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Reevaluating the Causes of Observed Changes in Indian Ocean Water Masses

SHEILA STARK,RICHARD A. WOOD, AND HELENE T. BANKS Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon, United Kingdom

(Manuscript received 16 February 2005, in final form 8 December 2005)

ABSTRACT

The consistency between observed changes in Subantarctic Mode Water (SAMW) properties at 32°S in the Indian Ocean and model simulations is explored using the Third Hadley Centre Coupled Ocean– Atmosphere GCM (HadCM3). Hydrographic data collected in 2002 show that the water mass is warmer and saltier on isopycnals than in 1987, in contrast to the isopycnal freshening observed between 1962 and 1987. The response of HadCM3 under a range of forcing scenarios is explored and the observed freshening is only seen in experiments that include greenhouse gas forcing; however, there is no subsequent return to more saline conditions in 2002. The response of the model to greenhouse gas forcing is dominated by a persistent freshening trend, the simulated water mass variability agrees well with that suggested by the limited observations. Comparing model isopycnal changes from the forced experiments with a control run shows that the changes from the 1960s to 2002 are best explained by internal variability. This is in contrast to earlier work, which attributed the observed isopycnal freshening to anthropogenic forcing. Although the model shows that at present an anthropogenic climate change signal is not detectable in SAMW, the model water mass freshens on isopycnals during the twenty-first century under increased greenhouse gas forcing. This is consistent with recent heat content observations, which suggest that the salting is unlikely to persist. In HadCM3, this freshening is due to an increasing surface heat flux and Ekman heat and freshwater flux into the water mass formation region. This paper emphasizes the importance of higher-frequency observations of SAMW if detection and attribution statements are to be made.

1. Introduction to global climate through the large-scale transport of heat and freshwater in the oceans. Despite the impor- The hydrological cycle is a fundamental component tant role of the ocean in the global hydrological cycle, of the planetary energy budget yet it remains one of the direct measurements of oceanic precipitation and least understood elements of the climate system. Over evaporation are too sparse to detect any patterns of recent years there has been a growing body of evidence change or variability in these fluxes. Interior water for a global-scale shift in the distribution of freshwater, masses, which are directly ventilated at the ocean sur- which may be linked to global warming and a possible face, are an attractive way to gauge changes in surface strengthening in the hydrological cycle [e.g., Curry et al. fluxes as they act to integrate highly variable surface (2003) in the Atlantic and Wong et al. (1999) in the changes in heat and freshwater improving the signal to Indo-Pacific]. Most ocean model simulations, under in- noise ratio. Despite this potential, the sparsity of oce- creasing greenhouse gas emissions, predict a weakening anic measurements makes it difficult to determine North Atlantic thermohaline circulation due to fresh- whether changes in water mass properties reflect inter- ening and warming in the subpolar seas (Houghton et nal climate variability and are “normal,” or reflect, for al. 2001), so an intensified hydrological cycle character- example, anthropogenic climate change, and are “un- ized by increased high-latitude precipitation could have usual.” For this reason, general circulation models pro- significant climatic impacts. vide a unique opportunity to understand the past and to The study of ocean water masses is intimately linked predict future ocean climatic changes. Subantarctic Mode Water (SAMW) is a globally important water mass formed in large quantities in the Corresponding author address: Sheila Stark, Hadley Centre for Climate Prediction and Research, Met Office, Fitzroy Road, Ex- Southern Ocean (McCartney 1977). Along with Ant- eter, Devon EX1 3PB, United Kingdom. arctic Intermediate Water (AAIW), SAMW exported E-mail: [email protected] from the Southern Ocean forms an upper limb of the

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Ϫ FIG. 1. Zonal mean isopycnal salinity in the range of the modeled mode waters (25.5–26.5 kg m 3) for 32°Sin the Indian Ocean for the 400 yr of the CTL run over which the ensemble members were run (a) before and (b) after the linear trend was removed from each isopycnal surface. global overturning circulation (Sloyan and Rintoul during the twentieth century, focusing primarily on the 2001). An observed cooling and freshening of SAMW last 50 yr. HadCM3 is a fully coupled ocean–atmo- on isopycnals between the 1960s and 1990s has been sphere model, without flux adjustments, which has been well documented (Bindoff and McDougall 2000; described in detail elsewhere (Gordon et al. 2000). All Johnson and Orsi 1997; Wong et al. 1999) and attrib- of the forced model runs are compared to a control uted to changes in surface dynamic forcing and the experiment (CTL), which was initialized with the hy- warming or freshening of surface waters. A coupled drography of the ocean given by Levitus and Boyer climate model study using the Third Hadley Centre (1994) and run with a fixed atmospheric composition ϭ Coupled Ocean–Atmosphere GCM (HadCM3) by representative of 1860 (pCO2 290 ppm). The first Banks et al. (2000) found that with anthropogenic forc- member of each forced ensemble is started from year ing SAMW became both cooler and fresher on isopy- 370 of CTL, by which time both the heat and freshwater cnals, and Banks and Bindoff (2003) classify the cooling budgets have come into balance (Pardaens et al. 2003). and freshening of midlatitude isopycnals in the Indo- We analyze ensembles forced with natural forcing Pacific as a possible fingerprint of anthropogenic cli- (NAT) from historical records of volcanic emissions mate change in the ocean. The most recent occupation and solar irradiance, with anthropogenic forcing (ANT) of the 32°S section in the Indian Ocean, however, has consisting of imposed historical changes in greenhouse shown the properties of SAMW return to near-1960s gases, ozone and sulfur, and with a combination of the conditions (Bryden et al. 2003; McDonagh et al. 2005), two (ALL). For each ensemble, four simulations are a reversal not seen in the HadCM3 simulations. In this carried out with the same forcing applied but different paper, the work of Banks et al. (2000) is continued, with initial conditions, generated by starting 100 yr apart in a more detailed analysis of SAMW in HadCM3 using a the control integration. Each ensemble covers different larger number of experiments than was available for time periods and we examine in detail the NAT en- the earlier study, and taking into account the recent semble from 1900 to 1996, the ANT ensemble from Bryden et al. (2003) results. The consistency between 1900 to 1999, and ALL from 1900 to 2002. the HadCM3 simulations and the observations is examined using a range of forcing scenarios with a view to b. Control drift addressing what is driving the water mass changes and Examination of zonal mean isopycnal salinity at 32°S whether the observed isopycnal changes reflect anthro- in CTL at mode water densities reveals a freshening pogenic forcing. trend on all levels, as illustrated by Fig. 1. It is found that the drift on each isopycnal surface is linear (R2 Ͼ 2. Methods 0.95 for each level) and a unique trend is removed from each isopycnal individually. On the 25.7 kg mϪ3 isopy- a. Model runs cnal, the core of the model SAMW in the western A suite of ensemble runs using HadCM3 are used to mode, the drift has a magnitude of 0.0005 psu yrϪ1; the look at the changing properties of SAMW along 32°S trend in the eastern mode is of a similar magnitude.

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FIG. 2. (a) Potential vorticity along 32°S in the HadCM3 control run. Note that the plot shows the magnitude of PV, with the minimum values being those closest to 0, though for stable stratification PV is negative in the Southern Hemisphere. SAMW is clearly visible as a band of low PV between 25.5 and 26.0 kg mϪ3. (b) The temperature and salinity of the mid-depth PV minimum.

From Fig. 1, it is evident that this procedure success- in HadCM3 at densities ranging from 25.5 to 26.0 kg fully removes the freshening drift on the mode water mϪ3. The modeled SAMW is both fresher and lighter isopycnals. To ensure that any patterns and trends in than observations because of the initial drift, as the each of the forced ensemble members represents water masses adjust to balance the freshwater budgets, changes due to the applied forcing the linear trend in during the model spinup (see Pardaens et al. 2003 for a CTL on each isopycnal is removed prior to all analyses. full discussion). Examination of temperature and salinity on the PV minimum (Fig. 2b) reveals that the model c. Differences from Banks et al. (2000) thermocline along this section contains modes whose The work presented here is complimentary to the characteristics fall into two broad categories. The west- earlier analysis described in Banks et al. (2000) where ern half of the section, to approximately 75°E, is occu- the observed isopycnal freshening of SAMW between pied by a water mass with an average temperature of 1962 and 1987 was compared with HadCM3 simula- 13.8°C while the mode water in the east of the section tions. This study incorporates the subsequent isopycnal is denser, fresher, and colder than that found in the salting observed in 2002 and the analysis differs from west with an average temperature of 11.4°C. Both the earlier work in the following ways. First, the Banks modes are slightly warmer than those observed in 2002 et al. (2000) analysis included only one ANT and one (McDonagh et al. 2005) where the western mode fol- NAT run; no ALL runs were available at the time. In lows the 13°C isotherm and the temperature of the PV the new work, an ensemble of four experiments is used minimum in the east decreases from 11°Cat75°Eto for each forcing scenario, the ANT and NAT experi- 9°C at the eastern end of the section. The transition ments in the earlier work are the first ensemble mem- between the two is less well defined in HadCM3 than in ber in each case. Second, the drift in the control runs, the observations. In the observations, there is a clear described above, is not removed in the Banks et al. separation of the modes because the warmer mode is (2000) study, and is therefore included in the freshening contained by the anticyclonic subgyre of the southwest on isopycnals seen in the forced experiments. The mag- Indian Ocean, which extends to approximately 70°E nitude of this drift (0.013 psu), however, is not large between 30° and 40°S (Stramma and Lutjeharms 1997). enough to significantly change the results of the earlier This subgyre is poorly represented by HadCM3. work. Finally, we take a different approach to explain- There is no evidence in the model of the 17°C Sub- ing the observed changes to incorporate the new data tropical Mode Water found on the western edge of the and the dependency of the two observed SAMW salin- section in 2002 (McDonagh et al. 2005) and in 1987 ity changes. (Toole and Warren 1993). For the subsequent analyses, the warm mode between 40° and 75°E is considered the western mode and the cool mode the eastern mode. 3. Model Subantarctic Mode Water in CTL Waters farther west than 40°E are not considered to be SAMW, which is most often identified by a maxi- recently ventilated because of their high PV and loca- mum in isopycnal thickness or a minimum in potential tion in the Aghulas Current, where the relative vorticity vorticity (PV) as shown in Fig. 2a, is found along 32°S can no longer be neglected.

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TABLE 1. Observed changes in SAMW salinity on isopycnals from hydrographic cruises in 2002, 1987, and historical data with a median date of 1962 (Bindoff and McDougall 2000).

Time period Isopycnal salinity change (psu) Reference 1962–87 Ϫ0.13 Bindoff and McDougall (2000) 1987–2002 40°–70°E (western mode) 80°E to Australia (eastern mode) McDonagh et al. (2005) ϩ0.09 ϩ0.07

4. Subantarctic Mode Water 1962–2002 house gas forcing results in a freshening trend and second that South Indian Ocean mode waters in HadCM3 a. Comparison to observations exhibit significant oscillations in their properties. Al- The observed changes in SAMW isopycnal salinity at though the available observations are too few to define 32°S in the Indian Ocean between 1962 and 2002 are SAMW variability, they indicate changes in isopycnal summarized in Table 1. The changes are representative salinity of the order of 0.1 psu over 15–25-yr time scales. of hydrographic cruises, which took place in 1987 and This decadal variability is reproduced well by the 2002 and historical hydrographic data with a median model, particularly in the ALL experiment that, despite date of 1962 (Bindoff and McDougall 2000). The east- using an ensemble mean, exhibits decadal freshenings ern and western ends of the section were also sampled of more than 0.1 psu, with corresponding, but weaker as part of the World Ocean Circulation Experiment saltings throughout the experiment. Figure 4 also re- (WOCE) in 1995, but these changes are not examined veals that the variability of the magnitude indicated by here in detail, as the whole transindian section was not the observations is reproduced by CTL though over undertaken. Inclusion of this data does not change the conclusions presented in this paper. In summary, a freshening of 0.13 psu has been observed on SAMW isopycnals between 1962 and 1987 along the entire 32°S section with a subsequent isopycnal salting of 0.09 and 0.07 psu observed in 2002 in the western and eastern modes, respectively. Figure 3 shows the time series of ensemble mean zonally averaged isopycnal salinity anomalies with re- spect to the CTL for each of the experiments; the control drift has been removed from each run. Considering the western mode, it is evident that with only natural forcing there is significant variability in the model mode water, it shows oscillating periods of fresher and more saline isopycnal conditions. NAT does not show a freshening of SAMW between 1962 and 1987, whereas runs where greenhouse gas forcing is included (ANT and ALL) exhibit both fresher water masses overall and decreasing salinity between the early 1960s and the late 1980s, though it is more pronounced in the western than the eastern mode (not shown). It is also clear, that to the limit of the ensemble data (1999 for ANT and 2002 for ALL), there is no evidence of a return to the more saline conditions of the 1960s, and the freshening trend persists. This result makes it hard to explain the observed isopycnal salinity changes as a forced response. Close examination of the mode water proper- FIG. 3. Ensemble mean zonally averaged isopycnal salinity ties (Fig. 4) reveals that there is a less distinct freshen- anomalies along 32°S for the western mode for NAT, ANT, and ing trend in the ANT run at the eastern than western ALL. In each case, the anomalies are generated relative to a mean of 400 yr of CTL, with the linear trend removed from all experi- end of the section. It is also clear that both ANT and ments. Note the different time scales for each ensemble; only the ALL are typified by oscillations in mode water salinity, all-forcing ensemble has data that extends to 2002 for direct com- as in NAT, which suggests two things, first that green- parison with recent observations.

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FIG. 4. Time series of salinity anomaly relative to CTL at the core of the model mode water, on the (top left) 25.7 kg mϪ3 isopycnal in the west and (top right) the 26.0 kg mϪ3 isopycnal in the east, noting that for clarity different scales have been used. The bottom plot shows the western mode salinity on the 25.7 kg mϪ3 isopycnal for 400 yr of CTL, with a close up on yr 150–200, which shows events of the magnitude observed. shorter time scales. The highlighted 50-yr period shown by comparing this 25-yr change with that from every has a freshening of 0.14 psu over an 8-yr period, one of overlapping 25-yr period from 820 yr of CTL. By as- 0.08 psu over 9 yr, and one of 0.1 psu over 5 yr. The suming that CTL is fully representative of internal vari- same period also exhibits increases of salinity of 0.12 ability and applying a two-tailed test they found that a psu over 4 yr, one of 0.07 psu over 8 yr, and one of 0.06 freshening greater than 0.1 psu over 25 yr was signifi- psu over 7 yr. Overall the variability of SAMW prop- cant at the 5% level. It was therefore concluded that the erties in the model is consistent with the magnitude of observed freshening of 0.13 psu was unlikely to be due changes seen in the observations, but the model results to internal variability alone and was a signal of anthro- suggest significant variability on decadal time scales, em- pogenic climate change. Here we consider the two suc- phasizing the possibility of aliasing in the observations. cessive observed SAMW isopycnal salinity changes to- None of the forced model runs show SAMW return- gether and assess how unusual it is to have a western ing during the late 1990s to the properties it had in the mode 25-yr freshening of 0.13 psu followed by a salting 1960s. It is evident, however, that oscillations in mode of 0.09 psu. water properties are exhibited by the model and that Figure 5 shows the distribution of 40-yr changes de- the freshening in the ALL and ANT runs is not unidi- composed into a 25-yr change and its subsequent 15-yr rectional. change for each overlapping 40-yr period of CTL and for each of the forced ensembles. By considering only b. Explanation of observed changes the results on the x axis the plots are equivalent to the Banks et al. (2000) determine the significance of the distribution of 25-yr isopycnal salinity changes shown in isopycnal freshening observed between 1962 and 1987 Fig. 4 of Banks et al. (2000), which showed that a fresh-

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FIG. 5. Distribution of 40-yr changes (black crosses) in CTL, ANT, ALL, and NAT at the core of the model SAMW for the western mode (25.7 kg mϪ3 isopycnal). The observed changes are shown by the red square and the comparable changes (the last five 40-yr periods) in each forced run are highlighted by red crosses for the last three ensemble members. Comparable changes from the first ensemble member, used in the Banks et al. (2000) analysis, are shown by green crosses. The blue ellipsoid illustrates 2.5 standard deviations from the mean of the distribution assuming that any 25-yr salting is followed by a 15-yr freshening of the same magnitude or vice versa [Eq. (1)]. Similar patterns are seen for the eastern mode, which is not shown. ening of the right magnitude appears to be more com- change, but we do not know the exact nature of this mon in the ANT run than in NAT or CTL. Correcting dependence. We draw a 2.5 standard deviation ellipsoid for the isopycnal salinity drift in CTL in Banks et al. for the CTL distribution by assuming that as there is no (2000) shifts their 25-yr change distribution to the right trend in the SAMW salinity (Figs. 1 and 4), any salting resulting in more saline changes as discussed in section is followed by a freshening and vice versa such that 2c. We might have expected that the new results would E͑y|x ϭ N ͒ ϭϪN, ͑1͒ show more ANT 25-yr freshenings of at least 0.13 psu as there are now four ensemble members rather than where x and y are the salinity change after 25 and 15 yr, just one. It appears from Fig. 5, however, that the first respectively. We consider an observation to be an out- ANT ensemble member, which was used by Banks et lier if it falls outside of this ellipsoid. Using this defini- al. (2000), has more 25-yr freshenings of the magnitude tion the observed changes are outliers in the CTL dis- observed than the other ensemble members. Interest- tribution for both the eastern and western modes, ingly, it is also this experiment that exhibits 15-yr salt- though the eastern distribution exhibits one 40-yr pe- ings after such freshenings. This emphasizes the impor- riod that reproduces the observed changes exactly. tance of initial conditions and internal variability and With the exception of the western mode ANT distribu- highlights the need for ensemble-based prediction. tion, there are no highlighted 40-yr periods in the It is evident from Fig. 5 that the observed changes are forced distributions that fall outside of the CTL ellip- unusual in the control distribution, the same being true soid. This suggests that an oscillation in isopycnal sa- for the eastern mode (not shown). The subsequent linity comparable to that observed is no more likely to 15-yr change is not independent from the initial 25-yr occur in the forced experiments than the HadCM3 con-

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Fig 5 live 4/C 15 AUGUST 2006 S TARK ET AL. 4081 trol. The western mode ANT distribution is dominated by a freshening trend, which results in some comparable periods that have a realistic 25-yr change though the subsequent salting is either underestimated or ab- sent. In the east the freshening trend is much weaker than in the west but there are periods comparable to the observations that fall outside of the ellipsoid though both freshening and salting changes are underestimated. The ALL ensemble also exhibits a freshening trend but it is weaker than in ANT. It is difficult to assess this model response to greenhouse gas forcing, as the three observations are too few to reveal any possible trend. The NAT experiments, for both the eastern and western modes, are characterized by oscillating SAMW isopycnal salinity. In the west, there are two highlighted periods that show changes very similar to FIG. 6. Distribution of the salinity change at the core of the Ϫ3 the observations at the right time; in the east, all of the model SAMW (25.7 kg m isopycnal) for every overlapping 40- yr period of CTL. The western mode is shown but the same result comparable periods lie within the ellipsoid. is seen if the eastern mode is considered. The observed change is In summary, the CTL distribution, and NAT in the shown by the solid vertical line and the 5% level for the distribu- western mode, show oscillating mode water isopycnal tion is shown by the dotted line. salinity, which is consistent with the available observations. Therefore, we have no reason to conclude that the changes in SAMW properties observed are not due formation region to be assessed. The range in model to internal variability. The lack of higher-frequency ob- temperature and salinity properties (2.5°C and 0.45 servations makes it extremely difficult to make a more psu) is somewhat higher than observed, which is to be definite attribution statement than this. If the model expected as the model run covers a much longer period internal variability is representative of real-world than the 6 yr of observations. The result suggests that SAMW then the observations would be a detectable the variability in model SAMW properties is not un- change. At present, however, there are not enough ob- derestimated in the deep-winter mixed layers south of servations available to assess the true variability, and Australia. As HadCM3 captures SAMW variability hence, how realistic the model internal variability is. well along SR3, we have no reason to expect the model New observations that better characterize the variabil- to perform worse farther west, yet until additional ob- ity of SAMW may lead us to revise our conclusions. It servations are available it is important to be aware that is interesting to note that if the 1987 observations are Fig. 5 may also suggest that the internal variability of ignored the result is a 40-yr freshening of 0.04 and 0.06 the model is too weak, as the observed oscillation in psu in the western and eastern modes, respectively. Fig- 40-yr salinity changes are clearly an outlier in the dis- ure 6 illustrates that such changes lie well within the tribution. model internal variability. This emphasizes the need for more data before attributing water mass changes to any 4. SAMW in the twenty-first century individual forcing scenario. a. Isopycnal changes The lack of high-frequency oceanic measurements at depth makes the true internal variability of SAMW Having examined the twentieth-century SAMW sa- hard to diagnose. However, six occupations of the linity changes we now consider what is likely to happen WOCE SR3 section between Australia and Antarctica, as greenhouse gases continue to increase. We use a crossing the formation zones in the Subantarctic Zone two-member ensemble forced with the Intergovern- (SAZ), between 1991 and 1996, allow the variability of mental Panel on Climate Change (IPCC) B2 scenario the dense SAMW found here to be well characterized emissions (Houghton et al. 2001). These experiments (Rintoul and England 2002). The range of SAMW are continuations of the first two members of the ANT properties exceeded 1.5°C and 0.3 psu and the changes ensemble. The B2 experiment is described in more de- were seen to be density compensating on a year-to-year tail in Johns et al. (2003) and has been used to study basis. Examination of water mass properties at 145°E, water mass properties by Banks et al. (2000), Banks, near the SR3 line in HadCM3 over 400 yr of CTL al- and Bindoff (2003). lows the internal variability of the model SAMW in the Examination of the B2 ensemble in Fig. 7 shows that

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FIG. 7. Western mode zonally averaged SAMW (25.5–26.0 kg Ϫ3 m ) isopycnal salinity for the two B2 scenario runs. The core of FIG. 8. Distribution of 40-yr changes in the two B2 simulations Ϫ3 the SAMW for each experiment (25.7 kg m ) is highlighted in at the core of the model SAMW for the western mode (25.7 kg bold. The results for the eastern mode are very similar. mϪ3 isopycnal). The twentieth-century results are shown in black and the twenty-first-century results are colored. The CTL 2.5 sigma ellipsoid is also shown and the observed changes are shown all of the isopycnals in the SAMW range freshen over by the red square. Similar patterns are seen for the eastern mode, the course of the twenty-first century. This is true for which is not shown. both the western and eastern mode (not shown). Su- perimposed on the freshening trend are decadal time- shown by Fig. 9 the formation of the cold, dense eastern scale fluctuations between fresher and more saline con- mode is centered farther south than that of the warmer ditions, there is a salting of 0.08 psu between 2037 and western mode. This is in general agreement with Mc- 2045 in the first ensemble member. The distribution of Cartney (1982) who found that the formation of 13°C 40-yr changes for these two ensemble members (Fig. 8) water was centered at 40°S, 70°E and that of 10°C water emphasizes this freshening trend; many of the periods at 45°S, 110°E. The two B2 experiments are used to are outside the CTL 2.5 sigma ellipsoid. In HadCM3, deduce the mechanisms linking surface forcing to sub- the observed SAMW changes in the twentieth century surface changes as they have a higher signal-to-noise are even less likely to occur as greenhouse gas levels ratio than is seen in the twentieth century. Throughout increase. Both the 25- and 15-yr changes in Fig. 8 are this discussion results are presented from the first B2 skewed toward isopycnal freshening compared to the ensemble member though it should be noted that none twentieth-century results, but the trend is not constant. of the results or conclusions change if the second mem- The 40-yr periods, which exhibit the strongest isopycnal ber is used. freshening in these two experiments, start between During the twenty-first century with B2 scenario 2020 and 2040. Later in the experiment, some 15-yr forcing the winter mixed layers in the SAMW isopycnal saltings are seen and the 25-yr freshenings are less pro- outcrop region are seen to warm and freshen on z-level nounced than between 2020 and 2040. The isopycnal surfaces, consistent with the freshening on isopycnals changes seen in the twenty-first century are neither uni- seen in the interior. This is either the result of direct directional or of constant strength. surface forcing or due to Ekman processes. The tradi- tional view has been that the properties of SAMW are b. What drives the changes? determined by the air–sea exchange of heat and fresh- SAMW originates as wintertime deep mixed layers water in the SAZ, where the water mass isopycnals and as it is formed it takes the temperature and salinity outcrop. More recent analyses (England et al. 1993; properties of this layer into the ocean interior. It has Rintoul and England 2002; Ribbe 1999), however, have been shown (e.g., Church et al. 1991; Bindoff and Mc- highlighted the importance of northward Ekman trans- Dougall 2000) that a freshening of SAMW on isopyc- port carrying cool, fresh Antarctic Surface Water into nals, such as that seen in Fig. 7, can be the result of the SAZ. The thickness of mode water layers and the either warming or freshening on pressure surfaces of short time available in winter to modify their proper- the winter mixed layer where the water mass outcrops. ties, much of which is used eroding the seasonal ther- In HadCM3, the water mass formation regions for both mocline, means that large surface flux anomalies are the western and eastern modes are identified using po- needed to change mode water properties (Warren tential vorticity and winter mixed layer properties. As 1972). To drive the interannual SAMW variability seen

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FIG. 9. Contour plot of late-winter (September) mixed layer temperatures (°C) and outcrop regions for SAMW isopycnals, which were used to identify the formation regions for the western and eastern modes found at 32°S. Outcrop regions are shown by the black lines that have been contoured at 0.1 kg mϪ3 intervals between 25.5 and 26.0 kg mϪ3, the density range of the model SAMW, and the colors represent the temperature of the September mixed layer, with red illustrating the warm (salty) modes found on the western half of the section and blue the cold (fresh) modes found in the east. in HadCM3 would require anomalous fluxes over 3 decreases over the twenty-first century while that to the months of 5.2 m of freshwater or 165 W mϪ2 of heat. south increases whereas the heat flux increases over These values are large compared to the climatological both regions. Overall, the twenty-first century increases values of 0.5 m yrϪ1 (Baumgartner and Reichel 1975), in formation zone heat flux and Ekman heat and fresh- Ϫ18.0 W mϪ2 (da Silva et al. 1994), and 7.7 W mϪ2 water divergences will all contribute to the winter (Josey et al. 1996). This combined with the near-density mixed layer warming and freshening and hence the compensating nature of the changes make it likely that isopycnal freshening seen at 32°S. This result agrees as in Rintoul and England (2002), the Ekman advection well with Banks et al. (2002) who concluded that heat- of Antarctic Surface Water into the formation zones ing is the dominant factor driving isopycnal changes in makes a significant contribution to SAMW properties SAMW. From Fig. 10 it appears likely that freshwater on these time scales. will only contribute to isopycnal changes in SAMW via Over the twenty-first century the changes in SAMW Ekman advection. are not density compensating yet it is evident from Fig. 10 that Ekman processes also contribute to the chang- 5. Summary and conclusions ing properties of the model water mass over longer time scales. First, considering formation zone surface fluxes, The main aim of this paper was to evaluate SAMW in there is a clear upward trend evident in the net surface the twentieth century in light of the new observations at heat flux, there is an increase in heat to the ocean of 8.5 32°S in the Indian Ocean, as well as assessing what is WmϪ2 between the 1990s and the 2090s. Over the same likely to happen during the twenty-first century. The period the net balance of precipitation and evaporation experiments that included greenhouse gas forcing (PϪE) decreased by 43.7 mm yrϪ1, resulting in a salting (ANT and ALL) exhibit isopycnal freshening trends on isopycnals in contrast to the freshening that will re- that, despite reproducing the observed freshening be- sult from the positive heat flux change. It is also clear tween 1962 and 1987, failed to capture the subsequent that during the twenty-first century the Southern Ocean salting. This is in contrast to the Banks et al. (2000) heat flux and net PϪE increase strongly and these will study, which was based on fewer model runs (one mem- contribute to the formation zone wintertime properties ber only from the ANT and NAT ensembles) and sug- via Ekman advection. An increase in Ekman heat and gested that the observed freshening between 1962 and freshwater divergence of 0.03 PW and 0.04 Sv (1 Sv ϭ 1987 was most likely a signal of anthropogenic forcing. 1 ϫ 106 m3 sϪ1), respectively, is seen during the twenty- The inclusion of the 2002 observations at 32°S has led first century and as there is no trend in the westerly us to revise this statement. It should also be stressed wind stress these changes are driven by surface flux that if future observations, such as those from ARGO changes. There is a stronger upward trend in the fresh- floats, were to show that the model either underesti- water divergence as the PϪE within the formation zone mated or overestimated SAMW variability then we

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FIG. 10. Time series of the processes that are important in determining wintertime properties in the model SAMW formation zone as defined in Fig. 9 for the first B2 experiment from 1950 to 2100: PϪE in the (top left) Southern Ocean and (top right) the formation zone; surface heat flux in the (middle left) Southern Ocean and (middle right) the formation zone; and the Ekman (bottom left) freshwater and (bottom right) heat divergences. In each case, the Southern Ocean refers to the area immediately south of the formation zone as defined in Fig. 9 and the divergences are calculated as the differences between the Ekman transport into and out of the formation zone. The number quoted in each plot is the decadal mean difference in that property for the decades beginning 2089 and 1989. may conclude that HadCM3 is inconsistent with the twenty-first century evident in Fig. 7 confirms the pos- observations. sibility of aliasing in the observations. If this time series The work highlights the difficulty of detecting and is sampled randomly over 15–25-yr time scales, in a attributing a possible climate change signal in a water similar fashion to the hydrographic cruises of the twen- mass that is so poorly sampled. With only a handful of tieth century, the importance of higher-frequency ob- hydrographic cruises it is nearly impossible to charac- servations becomes apparent. For example, if the first terize the true internal variability of SAMW let alone B2 experiment were sampled in 2015, 2037, and 2048, detect an unusual or climate change signal. The contin- then SAMW would have freshened by 0.15 psu over ued variability on decadal time scales during the 22 yr and then increased in salinity over the next 11 yr

Unauthenticated | Downloaded 09/28/21 06:38 PM UTC 15 AUGUST 2006 S TARK ET AL. 4085 by 0.07 psu, which is not dissimilar to the changes ob- To improve the likelihood of detecting an “unusual” served in the second half of the twentieth century. With trend in water mass properties the true nature of no extra information, it is difficult to see how this could SAMW variability needs to be better characterized as be interpreted as anything other than internal variabil- do the water mass properties across the whole Indo- ity yet the model time series show a clear and strong Pacific basin, a region where large-scale freshening has freshening trend in response to the B2 forcing. Banks been observed (Wong et al. 1999) and identified as a and Wood (2002) showed using the first B2 experiment possible fingerprint of anthropogenic climate change in HadCM3 that two single observations of SAMW at (Banks and Bindoff 2003). The ARGO profiling float 32°S, or hydrographic cruises, would have to be col- network offers for the first time an opportunity to ob- lected 55 yr apart for the signal to be significantly dif- serve water mass variability on both annual and inter- ferent from internal variability at the 5% level. For annual time scales and reduce the uncertainties associ- clear detection or attribution statements to be made ated with both observational and simulated results. It is about SAMW higher-frequency observations, such as only by characterizing the real water mass variability those offered by the ARGO float network, are vital. that model responses can be better understood and Anthropogenic climate change, with an associated more reliable projections of future changes be made. rise in global mean temperature, is expected to mani- fest itself as a freshening in those mode waters that Acknowledgments. This work was funded by the De- outcrop at high southern latitudes. When HadCM3 is partment of Environment, Food, and Rural Affairs un- forced with the IPCC B2 scenario emissions such an der the Climate Prediction Program (PECD 7/12/37). isopycnal freshening response is seen due to an in- We thank Yvonne Searl and Peili Wu for their valuable creased surface heat flux and increased Ekman advec- comments. tion of both heat and freshwater into the formation region. It is also clear from Fig. 7 that the long-term REFERENCES freshening trend will be punctuated by decadal time- Banks, H. T., and R. Wood, 2002: Where to look for anthropo- scale oscillations between more saline and fresher con- genic climate change in the ocean. J. Climate, 15, 879–891. ditions. The observed twentieth-century oscillation in ——, and N. L. 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