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

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-