Winter seal-based observations reveal glacial meltwater surfacing in the southeastern Amundsen Sea

Yixi Zheng,Brice Loose,Benjamin G M Webber,Louise C Biddle,Lars Boehme,Karen J Heywood,David P Stevens

doi:10.1038/s43247-021-00111-z

Abstract

Determining the injection of glacial meltwater into polar oceans is crucial for quantifying the climate system response to ice sheet mass loss. However, meltwater is poorly observed and its pathways poorly known, especially in winter. Here we present winter meltwater distribution near Pine Island Glacier using data collected by tagged seals, revealing a highly variable meltwater distribution with two meltwater-rich layers in the upper 250 m and at around 450 m, connected by scattered meltwater-rich columns. We show that the hydrographic signature of meltwater is clearest in winter, when its presence can be unambiguously mapped. We argue that the buoyant meltwater provides near-surface heat that helps to maintain polynyas close to ice shelves. The meltwater feedback onto polynyas and air-sea heat fluxes demonstrates that although the processes determining the distribution of meltwater are small-scale, they are important to represent in Earth system models.

Highlights

Determining the injection of glacial meltwater into polar oceans is crucial for quantifying the climate system response to ice sheet mass loss
The strongest melt has been reported in West Antarctic ice shelves such as the Pine Island Ice Shelf (PIIS) and Thwaites Ice Shelf[2,3] where the deep intrusion of warm modified Circumpolar Deep Water transports heat southward via bathymetric troughs crossing the continental shelf[4]
The warm modified Circumpolar Deep Water (mCDW) enters the ice-shelf cavity, circulates beneath the ice shelf providing heat for basal melting, and forms a relatively fresh meltwater-rich water mass that is colder than mCDW but warmer than the surrounding Winter Water (WW)[4,5,6]

Summary

Introduction

Determining the injection of glacial meltwater into polar oceans is crucial for quantifying the climate system response to ice sheet mass loss. As solar warming in the near-surface layer (upper 200 m) can be mistaken for the signal of relatively warm meltwater exiting the ice-shelf cavity, the apparent near-surface meltwater-rich layer deduced from austral summer hydrographic observations is usually thought to be an artefact of solar warming and is neglected[5,6,18,19]. Our winter observations reveal clear signals of meltwater both at depth and near surface, connected by distinct meltwater-rich columns, while much of the WW layer remains meltwater-poor. This spatial heterogeneity is in contrast to the relatively high and horizontally-uniform upper-ocean meltwater content indicated by summer ship-based hydrography and previously-unpublished near-surface noble gas measurements. The winter processes revealed by our study are likely important for bringing nutrients to the near-surface layer prior to the spring bloom, and for bringing heat to the surface to prevent sea ice from forming and maintaining the polynyas in front of the ice shelves

Methods

Results

Conclusion