High-resolution surveys of internal tidal waves in Monterey Bay, California, using an autonomous underwater vehicle

François Cazenave, Yanwu Zhang, Erika McPhee-Shaw, James G. Bellingham, Timothy P. Stanton

Limnol. Oceanogr. Methods 9:571-581 (2011) | DOI: 10.4319/lom.2011.9.571

ABSTRACT: Temperature measured at the Monterey Inner Shelf Observatory (MISO) in southern Monterey Bay reveals frequent solitons and semidiurnal internal tidal bores (ITBs). A field study was undertaken to determine whether these features originate offshore and propagate shoreward over the continental shelf, and to characterize their evolution as they propagate. We developed a new survey method of using an autonomous underwater vehicle (AUV) to tightly track the thermocline along a 4.5 km transect, repeated 18 times over 18 hours, thereby measuring the internal-wave induced isotherm displacement at very high resolution. AUV measurements were compared with measurements at MISO, which is closer to shore. Internal waves of various scales were captured. An ITB of amplitude greater than 20 m propagating shoreward from the outer bay was the dominant feature. Rapid changes in stratification, possibly due to horizontal advection, were also observed. Technical challenges, advantages, and future improvements of this new AUV survey method are discussed. This study demonstrates that AUVs are efficient tools for surveying internal tidal waves.