Geophysical Research Letters RESEARCH LETTER The Ice-Ocean Governor: Ice-Ocean Stress Feedback Limits 10.1029/2018GL080171 Beaufort Gyre Spin-Up Special Section: The Arctic: An AGU Joint Gianluca Meneghello1 , John Marshall1 , Jean-Michel Campin1 , Edward Doddridge1 , Special Collection and Mary-Louise Timmermans2 Key Points: 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA, • Spin-up of the Beaufort Gyre is 2Department of Geology and Geophysics, Yale University, New Haven, CT, USA regulated by a negative feedback between ice-ocean stress and surface currents: an ice-ocean stress governor • In the present Arctic system, the Abstract The Beaufort Gyre is a key circulation system of the Arctic Ocean and its main reservoir of governor is likely a key process freshwater. Freshwater storage and release affects Arctic sea ice cover, as well as North Atlantic and global controlling the equilibration of climate. We describe a mechanism that is fundamental to the dynamics of the gyre, namely, the ice-ocean the gyre • Continued sea ice loss will likely stress governor. Wind blows over the ice, and the ice drags the ocean. But as the gyre spins up, currents lead to reduced effectiveness of the catch the ice up and turn off the surface stress. This governor sets the basic properties of the gyre, such as governor and change the its depth, freshwater content, and strength. Analytical and numerical modeling is employed to contrast the fundamental internal dynamics of the gyre equilibration processes in an ice-covered versus ice-free gyre. We argue that as the Arctic warms, reduced sea ice extent and more mobile ice will result in a deeper and faster Beaufort Gyre, accumulating more freshwater that will be released by Ekman upwelling or baroclinic instability. Supporting Information: • Supporting Information S1 Plain Language Summary The Beaufort Gyre, located north of Alaska and Canada, is a key circulation system of the Arctic Ocean. Changes in its depth and circulation influence the evolution of the Correspondence to: Arctic sea ice cover, the North Atlantic circulation, and the global climate. The gyre is driven by persistent, G. Meneghello, [email protected] ice-mediated winds, accumulating surface freshwater toward the center, deepening the gyre, and spinning up its currents. We describe a mechanism, dubbed here the ice-ocean governor, in which the interaction of surface currents with the ice regulates the depth of the Beaufort Gyre: The spinning up of the gyre reduces Citation: Meneghello, G., Marshall, J. C., the relative speed between the ocean and the ice, and hence the freshwater accumulation. This competes Campin, J.-M., Doddridge, E., & with, and we argue is more important than, the release of freshwater by flow instability, which moves water Timmermans, M.-L. (2018). The from the center toward the periphery. In the current climate the depth and speed of the Beaufort Gyre are ice-ocean governor: Ice-ocean stress feedback limits Beaufort Gyre spin-up, mainly set by the governor, but this may change in a warming world where reduced ice cover will render the Geophysical Research Letters, 45. ice-ocean governor less effective. The resulting deeper, swifter gyre will likely exhibit more variability in its https://doi.org/10.1029/2018GL080171 freshwater storage and flow speeds. Received 23 AUG 2018 Accepted 29 SEP 2018 1. Introduction Accepted article online 17 OCT 2018 Anticyclonic winds centered over the Arctic Ocean’s Beaufort Gyre (BG) force a lateral Ekman transport bring- ing surface freshwater toward the center of the gyre and steepening isopycnals. This convergence increases the freshwater content of the BG and spins up its geostrophic current (McPhee et al., 2009; Proshutinsky et al., 2002, 2015; Proshutinsky & Johnson, 1997; Proshutinsky et al., 2009; Timmermans et al., 2011; Zhang et al., 2016). Freshwater accumulation, storage, and release from the BG, controlled by these wind-driven dynamics, have far-reaching influence on Arctic and global climate (Proshutinsky et al., 2015). However, wind variability alone cannot explain the variability in freshwater content (Giles et al., 2012). Gyre spin-up and freshwater increase, proportional to the curl of the ocean surface stress, are complicated by the presence of sea ice cover, which acts to decouple wind and surface ocean stresses through internal lateral stresses and interacts with surface currents. Here we present a theoretical framework that illustrates how the interaction between under-ice geostrophic ocean currents and sea ice cover (described by the observational studies of Dewey et al., 2018; Kwok & Morison, 2017; Meneghello et al., 2017, 2018; Zhong et al., 2018) plays a key role in equilibrating the freshwater content of the BG. Inspired by self-limiting speed regulators (or governors) in mechanical systems (Bennet, 1993; Maxwell, 1867), we call this mechanism the ice-ocean stress governor. More specifically, the term governor refers to a mechanism for regulating the speed of a system by automatically ©2018. American Geophysical Union. restricting the flow of water, air, fuel, etc., when the speed increases and increasing the flow when it decreases All Rights Reserved. (Murray et al., 2018). MENEGHELLO ET AL. 1 Geophysical Research Letters 10.1029/2018GL080171 Figure 1. Ekman pumping climatology. (a) Mean Ekman pumping over 2003–2014; negative (blue) indicates downwelling and positive (red) upwelling. (left) Downwelling estimates locally exceed 30 m/year if the geostrophic current is neglected; (middle) inclusion of the geostrophic current results in an upwelling effect, largely compensating the ice-driven downwelling; (right) the net Ekman pumping, the sum of the previous two panels, yields moderate downwelling together with patches of upwelling. The Beaufort Gyre Region is marked by a red line in the inset. (b) Monthly Ekman pumping climatology integrated overthe Beaufort Gyre Region and its partitioned contributions, where negative indicates downwelling. Black bars show total Ekman pumping, equivalent to the right panel in (a). Red and orange bars show pumping induced by winds over ice-free regions and by ice in ice-covered regions, respectively. Within ice-covered regions, downward pointing empty green bars show downwelling induced by the ice if the geostrophic current is neglected, while the upward directed blue bars show upwelling induced by the geostrophic currents flowing under the sea ice. Blue and green bars largely balance each other and exactly balance if urel = 0. Gray dots represent ice concentration. All data and methods are described in Meneghello et al. (2018). The total stress at the ocean surface is a combination of ice-ocean stress i and air-ocean stress a, each of which may be estimated by a quadratic drag law, weighted by the sea ice concentration : ( ) ( ) | | | | = C |u | u + (1 − ) C |u | u . (1) ⏟⏞⏞⏞⏞⏞⏞⏞⏟⏞⏞⏞⏞⏞⏞⏞⏟Di rel rel ⏟⏞⏞⏞⏞⏞⏞⏞⏟⏞⏞⏞⏞⏞⏞⏞⏟a Da a a i a . CDi = 0 0055 and CDa = 0 00125 are dimensionless drag coefficients for the ice-ocean and air-ocean stress 3 3 respectively, = 1,028 kg/m is water density, and a = 1.25 kg/m is air density. In the computation of a, the surface ocean velocity, of a few centimeters per second, is considered negligible with respect to a wind velocity ua of a few meters per second. However, surface ocean velocity cannot be neglected in the estimation of i: The ice-ocean relative velocity urel is expressed as the difference between the ice velocity ui and the i geostrophic surface ocean velocity ug.Thatis,urel =(ui −ug)e , where is a turning angle taking into account the Ekman layer. Figure 1, modified from Meneghello et al. (2018), shows how the intensity of the ocean surface currents plays a central role in modulating the Ekman pumping in an ice-covered gyre (Dewey et al., 2018; Meneghello et al., 2017, 2018; Zhong et al., 2018). Estimates of wind- and ice-induced downwelling can exceed 30 m/year locally if the geostrophic current is neglected (Figure 1a, blue region in the left panel; see also Yang, 2006, 2009). However, this is largely compensated by the upwelling effect of the surface current flowing below the ice (red region in the central panel), acting as a negative feedback and reducing the downwelling. That is, the governor drives the system toward urel = 0. Consequently, the net Ekman downwelling is strongly reduced MENEGHELLO ET AL. 2 Geophysical Research Letters 10.1029/2018GL080171 (right panel). A monthly climatology of the 2003–2014 Ekman pumping and its components averaged over the Beaufort Gyre Region (Figure 1b) shows how the total Ekman pumping is reduced by the geostrophic current and even reversed during the months of January, February, and March (Meneghello et al., 2018). Through analytical calculations and an idealized numerical model, we demonstrate here how the governor acts as a mechanism to equilibrate the freshwater content of the gyre. For example, should the anticy- clonic ice-ocean stress curl —and freshwater accumulation rate—intensify, the geostrophic flow of the gyre will strengthen, reducing the surface stress until the governor turns off the ice-ocean stress. This is a dis- tinct alternative to the eddy equilibration mechanism first proposed for the Southern Ocean (Karsten et al., 2002; Marshall & Radko, 2003) and more recently extended to the BG (Davis et al., 2014; Lique et al., 2015; Manucharyan & Spall, 2016; Manucharyan et al., 2016; Meneghello et al., 2017; Yang et al., 2016). To explore the governor mechanism and test our theoretical model, we analyze the response of an idealized gyre under two different limit-case scenarios: (i) an ice-driven gyre ( = 1 in equation (1), in which forcing depends purely on gradients of = i) and (ii) an ice-free, wind-driven gyre ( = 0, in which forcing depends purely on gradients of = a). We conclude with a discussion of the implications of the governor for the Arctic Ocean’s circulation and its freshwater content.