Trends in the ocean mixed-layer depth in the western North Pacific tropical cyclone basin

The upper ocean mixed-layer depth (MLD) has been recognized as a crucial factor in understanding the impact of climate change on tropical cyclone (TC) intensity, while the specific MLD variations in the western North Pacific (WNP) basin have received comparatively little attention. Using two ocean reanalysis datasets and the output of 28 CMIP6 models, the climatological mean MLD and its trend in the TC active region are examined. Compared to the ORAS4 and EN4 datasets, the spatial pattern of the climatological mean MLD can be well simulated in the multi-model ensemble (MME). The mean MLD decreases with increasing latitude and maintains the maximum in the equatorial region. The spatial patterns of MLD long-term (1958-2017) and short-term (1979-2017) trends are deepening in the TC active region, with the relatively large positive trends occurring to the southeastern WNP primarily between 5 degrees N and 15 degrees N. However, the simulated trend in the MME is dominant with shoaling MLD, and the shoaling of the MLD is also projected under the SSP5-8.5 scenario. It is suggested that the bias on the simulated shoaling of the MLD in CMIP6 models may result from the overestimated enhancement of the upper-ocean stratification. Thus, there is a discrepancy between the observed and simulated MLD trends in the WNP basin, which may have implications for TC intensity change under global warming scenarios. The historical simulated changes of the mixed-layer depth (MLD) are dominant with negative trends in the TC active region in CMIP6 models, while it shows positive trends in observations. The projected shoaling of the MLD in models may result from the overestimated enhancement of the upper-ocean stratification. image