David Marshall

Abstract:

Observations indicate that symmetric instability is active in the East Greenland Current during strong northerly wind events. Theoretical considerations suggest that mesoscale baroclinic instability may also be enhanced during these events. An ensemble of idealized numerical ocean models forced with northerly winds shows that the short time-scale response (from 10 days to 3 weeks) to the increased baroclinicity of the flow is the excitation of symmetric instability, which sets the potential vorticity of the flow to zero. The high latitude of the current means that the zero potential vorticity state has low stratification, and symmetric instability destratifies the water column. On longer time scales (greater than 4 weeks), baroclinic instability is excited and the associated slumping of isopycnals restratifies the water column. Eddy-resolving models that fail to resolve the submesoscale should consider using submesoscale parameterizations to prevent the formation of overly stratified frontal systems following down-front wind events. The mixed layer in the current deepens at a rate proportional to the square root of the time-integrated wind stress. Peak water mass transformation rates vary linearly with the time-integrated wind stress. Mixing rates saturate at high wind stresses during wind events of a fixed duration which means increasing the peak wind stress in an event leads to no extra mixing. Using ERA5 reanalysis data we estimate that between 0.9 Sv and 1.0 Sv of East Greenland Coastal Current Waters are produced by mixing with lighter surface waters during wintertime due to down-front wind events. Similar amounts of East Greenland-Irminger Current water are produced.

Abstract:

Ocean ventilation translates atmospheric forcing into the ocean interior. The Southern Ocean is an important ventilation site for heat and carbon and is likely to influence the outcome of anthropogenic climate change. We conduct an extensive backwards-in-time trajectory experiment to identify spatial and temporal patterns of ventilation. Temporally, almost all ventilation occurs between August and November. Spatially, “hotspots” of ventilation account for 60% of open-ocean ventilation on a 30 years timescale; the remaining 40% ventilates in a circumpolar pattern. The densest waters ventilate on the Antarctic shelf, primarily near the Antarctic Peninsula (40%) and the west Ross sea (20%); the remaining 40% is distributed across East Antarctica. Shelf-ventilated waters experience significant densification outside of the mixed layer.

Abstract:

​​​​​​​The ability to detect and quantify methane emissions from offshore platforms is of considerable interest in providing actionable feedback to industrial operators. While satellites offer a distinctive advantage for remote sensing of offshore platforms which may otherwise be difficult to reach, offshore measurements of methane from satellite instruments in the shortwave infrared are challenging due to the low levels of diffuse sunlight reflected from water surfaces. Here, we use the GHGSat satellite constellation in a sun glint configuration to detect and quantify methane emissions from offshore targets around the world. We present a variety of examples of offshore methane plumes, including the largest single emission at (84 000 ± 24 000) kg h−1 observed by GHGSat from the Nord Stream 2 pipeline leak in 2022 and the smallest offshore emission measured from space at (180 ± 130) kg h−1 in the Gulf of Mexico. In addition, we provide an overview of the constellation's offshore measurement capabilities. We measure a median column precision of 2.1 % of the background methane column density and estimate a detection limit, from analytical modelling and orbital simulations, that varies between 160 and 600 kg h−1 depending on the latitude and season.

Abstract:

Particle Image Velocimetry (PIV) measurement data and numerical simulation data for the paper "Energetic Constraints on Baroclinic Eddy Heat Transport with a Beta Effect in the Laboratory".

The experimental data (dataGCM.zip) used the MITgcm (Marshall et al., 1997) with the input adapted from its rotating tank tutorial (model version checkpoint67c, see the manual from Adcroft et al., 2018). The sample is also for the flow regime at 1 rad/s rotation rate and 4 K temperature contrast, including the beta-plane and f-plane cases. The dataset includes temperature and velocity fields, etc. In the cylindrical coordinate, there are 120 zonal grids, 180 radial grids, and 486 vertical grids for the beta-plane case. This data set is limited in the zonal resolution. This limitation should be considered to validate results derived from the simulation data.

The experimental data (dataPIV.zip) comprises velocity fields from the multi-level PIV measurements, see the method of Wordsworth et al., 2008. The data sample is for the flow regime with the experimental control of 1 rad/s rotation rate and 4 K temperature contrast. Both samples of the beta-plane and f-plane cases are included. The software package UVMAT (Sommeria, 2013) had been applied to process PIV images in time series. For further processing, data should be re-scaled in both the spatial and time dimensions for the annulus diameter of 16 cm and the sampling time step of 0.33 seconds. The multi-level information is indexed for files with 1 to 5 at the lower height level 1, 6 to 10 at the lower level 2, 11 to 15 for middle level 3, etc. Indexes in the multiples of 5 are velocities derived from image pairs between distinct height levels and should not be included for further processing, e.g. indexes 5 or 10. Indexes for upper height level 5 in the beta-plane case should not be directly used for further processing because the sloping topography interferes with the sampling horizontal plane. This data set is limited by the 6-second delay in the sampling between the discrete height levels. This limitation should be considered to validate results derived from the experimental data.

References:

Adcroft, A., Campin, J.-M., Dutkiewicz, S., Evangelinos, C., Ferreira, D., Forget, G., . . . Molod, A. (2018). MITgcm user manual. (https://dspace.mit.edu/handle/1721.1/117188, https://mitgcm.readthedocs.io/en/latest/, [Last Accessed: 25 September 2023])

Marshall, J., Adcroft, A., Hill, C., Perelman, L., & Heisey, C. (1997). A finite-volume, incompressible Navier Stokes model for studies of the ocean on parallel computers. Journal of Geophysical Research: Oceans, 102 (C3), 5753-5766. doi: https://doi.org/10.1029/96JC02775

Sommeria, J. (2013). UVMAT. (http://servforge.legi.grenoble-inp.fr/projects/soft-uvmat, [Last Accessed: 29 September 2023])

Wordsworth, R. D., Read, P. L., & Yamazaki, Y. H. (2008). Turbulence, waves, and jets in a differentially heated rotating annulus experiment. Physics of Fluids, 20 (12), 126602.

Abstract:

There is an increasing interest in mesoscale eddy parameterizations that are scale-aware, normally interpreted to mean that a parameterization does not require parameter recalibration as the model resolution changes. Here we examine whether Gent–McWilliams (GM) based version of GEOMETRIC, a mesoscale eddy parameterization that is constrained by a parameterized eddy energy budget, is scale-aware in its energetics. It is generally known that GM-based schemes severely damp out explicit eddies, so the parameterized component would be expected to dominate across resolutions, and we might expect a negative answer to the question of energetic scale-awareness. A consideration of why GM-based schemes damp out explicit eddies leads a suggestion for what we term a splitting procedure: a definition of a “large-scale” field is sought, and the eddy-induced velocity from the GM-scheme is computed from and acts only on the large-scale field, allowing explicit and parameterized components to co-exist. Within the context of an idealized re-entrant channel model of the Southern Ocean, evidence is provided that the GM-based version of GEOMETRIC is scale-aware in the energetics as long as we employ a splitting procedure. The splitting procedure also leads to an improved representation of mean states without detrimental effects on the explicit eddy motions.