Interdecadal water mass changes in the Southern Ocean ... - CiteSeerX

GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L07607, doi:10.1029/2004GL022220, 2005

Interdecadal water mass changes in the Southern Ocean between 30°E and 160°E Shigeru Aoki,1 Nathaniel L. Bindoff,2,3 and John A. Church3,4 Received 13 December 2004; revised 10 March 2005; accepted 17 March 2005; published 14 April 2005.

[1] Interdecadal water mass changes in the Indian - Western Pacific sectors of the Southern Ocean were investigated using the Japanese Antarctic Research Expeditions and historical hydrographic observations from the 1950s to 1990s. Freshening and cooling occurred on the neutral density surfaces of 27.0 kg
m 3 equatorward of SubAntarctic Front. Results for the area south of the Polar Front show warm and saline anomalies and oxygen decreases on the surfaces around 27.9 kg
m 3, which correspond to the Upper Circumpolar Deep Water. These latter anomalies are most simply explained by the mixing of these shallow waters with warmer and fresher surface waters. Steric sea level has also increased with an average change of 1mm
yr 1 from the 1970s to 1990s. The changes are larger north of the Sub-Antarctic Front, implying a strengthening of the Antarctic Circumpolar Current. It appears that the observed changes are consistent with the results from coupled climate model results for a similar period. Citation: Aoki, S., N. L. Bindoff, and J. A. Church (2005), Interdecadal water mass changes in the Southern Ocean between 30
E and 160
E, Geophys. Res. Lett., 32, L07607, doi:10.1029/2004GL022220.

1. Introduction [2] The Southern Ocean plays a key role in the global ocean circulation and its variations. Recently, long-term differences in water mass properties have been detected in the southwest Pacific Ocean [Bindoff and Church, 1992; Johnson and Orsi, 1997] and the Indian Ocean [Bindoff and McDougall, 2000; Wong et al., 1999, 2001]. A cooling and freshening of Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are diagnosed on the isopycnal surfaces for about 20 years from the 1960s, while a saltier shift is found from 1987 to 2002 for the upper thermocline in the Indian sector [Bryden et al., 2003]. Gille [2002] showed a warming of 0.17
C centered on the 27.5 kg
m 3 density surface (1000 db) between the 1950s and 1990s over the Southern Ocean. A warming trend of 0.01
C
yr 1 has been found in the deep and bottom waters in the Weddell Sea from continuous observations [Fahrbach et al., 1998; Robertson et al., 2002]. While the presence of a significant long-term variability of 1 Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan. 2 Institute of Antarctic and Southern Ocean Studies, University of Tasmania, Hobart, Tasmania, Australia. 3 CSIRO Marine Research, Hobart, Tasmania, Australia. 4 Antarctic Climate and Ecosystems Cooperative Research Centre, Hobart, Tasmania, Australia.

Copyright 2005 by the American Geophysical Union. 0094-8276/05/2004GL022220$05.00

the polar water masses is indicated, the number and spatial coverage of deep hydrographic observations available to describe the temporal variations are still limited in the Southern Ocean. [3] In the Indian and Western Pacific sectors of the Southern Ocean, continuous oceanographic observations have been conducted under the auspices of the Japanese Antarctic Research Expedition (JARE). Aoki [1997] and Aoki et al. [2003] examined JARE oceanographic observations to investigate long-term variations of the subsurface temperature. Warming trends of 0.004 – 0.01
C
yr 1 were found to depths of 900m. However, these two studies were limited to the temperature profiles and in their meridional data coverage. To fully investigate water mass changes for the broader area, we have used all the bottle temperature, salinity and oxygen data obtained on JARE voyages and historical data archived by Gordon et al. [1982] (abbreviated GMB hereafter). If we use salinity as well as temperature, we are able to estimate surface steric height. One of the major sources of uncertainty in estimating global sea level rise is the inadequate information on the change in the Southern Hemisphere [Church et al., 2001]. We attempt to investigate the long-term variation of the steric height component in the recent sea level change in the Southern Ocean.

2. Data and Method [4] From the mid-1960s to the present, JARE has made observations of water temperature, salinity, and dissolved oxygen in the Southern Ocean every year [see Aoki et al., 2003]. Analysis domains were chosen as four regions centred on the ship tracks (Figure 1); meridional regions around 45
E (30
– 55
E), 110
E (100
– 120
E), and 150
E (140
– 160
E), and a southern zonally oriented region south of 60
S (30
– 80
E). Only bottle data were examined to avoid a possible salinity bias between bottle and CTD/O2 measurements. Salinity was routinely observed and analyzed by Hydrographic and Oceanographic Department/ Japan Coast Guard. No significant bias or trend is reported for the consistent salinity analysis. Dissolved oxygen observations were made using the Winkler’s method. The data profiles were visually examined to eliminate obvious outliers. A total of 259 JARE casts were used in this analysis. [5] Historical GMB data, mainly from Nansen bottle observations, were also used to augment temporal and spatial coverage, especially for the earlier period. These data were obtained from the 1950s to late-70s and were mainly taken in the 1960s. A total of 587 historical casts were used. [6] From the data set, potential temperature, salinity, and pressure were derived on the neutral density (g) surfaces

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Figure 1. Distributions of the observation stations for JARE (dots) and GMB (open circles). Overlaid solid lines denote positions of fronts after Orsi et al. [1995]. using the scheme developed by Jackett and McDougall [1997]. The dissolved oxygen on g surfaces was estimated using cubic spline interpolation of the original data. To eliminate erroneous values, the data with density inversions were excluded from the data set. [7] The sampling points are different from year to year and the temporal mean fields are different by region; consequently spatial aliasing of different sampling points can occur. The spatial aliasing can be significantly reduced by subtracting the temporal mean field during the observation period. Since we do not know the mean field, the climatological fields of the WHP-SAC data [Gouretski and Jancke, 1998] were used. The anomalies in potential temperature, salinity, pressure and oxygen were derived by subtracting the climatological mean field from each of the observations.

3. Results 3.1. Water Mass Variability [8] Long-term variations in water mass properties were examined on the g surfaces. For the zonal region south of 60
S, we have relatively continuous data sampling in time. For the g of 27.8 –27.9 kg
m 3, which corresponds to the Upper Circumpolar Deep Water (UCDW) layer, time series of the anomalies indicate that potential temperature and salinity increased with the maximum rate of 0.030 ± 0.015
C
yr 1 and 0.0022 ± 0.0010 psu
yr 1 (Figure 2). There was no significant bias between the data obtained from JARE and GMB. [9] To investigate long-term variations for the meridional regions, the anomalies were combined into two periods (1960 – 80 and 1980– 96), and the differences between these periods were examined. Along the 150
E section, warm and saline anomalies were found between the temperature minimum and oxygen minimum (27.5 – 28.0 kg
m 3 , corresponding to the base of the winter mixed-layer to the core of the UCDW) south of 58
S near the Polar Front (PF) (Figure 3c). North of Sub-Antarctic Front (SAF), cool and fresh anomalies with deeper density surfaces were detected at 26.8 – 27.2 kg
m 3 surfaces above the salinity minimum (the core of the AAIW). Deepening of the density surfaces was detected below the oxygen minimum south of 60
S and shoaling above the oxygen minimum north of 50
S. Decreases in oxygen concentration was detected at around 27.8 kg
m 3 near the oxygen minimum and above the salinity maximum in the Polar Frontal Zone (PFZ) for 54
– 62
S.

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[10] Along the 110
E section similar features were detected; warm and saline anomalies at 27.6– 28.0 kg
m 3 south of 48
S near SAF, and cool and fresh anomalies with deeper density surfaces at 27.0 –27.4 kg
m 3 north of SAF (Figure 3b). Deepening of the density surfaces was detected below the oxygen minimum around 60
S. At 27.6 kg
m 3, a decrease in oxygen was detected for 52
– 56
S south of PF. [11] Along the 45
E section, warm and saline anomalies were again observed at 27.7– 28.0 kg
m 3 south of 56
S and south of PF (Figure 3a). North of 40
S and north of the Sub-Tropical Front (STF), cool and fresh anomalies were found above the salinity minimum (27.0– 27.4 kg
m 3), although they were not statistically significant. Deepening of the density surfaces were observed below the oxygen minimum south of 60
S and also near 50
S. A weak decrease in oxygen was detected at around 52
S between the temperature minimum and oxygen minimum (27.6– 27.8 kg
m 3). [12] Thus, a similar spatial pattern of change occurred on all three meridional sections (Figure 4). Cool and fresh anomalies were common to the three sections around the 27.0 kg
m 3 surface north of SAF. Warm, saline, lowoxygen anomalies over 27.7 – 28.0 kg
m 3 and deeper density surfaces below range south of the PF. 3.2. Steric Height Change [13] To estimate the effect of these changes on sea level, geopotential anomalies were calculated and meridionally averaged in four degree bins using the raw data on pressure surfaces. Then the differences between the periods of 1960– 80 and 1980– 96 were examined (Figure 5). For all three sections almost all of the geopotential anomlies is confined to the top 500 dbar of the water column. At 45
E, the differences show an increase in geopotential anomalies relative to 1500 dbar of about 0.04 m north of 52
S; i.e. mostly north of the SAF. Between 52
and 62
S the anomalies are less than 0.02 m. At 110
E, the anomalies are about 0.03 m and fairly uniform with latitude until they fall to zero south of 60
S. At 150
E, the anomalies are larger, up to 0.06 m north of the SAF and also at latitudes of about 60
S and are smaller for 54
– 58
S and south of 64
S.

Figure 2. Time series of anomalies of (a) salinity (psu), (b) potential temperature (
C), and (c) pressure (db) on the g surface of 27.90 kg
m 3 for the zonal region [30
– 80
E, 70
–60
S]. Dots (open circles) denote the JARE (GMB) observations. The thin lines show the least squares trend through the anomalies.

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Figure 3. Meridional distribution of salinity, potential temperature, pressure, and dissolved oxygen anomalies on neutral density surfaces for (a) 45
E, (b) 110
E, and (c) 150
E sections. Solid lines denote positive anomalies and dotted lines denote negative anomalies. Contour intervals are 0.02 psu, 0.2
C, 50 db, and 0.5 ml/l, respectively. The white lines denote the salinity minimum, blue lines the temperature minimum, black lines the oxygen minimum, and red lines denote the salinity maximum. Overall, the changes are consistent with an increase in geopotential anomaly gradient across the ACC.

4. Discussion [14] North of SAF, cool and fresh anomalies were found on the isopycnals at 27.0 kg
m 3. This trend is consistent with the previous results in the lower-latitudes of the Indian Ocean and Southwest Pacific Ocean [e.g., Bindoff and Church, 1992] and for the lower thermocline water of the Indian Ocean [Bryden et al., 2003]. This cooling and freshening signal can be explained by the warming of the surface waters and their subsequent subduction [Bindoff and McDougall, 1994]. [15] South of PF, warm and saline anomalies were significant in UCDW on isopycnals. This pattern of change south of PF (and also with the changes north of SAF) is the same as the ‘‘fingerprint’’ of climate change found in the Indian and Pacific sectors of the Southern Ocean for a

Figure 4. A schematic diagram of the observed changes on density surfaces. Cool and fresh anomalies were found for around the 27.0 kg
m 3 surface north of SAF. Warm, saline, low-oxygen anomalies over 27.7– 28.0 kg
m 3 and deeper density surfaces below range south of the PF.

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lies for the whole latitude range of the observations is about 0.02 m over the two decades. The present steric height changes of about 1 mm
yr 1 in this region of the Southern Ocean, a data sparse region in previous global analyses, imply a larger rate of ocean heat uptake in global average sea level rise [Church et al., 2004] than the global average steric height changes of 0.55 mm
yr 1 implied by the ocean heat uptake of Levitus et al. [2000]. However, the observations are still scarce and given that a large decadal oscillation or interannual variability has been detected for the shallower SAMW in the lower latitudes [Bryden et al., 2003], continuous observations are crucial to monitor and predict the present and future of these changes. Figure 5. Differences in geopotential anomaly (dyn-cm) between 1960 – 80 and 1980 – 96 referred to 500 db (triangles), 1000 db (open circles), and 1500 db (squares) for 45
E (top), 110
E (middle), and 150
E (bottom). similar period in the Hadley Centre coupled ocean atmosphere model (HadCM3) [Banks and Bindoff, 2003]. The observed warm and saline anomalies are consistent with a warming and freshening of the surface water at the base of the mixed layer. This slightly counter-intuitive result occurs because the surface waters are fresher and colder than the underlying CDW and freshening of surface waters appears as an increase in salinity on density surfaces (for 0 < Rr