Thin ice, deep snow and surface flooding in Kotzebue Sound: landfast ice mass balance during two anomalously warm winters and implications for marine mammals and subsistence hunting

Abstract

The inaugural data from the first systematic program of sea-ice observations in Kotzebue Sound, Alaska, in 2018 coincided with the first winter in living memory when the Sound was not choked with ice. The following winter of 2018–19 was even warmer and characterized by even less ice. Here we discuss the mass balance of landfast ice near Kotzebue (Qikiqtaġruk) during these two anomalously warm winters. We use in situ observations and a 1-D thermodynamic model to address three research questions developed in partnership with an Indigenous Advisory Council. In doing so, we improve our understanding of connections between landfast ice mass balance, marine mammals and subsistence hunting. Specifically, we show: (i) ice growth stopped unusually early due to strong vertical ocean heat flux, which also likely contributed to early start to bearded seal hunting; (ii) unusually thin ice contributed to widespread surface flooding. The associated snow ice formation partly offset the reduced ice growth, but the flooding likely had a negative impact on ringed seal habitat; (iii) sea ice near Kotzebue during the winters of 2017–18 and 2018–19 was likely the thinnest since at least 1945, driven by a combination of warm air temperatures and a persistent ocean heat flux.

The Winter Heat Budget of Sea Ice in Kotzebue Sound: Residual Ocean Heat and the Seasonal Roles of River Outflow

“…In the winter, river outflow was consistently at its freezing point, while salty ocean water contained some residual sensible heat, which is likely the source of heat responsible for retarding sea ice growth in the vicinity of the WITTE ET AL. ITO (Mahoney et al, 2021). This also points to heat storage in the ocean as the primary mechanism for the unprecedented sea ice losses seen in Kotzebue Sound as well as the Bering and Chukchi Seas in the past few years, echoing other recent analyses (Huntington et al, 2020;Thoman et al, 2020).…”

“…Table 1 details the automated instrumentation used on both stations, including accuracy specifications as well as deployment height/depth. The ITO also included snow stakes and hotwire gauges (Mahoney et al, 2009(Mahoney et al, , 2021 where snow depth and ice thickness were measured on an approximately weekly basis by residents of Kotzebue, including co-authors Robert J. Schaeffer and Alex Whiting. As a complement to the ITO, downwelling shortwave and longwave radiation were also measured by a heated pyranometer & heated pyrgeometer installed on a tower above the Fish & Wildlife Service building in Kotzebue, ∼720 m ESE of the ITO.…”

“…Calculation of the conductive flux across the snow and ice was complicated by surface flooding that began on February 14, when the weight of newly deposited snow pushed the surface of the ice below the waterline. In a companion paper to this study, Mahoney et al (2021) present both observational and model-based evidence for the evolution of a flooded layer between the ice and dry snow. In short, the height of the ice surface above the waterline (i.e., freeboard) was determined using Archimedes' principle, our measurements of ice thickness and snow depth, and assumptions about the densities of the snow, ice, and seawater.…”

Ringed seal (Pusa hispida) breeding habitat on the landfast ice in northwest Alaska during spring 1983 and 1984

“…Until the early 21 st century, Kotzebue Sound was consistently covered with stable landfast ice during at least January-April, extending in a continuous expanse along the shore and seaward to the entrance of the Sound [ 53 ]. Landfast ice has not completely covered Kotzebue Sound during winter for approximately the last decade [ 54 ]. Only relatively narrow margins of landfast ice attached to the periphery were formed in several recent years.…”

“…Future projections suggest that precipitation will increasingly fall as rain rather than snow, shortened sea ice-covered seasons with diminished spatial extent will result in decreased periods and areas for snow accumulation, and warmer air temperatures could result in earlier melt [ 30 , 32 , 59 ]. A recent study also indicated that the weight of accumulated snow on top of unusually thin landfast ice can contribute to widespread surface flooding [ 54 ]. If such conditions coincide with the pupping period, lairs used by pups still in lanugo may flood and compromise the pup’s ability to stay warm.…”