Observing near-surface meltwater exiting from beneath Pine Island Glacier, Antarctica

Yixi Zheng is a PhD student working on physical oceanography at the Centre for Ocean and Atmospheric Sciences, University of East Anglia, UK. She is interested in the interactions between sea ice, ice shelves and ocean in Antarctic continental shelf seas, and how they affect ice-shelf melting and our future climate through upper-ocean processes. In a recent Nature communication paper, she used MEOP data, combined with ship-based observations to reveal the wintertime meltwater distribution in front of Pine Island Glacier for the very first time. They show that the wintertime meltwater surfaces and provides near-surface heat that helps to maintain polynyas close to ice shelves.

Pine Island Glacier is melting rapidly and exporting glacial meltwater into the ocean. The glacial meltwater is thought to play a role in hydrography and sea ice distribution but it is poorly observed and its pathways poorly known due to the sparse observations, especially in winter. In this study we turn to MEOP observations to determine the seasonal distribution of near-surface meltwater exiting from beneath Pine Island Glacier in winter. In winter, the basal meltwater is much warmer (up to about 1 ºC above freezing) than other upper-ocean water masses which have temperatures near the freezing point (about -1.9 ºC). In this way we unambiguously distinguish ice shelf basal meltwater from the ambient water masses. 

Here we present, for the very first time, a set of wintertime full-depth measurements of 625 salinity and temperature profiles collected by sensors attached onto three seals. Our wintertime observations reveal clear signals of meltwater both at depth and near surface, connected by distinct meltwater-rich columns, while much of water above 450 m remains meltwater-poor. This spatial heterogeneity is in contrast to the relatively high and horizontally-uniform upper-ocean meltwater content indicated by summertime measurements. 

We argue that in winter, compared with the cold and dense ambient water, the meltwater-rich water has sufficient buoyancy to rise to near surface without undergoing intense lateral mixing. 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 thus maintaining the polynyas in front of the ice shelves.



Map of the study region. Positions of the seal-tag hydrographic profiles collected in winter (July to September) 2014 are indicated by solid dots and diamonds coloured by the conservative temperature, , above freezing at 2 dbar (upper colour scale in red and blue).

Fig. 2 Schematic representation of the meltwater pathways in winter and summer. a, Wintertime meltwater either spreads along pycnocline or rises through uniform water layers without undergoing intense lateral mixing. Wintertime meltwater rising to near surface melts sea ice and forms polynyas. b, Summertime meltwater spreads along pycnocline as well. However, in contrast to wintertime meltwater, summertime rising meltwater penetrating through stratified water mixes with ambient water intensively and spreads widely.

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