Figure 1. The ATOC acoustic array is superimposed on a map of root-mean-square (rms) sea level anomaly from 4 years (January 1993 to December 1996) of TOPEX/POSEIDON altimetric measurements. Red lines indicate the sections used in the present study and are referenced by letter labels. Yellow lines show additional sections along which the acoustic propagation has been observed, but for which the data were not employed here. Data assimilation was carried out in the region bounded by the outer white rectangle, and heat content estimates were obtained inside the inner white rectangle. Much, but not all, of the elevation anomalies represent seasonal thermal changes within the ocean, with the acoustic data. providing a stable spatial average that is otherwise difficult to obtain. The ATOC region, being on the eastern side of the ocean, shows comparatively weak variability. Nevertheless, it is evident that the different acoustic sections will, during any 10-day period. have potentially very different anomalies.
The travel times for section k (Fig. 2), and with one exception all other sections, decreased in the summer and increased in the winter, consistent with the expected seasonal heating and cooling of the surface layer. For section v1, the situation is different: maximum heat content was recorded in March 1996. From a comparison of sections o and v1 (Fig. 3), we infer that the winter surface layer cooling near the source, where the two sections overlap, was more than offset by a subsurface warming near receiver v1, where the rays do not sample the surface layer.
Altimetric Component. The TOPEX/POSEIDON altimeter,13 in a 10-day repeating orbit, produces precise and accurate measurements of the sea surface topography (Fig. 1). Sea surface topography can be used to determine motions deep into the oceanic interior,14 but its structure reflects a complex combination of many different physical processes, barotropic and baroclinic.
