Variation in the structure of the water column as captured by Slocum glider CTD and by CTD from a research vessel and assessment of internal waves

摘要

An autonomous underwater vehicle (AUV) "Slocum Glider" was deployed on the South coast of Newfoundland to collect oceanographic data from surface to 200 m depth. Between 7 March and 21 March 2012, the CTD mounted on the glider measured temperature, salinity, and dissolved oxygen at different depths along transects crossing Hermitage Bay, the area south of Connaigre Bay, Fortune Bay, and Belle Bay. Possible upwelling events were observed during the glider's survey suggesting a transport of water in and out of the area and deep water exchanges between basins. Two months later, on 5-7 May 2012, the area was revisited and the same physical parameters were measured on a lesser spatial resolution using a CTD on board of the R.V. CCGS Shamook. Variation in the physical parameters was observed with warming and freshening of the near-surface water. A subsurface layer with higher concentration of dissolved oxygen was also noted at around 50 m depth, suggesting enhanced local primary production due to internal mixing bringing nutrients towards the surface layer. The processes of heating of the surface layer and the appearance of relatively oxygen-rich water at mid-depth are especially of importance for the aquaculture activities occurring in the area. The present work first describes the glider data collection including the glider configuration and the expected planned tracks. It then details the data collected with this vehicle as well as those collected with the research vessel. Analysis of the differences between the two data sets is later presented with focus on added features that the glider provides. One interesting feature captured by the glider is the possible presence of internal waves. During the experiment, the glider deviated from the planned track by up to a couple of kilometers. During these track deviations noticeable change in the water column structure with vertical movement of the thermocline and halocline of the order of tens of meters was observed. Such v- rtical movement is thought to be associated with internal oscillations generated by tidal flow over topography or wind forcing disturbances. The second part of the work makes an attempt to analyse these vertical movements and estimate the characteristics of the internal oscillations including their vertical velocities and wave properties using the glider attitude. The result of the analysis will help in understanding the physical processes that are present in the waters of the South coast of Newfoundland and will be of interest for on-going development of a circulation model for the area.