Low frequency variability on the continental slope of the southern Weddell Sea

Abstract

One‐year long records of temperature, salinity, and currents show seasonally varying, energetic oscillations with a dominant period of approximately 35 h on the upper continental slope of the southern Weddell Sea. The data set is sampled by five moorings deployed on the slope of the Crary Fan, east of the main outflow site of the Filchner overflow plume. The characteristics of the observed oscillations are compared to idealized coastal trapped waves inferred from a numerical code. The variability at 35 h period is identified as mode 1 waves with wavelengths less than 200 km and group velocity opposing the phase speed, indicating energy propagation toward east. Filchner Depression and the nearby ridges on the slope are suggested as the generation site where the dynamics associated with the overflow plume can force the variability. Historical time series at the overflow site are revisited to identify the source of previously reported variability at 3 and 6 day time scales. Mode 2 waves at wavelengths of about 100 and 1000 km were found to bear resemblance to the 3 day and 6 day variability, respectively. The seasonal variation in the energy in the 35 h band shows small but significant correlation with the low frequency easterly winds. The presence of coastal trapped waves along the continental slope of the Weddell Sea can increase the heat exchange across the shelf break and affect the dense water production rates.

Seasonal resonance of diurnal coastal trapped waves in the southern Weddell Sea, Antarctica

“…3). High energy levels are additionally observed at semi-diurnal frequencies and around 35 h, as also found by Jensen et al (2013) and Darelius et al (2009).…”

“…at the ridge), the rotation angles have increased uncertainty (marked with an asterisk). More details on the moorings can be found in Foldvik et al (2004) and references therein, as well as in Jensen et al (2013) and Darelius et al (2016 to minimise the effects of the Ice Shelf Water plume. Temperature records suggest that plume water is rarely present at these levels.…”

“…where, following Jensen et al (2013), ω 1 and ω 2 correspond to periods of 26.9 and 21.3 h, respectively. Diurnal tidal KE has also been inferred using tidal predictions from the Circum-Antarctic Tidal Simulation version 2008b (CATS2008b), an updated version of the linear tidal inverse model described by Padman et al (2002).…”

“…tides or wind (Huthnance, 1995). Additionally, a connection between the generation of the waves and the outflow of dense shelf water through troughs has been suggested (Marques et al, 2014;Jensen et al, 2013). CTWs with subinertial frequencies propagate along a trapping boundary, e.g.…”

“…Studies by, e.g. Marques et al (2014), Jensen et al (2013), Brink (1991) and Wang and Mooers (1976) indicate that, in addition to the currents, seasonally varying hydrography alters the properties of CTWs.…”