The Arctic Ocean's Beaufort Gyre

Jamie set us up in the last blog describing the sense of the Arctic Ocean surface circulation since the 1990s. He takes the perspective that the overall circulation is best characterized by the strength and extent of a sea-surface height trough and the associated cyclonic circulation in the Eurasian Basin, instead of a characterization of the overall circulation that centers on the state of the Beaufort Gyre in the Canadian Basin. I look forward to discussing this perspective in the workshop, and I hope the debate will help us to converge on a framework that appreciates relationships and feedbacks between flow regimes in different sectors of the Arctic Ocean. This is particularly important to the extent that the metrics and characteristics we use for describing how the surface circulation of the Arctic Ocean works, and how it evolves on interannual and decadal timescales, guide our observing focus and strategies. To prepare for these important discussions, it might be helpful to review the changes that are presently underway in the Beaufort Gyre, under the influence of the atmospheric Beaufort High.

The anticyclonic (clockwise) Beaufort Gyre dominates the Canadian Basin circulation with a diameter of around 800 km and typical speeds near the ocean surface of several centimeters per second. The seawater that constitutes the gyre is fresher overall than anywhere else in the Arctic Ocean; the Beaufort Gyre is the Arctic's largest freshwater reservoir. Freshwater is accumulated in the gyre by the influence of the wind forcing associated with the Beaufort High. The winds drive surface convergence of freshwater from river discharge, ice melt, net precipitation and Pacific Water inflows. The amount of freshwater accumulated depends on the availability of freshwater from the various sources during times when wind forcing is most amenable to convergence. Since the early 2000s, we have been able to gain a better understanding of these processes through observations collected under the NSF-funded Beaufort Gyre Observing System (BGOS, Figure 1a). BGOS was established in 2003 as a collaborative program between US scientists and scientists from Fisheries and Oceans, Canada. Observations in the Beaufort Gyre region derive from moorings, yearly hydrographic surveys and drifting Ice-Tethered Profilers.

Observations indicate an almost 40% increase in Beaufort Gyre freshwater since the 1970s (from around 17 × 10³ km³ to 23.5 × 10³ km³ in 2018) (Proshutinsky et al., 2019, 2020), see Figures 1b and 2. These increases are attributed to a strengthening of the Beaufort Gyre circulation in response to anticyclonic wind forcing during a time of increased freshwater entering through Bering Strait, more freshwater available from sea-ice melt, and freshwater discharge from the Mackenzie River having more influence.

Figure 1 (from Proshutinsky et al., 2020): (a) Beaufort Gyre region with Beaufort Gyre Observational System (BGOS) mooring locations (stars), and hydrographic station locations where observations have been made from 2003-2020. (b) Time‐averaged summer freshwater content (meters, relative to reference salinity 34.8; colors and contours) in the Beaufort Gyre region for 1950s-1980s and 2013-2018.

Figure 2 (from Proshutinsky et al., 2019): Annual freshwater content in the Beaufort Gyre region inferred from satellite SSH (yellow bars), and calculated using Ice‐Tethered Profiler (ITP) data (blue bars) and BGOS mooring data (red bars). The black dotted, black dashed, and red dashed lines indicate linear freshwater content trends from ITP (455±232 km³/year), SSH (524±256 km³/year), and mooring data (534±153 km³/year), respectively. All trends are positive and statistically significant. Freshwater content estimated from mooring data does not include the upper 65 m of the water column.

Predicting the fate of Beaufort Gyre freshwater as it relates to continued sea-ice losses is a priority for future climate projections. At present, the Beaufort Gyre is controlled by sustained wind forcing, with both ocean eddy fluxes and stresses at the ice-ocean interface playing a role in balancing the wind forcing and regulating the gyre’s freshwater content (see e.g., Meneghello et al., 2020). The role of ice-ocean stresses will be greatly diminished in a future, seasonally ice-free Beaufort Gyre having a thinner, more mobile winter sea-ice cover. It is unclear whether this is already a factor in the recent build-up of Beaufort Gyre freshwater.

 Returning to the topic of Jamie's blog, changes in the strength of the Beaufort Gyre circulation, and its capacity to accumulate freshwater bear a clear relationship to the general circulation of the entire Arctic Ocean. Understanding this general circulation relies on our ability to predict the prevailing wind forcing, namely the two main atmospheric centers of action - the Beaufort High and the Icelandic Low. Will ongoing Arctic warming and sea-ice losses lead to a reduced Beaufort High (favoring freshwater release), and an intensified Icelandic Low (e.g., Moore et al., 2018)? In a warming Arctic, it is unclear which atmospheric circulation patterns will dominate and how these will influence the ocean dynamics and freshwater of the Beaufort Gyre and the Arctic as a whole. Our workshop will be most productive and stimulating if atmospheric dynamicists are among the participants deliberating anticipated changes in atmospheric forcing. Finally, we will need to consider how best to characterize ocean circulation and modes of variability as these metrics relate to decisions surrounding observing strategies. This is a topic for a future blog post.

References:

Meneghello, G., Doddridge, E., Marshall, J., Scott, J., & Campin, J.-M. (2020). Exploring the role of the “ice–ocean governor” and mesoscale eddies in the equilibration of the Beaufort Gyre: Lessons from observations. Journal of Physical Oceanography, 50(1), 269–277.

Moore, G., Schweiger, A., Zhang, J., & Steele, M. (2018). Collapse of the 2017 winter Beaufort High: A response to thinning sea ice? Geophysical Research Letters, 45, 2860–2869. https://doi.org/10.1002/2017GL076446.

Proshutinsky, A., Krishfield, R., Toole, J. M., Timmermans, M.-L.,Williams,W., Zimmermann, S., Yamamoto-Kawai, M., Armitage, T.W.K., Dukhovskoy, D., Golubeva, E., Manucharyan, G. E., Platov, G., Watanabe, E., Kikuchi, T., Nishino, S., Itoh, M., Kang, S.-H., Cho, K.-H., Tateyama, K., & Zhao, J. (2019). Analysis of the Beaufort Gyre freshwater content in 2003-2018. Journal of Geophysical Research: Oceans, 124, 9658–9689. https://doi.org/10.1029/2019JC015281

Proshutinsky, A., Krishfield, R., & Timmermans, M.-L. (2020). Introduction to special collection on Arctic Ocean Modeling and Observational Synthesis (FAMOS) 2: Beaufort Gyre phenomenon. Journal of Geophysical Research: Oceans, 125, e2019JC015400. https://doi.org/10.1029/2019JC015400.