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Quantifying internal variability of Subantarctic Mode Water and its response to anthropogenic forcing

PI: Yanli Jia
Institution: University of Hawaii
Additional Investigators: Richard Wood, and Sheila Stark
Mode waters, as they ventilate in the thermocline, carry information about surface conditions at their formation regions into the ocean interior. Variabilities in their properties are thus potential indicators of changes in surface forcing. For example, Banks et al. (2000) found that Subantarctic Mode Water (SAMW) is a potentially sensitive indicator of anthropogenic climate change. This deduction was based on a comparison of the observed changes of the water mass between two snapshots (1962 and 1987) with that simulated by a climate model (HadCM3) driven by anthropogenic forcing. The simulated freshening and cooling were comparable with that observed in both magnitude and pattern. A recent survey along 32 deg-S in the Indian Ocean provided another snapshot (2002), showing that there has been a reversal of the SAMW freshening trend with the water mass properties being almost identical to their 1962 values (Bryden et al. 2003). However, this reversal is not reproduced by any of the HadCM3 simulations. Instead, the freshening trend continues in model experiments that include anthropogenic forcing. A comprehensive analysis of the SAMW in HadCM3 recently conducted by Stark et al. (2004), making use of a large set of ensemble experiments with different atmospheric forcing scenarios, concludes that the observed decadal variability of the SAMW can only be explained by internal variability, and that the persistent freshening trend in simulations with anthropogenic forcing is due to a combination of shallow model mixed layers and elevated model wind stress, precipitation and Tasman inflow.

The modified conclusion described above emphasizes the dependence of our understanding of climate variability on direct ocean observations, as well as the benefits of extensive numerical experimentations. With the latter in mind, we propose to analyse the multi-model dataset to be collected for the IPCC AR4, our objective being to quantify internal variability of SAMW and its response to anthropogenic forcing. We shall evaluate each of the climate models in terms of ocean heat transport and the large scale ocean circulation as in Jia (2003), as well as the SAMW properties. Based on this initial evaluation, we shall select a subset of the models for a detailed investigation of the nature of the SAMW variability using control experiments and experiments with different forcing scenarios. The datasets required are listed below.

Ocean heat transport and meridional overturning streamfunction, globally and for individual basins
Barotropic streamfunction for ocean
Ocean temperature and salinity
Net surface freshwater flux (P-E)
Wind stress


Banks et al., 2000: Are observed decadal changes in intermediate water masses a signature of anthropogenic climate change? Geophys. Res. Lett., 27, 2961-2964.
Bryden et al., 2003: Changes in ocean water mass properties: oscillations or trends? Science, 300, 2086-2088.
Jia, 2003: Ocean heat transport and its relationship to ocean circulation in the CMIP coupled models. Clim. Dyn., 20, 153-174.

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