Comment on tc-2021-250

doi:10.5194/tc-2021-250-rc1

Abstract

Polynyas are key sites of ice production during the winter and are important sites of biological activity and carbon sequestration during the summer. The Amundsen Sea Polynya (ASP) is the fourth largest Antarctic polynya, has recorded the highest primary productivity and lies in an embayment of key oceanographic significance. However, knowledge of its dynamics, and of sub-annual variations in its area and ice production, is limited. In this study we primarily utilize Sentinel-1 SAR imagery, sea ice concentration products and climate reanalysis data, along with bathymetric data, to analyze the ASP over the period November 2016–March 2021. Specifically, we analyze (i) qualitative changes in the ASP's characteristics and dynamics, and quantitative changes in (ii) summer polynya area, (iii) winter polynya area and ice production. From our analysis of SAR imagery we find that ice produced by the ASP becomes stuck in the vicinity of the polynya and sometimes flows back into the polynya, contributing to its closure and limiting further ice production. The polynya forms westward off a persistent chain of grounded icebergs that are located at the site of a bathymetric high. Grounded icebergs also influence the outflow of ice and facilitate the formation of a 'secondary polynya' at times. Additionally, unlike some polynyas, ice produced by the polynya flows westward after formation, along the coast and into the neighboring sea sector. During the summer and early winter, broader regional sea ice conditions can play an important role in the polynya. The polynya opens in all summers, but record-low sea ice conditions in 2016/17 cause it to become part of the open ocean. During the winter, an average of 78 % of ice production occurs in April–May and September–October, but large polynya events often associated with high winds can cause ice production throughout the winter. While passive microwave data or daily sea ice concentration products remain key for analyzing variations in polynya area and ice production, we find that the ability to directly observe and qualitatively analyze the polynya at a high temporal and spatial resolution with Sentinel-1 imagery provides important insights about the behavior of the polynya that are not possible with those datasets.

Highlights

Introduction‘latent heat polynyas’ (and referred to as ‘polynyas’), are sites of open water surrounded by sea ice and land, glacier ice or fast ice (Armstrong, 1972; Tamura et al, 2008; Park et al., 2018)
‘latent heat polynyas’, are sites of open water surrounded by sea ice and land, glacier ice or fast ice (Armstrong, 1972; Tamura et al, 2008; Park et al, 2018)
Focusing on the summers of 2016/17 - 2020/21 and the winters of 2017 - 2020, we present the first detailed study of year-round variations in the Amundsen Sea Polynya’s behavior, area, and ice production

Summary

Introduction

‘latent heat polynyas’ (and referred to as ‘polynyas’), are sites of open water surrounded by sea ice and land, glacier ice or fast ice (Armstrong, 1972; Tamura et al, 2008; Park et al., 2018) These polynyas are distributed around the coast of Antarctica and are typically at fixed geographic locations each year. The combination of ice-free conditions, summer sunlight, and the availability of dissolved iron (e.g. Arrigo et al, 2008a; 2012; St-Laurent et al, 2017), enables large phytoplankton blooms to develop in polynyas during this summer period These phytoplankton blooms fix carbon from dissolved carbon dioxide, some of which sinks below the surface layer (Sweeney et al, 2003). The evolution of polynyas during the summer is considered a key factor in the primary productivity of the Southern Ocean, and their role in the sequestration of Carbon Dioxide (the ‘biological pump’) (Arrigo et al, 2008b)

Results

Discussion

Conclusion