Oceanographer at the Max Planck Institute for Meteorology

Publications

Destratifying and Restratifying Instabilities During Down-Front Wind Events: A Case Study in the Irminger Sea

Published in JGR: Oceans, 2024

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.

Recommended citation: Goldsworth, F. W., Johnson, H. L., Marshall, D. P., & Le Bras, I. A. (2024) Destratifying and Restratifying Instabilities During Down-Front Wind Events: A Case Study in the Irminger Sea. https://doi.org/10.1029/2023JC020365

Density staircases generated by symmetric instability in a cross-equatorial deep western boundary current

Published in Geophysical Research Letters, 2022

Density staircases are observed in an idealized model of a deep western boundary current upon crossing the equator. We propose that the staircases are generated by the excitement of symmetric instability as the current crosses the equator. The latitude at which symmetric instability is excited can be predicted using simple scaling arguments. Symmetric instability generates overturning cells which, in turn, cause the inhomogenous mixing of waters with different densities. The mixing barriers and well mixed regions in density profiles coincide, respectively, with the boundaries and centers of the overturning cells generated by the symmetric instability. This new mechanism for producing density staircases may require us to re-evaluate the origins of some of the density staircases observed in the Tropical Atlantic.

Recommended citation: Goldsworth, F. W., Johnson, H. L. , Marshall, D. P. (2022) Density staircases generated by symmetric instability in a cross-equatorial deep western boundary current. https://doi.org/10.1029/2022GL100961

The biological carbon pump in CMIP6 models: 21st century trends and uncertainties

Published in Proceedings of the National Academy of Sciences, 2022

The biological carbon pump (BCP) stores ∼1,700 Pg C from the atmosphere in the ocean interior, but the magnitude and direction of future changes in carbon sequestration by the BCP are uncertain. We quantify global trends in export production, sinking organic carbon fluxes, and sequestered carbon in the latest Coupled Model Intercomparison Project Phase 6 (CMIP6) future projections, finding a consistent 19 to 48 Pg C increase in carbon sequestration over the 21st century for the SSP3-7.0 scenario, equivalent to 5 to 17% of the total increase of carbon in the ocean by 2100. This is in contrast to a global decrease in export production of –0.15 to –1.44 Pg C y–1. However, there is significant uncertainty in the modeled future fluxes of organic carbon to the deep ocean associated with a range of different processes resolved across models. We demonstrate that organic carbon fluxes at 1,000 m are a good predictor of long-term carbon sequestration and suggest this is an important metric of the BCP that should be prioritized in future model studies.

Recommended citation: Wilson et al. (2022). The biological carbon pump in CMIP6 models: 21st century trends and uncertainties. Proceedings of the National Academy of Sciences. 119(29). https://doi.org/10.1073/pnas.2204369119

Symmetric Instability in Cross-Equatorial Western Boundary Currents

Published in Journal of Physical Oceanography, 2021

The upper limb of the Atlantic meridional overturning circulation draws waters with negative potential vorticity from the Southern Hemisphere into the Northern Hemisphere. The North Brazil Current is one of the cross-equatorial pathways in which this occurs: upon crossing the equator, fluid parcels must modify their potential vorticity to render them stable to symmetric instability and to merge smoothly with the ocean interior. In this work a linear stability analysis is performed on an idealized western boundary current, dynamically similar to the North Brazil Current, to identify features that are indicative of symmetric instability. Simple two-dimensional numerical models are used to verify the results of the stability analysis. The two-dimensional models and linear stability theory show that symmetric instability in meridional flows does not change when the nontraditional component of the Coriolis force is included, unlike in zonal flows. Idealized three-dimensional numerical models show anticyclonic barotropic eddies being spun off as the western boundary current crosses the equator. These eddies become symmetrically unstable a few degrees north of the equator, and their PV is set to zero through the action of the instability. The instability is found to have a clear fingerprint in the spatial Fourier transform of the vertical kinetic energy. An analysis of the water mass formation rates suggest that symmetric instability has a minimal effect on water mass transformation in the model calculations; however, this may be the result of unresolved dynamics, such as secondary Kelvin–Helmholtz instabilities, which are important in diabatic transformation.

Recommended citation: Goldsworth, F. W., Marshall, D. P., Johnson, H. L. (2021). Symmetric Instability in Cross-Equatorial Western Boundary Currents. Journal of Physical Oceanography, 51(6), 2049– 2067. https://doi.org/10.1175/JPO-D-20-0273.1. https://doi.org/10.1175/JPO-D-20-0273.1

The impact of ship emission controls recorded by cloud properties

Published in Geophysical Research Letters, 2019

The impact of aerosols on cloud properties is one of the leading uncertainties in the human forcing of the climate. Ships are large, isolated sources of aerosol creating linear cloud formations known as shiptracks. These are an ideal opportunity to identify and measure aerosol-cloud interactions. This work uses over 17,000 shiptracks during the implementation of fuel sulfur content regulations to demonstrate the central role of sulfate aerosol in ship exhaust for modifying clouds. By connecting individual shiptracks to transponder data, it is shown that almost half of shiptracks are likely undetected, masking a significant contribution to the climate impact of shipping. A pathway to retrieving ship sulfate emissions is demonstrated, showing how cloud observations could be used to monitor air pollution.

Recommended citation: Gryspeerdt, E., Smith, T. W. P., O'Keeffe, E., Christensen, M. W., & Goldsworth, F. W. (2019). The impact of ship emission controls recorded by cloud properties. Geophysical Research Letters, 46, 12547– 12555. https://doi.org/10.1029/2019GL084700