Acoustic response for the Deep Thermal Front variability off Brazilian southeastern coast: A preliminary study

Intrusions of relatively cold and fertile South Atlantic Central Water (SACW) are a typical oceanographic phenomenon observed on the continental shelf off the southeastern Brazilian coast. Among the different factors that drive the SACW intrusions, one prominent mechanism is water transport driven by NE-E wind conditions. These winds vary seasonally, and they are prevalent during the spring and summer months. During these months, the water column is generally stratified while the reverse is characteristic in winter, when the water column is almost homogeneous. This cold water forms a deep front, known as Deep Thermal Front (DTF). However, a range of other factors influence the SACW intrusions and there is a need to better understand them. Prediction and monitoring of DTF variability are issues of great interest given its importance, for example, on climate, fishing and for the oil industry. In this regard, acoustics can provide additional data that can supplement conventional oceanographic methods for tracking and monitoring the front movement. Acoustic methods present an interesting advantage because they are able to sample the water column over large three-dimensional distances on an effectively synoptic scale. Preliminary results presented in this paper are encouraging, because the acoustic propagation is strongly dependent on the front variability. Results show that SAWC intrusions are responsible for trapping the rays in the channel of subsurface cold waters, especially evident in the spring (SAWC maximum intrusion). The opposite is seen in winter, when the water column is practically homogeneous and there are no cold water intrusions, and the rays propagate throughout the whole water column.