By Randy Showstack
With the Arctic Ocean warming much more rapidly than the global average over the past several decades, the extent and thickness of sea ice in the Arctic has decreased dramatically. Climate models project that these trends will accelerate even more in the future. A new analysis finds that increased ocean heat transport (OHT) into the Arctic through the Barents Sea Opening (BSO) is the primary driver of this warming, with OHT from the Fram and Bering straits also contributing to the warming. (See Figure 1 for a map of the region.)
Figure 1. Map indicating the location of the Arctic gateways where the ocean heat transport is calculated. (Illustration by Natalie Renier, Woods Hole Oceanographic Institution)
The analysis also finds that the projected future OHT changes are driven primarily by warmer inflowing water rather than by changes in the volume of water transported through those three Arctic gateways, according to the journal article, “The Respective Roles of Ocean Heat Transport and Surface Heat Fluxes in Driving Arctic Ocean Warming and Sea Ice Decline,” published in the American Meteorological Society’s Journal of Climate in January 2024.
In addition, the analysis calculates the contributions of ocean-ice and atmosphere-ice heat fluxes to the sea ice heat budget changes (see Figure 2), and determines that the ocean is likely to become the most important source of heat for the ice and hence the most responsible for the melting. “Throughout the entire twentieth century as well as the early twenty-first century, the atmosphere is the main contributor to ice heat gain in summer, though the ocean’s role is not negligible. However, over the course of the 21st century, the ocean progressively becomes the main heat source for the ice as the ocean warms,” according to the article.
Figure 2. Illustration of the factors that contribute to the ocean and sea-ice heat budgets. Credit: Natalie Renier from WHOI Graphics.
“The ocean is expected to play a very important role in driving sea-ice loss in the future. In particular, the increased heat inputs from the different Arctic Ocean gateways, particularly the Barents Sea Opening, the Fram Strait, and the Bering Strait from the Pacific side of the Arctic are going to be very important for driving active ocean warming,” said journal article lead author Dylan Oldenburg, a postdoctoral investigator in physical oceanography at the Woods Hole Oceanographic Institution (WHOI). Co-authors are Young-Oh Kwon of WHOI; Claude Frankignoul with WHOI and Sorbonne University; and Gokhan Danabasoglu, Stephen Yeager, and Who M. Kim with the National Science Foundation’s National Center for Atmospheric Research.
For their heat budget analysis, the researchers defined the Arctic Ocean as the region bounded by the Arctic Ocean gateways, which include the Barents Sea Opening as well as the Barrow, Bering, Fram, and Nares straits. The analysis uses the 100-member Large Ensemble of the state-of-the-art fully coupled Community Earth System Model, version 2 (CESM2), focusing primarily on the period between 1920 and 2100. This Large Ensemble provides researchers with the ability to separate climate change signal variations from natural internal variability.
“The beauty of having so many ensemble members, or simulations, is that when you average them together, there is a lot less noise, and you can remove internal variability that shows up in each ensemble member,” said Oldenburg. “When you look at the Large Ensemble, you can isolate the response of the climate or the Arctic Ocean, for instance, to the greenhouse gas forcing without internal variability masking what is happening due to that forcing.”
Although the model has some minor drawbacks, such as underestimating the Fram Strait OHT relative to observations, overall CESM2 is a good model and the net volume transports through the Arctic gateways are well-represented relative to observations, the article noted. “For limited parts of the historical period, we can rely on observations. However, because observations in the Arctic Ocean are limited and we do not have observations of future conditions, we need to use models,” said Oldenburg. “Our results based on CESM2-LENS indicate an essential role for ocean heat transport in driving Arctic Ocean warming, in line with previous studies.”
The earlier studies have typically examined OHTs that are zonally integrated across a particular latitude, and not based on physical pathways into the Arctic Ocean, according to Oldenburg. The researchers here explicitly calculated the OHT across each of the Arctic gateways to determine the relative importance of OHTs across the gateways. All of the heat flux components are fairly constant in the ensemble mean until about 1980, when the total OHT into the Arctic starts to rapidly increase, eventually strengthening from 0.06 petawatts (PW) from 1961-1980 to 0.10 PW from 2081-2100. The total OHC change over the course of the simulations increases the mean Arctic Ocean temperature from 0.3° to 0.5° Celsius, according to the article. “The increase in the OHT into the Arctic in response to the external forcing results in an Arctic temperature increase which is partly damped by the enhanced heat loss through the sea surface, as found in other climate models.”
In terms of individual gateways, the BSO provides 41% of the total increase in OHT into the Arctic Ocean between 1961-1980 and 2081-2100. The OHT also increases through the Fram and Bering straits, while the Nares Strait OHT is fairly constant until about 2040 when it starts to decrease, and volume transport declines slightly across the Bering Strait.
While thermodynamic (i.e. temperature-driven) changes largely drive the net OHT increase into the Arctic, nonlinear components (i.e. anomalies due to concurrent changes in temperature and velocity) also play a major role. Of the net OHT increase from 1920–80 to 2081–2100, thermodynamic changes contribute 0.03 PW, with nonlinear changes contributing 0.01 PW; dynamic circulation changes act to decrease the net OHT after 2040, with an anomaly slightly smaller than -0.01 PW over the same period due to negative dynamic components from the Nares and Barrow Straits.
The research also looked into the impacts of shortwave (solar) and longwave radiation on the ocean and on sea ice, and looks at the heat budget from the perspective of the ice.
“Essentially, the heat transported into the Arctic Ocean via OHT is primarily used to melt the sea ice until the sea ice coverage declines substantially in the mid-21st century and the exposed ocean surface increases, after which heat begins to flux out to the atmosphere,” the researchers noted.
In terms of heat flux components, while absorbed shortwave radiation strengthens due to decreasing surface albedo with more open ocean and reduced cloud cover, starting in 2040 this increased heat flux is compensated by strengthened outgoing longwave radiation and stronger sensible and latent heat loss reflecting ocean warming and more extended open waters, according to the research. Though incoming longwave radiation also increases over the course of the simulations, that increase is compensated by the strengthened outgoing longwave radiation, which is consistent with the balance in the mean seasonal cycle and which explains the increased ocean cooling due to longwave radiation out to 2100.
“It is very important to understand why the Arctic Ocean is warming and why there are dramatic changes that are being predicted,” Oldenburg said. “This research, which reinforces what has already been found about extreme warming in the Arctic and the loss of sea ice, highlights the importance of ocean heat transport in driving Arctic warming and sea ice decline.”