Exceptionally Warm and Prolonged Flow of Warm Deep Water Toward the Filchner‐Ronne Ice Shelf in 2017

Svenja Ryan,Markus Janout,Lucie Vignes,Elin Darelius,Hartmut H Hellmer,Michael Schröder

doi:10.1029/2020gl088119

Abstract

AbstractThe Filchner‐Ronne Ice Shelf, fringing the southern Weddell Sea, is Antarctica's second largest ice shelf. At present, basal melt rates are low due to active dense water formation; however, model projections suggest a drastic increase in the future due to enhanced inflow of open‐ocean warm water. Mooring observations from 2014 to 2016 along the eastern flank of the Filchner Trough (76°S) revealed a distinct seasonal cycle with inflow if Warm Deep Water during summer and autumn. Here we present extended time series showing an exceptionally warm and long inflow in 2017, with maximum temperatures exceeding 0.5°C. Warm temperatures persisted throughout winter, associated with a fresh anomaly, which lead to a change in stratification over the shelf, favoring an earlier inflow in the following summer. We suggest that the fresh anomaly developed upstream after anomalous summer sea ice melting and contributed to a shoaling of the shelf break thermocline.

Highlights

The ice shelves fringing the Antarctic continent are central to the stability of the Antarctic Ice Sheet, whose mass balance is largely controlled by the interaction of the ice shelves with the ocean (Fürst et al, 2016)
Basal melt rates are low due to active dense water formation; model projections suggest a drastic increase in the future due to enhanced inflow of open‐ocean warm water
The exchange of water masses across the continental shelf break is largely controlled by dynamics and characteristics of the near‐circumpolar Antarctic Slope Front (ASF; Jacobs, 1991) and the associated Antarctic Slope Current (ASC; see Thompson et al, 2018, for a review)

Summary

Introduction

The ice shelves fringing the Antarctic continent are central to the stability of the Antarctic Ice Sheet, whose mass balance is largely controlled by the interaction of the ice shelves with the ocean (Fürst et al, 2016). Enhanced basal melt rates and ice shelf thinning have been observed widely during the last decade, in particular in the Bellingshausen and Amundsen Seas (e.g., Paolo et al, 2015; Rignot et al, 2013) These changes have mostly been attributed to an increased transport of warm middepth Circumpolar Deep Water (CDW) onto the continental shelf (Nakayama et al, 2013; Pritchard et al, 2012). The southeastern Weddell Sea, home of the large Filchner‐Ronne Ice Shelf (FRIS), is of great climatic importance as dense water production on the 400 km broad continental shelf contributes significantly to the formation of Weddell Sea Deep and Bottom Waters (Nicholls et al, 2009) Both water masses are precursors of the globally relevant Antarctic Bottom Water (Foldvik, 2004), which ventilates the abyssal ocean (Reid & Lynn, 1971) and supplies the lower limb of the global overturning circulation (Sloyan & Rintoul, 2001). SSMI‐SSMIS sea ice concentration data were obtained as a 5‐day median‐filtered and gap‐filled product from the Integrated Climate Data Center in Hamburg (ICDC), Germany

Prolonged and Exceptionally Warm Inflow in 2017

Fresh Anomaly

Modified Shelf Density Structure

Shoaled Thermocline

Thermocline Preconditioning due to Sea Ice Melt

Conclusions