Internal gravity waves in McMurdo Sound, Antarctica
Internal waves have an impact on many oceanic processes such as heat transport and mixing and have been studied intensively in mid and low latitudes. Their presence in polar regions has received less attention due to difficulties involved in data collection. High latitudes, beyond the critical latitude for the M2 tide (74.5°S) are particularly interesting and almost unstudied. The behaviour of internal waves is significantly changed in these regions as they cannot propagate freely. This thesis describes internal waves in McMurdo Sound, Antarctica. It is unique in three respects: first, the study site is beyond the critical latitude for the M2 tide and is the highest southern latitude (77.5°S) studied for internal waves, second, some of the data were collected during the Antarctic winter, and third, it is the first study of internal waves in McMurdo Sound. The spatial and temporal variation of internal waves, their frequency distribution and the vertical structure of the water column have been investigated during winter 2003 (April- September) and spring 2004 (November). Two different approaches have been taken. Profiles give a good vertical resolution for short time periods, while moored instruments offer the opportunity to study longer-term changes at particular depths. Vertical stratification changed significantly throughout the year. Temperature profiles are almost constant from May to November (mean temperature around -1.9°C). Simultaneously, density, which is mainly a function of salinity in this low temperature regime, increases and its variability with depth decreases. This is directly related to the annual cycle of sea-ice formation. Solar radiation in summer injects heat and fresh meltwater into the surface water layers, enhancing stratification. On the other hand, during winter, salt rejection from the local ice cover forces mixing. Internal tides in McMurdo Sound are sub-inertial. They are dominated by the diurnal frequency band with constituents having amplitudes of around 4 m followed by the semi-diurnal frequencies. Their energy levels decrease during the year, as expected, coinciding with the decrease of stratification of the water column that is necessary to support internal waves. Internal tides are most likely forced and decay as they propagate away from their generation site. Three possible generation sites emerge, local generation within the Sound, coastal internal Kelvin waves or semi-diurnal waves could have propagated from lower latitudes. Spectral slopes of higher frequency oscillations are consistent with the Garrett-Munk spectrum and Kolgomoroff's turbulence law. They indicate that both turbulent flows and internal waves could be responsible for the observations.
Advisor: Langhorne, Pat; Williams, Mike; Vennell, Ross
Degree Name: Master of Science
Degree Discipline: Physics
Publisher: University of Otago
Research Type: Thesis