Observations of Seasonal Upwelling and Downwelling in the Beaufort Sea Mediated by Sea Ice

Abstract

We present observational estimates of Ekman pumping in the Beaufort Gyre region. Averaged over the Canada Basin, the results show a 2003–14 average of 2.3 m yr−1 downward with strong seasonal and interannual variability superimposed: monthly and yearly means range from 30 m yr−1 downward to 10 m yr−1 upward. A clear, seasonal cycle is evident with intense downwelling in autumn and upwelling during the winter months, despite the wind forcing being downwelling favorable year-round. Wintertime upwelling is associated with friction between the large-scale Beaufort Gyre ocean circulation and the surface ice pack and contrasts with previous estimates of yearlong downwelling; as a consequence, the yearly cumulative Ekman pumping over the gyre is significantly reduced. The spatial distribution of Ekman pumping is also modified, with the Beaufort Gyre region showing alternating, moderate upwelling and downwelling, while a more intense, yearlong downwelling averaging 18 m yr−1 is identified in the northern Chukchi Sea region. Implications of the results for understanding Arctic Ocean dynamics and change are discussed.

Critical Role of Continental Slopes in Halocline and Eddy Dynamics of the Ekman‐Driven Beaufort Gyre

“…This corresponds to a uniform Ekman pumping velocity of about 10 m/year. This is potentially an overestimate of Ekman pumping rates in light of new observational studies that incorporate satellite‐derived surface geostrophic flow (Armitage et al, ) in calculations of the ocean surface stress (Dewey et al, ; Meneghello et al, ; Zhong et al, ). However, our results do not qualitatively depend on the magnitude of the Ekman pumping, a sensitivity to which has been explored in detail in Manucharyan and Spall () and Manucharyan et al (); here we focus on highlighting the critical impacts brought by the inclusion of continental slopes.…”

“…Hypotheses about the BG equilibration processes, among others, include (i) halocline flattening due to mesoscale eddies formed via baroclinic instability in order to release the available gravitational potential energy (APE) stored in a bowl‐shape halocline (Manucharyan & Spall, ), (ii) frictional dissipation of its geostrophic currents by rubbing against the sea ice (Dewey et al, ; Meneghello et al, ; Zhong et al, ), and (iii) a balance between vertical diffusion in the interior of the gyre and eddy fluxes from its boundaries (Spall, ). The vertical mixing hypothesis does not rely on the intensity of Ekman pumping and hence can only provide a partial explanation of the dynamics.…”

Retrieving Sea Ice Drag Coefficients and Turning Angles From In Situ and Satellite Observations Using an Inverse Modeling Framework

“…The choice of depth for represents the measurements available when forcing a coupled or stand‐alone sea ice climate model (Tsamados et al, ) or when analyzing observation data (such as in the studies discussed in section and by Meneghello et al, ). This study considered the case of both surface and geostrophic or geostrophic equivalent relative currents measured at the bottom of the ocean mixed layer.…”

A Three‐Way Balance in the Beaufort Gyre: The Ice‐Ocean Governor, Wind Stress, and Eddy Diffusivity

“…The eddy‐induced overturning circulation opposes the wind‐driven overturning circulation, thereby halting the accumulation of freshwater and equilibrating the BG. However, an alternative mechanism for equilibrating the BG has recently been proposed: the Ice‐Ocean Governor (Meneghello, Marshall, Timmermans, et al, ; Meneghello, Marshall, Campin, et al, ). The Ice‐Ocean Governor relies on the fact that the relative velocity between the sea ice and the ocean controls the surface stress at the ice‐ocean interface; if the ocean and the ice are moving at the same speed, there is no transfer of momentum, and the gyre is equilibrated.…”

“…This mechanism is dubbed the Ice‐Ocean Governor by analogy with mechanical governors that regulate the speed of engines and other devices through dynamical feedbacks (see, e.g., Maxwell, ). The ocean velocity has long been included when calculating the ice‐ocean stress in numerical models (see, e.g., Hibler, ), but appreciation of the importance of this effect has been very recent (Dewey et al, ; Kwok & Morison, ; Meneghello et al, ; Meneghello, Marshall, Timmermans, et al, ; Meneghello, Marshall, Campin, et al, ; Zhong et al, ) following the publication of a data set that estimates sea surface height and surface geostrophic velocity in sea ice‐covered areas (Armitage et al, ).…”