Feedbacks between ice cover, ocean stratification, and heat content in Ryder Bay, western Antarctic Peninsula

Abstract

A multiyear, all-season time series of water column physical properties and sea ice conditions in Ryder Bay, at the western Antarctic Peninsula (WAP), is used to assess the effects on the ocean of varying ice cover. Reduced ice cover leads to increased mixing and heat loss in the winter. The reduction in stratification persists into the following summer, preconditioning the water column to a greater vertical extent of surface-driven mixing. This leads to an increased amount of heat from insolation being mixed down, affecting approximately the top 100 m. The increased heat uptake in summer exceeds the heat lost the preceding winter, giving the initially counter-intuitive effect that enhanced winter cooling generates warmer temperatures in the following summer and autumn. This process is therefore a positive feedback on sea ice, as reduced sea ice leads to increased heat content in the ocean the following autumn. It also causes increased winter atmospheric temperatures due to the increased winter heat loss from the ocean. In the deeper part of the water column, heat and carbon stored in the Circumpolar Deep Water (CDW) layer are released by deep mixing events. At these depths, conditions are restored by advection and vertical mixing on multiyear time scales. In recent years, stronger deep mixing events in winter have led to a persistent reduction in CDW temperatures at the study site. Ocean glider data demonstrate the representativeness of these results across the wider region of Marguerite Bay, within which Ryder Bay is situated.

Interannual variability in phytoplankton biomass and species composition in northern Marguerite Bay (West Antarctic Peninsula) is governed by both winter sea ice cover and summer stratification

“…The monitoring site is situated in Ryder Bay, which is part of the northern Marguerite Bay region. Venables and Meredith () showed that the RaTS location in Ryder Bay is representative of the broader physical environment across northern Marguerite Bay. The distance between the nearest shore and the RaTS site is ∼2 km, and the maximum depth is ∼520 m. Data were collected between April 1997 and October 2012.…”

“…We define fast‐ice cover as days with ≥ 80% coverage of the area by fast‐ice. Even though multiple observers throughout the years can introduce internal heterogeneity in the series due to the observations' subjective nature, such errors were still smaller than the large interannual variability in physical changes within the region (Venables and Meredith ).…”

Assessing Drivers of Coastal Primary Production in Northern Marguerite Bay, Antarctica

“…thrived. Period 3 was characterized by increased wind mixing causing a more homogenous water column and deeper MLDs, typical for conditions at the end of summer and observed more frequently at the WAP Montes-Hugo et al, 2009;Venables and Meredith, 2014;Rozema et al, 2017b). Thereafter, the community consisted of large proportions of haptophytes, diatoms and cryptophytes (Figure 6).…”

“…Here, the year-round Rothera Oceanographic and Biological Time Series (RaTS) program, representing the larger northern Marguerite Bay region, provides the infrastructure and scientific context to understand the dynamics in phytoplankton productivity (Venables and Meredith, 2014). Previous studies have shown a variable, yet generally high, level of phytoplankton biomass dominated by diatoms during summers in this region (Annett et al, 2010;Kozlowski et al, 2011;Rozema et al, 2017a).…”

Southern Ocean Phytoplankton in a Changing Climate

“…They generally occur in shallow mixed layers where wind speeds are <5 m s −1 (Fitch and Moore, 2007). Storms, wind mixing, and waves deepen mixed depths in the MIZ, reducing the light availability and inhibiting bloom development ( Figure 7B) (Venables and Meredith, 2014). As a result, blooms only cover 17-24% of the MIZ over summer with maximum coverage of only 0.36 million km 2 in December (Savidge et al, 1996;Fitch and Moore, 2007).…”