Monitoring deep ocean temperatures using low-frequency ambient noise

Abstract

In order to precisely quantify ocean’s heat capacity and ocean’s influence on climate change, it is important to accurately monitor ocean temperature variations, especially in the deep ocean (i.e., at depths ~1000 m), which cannot be easily surveyed by satellite measurements. Indeed, to date, deep ocean temperatures are most commonly measured using free-drifting oceanographic floats (e.g., ARGO floats), but this approach only provides a limited spatial and temporal resolution because of the sparseness of the existing global network of oceanographic floats. On the other hand, acoustic thermometry has already been demonstrated as an important modality for measuring ocean temperature and its heat capacity using low-frequency sound. However, current implementations of acoustic thermometry requires the use of active sources; aside from the technology issues of deploying such sources, there is also the ongoing issue of the potential disturbance of marine animals. We will demonstrate a totally passive acoustic method of acoustic thermometry based only on coherent processing of low-frequency ocean noise (f<50Hz) and whose results are in agreement with classical point measurements obtained from oceanographic floats. We will discuss how passive acoustic thermometry could improve global monitoring of deep ocean temperature variations through implementation using a global network of hydrophone arrays.