EGU General Assembly 2020
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Variability of the Subantarctic Mode Water volume in the South Indian Ocean during 2004-2018

Yu Hong1,2, Yan Du1,2,3, Tangdong Qu4, and Wenju Cai5,6
Yu Hong et al.
  • 1South China Sea Institute of Oceanology, China (
  • 2Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou, China
  • 3University of Chinese Academy of Sciences, Beijing, China
  • 4Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, CA, 90095, USA.
  • 5Centre for Southern Hemisphere Oceans Research (CSHOR), CSIRO Oceans and Atmosphere, Hobart 7004, Tasmania, Australia
  • 6Key Laboratory of Physical Oceanography, Institute for Advanced Ocean Studies, Ocean University of China and Qingdao National Laboratory for Marine Science and Technology, Qingdao, China.

Analysis of the Argo data reveals that the Subantarctic Mode Water (SAMW) in the South Indian Ocean, characterized by a vertical potential vorticity minimum, decreases by 10% in volume from 2004 to 2015. Most of this volume decrease occurs in the density range 26.8-26.9 kg m-3 which forms southwest of Australia, while a slight volume increase occurs in 26.6-26.8 kg m-3. Further analysis of the data indicates that a reduction of subtropical high and westerly winds in the South Indian Ocean weakens (intensifies) the E-P, heat loss, Ekman pumping and shoals (deepens) the mixed layer southwest of Australia (west of 90°E), which leads the decrease in 26.8-26.9 kg m-3 (increase in 26.6-26.8 kg m-3) by 3 years (see the figure below). This result suggests that the subtropical wind system variation plays an important role in the volume variation of SAMW in the South Indian Ocean in the Argo period.

How to cite: Hong, Y., Du, Y., Qu, T., and Cai, W.: Variability of the Subantarctic Mode Water volume in the South Indian Ocean during 2004-2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4670,, 2020

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Presentation version 1 – uploaded on 04 May 2020
  • CC1: Implications of the SAMW volume decrease, Alexander Haumann, 15 May 2020

    Thank you for the nice display, Yu! Very interesting results. I have two related questions:

    1. You mention a slight volume increase in lighter densities that I presume compensates for some of the decrease in SAMW volume that you report over this period. Is there also an associated decrease in the underlying AAIW density layers to compensate for the change in SAMW volume?

    2. What is the implication of the decreased SAMW for the ocean heat uptake during this period? Is it larger or smaller?

    • AC1: Reply to CC1, Yu Hong, 16 May 2020

      Hi Alex,

      Thanks for your questions!

      1. For the first question, we did find a clear decreasing trend for the underlying the AAIW density layers in the South Indian part, which is consistent with the finding of a decreasing trend for the AAIW in the entire Southern Ocean (Portela E, Kolodziejczyk N, Maes C, et al. Interior water-mass variability in the Southern-Hemisphere oceans during the last decade[J]. Journal of Physical Oceanography, 2020.). However, we didn't check the reason for this trend. In Portela's study, they think the main process involved in the AAIW volume loss is the isopycnal transformation into the Antarctic Winter Waters which is connected with the eddy field. 

      2. For the second question, we notice the ocean heat content (OHC) of the SAMW is mainly controlled by the volume. An increasing trend is found for the South Indian OHC in the density range of σ0 = 25.6–27.1 (Figure S3b), which is in accordance with the increasing OHC trend inferred from a previous study (Llovel & Terray, 2016). We note that this increasing OHC is dominated by an increasing volume of water that is lighter than the SAMW. And yes, we would say the decreasing volume of the dense SAMW partially offsets the increase in the total SIO OHC and the heat uptake.