Evans
Dafydd Gwyn
Evans
Dafydd Gwyn
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ArticleMesoscale eddy dissipation by a "zoo" of submesoscale processes at a western boundary(American Geophysical Union, 2020-11-04) Evans, Dafydd Gwyn ; Frajka-Williams, Eleanor E. ; Naveira Garabato, Alberto C. ; Polzin, Kurt L. ; Forryan, AlexanderMesoscale eddies are ubiquitous dynamical features that tend to propagate westward and disappear along ocean western boundaries. Using a multiscale observational study, we assess the extent to which eddies dissipate via a direct cascade of energy at a western boundary. We analyze data from a ship‐based microstructure and velocity survey, and an 18‐month mooring deployment, to document the dissipation of energy in anticyclonic and cyclonic eddies impinging on the topographic slope east of the Bahamas, in the North Atlantic Ocean. These observations reveal high levels of turbulence where the steep and rough topographic slope modified the intensified northward flow associated with, in particular, anticyclonic eddies. Elevated dissipation was observed both near‐bottom and at mid depths (200–800 m). Near‐bottom turbulence occurred in the lee of a protruding escarpment, where elevated Froude numbers suggest hydraulic control. Energy was also radiated in the form of upward‐propagating internal waves. Elevated dissipation at mid depths occurred in regions of strong vertical shear, where the topographic slope modified the vertical structure of the northward eddy flow. Here, low Richardson numbers and a local change in the isopycnal gradient of potential vorticity (PV) suggest that the elevated dissipation was associated with horizontal shear instability. Elevated mid‐depth dissipation was also induced by topographic steering of the flow. This led to large anticyclonic vorticity and negative PV adjacent to the topographic slope, suggesting that centrifugal instability underpinned the local enhancement in dissipation. Our results provide a mechanistic benchmark for the realistic representation of eddy dissipation in ocean models.
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ArticleRecent wind-driven variability in Atlantic water mass distribution and meridional overturning circulation(American Meteorological Society, 2017-03-17) Evans, Dafydd Gwyn ; Toole, John M. ; Forget, Gael ; Zika, Jan D. ; Naveira Garabato, Alberto C. ; Nurser, A. J. George ; Yu, LisanInterannual variability in the volumetric water mass distribution within the North Atlantic Subtropical Gyre is described in relation to variability in the Atlantic meridional overturning circulation. The relative roles of diabatic and adiabatic processes in the volume and heat budgets of the subtropical gyre are investigated by projecting data into temperature coordinates as volumes of water using an Argo-based climatology and an ocean state estimate (ECCO version 4). This highlights that variations in the subtropical gyre volume budget are predominantly set by transport divergence in the gyre. A strong correlation between the volume anomaly due to transport divergence and the variability of both thermocline depth and Ekman pumping over the gyre suggests that wind-driven heave drives transport anomalies at the gyre boundaries. This wind-driven heaving contributes significantly to variations in the heat content of the gyre, as do anomalies in the air–sea fluxes. The analysis presented suggests that wind forcing plays an important role in driving interannual variability in the Atlantic meridional overturning circulation and that this variability can be unraveled from spatially distributed hydrographic observations using the framework presented here.
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ArticleMixing and air–sea buoyancy fluxes set the time-mean overturning circulation in the subpolar North Atlantic and Nordic Seas(European Geosciences Union, 2023-06-02) Evans, Dafydd Gwyn ; Holliday, N. Penny ; Bacon, Sheldon ; Le Bras, IsabelaThe overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm, salty Atlantic Water into cold, fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP array can therefore be linked to the transformation by air–sea buoyancy fluxes and mixing in the region north of the OSNAP array. Here, we estimate these water mass transformations using observational-based, reanalysis-based and model-based datasets. Our results highlight that air–sea fluxes alone cannot account for the time-mean magnitude of the overturning at OSNAP, and therefore a residual mixing-driven transformation is required to explain the difference. A cooling by air–sea heat fluxes and a mixing-driven freshening in the Nordic Seas, Iceland Basin and Irminger Sea precondition the warm, salty Atlantic Water, forming subpolar mode water classes in the subpolar North Atlantic. Mixing in the interior of the Nordic Seas, over the Greenland–Scotland Ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air–sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that, for ocean and climate models to realistically simulate the overturning circulation in the North Atlantic, the small-scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme.