The amplitude of the acoustically derived sea level anomaly is about half that of the altimeter sea level anomaly. Since the acoustic measurements are dominated by temperature variability, about half of the observed sea level variability appears to be due to temperature. Satellite altimetry is therefore not a robust sole surrogate for estimating heat content of the ocean. When combined with acoustic measurements, however, heat content can be more reliably derived.
Model + Observations
The next step in exploiting the combination of acoustic and altimetric data is to assimilate the observations into a GCM. Wunsch and the ATOC Consortium (1998) have done this recently, with promising results. There is no need to try to derive temperature from the altimeter, or sea level from acoustic thermometry. The GCM can accommodate each measurement type without further derivation. Acoustic thermometry observations constrain and improve the modeled temperature along ATOC's acoustic paths, and satellite altimetry improves its realisation of sea level at the modeled free surface along TOPEX/POSEIDON's ground tracks. Where there are no observations, the model provides the best estimate of the ocean state. Wunsch notes that the combination of model and data produces a better estimate than either could alone.
Indian Ocean
The spatial scale of ocean climate variability dictates that each major ocean basin be monitored in some comprehensive way for changes in heat content and temperature structure. Acoustic thermometry, as ably demonstrated in the Pacific, is a technique that intrinsically matches the scale of climate variability. The Indian Ocean is a prime candidate for the next step in expansion towards a global acoustic network. It is unique in that it is the only major ocean basin that is completely bounded to the north by a continental landmass. Exchange takes place with the Southern Ocean, and to a limited extent, with the Pacific Ocean through the Indonesian Archipelago. Interannual variability of the heat content of the northern Indian Ocean influences the intensity of the NW Monsoon, which in turn has major consequences on Asian and Australian agriculture. A strong correlation has been demonstrated to exist between sea surface temperature anomalies of the Northeastern Indian Ocean and Australian agricultural production, linked by variations in rainfall. A 1-C variation from year to year corresponds to a variation in production of about $6B. Measuring the ocean temperature on regional to basin scales is therefore crucial to predicting rainfall for the agriculturally sensitive regions bordering the northern and eastern Indian Ocean.
