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An ocean modeling study to quantify wind forcing and oceanic mixing effects on the tropical North Pacific subsurface warm bias in CMIP and OMIP simulations

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Abstract

Sea surface temperature (SST) bias in the climate models has been a focus in the past, but subsurface temperature biases have not received much attention yet. In this study, subsurface temperature biases in the tropical North Pacific (TNP) are investigated by analyzing the CMIP6, CMIP5 and OMIP products, and by performing ocean model simulations. It is found that almost all the CMIP and OMIP simulations have a pronounced subsurface warm bias (SWB) in the northeastern tropical Pacific (NETP), and the model developments over the past decade do not indicate obvious improvements in bias pattern and magnitude from CMIP5 to the latest version CMIP6. This SWB is primarily caused by the model deficiencies in the simulated surface wind stress curl (WSC) in the NETP, which is too weak to produce a sufficient Ekman upwelling, a bias that also exists in OMIP simulations. The uncertainties in the parameterizations of the oceanic vertical mixing processes also make a great contribution, and it is demonstrated that the estimated oceanic vertical diffusivities are overestimated both in the upper boundary layer and the interior in the CMIP and OMIP simulations. The relationships between the SWB and the misrepresented oceanic vertical mixing processes are investigated by conducting several ocean-only experiments, in which the upper boundary layer mixing is modified by reducing the wind stirring effect in the Kraus-Turner type bulk mixed-layer scheme, and the interior mixing is constrained by using the Argo-derived diffusivity. By applying these modifications to oceanic vertical mixing schemes, the SWB is greatly reduced in the NETP. The consequences of this SWB are further analyzed. Because the thermal structure in the NETP can influence the simulations of oceanic circulations and equatorial upper-ocean thermal structure, the large SWB in the CMIP6 models tends to produce a weak equatorward water transport in the subsurface TNP, a weak equatorial upwelling and a warm equatorial upper ocean.

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Availability of data and material

Not applicable.

Code availability

The data and computer codes used in the paper are available from the corresponding author (e-mail: rzhang@qdio.ac.cn).

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Acknowledgements

The authors wish to thank the anonymous reviewers for their numerous comments that helped to improve the original manuscript. We thank Andreas M. Thurnherr at Lamont-Doherty Earth Observatory for providing the LADDER data. We acknowledge the World Climate Research Programme, which, through its Working Group on Coupled Modelling, coordinated and promoted CMIP6. We thank the climate modeling groups for producing and making available their model output, the Earth System Grid Federation (ESGF) for archiving the data and providing access, and the multiple funding agencies who support CMIP6 and ESGF. This research is supported by the National Natural Science Foundation of China (Grant Nos. 41906007, 41690122(41690120), 42030410, 41705082, 41421005), the National Key Research and Development Program of China (No. 2017YFC1404102(2017YFC1404100)), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB 40000000 and XDB 42000000), and the Shandong Taishan Scholarship.

Funding

This research is supported by the National Natural Science Foundation of China (Grant Nos. 41906007, 41690122(41690120), 42030410, 41705082, 41421005), the National Key Research and Development Program of China (No. 2017YFC1404102(2017YFC1404100)), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB 40000000 and XDB 42000000), and the Shandong Taishan Scholarship.

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Authors and Affiliations

  1. CAS Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, and Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao, 266071, China

    Yuchao Zhu, Rong-Hua Zhang & Delei Li

  2. Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao, China

    Yuchao Zhu, Rong-Hua Zhang & Delei Li

  3. Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xi’an, 710061, China

    Rong-Hua Zhang

  4. University of Chinese Academy of Sciences, Beijing, China

    Rong-Hua Zhang

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  1. Yuchao Zhu
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Correspondence to Rong-Hua Zhang.

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Zhu, Y., Zhang, RH. & Li, D. An ocean modeling study to quantify wind forcing and oceanic mixing effects on the tropical North Pacific subsurface warm bias in CMIP and OMIP simulations. Clim Dyn 58, 999–1014 (2022). https://doi.org/10.1007/s00382-021-05946-y

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