Response of the mixed layer depth and subduction rate in the subtropical Northeast Pacific to global warming
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Abstract: The response of the mixed layer depth (MLD) and subduction rate in the subtropical Northeast Pacific to global warming is investigated based on 9 CMIP5 models. Compared with the present climate in the 9 models, the response of the MLD in the subtropical Northeast Pacific to the increased radiation forcing is spatially non-uniform, with the maximum shoaling about 50 m in the ensemble mean result. The inter-model differences of MLD change are non-negligible, which depend on the various dominated mechanisms. On the north of the MLD front, MLD shallows largely and is influenced by Ekman pumping, heat flux, and upper-ocean cold advection changes. On the south of the MLD front, MLD changes a little in the warmer climate, which is mainly due to the upper-ocean warm advection change. As a result, the MLD front intensity weakens obviously from 0.24 m/km to 0.15 m/km (about 33.9%) in the ensemble mean, not only due to the maximum of MLD shoaling but also dependent on the MLD non-uniform spatial variability. The spatially non-uniform decrease of the subduction rate is primarily dominated by the lateral induction reduction (about 85% in ensemble mean) due to the significant weakening of the MLD front. This research indicates that the ocean advection change impacts the MLD spatially non-uniform change greatly, and then plays an important role in the response of the MLD front and the subduction process to global warming.
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Key words:
- mixed layer depth /
- mixed layer depth front /
- subduction /
- ocean advection /
- non-uniform
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Figure 1. The February–March mean MLD (shading, interval: 20 m) from the historical simulation of the 9 models (a–i) and ensemble mean result (j). The contours indicate the mean MLD difference (RCP8.5 minus historical), dashed lines (negative values) means the MLD shoals in a much warmer climate. Magenta points represent the location of the MLD front in historical simulation.
Figure 2. The MLD maximum (a), the intensity of the MLD front mean (b) during February–March in the 9 CMIP5 models and ensemble mean result. The colors represent historical simulation (blue), RCP8.5 experiment (red), and their differences (gray). Percentages represent changes compared to corresponding historical simulation.
Figure 3. Differences (RCP 8.5 minus historical) of Ekman pumping rate (shading, downward is positive) mean during February–March of the 9 models (a–i) and ensemble mean result (j). The positive values indicate the Ekman pumping trend to downward and contribute to deepening the MLD. The contours indicate the difference (RCP8.5 minus historical) in MLD mean during February–March (contours, interval: 10 m). The magenta points represent the location of the MLD front in historical simulation.
Figure 5. Differences (RCP 8.5 minus historical) of temperature advection (shading) mean during February–March of the 9 models (a–i) and ensemble mean result (j). The positive values indicate warm advection increased or cold advection decreased in the upper ocean and contribute to shoaling the MLD. The contours indicate the difference (RCP8.5 minus historical) in MLD mean during February–March (contours, interval: 10 m). The magenta points represent the location of the MLD front in historical simulation.
Figure 4. Differences (RCP 8.5 minus historical) of heat flux (shading) mean during February–March of the 9 models (a–i) and ensemble mean result (j). The positive values indicate the ocean release more heat to the atmosphere and contribute to deepening the MLD. The contours indicate the difference (RCP8.5 minus historical) in MLD mean during February–March (contours, interval: 10 m). The magenta points represent the location of the MLD front in historical simulation.
Figure 6. Differences (RCP 8.5 minus historical) of the subduction rate (shading, downward is positive) and subduction in historical simulation (contours, only over
$ 2\!\times\! {10}^{6} $ m/s are shown) during February–March in the 9 models (a–i) and ensemble mean (j) result. The magenta points represent the location of the MLD front in historical simulation.Table 1. The 9 models from CMIP5 analyzed in this study
Model Institution Country CCSM4 National Center for Atmospheric Research USA CNRM-CM5 Centre National de Recherches Meteorologiques France GFDL-ESM2G NOAA/Geophysical Fluid Dynamics Laboratory USA GFDL-ESM2M NOAA/Geophysical Fluid Dynamics Laboratory USA IPSL-CM5A-LR Institute Pierre-Simon Laplace France IPSL-CM5A-MR Institute Pierre-Simon Laplace France MIROC-ESM University of Tokyo Japan MPI-ESM-LR Max Planck Institute for Meteorology Germany MRI-CGCM3 Meteorological Research Institute Japan 
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Table 2. Qualitative comparison of the contribution of the three major factors to MLD change after global warming
ID Model Northern region Southern region Contribution of factor MLD
trendContribution of factor MLD
trend$ {w}_{\rm{e}} $ Q Tad $ {w}_{\rm{e}} $ Q Tad a CCSM4 + – – $ \downarrow $ + + + $ \uparrow $ b CNRM-CM5 + – + $ \downarrow $ + – – $ \downarrow $ c GFDL-ESM2G + – + $ \downarrow $ + + + $ \uparrow $ d GFDL-ESM2M + – – $ \downarrow $ + + – $ \downarrow $ e IPSL-CM5A-LR – + + $ \downarrow $ – – + $ \uparrow $ f IPSL-CM5A-MR – + + $ \downarrow $ – – + $ \uparrow $ g MIROC-ESM – + – $ \downarrow $ – + – $ \downarrow $ h MPI-ESM-LR + + – $ \downarrow $ + + – $ \downarrow $ i MRI-CGCM3 – + + $ \downarrow $ – + – $ \downarrow $ Note: $ {w}_{\rm{e}} $, Q, and Tad represent the Ekman pumping, net heat flux, and upper-ocean heat advection, respectively; + means it contributes to the MLD deepening, while – means it helps to shallow the MLD; the red color represents that factor is dominant to the actual MLD change; MLD trend represents the final change of MLD, $ \uparrow $ means the MLD deepened, and $ \downarrow $ means the MLD shallowed.
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