The 2007 Bering Strait oceanic heat flux and anomalous Arctic sea-ice retreat
2009, Woodgate et al., Geophysical Research Letters
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Abstract
To illuminate the role of Pacific Waters in the 2007 Arctic sea-ice retreat, we use observational data to estimate Bering Strait volume and heat transports from 1991 to 2007. In 2007, both annual mean transport and temperatures are at record-length highs. Heat fluxes increase from 2001 to a 2007 maximum, 5–6 × 1020 J/yr. This is twice the 2001 heat flux, comparable to the annual shortwave radiative flux into the Chukchi Sea, and enough to melt 1/3rd of the 2007 seasonal Arctic sea-ice loss. We suggest the Bering Strait inflow influences sea-ice by providing a trigger for the onset of solar-driven melt, a conduit for oceanic heat into the Arctic, and (due to long transit times) a subsurface heat source within the Arctic in winter. The substantial interannual variability reflects temperature and transport changes, the latter (especially recently) being significantly affected by variability (> 0.2 Sv equivalent) in the Pacific-Arctic pressure-head driving the flow.
Excerpts
How significant was heat flux from Pacific Waters (PW) in the extreme 2007 Arctic sea-ice retreat? Woodgate et al. [2006] estimated that Bering Strait oceanic heat fluxes from the early 2000s were substantial, 2–4 x 1020 J/yr, and that the extra heat input from 2001 to 2004 was enough to melt 640,000 km2 of 1 m thick ice, comparable to the change in summer ice-extent (700,000 km2) between 2001 and 2004 [Stroeve et al., 2005]. Shimada et al. [2006] propose a positive feedback where heat carried by northward flowing PW weakens the ice-pack thereby promoting more sea-ice motion in response to wind, which in turn enhances the wind-driving of PW into the Arctic.
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The Bering Strait - 85 km wide, 50 m deep and divided into two channels by the Diomede Islands - is the only oceanic gateway between the Pacific and the Arctic Oceans. Year-round near-bottom oceanographic moorings (Figure 1a) have been deployed in the region almost continuously since 1990, generally at three locations – one at a mid-strait site (A3, 66.3N 169W) 60 km north of the Diomede Islands, and one at the centre of each channel (A2, eastern channel; A1, western channel).
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Annual mean heat fluxes (Figure 2e) increase almost monotonically from 2001 to 2007. Heat flux variations are driven almost equally by changes in volume transport and in temperature, and the highest heat flux years (2004 and 2007) are those where both transport and temperature are high. Year 2007 yields a clear record-length maximum, estimated at 3.5 x 1020 J/yr from A3 data alone and at 4–4.7 x 1020 J/yr including a 10–20 m surface layer. Adding 1 x 1020 J/yr for the ACC yields a total heat flux of 5–5.7 x 1020 J/yr. This is almost a doubling of the total 2001 heat flux (2.6– 2.9 x 1020 J/yr), and 1 x 1020 J/yr greater than the previous high in 2004 (4.3–4.8 x 1020 J/yr).
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How relevant is this amount of heat (3–6 x 1020 J/yr, i.e., 10–20 TW) in the Arctic?
This much heat could melt 1–2 x 106 km2/yr of 1 m thick ice, maybe 1/3rd of annual arctic sea-ice retreat and comparable to interannual variability in the September ice extent – winter extent is 10 x 106 km2,; the 2006 and 2007 September minima were 6 and 4 x 106 km2 respectively (National Snow and Ice Data Center data). Pacific Waters (PW) are found over roughly half the Arctic Ocean. Averaged over that area (5 106 km2), the Bering Strait heat flux is 2–4 W/m2, a significant fraction of Arctic annual mean net surface heat fluxes (2 to 10 W/m2, ERA-40 atmospheric reanalysis [Serreze et al., 2007, Figure 5]).
The Bering Strait heat flux is also comparable to the solar input to the Chukchi Sea, 4 x 1020 J/yr (1300 MJm2 yr1, 1998–2007 range [Perovich et al., 2007] (and subsequent extension), Chukchi Sea area 350 x 103 km2). In fact, the Bering Strait heat flux is surprisingly large for its net volume. Although the Fram Strait inflow is about 10 times greater in volume, its estimated net heat input to the Arctic is 30–50 TW [Schauer et al., 2008], only about 3 times our Bering Strait estimate. Thus, purely in terms of heat, the Bering Strait contribution is large enough to be a significant player in sea-ice retreat.
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There is little understanding of Bering Strait temperatures, which are presumably a complex result of atmospheric and solar effects on the northward moving waters of the Bering Sea. For transport, however, it is generally thought that the flow has two main drivers – a Pacific-Arctic pressure-head (of debated origin, often assumed constant) and local wind forcing (see Woodgate et al. [2005b] for discussion).
Conclusions
Using year-round data from in situ moorings and satellite-sensed sea surface temperatures, we quantify oceanic fluxes of volume and heat from the Pacific to the Arctic via the Bering Strait between 1991 and 2007 with special focus on 1998 to 2007. We find heat flux increases almost monotonically from 2001 to 2007. Reflecting both high volume transports and high temperatures, the estimated 2007 heat flux was the greatest recorded to date, 5–6 x 1020 J/yr (range reflecting uncertainty in depth of the summer surface layer). This is almost a doubling of the total 2001 heat flux and somewhat greater than the incoming shortwave solar input into the Chukchi Sea. Moreover, the interannual variability in the Bering Strait heat flux is slightly larger than that of shortwave solar input to the Chukchi.
We suggest that PW flow (both in terms of the transport of ice and of far-field and locally gained heat) acts initially as a trigger for the onset of the seasonal melt back of ice, and subsequently may drive a year-round modest thinning of the western Arctic ice, as it feeds a subsurface temperature maximum under the ice-pack in winter. Factors such as large interannual variability in heat flux, timing of the delivery of heat to the Arctic Ocean, and volume flux are also important. Thus, overall, the Bering Strait’s leverage on the Arctic system may be greater than its comparatively small volume may suggest.
The data suggest that change in volume flux, which drives about half of the change in heat flux, is due not just to varying winds but also to significant (> 0.2 Sv equivalent) variation in the large-scale pressure-head forcing between the Pacific and the Arctic. This suggests that estimating the flow from wind-data alone will underestimate flux variability. Similarly, sea-surface temperatures show more interannual variability than the temperatures of the bulk of the water column, suggesting that flux estimates that are tightly coupled to SST may overestimate flux variability.
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