Journal Article

Asymmetrically Stratified Beaufort Gyre: Mean State and Response to Decadal Forcing

Steele, Michael, Zhang, Jiaxu, Cheng, Wei, Weijer, Wilbert

2023

Overview

Recent progress in understanding Beaufort Gyre (BG) dynamics reveals an important role of ice‐ocean stress in stabilizing BG freshwater content (FWC) over seasonal to interannual timescales. But how the BG's stratification and FWC respond to surface forcing over decadal timescales has not been fully explored. Using a global ocean‐sea ice model, we partition the BG into upper, middle (halocline), and lower (thermocline) layers and perform a volume budget analysis over 1948–2017. We find that the BG's asymmetric geometry (with steep and tight isohalines over continental slopes relative to the deep basin) is key in determining the mean volume transport balance. We further find that a net Ekman suction during 1983–1995 causes the upper and middle layers to deflate isopycnally, while an enhanced Ekman pumping during 1996–2017 causes these layers to inflate both isopycnally and diapycnally, the latter via anomalous flux from the upper to the middle layer. Plain Language Summary The Beaufort Gyre (BG) has increased its liquid freshwater content (FWC) by 40% in the past two decades. If released and transported downstream to the subpolar North Atlantic Ocean, the excess water might affect the ocean circulation via suppression of deep‐water formation. However, which layer is responsible for BG freshwater accumulations and releases over decadal timescales and the corresponding physical processes remain unclear, hampering our attempts to make future predictions. Here we use an ocean‐sea ice model to explore such changes in its three characteristic layers (upper mixed‐layer water, Pacific Water in the middle layer, and Atlantic Water in the lower layer). We find that the asymmetry of the BG, which has been simplified as a symmetric bowl shape in most previous studies, is important in determining the BG's layered mean state. Over decadal timescales, changes in BG volume are controlled by annual‐mean Ekman pumping/suction resulting from combined wind and ice‐ocean stresses. This study emphasizes the role of asymmetric geometry in determining the BG mean volume balance. It also explores the role of mean flow across the gyre's lateral boundaries in regulating BG's volume and FWC over decadal timescales. Key Points The Beaufort Gyre's asymmetric geometry is the key to explain a net mean lateral outflow in the upper layer, despite Ekman convergence In the mean, the Gyre is fed by a northeast inflow into its middle layer, with outflow to its southwest from both upper and middle layers Deflation/inflation processes are asymmetric in response to anomalous Ekman suction/pumping on decadal timescales

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Asymmetry / Beaufort Gyre / Continental slope / eddy‐permitting model / Ekman pumping / Freshwater / freshwater content / Halocline / Inland water environment / Isohalines / Ocean circulation / Ocean currents / Oceans / Pumping / Sea ice / Sea ice models / Stabilizing / Stratification / Suction / Thermocline / volume budget / Volume transport / Water circulation