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Abstract: Antarctic Bottom Water (AABW) plays an important role in the meridional overturning circulation and contributes significantly to global heat transport and sea level rise (SLR). Based on the Global Ocean (1/12)° Physical Reanalysis (GLORYS12V1) products and conductivity-temperature-depth instrument data from the World Ocean Circulation Experiment hydrographic program, we analyzed the trends in the thickness, volume, temperature, salinity, and neutral density of the AABW in the Amundsen Sea from 1993 to 2017. Over the past 25 years, the volume has decreased by 3.45×1012 m3/a, thinning at a rate of 5 m/a. In the vertical direction, the contraction of the AABW is compensated by the volume expansion of the Circumpolar Deep Water. As the volume of AABW decreases, the temperature of the AABW increases by about 0.002°C/a. This warming is equivalent to a heat flux of 0.27 W/m2. A local SLR is produced due to thermal expansion of 0.35 mm/a. During the study period, the neutral density decreased by 0.000 3 kg/(m3∙a) due to warming. In the horizontal direction, the volume of AABW flowing from the Ross Sea into the Amundsen Sea gradually decreases and the temperature of the AABW increases continuously. The horizontal transport loss of the AABW volume is 4.07×1014 m3 and the horizontal heat transport results in a 0.03°C increase in the temperature of the AABW.
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Key words:
- Antarctic Bottom Water /
- Amundsen Sea /
- thickness /
- volume /
- hydrographic property
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Figure 1. The bathymetry and station locations in the main study area, with purple dots for the P18 (including 1994, 2007, and 2016), yellow dots for the S04P (including 2011 and 2018), and the Amundsen Sea within the black box. The red and orange arrows represent the schematic pathway of the Antarctic Bottom Water and Ross Gyre, respectively.
Figure 2. Comparison of conductivity temperature depth (CTD) section with GLORYS12V1 section in the P18, where the black solid line represents the contour; the black dashed line in a represents the 0.08°C contour; the red solid line represents the 1994 Antarctic Bottom Water (AABW) upper boundary position; the green solid line represents the 2007 AABW upper boundary position; and the yellow solid line represents the 2016 AABW upper boundary position. a, b, and c show the temperature (θ), salinity (S), and neutral density (γn) comparisons, respectively.
Figure 3. Comparison of conductivity temperature depth (CTD) section with GLORYS12V1 section in the S04P, where the black solid line represents the contour, the black dashed line in a represents the 0.08°C contour; the red solid line represents the 2011 Antarctic Bottom Water (AABW) upper boundary position; and the green solid line represents the 2018 AABW upper boundary position. a, b, and c show the temperature (θ), salinity (S), and neutral density (γn) comparisons, respectively.
Figure 4. The average thickness of the Antarctic Bottom Water (AABW) (a) and average thickness variation of the AABW (b) from 1993 to 2017. The green range in b represents the area where the trend is not statistically significant at the 95% confidence level (same as in Figs 7b, 7d and 9b).
Figure 5. The annual variation of the thickness in the Antarctic Bottom Water (AABW). In a, blue squares indicate the thickness in the Amundsen Sea and red squares indicate the thickness in the Ross Sea. R2 and p in a denote the coefficient of determination and the statistical p value (same as in Figs 6, 8a, 8d, 10a and 11). b. The latitudinal annual variation in the thickness of the AABW in the Amundsen Sea was derived by meridional mean. c. The longitudinal annual variation in the thickness of the AABW in the Amundsen Sea was derived by zonal mean. The different colors in b and c represent different years.
Figure 6. The annual variation in the volume of the three water masses in the Amundsen Sea. Blue squares represent the Antarctic Bottom Water (AABW), red squares represent the Circumpolar Deep Water (CDW), yellow squares represent the Antarctic Surface Water (AASW).
Figure 7. The average temperature of the Antarctic Bottom Water (AABW) (a) and average temperature variation of the AABW (b) from 1993 to 2017. The Amundsen Sea is within the black box. The black solid line in a indicates the 0°C contour. c, d are same as a and b but for salinity.
Figure 8. The annual variations of the temperature and salinity in the Antarctic Bottom Water (AABW). In a, blue squares represent the temperature in the Amundsen Sea and red squares represent the temperature in the Ross Sea. b. The latitudinal annual variation in the temperature of the AABW in the Amundsen Sea was derived by meridional mean. c. The longitudinal annual variation in the temperature of the AABW in the Amundsen Sea was derived by zonal mean. The different colors in b and c represent different years. d–f are same as a–c but for salinity.
Figure 9. The average neutral density of the Antarctic Bottom Water (AABW) (a) and the average neutral density variation of the AABW (b) from 1993 to 2017. The black solid line in a indicates the 28.305 0 kg/m3 contour.
Figure 10. The annual variation of the neutral density in the Antarctic Bottom Water (AABW). In a, the blue squares represent the neutral density in the Amundsen Sea and the red squares represent the neutral density in the Ross Sea. b. The latitudinal annual variation in the neutral density of the AABW in the Amundsen Sea was derived by meridional mean. c. The longitudinal annual variation in the neutral density of the AABW in the Amundsen Sea was derived by zonal mean. The different colors in b and c represent different years.
Figure 11. Volume transport (a) and heat transport (b) in the three boundary sections of the Amundsen Sea. Blue squares indicate the inflow into the Amundsen Sea through 126°W, red squares indicate the outflow from the Amundsen Sea through 98°W, yellow squares indicate the outflow from the Amundsen Sea through 60°S, and purple squares indicate the sum of the transport (total transport) in the three sections. The shaded area in a indicates a volume transport less than 0 m3/s and the shaded area in b indicates a heat transport more than 0 TW.
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