Discussion and Conclusion

Upper ocean temperature variability within the Indonesian seas and southeast Indian Ocean has been described on the basis of repeat high-quality XBT data collected since 1983. The observations have been compared to results from an eddy-permitting ocean circulation model. Besides strong annual variability in the seasonal thermocline and in the thermoclines off the Arafura and Java shelf breaks in both data sets, interannual variability dominates much of the thermocline variability elsewhere.

Next, a lagged multiple regression technique was performed on the basis of satellite altimetry collected since 1992. The results reveal that interan-nual sea-level variability in the southeast Indian Ocean and Indonesian seas is largely driven by remote equatorial winds, as predicted by Clarke and Liu (1994). The dominating effect of ENSO, through its modulation of Pacific equatorial winds and associated equatorial Rossby wave response, has been proposed and recognized in a series of studies (Clarke and Liu, 1994; Meyers, 1996; Ffield et al., 2000; Potemra, 2001; Wijffels and Meyers, 2004). The penetrating wave signal is visible in both in the coastal wave guides and as free Rossby waves propagating westward into the southeast Indian Ocean. Equatorial Indian Ocean winds have different timescales than Pacific winds. Indian Ocean wind energy penetrates along the Lesser Sunda island arc and into the internal Indonesian seas, controlling both sea level and thermocline depth along the eastern Java Sea shelf break and within Makassar Strait.

We have identified the presence of many of these features in our ocean circulation model but differences to observations exist in the amplitudes of the signals and, to a lesser extent, also in the phase of the signals. The most significant problem of this model (in common with many other ocean general circulation models) is the deeper than observed thermocline. The near-surface circulation of the ocean is determined by model physics and exterior forcing (surface heat and freshwater fluxes, wind stresses). Both of these components contain errors irrespective of the source of the surface forcing (from observations as used in this study or from an atmospheric circulation model in a coupled mode), which hampers identification and rectification of model errors. Nevertheless, model errors are the likely cause for the broad-scale overestimate of thermocline depth and associated misrepresentations of water temperature and salinity. A more realistic representation of interior mixing processes is required, and isopycnal mixing might help here.

Another limitation of the model used in this study relates to the moderate resolution (0.5° x 0.33°, 36 vertical levels). This resolution is too coarse to resolve complicated details of currents and water-mass properties in the ITF region. The primary source of vertical mixing for water-mass properties in the Indonesian seas could be internal waves generated by barotropic flow over variable bottom topography as discussed by Ffield and Gordon (1992, 1996). The effect of internal tides and their breaking on vertical mixing has been parameterized in our model by shear-dependent mixing based on a gradient Richardson number and therefore represents a potential source of error in simulating vertical structures.

The relatively weak response to changes in ENSO winds of the model compared to observations is intriguing and requires further investigation. One possible explanation for some of the observed discrepancies could be due to a poorly resolved coastal wave guide in the model. In this region, the remote response to both ENSO and Indian Ocean equatorial wind changes occur outside of the equatorial wave guide. While the response within the wave guide (latitudes approximately <8°) is quite well simulated, those outside are not. The signals must travel through coastal wave guides to reach the Ombai Strait, the Bay of Bengal, or the southwestern coast of Australia. We note that these signals are weak in the model, suggesting an unrealistic partition of energy between reflected equatorial waves and coastally trapped waves. Further work is needed to confirm this notion, but it may have implications for where high model resolution is required to resolve these scattering regions.

To help fully resolve the temporal variability and characteristics of the ITF transport and property fluxes, a multiyear comprehensive measurement program is required. A serious shortcoming in previous observational measurement programs within the Indonesian region is the lack of temporal coherence: the data cover different time periods and depths in the different passages of the complex pathways toward the Indian Ocean. This has lead to ambiguity of the mean and variable nature of the ITF, and the transformation of the thermohaline and transport profiles within the interior seas. For this reason, and in keeping with the recommendations stemming from the full community deliberation (OCEANOBS October 1999 meeting in St. Raphael, see Imawaki et al., 1999; Workshop on Sustained Observations for Climate of the Indian Ocean, Perth, November 2000, see http://www. marine.csiro.au/conf/socio/socio.html), an international co-operative effort commenced in August 2003 with deployments of multiyear moorings for direct throughflow velocity, salinity, and temperature measurements simultaneously in Makassar Strait, Lifamatola Passage, Lombok Strait, Ombai Strait, and Timor Passage (Fig. 1). The program that is called INSTANT (International Nusantara STratification ANd Transport) involves five nations: Australia, France, Indonesia, Netherlands, and USA. INSTANT is designed to provide a time series of ITF transport and property fluxes, and their variability from intraseasonal to annual timescales, along the ITF pathway from the intake of Pacific water at Makassar Strait and Lifamatola Passage to the Lesser Sunda exit channels into the Indian Ocean. The initial deployment of all moorings commenced in August 2003, with redeployment in February 2005, and recovery in July 2006. The resulting simultaneous, multiyear direct measurements in both the inflow and the outflow ITF passages will allow us, for the first time, to unambiguously determine the magnitude of the interocean transport, its properties, and their evolution between the Pacific and the Indian Oceans. Finally, an important outcome of the program will be to recommend an efficient set of ongoing observations that may offer a proxy for a long-term monitoring strategy of the ITF transport and property fluxes.

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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