Interannual Variability of Upper Ocean Water Masses as Inferred From Argo Array

Kolodziejczyk, Nicolas; Llovel, William; Portela, Esther

doi:10.1029/2018jc014866

Cited by 50 publications

(77 citation statements)

References 79 publications

(173 reference statements)

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“…A prominent water mass of the region, the Sub-Antarctic Mode Water (SAMW), is identified around the southern subtropical gyres as a layer of low potential vorticity (PV) acquired during its formation (McCartney, 1977). Variability of the SAMW properties on decadal and longer time scales has been reported by recent studies (e.g., Bindoff & McDougall, 2000;Gille, 2002;Bryden et al, 2003;Gao et al, 2018;Kolodziejczyk et al, 2019;Portela et al, 2020). Among others, Gao et al (2018) showed that the SAMW has significantly thickened, deepened, and warmed during the period from 2005 to 2015.…”

Section: Introductionsupporting

confidence: 54%

Variability of the Sub‐Antarctic Mode Water Subduction Rate During the Argo Period

Qu¹,

Gao

Fine

2020

Geophysical Research Letters

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Both a quasi‐biennial variability and an overall linearly increasing trend are identified in the Sub‐Antarctic Mode Water (SAMW) subduction rate across the Southern Hemisphere ocean, using the Argo data during 2005–2019. The quasi‐biennial variability is mainly due to variability of the mixed layer depth. Variability of wind stress curl in the SAMW formation regions associated with the Southern Annular Mode plays a critical role in generating the quasi‐biennial variability of the mixed layer depth and consequently the SAMW subduction rates. The SAMW subduction rate across the Southern Hemisphere ocean, long‐term mean totaling 56 Sv, has increased at 0.73 ± 0.65 Sv year−1 over the past 15 years. The increase has directly contributed to the observed increase in the total SAMW volume. Much of this increasing trend can be explained by the deepening mixed layers, which in turn are primarily forced by the strengthening westerly winds under an increasing Southern Annular Mode.

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Section: Introductionsupporting

confidence: 54%

Variability of the Sub‐Antarctic Mode Water Subduction Rate During the Argo Period

Qu¹,

Gao

Fine

2020

Geophysical Research Letters

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“…Note that the medium SAMW trend is small and nonsignificant. The volume trend of the dense and light SAMW is coincident with the two‐layer opposite thickness trend for the SIO SAMW over 2006–2015 (Figure 5 in Kolodziejczyk et al, 2019). There appears to be a decadal signal associated with this volume variation (Figures 2a and 2b), which needs to be confirmed by longer time observations.…”

Section: Samw Volume Change and Spatial Distributionmentioning

confidence: 53%

Variability of the Subantarctic Mode Water Volume in the South Indian Ocean During 2004–2018

Yan

Qu³

et al. 2020

Geophysical Research Letters

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An analysis of Argo data reveals that the subantarctic mode water (SAMW) in the South Indian Ocean (SIO), characterized by a vertical potential vorticity (PV) minimum, decreases by 10% in volume from 2004 to 2018. Most of this decrease occurs at the 26.8-26.9 kg m −3 density range which forms southwest of Australia, while a slight volume increase occurs at 26.6-26.8 kg m −3 . Further analysis indicates that the weakening of the Mascarene High and westerly winds in the SIO reduces the evaporation-precipitation, surface heat flux, and Ekman pumping and shoals the mixed layer southwest of Australia, which leads to a volume decrease at 26.8-26.9 kg m −3 in approximately 3 years. West of 90°E, the parameters exhibit the opposite change, leading to a volume increase at 26.6-26.8 kg m −3 . This result suggests that surface winds play an important role in the variability of the SIO SAMW volume during the Argo period.Plain Language Summary During austral winter in the subantarctic zone, a water mass called the subantarctic mode water with homogeneous properties (temperature, salinity) forms, sinks, and moves to the interior of the ocean. The formation and transport of the mode water are crucial for the climate, as the water takes up heat, freshwater, carbon, oxygen, and nutrients into the ocean interior. We find a distinct volume loss of the mode water in the South Indian Ocean from 2004 to 2018. Although this mode water forms in broad density ranges in different regions, the volume loss only occurs in the dense mode water south of Australia. Moreover, the volume of the light mode water, which forms in the center of the South Indian Ocean, slightly increases. We find that surface winds control the surface heat flux and water convergence, which determine the volume change here. These winds tend to vary oppositely in the two different mode water formation regions and make the volume vary in opposition phases. This study reveals the layer dependence of the mode water volume variation and reinforces the importance of atmospheric forcing, which would possibly help interpret the heat uptake in the Southern Ocean. Key Points: • Argo data reveal a subantarctic mode water volume loss in the South Indian Ocean over the period 2004-2018 • Most of the volume loss occurs in the density range of 26.8-26.9 kg m −3 , while a volume increase occurs at 26.6-26.8 kg m −3 • Changes in the SAMW volume are controlled by surface forcing, which is closely related to the Mascarene High variation Supporting Information: • Supporting Information S1 Correspondence to: Y. Du,

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“…In the past decade, most of the ocean warming measured by the near-global array of Argo floats occurred in the Southern Hemisphere, largely associated with changes in SAMW in the southern Pacific and Indian Oceans (Desbruyères et al 2017;Gao et al 2018;Häkkinen et al 2016;Kolodziejczyk et al 2019;Llovel and Terray 2016;Portela et al 2020;Roemmich et al 2015). The SAMW formation is vigorous in the south Indian Ocean (SIO), attracting much attention recently.…”

Section: Introductionmentioning

confidence: 99%

Changes in the Subantarctic Mode Water Properties and Spiciness in the Southern Indian Ocean based on Argo Observations

Zhang

Qu³

et al. 2021

Journal of Physical Oceanography

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The Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the Southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004-2018. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking at σθ=26.7-26.8 kg m−3in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004-2018. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy loss. An increase in the mixed-layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre.

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Interannual Variability of Upper Ocean Water Masses as Inferred From Argo Array

Cited by 50 publications

References 79 publications

Variability of the Sub‐Antarctic Mode Water Subduction Rate During the Argo Period

Variability of the Sub‐Antarctic Mode Water Subduction Rate During the Argo Period

Variability of the Subantarctic Mode Water Volume in the South Indian Ocean During 2004–2018

Changes in the Subantarctic Mode Water Properties and Spiciness in the Southern Indian Ocean based on Argo Observations

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