatorial Pacific

Climatological equatorial Pacific upwelling has been quantified observationally and reproduced in numerical simulations. However, the fine-scale structure and the processes that drive it remain unclear. A 1/208-resolution regional ocean simulation of the equatorial Pacific cold tongue encompassing 958–1708W from 1999 through 2018 is used to investigate these physical processes. The simulated upwelling at 50 m is asymmetric across the equator and stronger to the north than to the south, consistent with simulated and observed meridional divergence at 15 m. A two-dimensional Eliassen model of the meridional circulation is formulated to investigate the linear response to disruptions of the dominant thermal wind balance. The linearity of the diagnostic model is then exploited to separate and quantify the circulation owing to eddy fluxes from the dominant wind-driven circulation. A tripolar eddy-driven circulation is found in the top 100 m with upwelling of 0.7 m day²¹ on average near 28N (almost half the peak upwelling velocity at 50 m on the equator due to wind) compensated by weaker downwelling at about 28S and 58N. This eddy-driven meridional circulation largely explains the meridional asymmetry in climatological mean equatorial Pacific upwelling.