Comparison of Observed Temperature and Salinity Changes in the ...

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1JANUARY 2003 BANKS AND BINDOFF 157 TABLE 1. Zonal section 47N 24N 10N 17S 32S 43S Modern and historical data used in the comparison. Modern (yr) 1985 1985 1989 1994 1987 1989 No. of casts 299 156 213 294 109 72 Historical (yr) 1966 1970 1969 1967 1962 1967 No. of casts 299 509 400 706 451 18 Std dev (yr) 5.0 5.5 6.0 5.0 10.0 1 Time difference (yr) 19 15 20 30 25 22 can produce the observed changes in temperature and salinity on isopycnals. Beyond validation, in a coupled model it is possible to diagnose surface forcing and, therefore, to try to understand the processes and mechanisms by which subsurface oceanic changes may occur. This paper aims to address three questions: 1) Can a coupled climate model reproduce observed water mass changes in the Indo-Pacific? 2) Are observed changes anthropogenically forced or a signal of internal variability? 3) What atmospheric changes drive the observed subsurface water mass changes? We will first describe observations from the Indo-Pacific over the last century and simulations of a state-of-the-art coupled climate model, the third Hadley Centre Coupled Ocean– Atmosphere General Circulation Model (HadCM3; section 2). We will then compare observed changes in temperature and salinity with those simulated by the coupled climate model (section 3). In section 4 we will find the pattern of change in the coupled model that may be thought of as a fingerprint of anthropogenic forcing, and in section 5 we will relate the subsurface fingerprint to the changes in surface fluxes. Finally, in section 6 we will draw conclusions. 2. Data and methods a. The data We compare the output of the model with the observed differences in temperature and salinity in the deep ocean (Table 1). These differences are constructed from a variety of datasets and analyses (Bindoff and Church 1992; Bindoff and McDougall 1994, 2000; Wong et al. 2001). The historical data are primarily obtained from J. Reid and A. Mantyla (1994, personal communication) and from the Southern Ocean Atlas (Gordon et al. 1982) for both the Indian and Pacific Oceans. The hydrographic data from these atlases are mainly Nansen bottle data. The recent hydrographic data come from the World Ocean Circulation Experiment (WOCE) and are available from the World Hydrographic Program Office (WHPO; see online at http:// whpo.ucsd.edu). These WOCE data are all CTD data calibrated against salinity analyses from water samples taken through the whole water column. Typical accuracies of the CTD temperature and salinity data are 0.002C and 0.003 psu. The historical data are interpolated to the location of each cast from the recent hydrographic profiles using objective methods (Bretherton et al. 1976; Roemmich 1983; Bindoff and Wunsch 1992). The observed differences along these sections (Table 1) are distributed over both the Pacific and Indian Oceans (Fig. 1). The best coverage is of the Pacific basin, although the South Pacific is less well covered. These observed differences have been interpreted in earlier papers (listed above) but are now compared with the HadCM3 model for the first time. FIG. 1. The locations of the historical and modern data used to compare with the results from the coupled ocean–atmosphere model (HadCM3).
158 JOURNAL OF CLIMATE VOLUME 16 b. Numerical model The model we will look at here is the coupled ocean– atmosphere model, HadCM3 (Gordon et al. 2000), developed at the Hadley Centre. The major differences between this coupled model and the previous version (Johns et al. 1997) occur in the ocean component, which has higher horizontal resolution and employs the Gent and McWilliams (1990) scheme to represent mixing due to eddies. The introduction of these and other improvements meant that the coupled model no longer needed artificial flux adjustments to prevent the model drifting to an unrealistic state and exact fluxes are exchanged between the atmosphere and the ocean on a daily basis. The ocean component of the model is based on the Cox (1984) model with a horizontal resolution of 1.25 in both the north–south and east–west direction and 20 vertical levels. The reader is referred to Gordon et al. (2000) for details of the model. Here we will examine two experiments; firstly, the control experiment (CTL) and secondly an anthropogenic climate change experiment B2. CTL is initialized with the temperature and salinity of the ocean given by the climatology of Levitus and Boyer (1994) and with both the atmosphere and ocean at rest. While the oceanic and atmospheric heat budgets come into balance within 10 yr in CTL (Gordon et al. 2000), this is not the case for the freshwater budgets, which require a timescale of the order of 300–400 yr to come into balance (Pardaens et al. 2001, manuscript submitted to Climate Dyn., hereafter PBGR). Since any imbalance will directly affect the salinity structure of the ocean, this means that the water mass properties must have a similar timescale to the salinity field to reach a quasi equilibrium. For this reason, in this paper we will only look at water mass properties from year 370 onward. Experiment B2 is initialized at year 370 of CTL. The B2 is a climate change experiment with imposed anthropogenic changes of well-mixed greenhouse gases, tropospheric and stratospheric ozone and sulphate aerosols, and is described in detail by Johns et al. (2001). The B2 is forced using historical emissions and concentrations from 1859 until the present, then the IPCC B2 scenario until 2100. Experiment B2 has previously been used by Banks et al. (2000) and Banks and Wood (2002) to look at changes in water mass properties. A simpler experiment with HadCM3 based on 2% increase per year in carbon dioxide was used by Banks et al. (2002) to understand the causes of change in Subantarctic Mode Water in the Indian Ocean. The present paper is the first to quantitatively compare HadCM3 and observations throughout the Indo-Pacific and in its examination of the basinwide modes of the Indo-Pacific. FIG. 2. Temperature–salinity diagrams showing the Southern Hemisphere section average water mass changes found in both (a), (c), (e) the observations and (b), (d), (f) in the HadCM3 anthropogenic climate change experiment (B2) of HadCM3 for the period shown in Table 1. (a), (b): 17S; (c), (d): 32S; and (e), (f): 43S. The continuous lines are the old water mass characteristics, and the dashed lines are the new characteristics. The reader is referred to Fig. 4 to examine the changes in detail. 3. Comparison of observed changes and coupled model changes a. Qualitative comparison of modeled and observed temperature–salinity properties Figure 1 shows the location of the observations used in this study (Wong et al. 1999). The dates of the observations are described in Table 1. It should be noted that the historical data are interpolated onto the location of the modern sections and have an associated error bar to account for the effects of mesoscale variability. We will use the uncertainties later. For the model data we have extracted zonal sections from the annual means of the years of the historical and modern observations at the same latitudes as the observed sections. We have calculated error bars for the model to account for the uncertainty in the date of the historical data and shortterm variability of the model. This is based on the standard deviation of 5-yr differences in B2. Figure 2 shows temperature–salinity diagrams from both the modern and historical times for both the observational data and the modeled data at the three southern sections; 43, 32, and 17S. The bold lines denote the historical temperature–salinity profile, while the dashed lines denote the modern profile. In each case, the modeled and observed temperature–salinity profiles are qualitatively similar; in each profile there is a well-
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