Eddies Drive Meridionally Asymmetric Upwelling in the Equatorial Pacific

Abstract 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 1/20°-resolution regional ocean simulation of the equatorial Pacific cold tongue encompassing 95°–170°W 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 day −1 on average near 2°N (almost half the peak upwelling velocity at 50 m on the equator due to wind) compensated by weaker downwelling at about 2°S and 5°N. This eddy-driven meridional circulation largely explains the meridional asymmetry in climatological mean equatorial Pacific upwelling. Significance Statement Global coupled atmosphere–ocean models have difficulty forecasting subseasonal to seasonal weather, in part due to difficulties simulating and observing upwelling in the equatorial Pacific Ocean. High-resolution regional ocean models and observations reveal a previously unknown meridional asymmetry in the upwelling in the Pacific as well as its physics. This discovery has the potential to guide future observations and model development that could improve subseasonal to seasonal predictions of the ocean and the weather.