Controls on turbulent mixing on the West Antarctic Peninsula shelf

Abstract

The ocean-to-atmosphere heat budget of the West Antarctic Peninsula is controlled in part by the upward flux of heat from the warm Circumpolar Deep Water (CDW) layer that resides below ~200 m to the Antarctic Surface Water (AASW), a water mass which varies strongly on a seasonal basis. Upwelling and mixing of CDW influence the formation of sea ice in the region and affect biological productivity and functioning of the ecosystem through their delivery of nutrients. In this study, 2.5-year time series of both Acoustic Doppler Current Profiler (ADCP) and conductivitytemperature-depth (CTD) data are used to quantify both the diapycnal diffusivity κ and the vertical heat flux Q at the interface between CDW and AASW. Over the period of the study, a mean upward heat flux of ~1 W m-2 is estimated, with the largest heat fluxes occurring shortly after the loss of winter fast ice when the water column is first exposed to wind stress without being strongly stratified by salinity. Differences in mixing mechanisms between winter and summer seasons are investigated. Whilst tidally-driven mixing at the study site occurs year-round, but is likely to be relatively weak, a strong increase in counterclockwise-polarized near-inertial energy (and shear) is observed during the fast-ice-free season, suggesting that the direct impact of storms on the ocean surface is responsible for much of the observed mixing at the site. Given the rapid reduction in seaice duration in this region in the last 30 years, a shift towards an increasingly wind-dominated mixing regime may be taking place.

Shelf–ocean exchange and hydrography west of the Antarctic Peninsula: a review

“…A recent observational study made significant progress in understanding the sources and temporal variability of mixing on the shelf [ 73 ]. Using a 2.5 year-long time series of ocean velocity profiles, hydrographic profiles and wind velocity from Ryder Bay ( figure 1 ), wind-driven currents are shown to be a key source for mixing, with the time variability and intensity of this process being heavily modulated by the timing of ice retreat.…”

Changes in Phytoplankton Communities Along the Northern Antarctic Peninsula: Causes, Impacts and Research Priorities

“…Nevertheless, some of the regional flows and physical processes that impact the water mass mixing, hydrography, and the advection of surface, intermediate and shelf waters in the region are still undetermined. For example, new information is needed for a better understanding of (i) intrusions of Warm Deep Water (a Weddell Sea local water mass sourced by CDW) at intermediate levels from the Powell Basin toward the Bransfield Strait (e.g., Azaneu et al, 2017); (ii) the periodicity and frequency of CDW intrusions along the western continental shelf of the NAP (e.g., Moffat et al, 2009;Couto et al, 2017;McKee et al, 2019); (iii) the rate of water masses mixing along the deep Bransfield basins (e.g., Brearley et al, 2017); and (iv) the rate of Weddell Sea shelf waters advection and renewal from the east (e.g., Renner et al, 2012;Dotto et al, 2016). All these unknown mechanisms directly impact the vertical stratification of the upper water column, consequently, impacting the development and distribution of the phytoplankton groups due the local and regional changes of the physical and chemical properties.…”

Modeling of the Influence of Sea Ice Cycle and Langmuir Circulation on the Upper Ocean Mixed Layer Depth and Freshwater Distribution at the West Antarctic Peninsula

“…Although ERA-Interim is seen as the most reliable reanalysis over this area (Bracegirdle, 2013), Rodrigo et al (2013) argue that spatial resolution in the ERA-Interim dataset is not high enough to capture synoptic high-wind events and observe that ERA-Interim underestimates wind speed in the ice shelf areas around Ross Ice Shelf. Lower wind speed could also lead to shallower MLD in the model results, as Brearley et al (2017) have found that direct impact of storms on ocean surface is responsible for a large portion of the surface ocean mixing in the WAP shelf.…”

“…The biases towards shallower MLD could also be related to other limitations in the model configuration. A higher horizontal resolution could improve the model by better resolving eddy activity and coastal circulation; and adding tides could also have an impact in the vertical mixing, although Brearley et al (2017) find that tidally driven mixing is weak in the WAP shelf. Although ERA-Interim is seen as the most reliable reanalysis over this area (Bracegirdle, 2013), Rodrigo et al (2013) argue that spatial resolution in the ERA-Interim dataset is not high enough to capture synoptic high-wind events and observe that ERA-Interim underestimates wind speed in the ice shelf areas around Ross Ice Shelf.…”