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November 7, 2020

Quantitative Analysis of the Impact of Riverine Heat on Declining Arctic Sea Ice and Atmospheric Warming:
Increasing Riverine Heat Triggers the Arctic Warming

1. Key points

This is the first quantitative analysis on the impact of riverine heat flux on declines in Arctic sea ice and oceanic and atmospheric warming.
Riverine heat flux decreased regional Arctic sea-ice thickness by a maximum of more than 10%.
Riverine heat flux amplified atmospheric and ocean warming in the Arctic.

2. Overview

An international research group led by Hotaek Park and Eiji Watanabe of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Institute for Arctic Climate and Environmental Research (IACE) performed the first quantitative analysis on the impact of influxes of riverine heat into the Arctic Ocean. Such influxes have accompanied recent trends in global climate change, specifically declines in Arctic sea ice and both oceanic and atmospheric warming. Riverine heat flux, which is derived from the volume and temperature of water discharged into the Arctic Ocean, reaches its maximum in early summer (July), when coastal sea ice begins to retreat. In the Lena River of eastern Siberia, August water temperature increased from 12-13℃ in the 1960s, when the effects of global warming were not yet conspicuous, to nearly 20℃ in recent years, with a high amount of riverine heat entering into the Arctic Ocean.

Because most river discharge in early summer inflows beneath sea-ice cover to mingle with marine surface waters, it is difficult to observe the spread of heat from river water by ship or satellite. Moreover, because on-site observations of river discharge and water temperature are limited to major rivers due to difficulties of access and other factors, estimates of riverine heat fluxes covering the pan-Arctic regions have not been performed until now. To address this gap in knowledge, this research group used the land surface process model, CHANGE (※1), incorporated calculations of river water temperature, to quantify changes in riverine heat fluxes into the Arctic Ocean. These data were then used as boundary conditions in a coupled sea ice-ocean model, COCO (※2), for conducting numerical experiments.

An analysis from 1980 to 2015 provides the first quantitative evidence showing that riverine heat fluxes contributes a maximum of more than 10% to regional sea ice thinning in the Arctic Ocean. (Figure 1). The result suggests not only the melting of the underside of sea ice by warm river water flowing into the Arctic Ocean, but also feedbacks due to reductions in ice albedo(※3). A quantitative evaluation also revealed increases in sensible and latent heat energy released into the atmosphere from the warmed ocean surface following the retreat of sea ice, causing a 0.1℃ increase in summer air temperature during the past 36 years (Figures 24). These results demonstrate that the proposed feedback process, whereby in addition to warming climate, riverine heat further reduces Arctic sea ice, thus increasing ocean-atmosphere heat fluxes and resultant atmospheric temperatures, is partially responsible for amplifying Arctic warming. In the future, this research group plans to pursue more comprehensive studies into the impacts of (1) increased evaporation from the sea surface caused by the retreat of sea ice on precipitation/snowfall over land via atmospheric circulation and (2) river discharge into the Arctic Ocean on biogeochemical cycles, including freshwater and carbon cycles.

This research was supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (basic research grants JP26340018 and JP17H01870; Arctic Challenge for Sustainability Project grant JPMXD1300000000; Arctic Challenge for Sustainability II grant JPMXD1420318865).

The results of this study were published in Science Advances on November 7th (JST).

Increasing riverine heat influx triggers Arctic sea-ice decline and oceanic and atmospheric warming
Hotaek Park1,6、Eiji Watanabe1、Youngwook Kim 2、Igor Polyakov 3、Kazuhiro Oshima4、Xiangdong Zhang 3、John S. Kimball 2、Daqing Yang 5
1. Institute of Arctic Climate and Environmental Research, JAMSTEC
2. Numerical Terradynamic Simulation Group, WA Franke College of Forestry and 11 Conservation, The University of Montana
3. International Arctic Research Center and College of Natural Science and Mathematics, 13 University of Alaska Fairbanks
4. Faculty of Software and Information Technology, Aomori University
5. Environment and Climate Change Canada
6.Institute for Space-Earth Environmental Research, Nagoya University

【Supplemental information】

Land Surface Process Model, CHANGE: A model for calculating the cycling of water, heat, and carbon dioxide in atmosphere-vegetation-soil systems on land, based on physical laws and empirical formulae. By integrating heat balance, the ecological and physiological processes of plants, photosynthesis, and water cycle, the CHANGE model can quantitatively evaluate interactions and feedbacks in terrestrial systems caused by changing processes and elements.
Coupled Sea Ice-Ocean Model, COCO: A tool for calculating sea ice thickness, ocean currents, seawater temperature, and other physical oceanographic parameters, based on physical laws and empirical formulae. The COCO model was developed by the University of Tokyo Center for Climate System Research (now the Atmosphere and Ocean Research Institute), and was improved in partnership with JAMSTEC. The JAMSTEC supercomputer, “Earth Simulator,” was used in this study to perform numerical experiments for the years between 1979 and 2015 under different riverine discharge conditions.
Ice Albedo Feedback: The positive feedback loop wherein decreases in snow and ice cover on the surface of the Earth reduces the reflection of solar radiation (albedo), causing more heat to be absorbed, thereby warming land masses and oceans and further decreasing snow and ice cover. The albedo effect is thought to be one important mechanism in regulating climate change on Earth. In the study reported here, it was demonstrated that riverine heat influx causes the retreat of the Arctic sea ice, increasing the amount of solar radiation absorbed by the sea surface, and thus promoting further melting of sea ice and increases in ocean surface temperature.

Figure 1 (a)Major Actic river basins and riverine heat influx into the Arctic Ocean (Qrh, brown) and declines in sea ice caused by riverine heat (conversion to the amount of heat necessary for melting: AIV, gray)shown as mean values for 1980-2015.(b) Sea ice decline from experiments, including riverine heat influx, as compared to the results of experiments in which riverine heat influx was not included (%); all values shown are means for 1980-2015.


Figure 2 Increases in ocean water temperature caused by riverine heat(℃). The x-axis shows the north-south longitudinal line at 127ºE in the Laptev Sea (72–80ºN) and the y-aixs shows water depth (0–70m). The difference (right) between the 1980s (1981−1990, left), and the present (2006−2015, center), shows how ocean water temperature has increased in recent years due to riverine heat influx.


Figure 3 Schematic of changes in the Arctic atmosphere-sea ice-ocean water heat balance caused by riverine heat influx. Influxes in riverine heat are connected to the ice albedo feedback, promoting sea ice melting and increases in ocean water temperature. Increases in the volume of heat stored in the ocean cause the sensible and latent heat released into the atmosphere and therefore temperatures to rise.


Figure 4 Summer (June-September) temperature increases caused by riverine heat. The stippled line shows the overall trend (line of best fit) between 1980 and 2015.


(For this study)
Hotaek Park, Senior Researcher
Research Institute for Global Change, Institute of Arctic Climate and Environment Research, Arctic Ocean and Climate System Research Group(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
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