What induced the trend shift of mixed-layer depths in the Antarctic Circumpolar Current region in the mid-1980s?
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Abstract: An obvious trend shift in the annual mean and winter mixed layer depth (MLD) in the Antarctic Circumpolar Current (ACC) region was detected during the 1960–2021 period. Shallowing trends stopped in mid-1980s, followed by a period of weak trends. The MLD deepening trend difference between the two periods were mainly distributed in the western areas in the Drake Passage, the areas north to Victoria Land and Wilkes Land, and the central parts of the South Indian sector. The newly formed ocean current shear due to the meridional shift of the ACC flow axis between the two periods is the dominant driver for the MLD trends shift distributed in the western areas in the Drake Passage and the central parts of the South Indian sector. The saltier trends in the regions north to Victoria Land and Wilkes Land could be responsible for the strengthening mixing processes in this region.
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Key words:
- mixed layer depth /
- trend shift /
- Antarctic Circumpolar Current (ACC) /
- flow axis
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Figure 1. Annual mean MLD (unit: m) (a) and SST (unit: ℃) (b) anomaly time series in the ACC region (35°–65°S) for the period of 1960–2021 (taking 1991 to 2020 as the climatology). The blue and yellow lines are the linear trends for the periods of 1960–1985 and 1986–2021, and the red lines are the 11-year running mean of the corresponding black lines. The long-term mean (1960−2021) values are shown in the right up corners. (c, d) and (e, f) are the same as (a, b), but for austral winter (August−October) and summer (January−March) respectively.
Figure 2. Spatial patterns of the linear MLD trend (shaded areas) and the MLD long-term mean (black contours) for periods 1960–1985 (a) and 1986–2021 (b). c is the trend difference between a and b. d–f are the same as a–c, but for SST and its trends. g–i and j–l are the spatial patterns of MLD and SST in austral winter. The “ave” values are their respective trend mean. The dotted areas exceed the 95% confidence level.
Figure 3. Spatial patterns of the linear wind speed trend in the ACC region for 1960−1985 (a) and 1986−2021 (b). c is the difference between a and b. d–f and g–i are the same as a–c but for surface ocean current speed and wind stress curl. The black contours in a and b are the climatic core position of the ACC (1991−2020). The blue and purple contours in a–c are the trends circumpolar contours for period 1960–1985 (a) and 1986–2021 (b). The contours in d–f and g–i are the same as a–c but for ocean current and wind stress vorticity.
Figure 4. Spatial patterns of the linear wind speed trend in the ACC region for austral winter in 1960−1985 (a) and 1986−2021 (b). c is the difference between a and b. d–f and g–i are the same as a–c but for surface ocean current speed and wind stress curl. The black contours in a and b are the climatic core position of the ACC (1991−2020). The blue and purple contours in a, b and c are the trends circumpolar contours for austral winter in 1960–1985 (a) and 1986–2021 (b). The contours in d–f and g–i are the same as a–c but for ocean current and wind stress vorticity.
Figure 5. The annual mean (solid lines) and winter (dashed lines) zonal averaged profiles for climatology (black lines) and trends (blue and purple lines) of surface wind speed (a), ocean current speed (b), wind stress vorticity (c) and MLD (d) from 40°S to 65°S in the SP sector (70°W–180°). e, f, g and h are the same as a, b, c and d but for the SI sector (0°–90°E). The climatological values are based on the period of 1991–2020. The blue and purple lines are the corresponding zonal mean trends in the periods 1960–1985 (P1 in the legend) and 1986–2021 (P2 in the legend), respectively.
Figure 6. The annual mean and winter standardized time series of MLD (black solid line), wind speed (blue dashed line), ocean current speed (orange dashed line) and wind stress curl meridional gradient (red dashed line) in K1 (60°−68°S, 76°−150°W) (a) and K2 (40°−50°S, 50°−90°E) (b) regions. The correlations between MLD and the corresponding variables are given in the following brackets. c and d are the same as a and b, but for winter mean.
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