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1. The Cyclonic Mode of Arctic Ocean Circulation

   https://doi.org/10.1175/JPO-D-20-0190.1  

   Morison, James; Kwok, Ron; Dickinson, Suzanne; Andersen, Roger; Peralta-Ferriz, Cecilia; Morison, David; Rigor, Ignatius; Dewey, Sarah; Guthrie, John (April 2021, Journal of Physical Oceanography)

   null (Ed.)

   Abstract Arctic Ocean surface circulation change should not be viewed as the strength of the anticyclonic Beaufort Gyre. While the Beaufort Gyre is a dominant feature of average Arctic Ocean surface circulation, empirical orthogonal function analysis of dynamic height (1950–89) and satellite altimetry–derived dynamic ocean topography (2004–19) show the primary pattern of variability in its cyclonic mode is dominated by a depression of the sea surface and cyclonic surface circulation on the Russian side of the Arctic Ocean. Changes in surface circulation after Arctic Oscillation (AO) maxima in 1989 and 2007–08 and after an AO minimum in 2010 indicate the cyclonic mode is forced by the AO with a lag of about 1 year. Associated with a one standard deviation increase in the average AO starting in the early 1990s, Arctic Ocean surface circulation underwent a cyclonic shift evidenced by increased spatial-average vorticity. Under increased AO, the cyclonic mode complex also includes increased export of sea ice and near-surface freshwater, a changed path of Eurasian runoff, a freshened Beaufort Sea, and weakened cold halocline layer that insulates sea ice from Atlantic water heat, an impact compounded by increased Atlantic Water inflow and cyclonic circulation at depth. The cyclonic mode’s connection with the AO is important because the AO is a major global scale climate index predicted to increase with global warming. Given the present bias in concentration of in situ measurements in the Beaufort Gyre and Transpolar Drift, a coordinated effort should be made to better observe the cyclonic mode. 

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2. On the Role of Bottom Pressure Torques in Wind-Driven Gyres

   https://doi.org/10.1175/JPO-D-20-0147.1  

   Stewart, Andrew_L; McWilliams, James_C; Solodoch, Aviv (May 2021, Journal of Physical Oceanography)

   Abstract Previous studies have concluded that the wind-input vorticity in ocean gyres is balanced by bottom pressure torques (BPT), when integrated over latitude bands. However, the BPT must vanish when integrated over any area enclosed by an isobath. This constraint raises ambiguities regarding the regions over which BPT should close the vorticity budget, and implies that BPT generated to balance a local wind stress curl necessitates the generation of a compensating, nonlocal BPT and thus nonlocal circulation. This study aims to clarify the role of BPT in wind-driven gyres using an idealized isopycnal model. Experiments performed with a single-signed wind stress curl in an enclosed, sloped basin reveal that BPT balances the windsonlywhen integrated over latitude bands. Integrating over other, dynamically motivated definitions of the gyre, such as barotropic streamlines, yields a balance between wind stress curl and bottom frictional torques. This implies that bottom friction plays a nonnegligible role in structuring the gyre circulation. Nonlocal bottom pressure torques manifest in the form of along-slope pressure gradients associated with a weak basin-scale circulation, and are associated with a transition to a balance between wind stress and bottom friction around the coasts. Finally, a suite of perturbation experiments is used to investigate the dynamics of BPT. To predict the BPT, the authors extend a previous theory that describes propagation of surface pressure signals from the gyre interior toward the coast along planetary potential vorticity contours. This theory is shown to agree closely with the diagnosed contributions to the vorticity budget across the suite of model experiments. 

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3. Ice front retreat reconfigures meltwater-driven gyres modulating ocean heat delivery to an Antarctic ice shelf

   https://doi.org/10.1038/s41467-022-27968-8  

   Yoon, Seung-Tae; Lee, Won Sang; Nam, SungHyun; Lee, Choon-Ki; Yun, Sukyoung; Heywood, Karen; Boehme, Lars; Zheng, Yixi; Lee, Inhee; Choi, Yeon; et al (December 2022, Nature Communications)

   Abstract Pine Island Ice Shelf (PIIS) buttresses the Pine Island Glacier, the key contributor to sea-level rise. PIIS has thinned owing to ocean-driven melting, and its calving front has retreated, leading to buttressing loss. PIIS melting depends primarily on the thermocline variability in its front. Furthermore, local ocean circulation shifts adjust heat transport within Pine Island Bay (PIB), yet oceanic processes underlying the ice front retreat remain unclear. Here, we report a PIB double-gyre that moves with the PIIS calving front and hypothesise that it controls ocean heat input towards PIIS. Glacial melt generates cyclonic and anticyclonic gyres near and off PIIS, and meltwater outflows converge into the anticyclonic gyre with a deep-convex-downward thermocline. The double-gyre migrated eastward as the calving front retreated, placing the anticyclonic gyre over a shallow seafloor ridge, reducing the ocean heat input towards PIIS. Reconfigurations of meltwater-driven gyres associated with moving ice boundaries might be crucial in modulating ocean heat delivery to glacial ice. 

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4. Unraveling the Extensive Impact of Subthermocline Eddies on the Western Pacific Undercurrent System

   https://doi.org/10.1029/2024GL108395  

   Azminuddin, Fuad; Jang, Chan_Joo; Susanto, Raden_Dwi; Mubarrok, Saat (September 2024, Geophysical Research Letters)

   Abstract Subthermocline eddies (SEs) influencing ocean circulation are progressively known, yet their extensive impact on the western Pacific undercurrent system remains largely unexplored and, in some regions, often underestimated. Okubo‐Weiss parameter analysis reveals a distinctive meridional pattern of cyclonic and anticyclonic SE distribution in the interior western Pacific basin that aligns with zonally elongated mean flows. These westward‐propagating SEs play a pivotal role in regulating the formation of zonal undercurrents, particularly off‐equatorial regions, through the convergence of eddy potential vorticity flux. Along the Pacific western boundary region, anticyclonic SEs typically enhance (reverse) the velocity of boundary currents flowing northward (southward), primarily through barotropic energy conversion, while cyclonic SEs do the opposite. To summarize, we provide a schematic map of the circulation system in the western Pacific and emphasize the interconnected framework of undercurrents, particularly in relation to SEs. 

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5. An Overlooked Component of the Meridional Overturning Circulation

   https://doi.org/10.1175/JPO-D-24-0019.1  

   Spall, Michael_A (September 2024, Journal of Physical Oceanography)

   Abstract Upwelling along the western boundary of the major ocean basin subtropical gyres has been diagnosed in a wide range of ocean models and state estimates. This vertical transport isO(5 × 10⁶) m³s⁻¹, which is on the same order of magnitude as the downward Ekman pumping across the subtropical gyres and zonally integrated meridional overturning circulation. Two approaches are used here to understand the reason for this upwelling and how it depends on oceanic parameters. First, a kinematic model that imposes a density gradient along the western boundary demonstrates that there must be upwelling with a maximum vertical transport at middepths in order to maintain geostrophic balance in the western boundary current. The second approach considers the vorticity budget near the western boundary in an idealized primitive equation model of the wind- and buoyancy-forced subtropical and subpolar gyres. It is shown that a pressure gradient along the western boundary results in bottom pressure torque that injects vorticity into the fluid. This is balanced on the boundary by lateral viscous fluxes that redistribute this vorticity across the boundary current. The viscous fluxes in the interior are balanced primarily by the vertical stretching of planetary vorticity, giving rise to upwelling within the boundary current. This process is found to be nearly adiabatic. Nonlinear terms and advection of planetary vorticity are also important locally but are not the ultimate drivers of the upwelling. Additional numerical model calculations demonstrate that the upwelling is a nonlocal consequence of buoyancy loss at high latitudes and thus represents an integral component of the meridional overturning circulation in depth space but not in density space. Significance StatementThe purpose of this study is to better understand what is forcing water to upwell along the western boundary at midlatitudes of the major ocean basins. This is a potentially important process since upwelling can bring heat and nutrients closer to the surface, where they can be exchanged with the atmosphere. Also, since ocean currents vary with depth, pathways followed in the upper ocean are different from those found for the deeper ocean, so the amount and location of upwelling influence where these waters go. Idealized numerical models and theory are used to demonstrate that the upwelling is ultimately driven by density changes along the western boundary of the basin that result from heat loss at high latitudes. 

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