Abstract. Long-term (2003Long-term ( -2015 satellite-derived sea-ice extent, sea surface temperature (SST), and lower 10 tropospheric methane (CH 4 ) of the Barents and Kara Seas (BKS) were analyzed for statistically significant 11 anomalies and trends for 10 focus areas and on a pixel basis that were related to currents and bathymetry. Large 12 positive CH 4 anomalies were discovered around Franz Josef Land (FJL) and offshore west Novaya Zemlya in early 13 fall. Far smaller CH 4 enhancement was around Svalbard, downstream of known seabed seepage. 14 Strongest SST increase was southeast Barents Sea in June due to strengthening of the warm Murman Current 15 (MC) and in the south Kara Sea in September, when the cold Percey Current weakens and the MC strengthens. 16These regions and around FJL exhibit the strongest CH 4 growth. Likely sources are CH 4 seepage from subsea 17 permafrost and hydrates and the petroleum reservoirs underlying the central and east Barents Sea and the Kara Sea. 18The spatial pattern was poorly related to depth and better explained by shoaling. Peak CH 4 anomaly is several 19 months after peak SST, consistent with a several month delay between SST and seabed temperature. Continued MC 20 strengthening will increase heat transfer to the BKS, rendering the Barents Sea ice-free in about 15 years. The Cryosphere Discuss., https://doi

Supplementary Material S1. Review of Airborne Arctic Methane measurements CH4 concentration profiles over the Arctic Ocean were measured on five flights during the HIAPER Pole-to-Pole Observations (HIPPO) campaign (Kort et al., 2012; Wofsy, 2011) and produced evidence of sea surface CH4 emissions from the northern Chukchi and Beaufort Seas in most profiles, up to 82°N. Enhanced concentrations near the sea surface were common over fractured floating ice in sample profiles collected on 2 Nov.

Supplementary Material S1. Detailed Currents and Bathymetry Figure S1. Bathymetry and currents around Svalbard. Bathymetry from Norwegian Petroleum Directorate (2016). Currents adapted from Stiansen et al. (2009). Dashed black line shows the Barents Front location, Dashed currents are submerged; blue-cold, yellow-warm. Currents and flows around Svalbard Archipelago are complex (Supp. Fig. S1), dominated by the West Spitsbergen Current (WSC), which is the northerly fork of the Norwegian Atlantic Current (NAC), and flows northwards off the west coast of Spitsbergen. The cold, Percy Current (PC) flows southwest off the eastern shores of the Svalbard Archipelago. The cold East Spitsbergen Current (ESC) flows through the Hinlopen Strait and then joins the PC to flow around the south cape of Spitsbergen as the Sørkapp Current (SC), following the coast northwards as the Spitsbergen Coastal Current (SCC) (Svendsen et al., 2002). The cold SCC flows inshore of the WSC, and flows up Svalbard's western coast, inshore and shallower than the warm, Atlantic WSC. The interface between these two currents off west Spitsbergen forms a part of the Barents Front. Thus, coastal waters offshore West Spitsbergen are of Arctic Ocean origin, whereas further offshore lies Barents Sea water (origin Atlantic Ocean). The location of the Barents Sea Polar Front (Oziel et al., 2016) is semi-permanent and controlled by seabed topography (Fig. S1), particularly the Svalbard Bank and Grand Bank and the trough to the southwest of Svalbard.