The Thermocline Biases in the Tropical North Pacific and Their Attributions

Type Article
Date 2021-03
Language English
Author(s) Zhu Yuchao1, 2, 3, Zhang Rong-Hua1, 2, 3, 4, 5, Li Delei1, 2, 3, Chen Dake6, 7
Affiliation(s) 1 : CAS Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China
2 : Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao, China
3 : Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao, China
4 : Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xian, China
5 : University of Chinese Academy of Sciences, Beijing, China
6 : State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China
7 : Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China
Source Journal of Climate (0894-8755) (American Meteorological Society), 2021-03 , Vol. 34 , N. 5 , P. 1635-1648
DOI 10.1175/JCLI-D-20-0675.1
Keyword(s) Climate models, Model errors, Model evaluation/performance
Abstract

The tropical thermocline plays an important role in regulating equatorial sea surface temperature (SST); at present, it is still poorly simulated in the state-of-the-art climate models. In this paper, thermocline biases in the tropical North Pacific are investigated using the newly released CMIP6 historical simulations. It is found that CMIP6 models tend to produce an overly shallow thermocline in the northwestern tropics, accompanied by a deep thermocline in the northeastern tropics. A pronounced thermocline strength bias arises in the tropical northeastern Pacific, demonstrating a dipole structure with a sign change at about 8°N. These thermocline biases are accompanied with biases in the simulations of oceanic circulations, including a too weak North Equatorial Countercurrent (NECC), a reduction in water exchanges between the subtropics and the equatorial regions, and an eastward extension of the equatorward interior water transport. The causes of these thermocline biases are further analyzed. The thermocline bias is primarily caused by the model deficiency in simulating the surface wind stress curl, which can be further attributed to the longstanding double-ITCZ bias in the tropical North Pacific. Besides, thermocline strength bias can be partly attributed to the poor prescription of oceanic background diffusivity. By constraining the diffusivity to match observations, the thermocline strength in the tropical northeastern Pacific is greatly increased.

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