Abstract
AbstractDuring recent decades Arctic sea ice variability and retreat during winter have largely been a result of variable ocean heat transport (OHT). Here we use the Community Earth System Model (CESM) large ensemble simulation to disentangle internally and externally forced winter Arctic sea ice variability, and to assess to what extent future winter sea ice variability and trends are driven by Atlantic heat transport. We find that OHT into the Barents Sea has been, and is at present, a major source of internal Arctic winter sea ice variability and predictability. In a warming world (RCP8.5), OHT remains a good predictor of winter sea ice variability, although the relation weakens as the sea ice retreats beyond the Barents Sea. Warm Atlantic water gradually spreads downstream from the Barents Sea and farther into the Arctic Ocean, leading to a reduced sea ice cover and substantial changes in sea ice thickness. The future long-term increase in Atlantic heat transport is carried by warmer water as the current itself is found to weaken. The externally forced weakening of the Atlantic inflow to the Barents Sea is in contrast to a strengthening of the Nordic Seas circulation, and is thus not directly related to a slowdown of the Atlantic meridional overturning circulation (AMOC). The weakened Barents Sea inflow rather results from regional atmospheric circulation trends acting to change the relative strength of Atlantic water pathways into the Arctic. Internal OHT variability is associated with both upstream ocean circulation changes, including AMOC, and large-scale atmospheric circulation anomalies reminiscent of the Arctic Oscillation.
Highlights
The Arctic Ocean (Fig. 1) is currently losing sea ice in all regions during all seasons (Serreze et al 2007; Cavalieri and Parkinson 2012; Stroeve et al 2012; Onarheim et al 2018)
We first evaluate the ability of Community Earth System Model (CESM)-LE to simulate present-day ice–ocean interaction, focusing on the Barents Sea where long-term observations of ocean heat transport (OHT) are available (Årthun et al 2012)
In agreement with Oldenburg et al (2018), these results suggest that the response in poleward ocean heat transport to changes in the Atlantic meridional overturning circulation (AMOC) differs under internal variability and climate change, with the relationship between AMOC trends and Nordic Seas circulation changing sign depending on whether they are externally forced or a result of internal variability
Summary
The Arctic Ocean (Fig. 1) is currently losing sea ice in all regions during all seasons (Serreze et al 2007; Cavalieri and Parkinson 2012; Stroeve et al 2012; Onarheim et al 2018). To investigate the importance of ocean heat transport for future Arctic sea ice loss, and to disentangle the relative roles of internally and externally forced climate variability, we use the Community Earth System Model large ensemble simulation (CESM-LE; Kay et al 2015). To assess the relationship between internally driven trends in OHT and sea ice we regress the BSO heat transport trend from the individual ensemble members onto the wSIE. A detailed evaluation of Arctic–Atlantic ice and ocean properties in CESM is not performed here, the model appears to realistically simulate the present-day inflow of Atlantic heat to the Barents Sea and the associated response in sea ice cover, providing confidence in the model’s ability to assess future changes. CESM-LE has previously been used to study Arctic sea ice change (e.g., Swart et al 2015; Barnhart et al 2016; Jahn et al 2016; Labe et al 2018)
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