The Common Weak Biases of Pacific North Equatorial Countercurrent in Ocean Models Are Revealed


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The Pacific North Equatorial Countercurrent (NECC) is a major upper-ocean zonal flow of the wind-driven circulation in the tropical Pacific, flowing eastward across the Pacific Ocean basin between 2°N and 10°N. The NECC transport is about 10-30 Sv eastward out of the warm pool region to the relatively cold eastern Pacific on average. It plays important roles in the volume and heat budget of the warm pool and in shaping the tropical Pacific climate. However, the NECC is not well simulated in almost all stand-along ocean models in the recent second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The strength tends to be weaker than the observation generally and affect the modeled average status and variability of the tropical surface currents. This problem still exists in the Ocean Model Comparison Program (OMIP) of upcoming CMIP6.

Prof. Tseng and his international collaborators systematically investigated the fundamental causes for the model biases which may have a big impact on the simulation of the tropical ocean current. They identified the model biases come mainly from the direct wind forcing used in the CORE-II experiments. The biases may arise from the standard procedure in correcting the wind products using the QuikSCAT satellite winds. Particularly, the effect of the ocean currents on the calculation of momentum flux may be accounted twice during the correction procedure. This work provides important clues to correct the common biases of the climate and ocean models. It is expected to effectively improve the simulation of the tropical upper circulation in the stand-alone ocean models and reduce the impact of model deviation on climate simulation and prediction. This importance has been noted in (see Further information about the associated dynamics and details can be found in Prof. Tseng’s recent paper published in the J. Adv. Model. Earth Sy. (Sun et al., 2019).

Sun, Z., Liu*, H., Lin, P., Tseng*, Y.H., Small, J. and Bryan, F. (2019), “The modeling of the north equatorial countercurrent in the Community Earth System Model and its oceanic component,” J. Adv. Model. Earth Sy., 11, 531-544.

The upper 400 m vertically integrated zonal currents (Units: m2s-1) for (a) POP with CORE-II forcing, and (b) CESM in the equatorial Pacific. (c) The upper 400 m vertically integrated zonal current difference between CESM and POP. (d) The volume transports of NECC (Unit: Sv), which are defined as the meridionally integrated eastward transport between 3°N-10°N, for POP forced with CORE-II (red solid), CESM (blue solid) and Johnson et al. (2002) observation (black dots). The transports of the NECC masked by the observational sampling are also computed for both POP (red dash) and CESM (blue dash), donated as POP-J and CESM-J.