4.7 Article

The Thermocline Biases in the Tropical North Pacific and Their Attributions

Journal

JOURNAL OF CLIMATE
Volume 34, Issue 5, Pages 1635-1648

Publisher

AMER METEOROLOGICAL SOC
DOI: 10.1175/JCLI-D-20-0675.1

Keywords

Climate models; Model errors; Model evaluation/performance

Funding

  1. National Natural Science Foundation of China [41906007, 41690122(41690120), 41705082, 41421005]
  2. National Key Research and Development Program of China [2017YFC1404102(2017YFC1404100)]
  3. Strategic Priority Research Program of Chinese Academy of Sciences [XDB 40000000, XDB 42000000]
  4. Shandong Taishan Scholarship
  5. Earth System Grid Federation (ESGF)

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The study found thermocline biases in the tropical North Pacific in CMIP6 models, with a pronounced bias in the northeastern tropics, primarily due to deficiencies in simulating surface wind stress curl and oceanic background diffusivity.
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 degrees 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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