AUV Surveys of a Shallow Subtropical Water Column Beneath a Cold Atmospheric Front

The passage of a cold atmospheric front over a subtropical continental-self region such as southeast Florida is typically accompanied by cold 5 -10 m/s winds, with air temperatures dropping by 10-150C. The convective motion in the water column induced by the rapid surface cooling results in significant levels of small-scale turbulence and mixing in the water column. During such a front in fall, 1998, extensive measurements were made, in a shallow water column, of the distribution of currents, temperature and conductivity, as well as microstructure measurements of velocity and temperature, using a mobile autonomous underwater vehicle (AUV) platform, equipped with a 1200kHz acoustic Doppler current profiler (ADCP), a standard conductivity, temperature and depth (CTD) package and a custom small-scale turbulence package. A bottom-mounted 5-head ADCP recorded the vertical distribution of currents and Reynolds stress over an extended period during the passage of the front. Periodic CTD casts of the water column were conducted from a surface ship. Local atmospheric measurements included wind velocity, humidity, air temperature and solar radiation. The time series data from a nearby NOAA C-MAN atmospheric buoy clearly show the passage of the front. The corresponding distribution of TKE in the water column appears as a burst during the passage of the front. The AUV platforms (Figure 1) were utilized during Dec 17, 18 and 21, 1998. The Ocean Explorer (OEX) is a 2.1m long, 0.53m maximum diameter vehicle while the MADDOG is about the same length but with a maximum diameter of around 0.15m. The OEX carried out 'box-pattern' surveys at various depths. The 5-head ADCP was located in the center of the 'box-pattern' survey over a number of depths. Measurements from the mobile platforms have been utilized to construction of maps of distribution of currents, temperature, salinity and rates of dissipation of the kinetic energy in the water column. These will be presented, together with comparisons of the current measurements from the AUV with those of the 5-head ADCP. The Dec. 1998 experiment is part of a series of experiments being conducted at South Florida Measurement Center to characterize the impact of such cold fronts on the water column. The aim is to parameterize the physical processes involved in the associated air-sea interaction. Such parameterizations aid the development of subgrid scale models for large eddy simulation of the interaction. Planned future missions include measurement of bubble distribution beneath breaking waves using AUVs. The merits of the use of AUV platforms for oceanographic measurements will be discussed.

The work is funded by the Office of Naval Research (Program Manager: Dr Thomas Curtin).