JOURNAL OF GEOPHYSICAL RESEARCH
Supporting Information for ”Pathways of inter-basin
exchange from the Bellingshausen Sea to the
Amundsen seas”
M. Mar Flexas,
1
, Andrew F. Thompson
1
, Megan L. Robertson
1
, Kevin
Speer
2
, Peter M. F. Sheehan
3
, Karen J. Heywood
3
1
California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
2
Florida State University, Tallahassee, FL 32306, USA
3
University of East Anglia, Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, Norwich, Norfolk, NR4
7TJ, UK
Contents of this file
1. Text S1
2. Figures S1 to S4
Introduction
This file contains information about meltwater fraction calculations used
in this study (Text S1 and Figures S1 and S2), composites of seal hydrographic properties
at Thurston Plateau (Figure S3) and a detailed map of dynamic height from seal profiles
over Thurston Plateau (Figure S4).
February 27, 2024, 5:03pm
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:
Text S1.
To estimate the uncertainty of meltwater (MW) fractions obtained using the Optimum
Multiparameter (OMP) analysis (Tomczak, 1981; Tomczak & Large, 1989; Biddle et al.,
2017), we performed multiple MW fraction analyses that covered the full range of water
mass end member variability observed in the glider data. The OMP analysis uses weight
functions to account for uncertainty in the choice of end members. Following Biddle
et
al.
(2017) and Schulze Cretien
et al.
(2021) we used substantially large weight functions
for temperature (1.2
◦
C) and salinity (0.1), and we accounted for the fact that the glider
dissolved oxygen sensor was calibrated only pre- and post-cruise (and not
in situ
, during
the expedition, using Winkler titration methods) by attributing it a particularly large
weight function (of 400
μ
mol kg
−
1
).
Each glider section (sections shown in Figure 1) covered an “offshore” and an “onshore”
pair of WW and MCDW with distinct temperature, salinity and dissolved oxygen values
(Figure S1a and Figure S1b). Application of “offshore” end members (Figure S1c) al-
ways resulted in higher MW fractions than using “onshore” end members (Figure S1d).
Changing the water mass end members from onshore to offshore values changed the MW
fractions by a factor of 2. However, the vertical structure of MW distribution remained
qualitatively similar, with MW peaks co-located in both horizontal and vertical directions.
To err on the conservative side, we selected the water mass end members that provided
the smallest MW fraction. The most restrictive MW fractions were obtained using the
end members of the “onshore” WW/MCDW pair along Section 89
◦
W (Figure S1d). Ac-
cordingly, we run our MW calculations for the entire data set with that choice of water
mass end members.
February 27, 2024, 5:03pm
:
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To trace MW content from the coastal ice shelves to the continental shelf break we
took advantage of seal observations. However, seal observations do not contain dissolved
oxygen measurements. Therefore, it is of interest to compare MW calculations obtained
from glider data both with and without oxygen values. Additionally, we calculated MW
fractions using S and DO and T and DO.
We found minimal difference between meltwater fractions obtained with T, S and DO
(Figure S2a) and meltwater fractions obtained with S and DO (Figure S2b). Our interpre-
tation is that because the salinity of MCDW at onshore and offshore stations was similar
(34.72 onshore, 34.73 offshore; see values in Figure S1c,d), in absence the of T data, the
WW/MCDW mixing lines for onshore and offshore stations remained close to each other.
Hence, the difference in meltwater estimates using S and DO, or T, S and DO was small.
When using T and DO, the offshore meltwater estimates became close to zero (Figure
S2c). Our interpretation is that the temperature of onshore MCDW (1.15
o
C; Figure S1d)
corresponded to water within the lower pycnocline at offshore stations. Therefore, the
T-DO approach set MW estimates at temperatures above the 1.15
o
C isotherm to zero,
and thus, offshore stations were diagnosed to not contain any MW. We also observed
that when salinity was not included in the MW estimates the onshore meltwater fraction
increased up to 50% with respect to MW estimates obtained with T, S and DO. However,
the spatial structure of MW fractions remained qualitatively similar to MW estimates
obtained with T, S and DO (Figures S2a and S2c).
In general, MW fractions from glider data calculated without oxygen were larger by a
factor of 2 (Figure S2d) than those calculated with oxygen data (Figure S2a). Compar-
ing MW fractions calculated with and without oxygen data, the maxima of MW fractions
February 27, 2024, 5:03pm
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:
(what we call in the main text “MW cores”) were not always horizontally co-located along
a given section: without oxygen data, there was an increase in MW offshore and a de-
crease in MW onshore. Without oxygen data, there was also an increase in the horizonal
gradients of MW. In turn, when oxygen was included in the calculations, the distribution
of MW was more homogeneous (with less abrupt changes). However, MW fractions cal-
culated with and without the inclusion of oxygen data were located at similar depths and
density surfaces. These results provide confidence in the (qualitative) comparison of MW
fractions estimated from glider data with those obtained from seal observations. However,
in the absence of more robust estimations of the magnitude of MW content that could be
determined using for example, noble gases (Beaird et al., 2015; Biddle et al., 2019), we use
our estimations to provide a qualitative evaluation of the distribution of MW properties
on the Bellingshausen Sea continental shelf.
References
Beaird, N., Straneo, F., & Jenkins, W. J. (2015). Spreading of greenland meltwaters
in the ocean revealed by noble gases.
Geophys. Res. Lett.
,
42
, 7705–7713. doi:
10.1002/2015gl065003
Biddle, L. C., Heywood, K. J., Kaiser, J., & Jenkins, A. (2017). Glacial meltwater
identification in the Amundsen Sea.
J. Phys. Oceanogr.
,
47
, 933—954.
Biddle, L. C., Loose, B., & Heywood, K. J. (2019). Upper ocean distribution of
glacial meltwater in the Amundsen Sea, Antarctica.
J. Geophys. Res. Oceans
,
124
,
6854–6870. doi: 10.1029/2019JC015133
Schulze Chretien, L. M., Thompson, A. F., Flexas, M. M., Speer, K., Swaim, N., Oelerich,
R., . . . LoBuglio, C. (2021). The shelf circulation of the Bellingshausen Sea.
J.
February 27, 2024, 5:03pm
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Geophys. Res.: Oceans
,
126
, e2020JC016871. doi: 10.1029/2020JC016871
Tomczak, M. (1981). A multi-parameter extension of temperature/salinity diagram
techniques for the analysis of non-isopycnal mixing.
Progress in Oceanography
,
10
,
147–171. doi: 10.1016/0079-6611(81)90010-0
Tomczak, M., & Large, D. G. B. (1989). Optimum multiparameter analysis of mixing in
the thermocline of the eastern Indian Ocean.
J. Geophys. Res.
,
94
, 16,141–16,149.
doi: 10.1029/JC094iC11p16141
February 27, 2024, 5:03pm
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:
MCDW
WW
MW
θ
1.55
-1.50
-90.8
S
34.73
34.00
0
DO
140
240
1125
O
ff
shore end members
Onshore end members
MCDW
WW
MW
θ
1.15
-1.80
-90.8
S
34.72
34.00
0
DO
135
220
1125
(a)
(b)
(c)
(d)
O
ff
shore MCDW
Onshore MCDW
O
ff
shore WW
Onshore WW
O
ff
shore WW
Onshore WW
O
ff
shore MCDW
Onshore MCDW
Figure S1.
(a) Potential temperature (
θ
) vs. salinity (S) for glider data in Section 2 (see
Figure 1 for section location). (b) Potential temperature (
θ
) vs. dissolved oxygen (DO) for
glider data in Section 2. (c) Vertical section of meltwater fractions obtained using offshore end
member values in Section 2 (%, colored). Table inset: Offshore end member values for Modified
Circumpolar Deep Water (MCDW), Winter Water (WW), and meltwater (MW). (d) Same as
panel c, but for onshore end member values.
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:
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(a)
(b)
(c)
(d)
Figure S2.
Meltwater fraction estimates using different combinations of available hydrographic
variables: (a) Potential temperature (T), salinity (S) and dissolved oxygen (DO); (b) S and DO;
(c) T and DO; (d) T and S. The maximum meltwater fraction obtained using T and S (panel d)
is 1.1%.
February 27, 2024, 5:03pm
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:
(a)
(b)
(c)
(d)
Figure S3.
Composites of seal hydrographic properties (salinity, in color; neutral density, in
white contours) binned over one-degree longitude meridional sections at (a) 101
◦
W, (b) 100
◦
W,
(c) 99
◦
W and (d) 98
◦
W.
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:
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Figure S4.
Dynamic height (in color, m
s
2
s
−
2
) over Thurston Plateau from hydrographic seal
data.
February 27, 2024, 5:03pm