Dr Alex Megann

Job Title

Research Scientist – Climate Modelling

Site

Southampton

Email

apm@noc.ac.uk

Biography

Climate modelling, large-scale ocean circulation, climate uncertainty

I am an ocean modeller with a particular interest in the role of the ocean in climate variability and climate change.

I am an active contributor to the Joint Marine Modelling Project (JMMP), which is developing, jointly between NERC and the Met Office, the ocean component of the next-generation Earth System Model. The first deliverable of this project was the GO5.0 ocean configuration (Megann et al., 2014), with a ¼° global resolution, allowing the model to represent ocean eddies. The most recent deliverable is GO6.0 (Storkey et al, 2018), which forms the ocean component of the GC3.1 coupled climate model and the UKESM1 Earth System Model, both aimed at the IPCC Sixth Assessment Report (AR6) and the associated Coupled Model Intercomparison Project (CMIP6). GO6 includes a representation of icebergs, and a grid that is extended southwards to allow it to model the Antarctic ice sheets and their interactions with the ocean. Development of the next generation, GO8, is ongoing, and this will include several model improvement including interactive marine ice shelves as well as new features to reduce numerical mixing. I am leading WP3.2, which is concerned with the model physics improvements.

A current active research project is to diagnose the numerical mixing (mixing arising from truncations in the model advection scheme) in the NEMO ocean model. This mixing is in addition to that imposed by the mixing scheme of the model, the latter aiming to reproduce the mixing effect of real ocean processes such as breaking internal waves and shear instabilities. My results (Megann, 2018) indicate that numerical diapycnal mixing (mixing of water of different densities) can be several times larger than the explicit physical mixing. This has potential implications for the performance of the model in long climate projections, in particular on its ability to represent the uptake and storage of heat associated with anthropogenic climate change.

I led the RENUMERATE project, funded under the CMEMS Service Evolution II programme. The aims of this are: (1) to add tidal forcing to the GO8 1/4° global model configuration; (2) to add the z~ flexible vertical coordinate, aimed at reducing numerical mixing from internal waves and tides; and (3) to evaluate the joint effects of these two additions on numerical mixing, watermass changes, large-scale ocean circulation and sea ice. This was the first implementation of either tidal forcing or z~ to a global 1/4° NEMO, and successfully demonstrated that z~ indeed has the intended effect in reducing spurious mixing in a tidal simulation.

I lead WP2.3 of the North Atlantic Climate System Integrated Study (ACSIS), which delivered integrations of the GO6 1/4° global NEMO ocean configurations from 1958 to the present day with three different forcing datasets. This showed a direct and unambiguous connection between recent changes in surface buoyancy fluxes in the Labrador and Irminger Seas and decadal changes in the Atlantic meridional overturning circulation (AMOC).

I led the modelling component of OSCAR, a large interdisciplinary project investigating the effect of geothermal heat input on ocean circulation. This was focused on the nearly enclosed Panama Basin in the eastern tropical Pacific, which is a region with a high level of hydrothermal activity, but also illuminated the influence of geothermal heating on global ocean circulation. OSCAR involved an observational campaign involving hydrographers and geophysicists, as well as a high-resolution regional implementation of the HYCOM ocean model to simulate the circulation in the basin.

I created the CHIME coupled climate model (Megann et al, 2010), using the HYCOM hybrid-coordinate isopycnic ocean coupled to the atmosphere component of the HadCM3 climate model. Isopycnic-coordinate ocean models, using layers of constant density, have the potential for much reduced numerical mixing compared with the default model type, which is based on constant depth levels. This project showed that the isopycnic ocean in CHIME had a much more realistic representation of climatically important water masses such as North Atlantic Deep Water, Subantarctic Mode Water and Antarctic Intermediate Water than the HadCM3 model, identical to CHIME apart from its depth-coordinate ocean component. CHIME also simulated the overflow of Arctic water over the sills between Greenland and Scotland into the North Atlantic without the unphysical convection and excessive entrainment of the overflow waters seen in HadCM3 and similar models.

I have a strong Interest in climate uncertainty, particularly that associated with the ocean. I was involved in a joint project between NOC and the University of Exeter on developing parameter ensembles of the NEMO ocean model at different grid resolutions, and the creation of Bayesian emulators. This technique generates optimum ranges for the model parameters, but in addition allows a relatively large ensemble of a computationally efficient low-resolution model to supplement a small ensemble of a more realistic (but more expensive) high-resolution model to optimise the parameters of the latter.

Committee Memberships

I am a member of the CLIVAR Ocean Modelling Development Panel: https://www.clivar.org/clivar-panels/omdp

Projects

Roles Provided

Ocean and climate modelling large-scale ocean circulation; Atlantic overturning circulation; numerical mixing

Publications

Authors

Guiavarc'h, Catherine; Storkey, David ORCID: https://orcid.org/0000-0001-8000-5335; Blaker, Adam T. ORCID: https://orcid.org/0000-0001-5454-0131; Blockley, Ed ORCID: https://orcid.org/0000-0002-0489-4238; Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317; Hewitt, Helene ORCID: https://orcid.org/0000-0001-7432-6001; Bell, Michael J. ORCID: https://orcid.org/0000-0003-1530-6371; Calvert, Daley; Copsey, Dan ORCID: https://orcid.org/0000-0002-9706-9033; Sinha, Bablu; Moreton, Sophia; Mathiot, Pierre; An, Bo. 2025 GOSI9: UK Global Ocean and Sea Ice configurations. Geoscientific Model Development, 18 (2). 377-403. 10.5194/gmd-18-377-2025

Publication year

2025

Publication type

Article

Authors

Publication year

2025

Publication type

Article

Authors

Moat, B.I. ORCID: https://orcid.org/0000-0001-8676-7779; Sinha, B.; Berry, D.I.; Drijfhout, S.S.; Fraser, N.; Hermanson, L.; Jones; Josey, S.A. ORCID: https://orcid.org/0000-0002-1683-8831; King, B. ORCID: https://orcid.org/0000-0003-1338-3234; Macintosh, C.; Megann, A. ORCID: https://orcid.org/0000-0003-0975-6317; Oltmanns, M. ORCID: https://orcid.org/0000-0002-8959-4938; Sanders, R.; Williams, S. ORCID: https://orcid.org/0000-0003-4123-4973. 2024 Ocean heat convergence and North Atlantic multidecadal heat content variability. Journal of Climate, 37 (18). 4723-4742. 10.1175/JCLI-D-23-0370.1

Publication year

2024

Publication type

Article

Authors

Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317. 2024 Quantifying numerical mixing in a tidally forced global eddy-permitting ocean model. Ocean Modelling, 188, 102329. 10.1016/j.ocemod.2024.102329

Publication year

2024

Publication type

Article

Authors

Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317; Chanut, Jérôme; Storkey, David. 2022 Assessment of the z ∼ time‐filtered arbitrary Lagrangian‐Eulerian coordinate in a global eddy‐permitting ocean model. Journal of Advances in Modeling Earth Systems, 14 (11). 10.1029/2022MS003056

Publication year

2022

Publication type

Article

Authors

Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317; Blaker, Adam ORCID: https://orcid.org/0000-0001-5454-0131; Josey, Simon ORCID: https://orcid.org/0000-0002-1683-8831; New, Adrian ORCID: https://orcid.org/0000-0002-3159-8872; Sinha, Bablu. 2021 Mechanisms for late 20th and early 21st century decadal AMOC variability. Journal of Geophysical Research: Oceans, 126 (12). 10.1029/2021JC017865

Publication year

2021

Publication type

Article

Authors

Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317; Storkey, David. 2021 Exploring viscosity space in an eddy‐permitting global ocean model: Is viscosity a useful control for numerical mixing? Journal of Advances in Modeling Earth Systems, 13 (5). 10.1029/2020MS002263

Publication year

2021

Publication type

Article

Authors

Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317; Chanut, Jerome; Storkey, Dave. 2021 Evaluation of the z-tilde vertical coordinate in a 1/4° global NEMO. In: EGU General Assembly 2021, Online, 19-30 April 2021.

Publication year

2021

Publication type

Conference item

Authors

Desbruyères, D. G.; Sinha, B.; McDonagh, E. L.; Josey, S. A. ORCID: https://orcid.org/0000-0002-1683-8831; Holliday, N. P. ORCID: https://orcid.org/0000-0002-9733-8002; Smeed, D. A. ORCID: https://orcid.org/0000-0003-1740-1778; New, A. L. ORCID: https://orcid.org/0000-0002-3159-8872; Megann, A. ORCID: https://orcid.org/0000-0003-0975-6317; Moat, B. I. ORCID: https://orcid.org/0000-0001-8676-7779. 2020 Importance of boundary processes for heat uptake in the Subpolar North Atlantic. Journal of Geophysical Research: Oceans, 125 (9), e2020JC016366. 10.1029/2020JC016366

Publication year

2020

Publication type

Article

Authors

Robson, Jon; Aksenov, Yevgeny ORCID: https://orcid.org/0000-0001-6132-3434; Bracegirdle, Thomas J. ORCID: https://orcid.org/0000-0002-8868-4739; Dimdore-Miles, Oscar; Griffiths, Paul T.; Grosvenor, Daniel P.; Hodson, Daniel L.R.; Keeble, James; Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317; Osprey, Scott; Povey, Adam C.; Schröder, David; Yang, Mingxi; Archibald, Alexander T.; Carslaw, Ken S.; Gray, Lesley; Jones, Colin; Kerridge, Brian; Knappett, Diane; Kuhlbrodt, Till; Russo, Maria; Sellar, Alistair; Siddans, Richard; Sinha, Bablu; Sutton, Rowan; Walton, Jeremy; Wilcox, Laura J.. 2020 The evaluation of the North Atlantic climate system in UKESM1 historical simulations for CMIP6. Journal of Advances in Modeling Earth Systems, 12 (9), e2020MS002126. 10.1029/2020MS002126

Publication year

2020

Publication type

Article

Authors

Storkey, David; Blaker, Adam ORCID: https://orcid.org/0000-0001-5454-0131; Mathiot, Pierre; Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317; Aksenov, Yevgeny ORCID: https://orcid.org/0000-0001-6132-3434; Blockley, Edward W.; Calvert, Daley; Graham, Tim; Hewitt, Helene T.; Hyder, Patrick; Kuhlbrodt, Till; Rae, Jamie G.L.; Sinha, Bablu. 2018 UK Global Ocean GO6 and GO7: a traceable hierarchy of model resolutions. Geoscientific Model Development, 11 (8). 3187-3213. 10.5194/gmd-11-3187-2018

Publication year

2018

Publication type

Article

Authors

Barnes, Jowan M.; Morales Maqueda, Miguel A.; Polton, Jeff A. ORCID: https://orcid.org/0000-0003-0131-5250; Megann, Alex P. ORCID: https://orcid.org/0000-0003-0975-6317. 2018 Idealised modelling of ocean circulation driven by conductive and hydrothermal fluxes at the seabed. Ocean Modelling, 122. 26-35. 10.1016/j.ocemod.2017.12.005

Publication year

2018

Publication type

Article

Authors

Williams, K. D.; Copsey, D.; Blockley, E. W.; Bodas-Salcedo, A.; Calvert, D.; Comer, R.; Davis, P.; Graham, T.; Hewitt, H. T.; Hill, R.; Hyder, P.; Ineson, S.; Johns, T. C.; Keen, A. B.; Lee, R. W.; Megann, A. ORCID: https://orcid.org/0000-0003-0975-6317; Milton, S. F.; Rae, J. G. L.; Roberts, M. J.; Scaife, A. A.; Schiemann, R.; Storkey, D.; Thorpe, L.; Watterson, I. G.; Walters, D. N.; West, A.; Wood, R. A.; Woollings, T.; Xavier, P. K.. 2018 The Met Office Global Coupled Model 3.0 and 3.1 (GC3.0 and GC3.1) Configurations. Journal of Advances in Modeling Earth Systems, 10 (2). 357-380. 10.1002/2017MS001115

Publication year

2018

Publication type

Article

Authors

Megann, Alex ORCID: https://orcid.org/0000-0003-0975-6317. 2018 Estimating the numerical diapycnal mixing in an eddy-permitting ocean model. Ocean Modelling, 121. 19-33. 10.1016/j.ocemod.2017.11.001

Publication year

2018

Publication type

Article

Authors

Banyte, D.; Morales Maqueda, M.; Hobbs, R.; Smeed, D. A. ORCID: https://orcid.org/0000-0003-1740-1778; Megann, A. ORCID: https://orcid.org/0000-0003-0975-6317; Recalde, S.. 2018 Geothermal Heating in the Panama Basin: 1. Hydrography of the Basin. Journal of Geophysical Research: Oceans, 123 (10). 7382-7392. 10.1029/2018JC013868

Publication year

2018

Publication type

Article

Authors

Banyte, D.; Morales Maqueda, M.; Smeed, D. A. ORCID: https://orcid.org/0000-0003-1740-1778; Megann, A. ORCID: https://orcid.org/0000-0003-0975-6317; Hobbs, R.; Recalde, S.. 2018 Geothermal heating in the Panama Basin. Part II: Abyssal water mass transformation. Journal of Geophysical Research: Oceans, 123 (10). 7393-7406. 10.1029/2018JC013869

Publication year

2018

Publication type

Article

Authors

Kjellsson, Joakim; Holland, Paul R. ORCID: https://orcid.org/0000-0001-8370-289X; Marshall, Gareth J. ORCID: https://orcid.org/0000-0001-8887-7314; Mathiot, Pierre; Aksenov, Yevgeny ORCID: https://orcid.org/0000-0001-6132-3434; Coward, Andrew C. ORCID: https://orcid.org/0000-0002-9111-7700; Bacon, Sheldon ORCID: https://orcid.org/0000-0002-2471-9373; Megann, Alex P. ORCID: https://orcid.org/0000-0003-0975-6317; Ridley, Jeff. 2015 Model sensitivity of the Weddell and Ross seas, Antarctica, to vertical mixing and freshwater forcing. Ocean Modelling, 94. 141-152. 10.1016/j.ocemod.2015.08.003

Publication year

2015

Publication type

Article

Authors

Megann, A.P. ORCID: https://orcid.org/0000-0003-0975-6317; Storkey, D.; Aksenov, Y. ORCID: https://orcid.org/0000-0001-6132-3434; Alderson, S.; Calvert, D.; Graham, T.; Hyder, P.; Siddorn, J. ORCID: https://orcid.org/0000-0003-3848-8868; Sinha, B.. 2014 GO 5.0: The joint NERC-Met Office NEMO global ocean model for use in coupled and forced applications. Geotechnical Model Development, 7 (3). 1069-1092. 10.5194/gmd-7-1069-2014

Publication year

2014

Publication type

Article

Authors

Persechino, A.; Marsh, R.; Sinha, B.; Megann, A.P. ORCID: https://orcid.org/0000-0003-0975-6317; Blaker, A.T. ORCID: https://orcid.org/0000-0001-5454-0131; New, A.L. ORCID: https://orcid.org/0000-0002-3159-8872. 2012 Decadal-timescale changes of the Atlantic overturning circulation and climate in a coupled climate model with a hybrid-coordinate ocean component. Climate Dynamics, 39 (3-4). 1021-1042. 10.1007/s00382-012-1432-y

Publication year

2012

Publication type

Article

Authors

Balan Sarojini, B.; Gregory, J.M.; Tailleux, R.; Bigg, G.R.; Blaker, A.T. ORCID: https://orcid.org/0000-0001-5454-0131; Cameron, D.R.; Edwards, N.R.; Megann, A.P. ORCID: https://orcid.org/0000-0003-0975-6317; Shaffery, L.C.; Sinha, B.. 2011 High frequency variability of the Atlantic meridional overturning circulation. Ocean Science, 7. 471-486. 10.5194/os-7-471-2011

Publication year

2011

Publication type

Article

Authors

Megann, A.P. ORCID: https://orcid.org/0000-0003-0975-6317; New, A.L. ORCID: https://orcid.org/0000-0002-3159-8872; Blaker, A.T. ORCID: https://orcid.org/0000-0001-5454-0131; Sinha, B.. 2010 The sensitivity of a coupled climate model to its ocean component. Journal of Climate, 23 (19). 5126-5150. 10.1175/2010JCLI3394.1

Publication year

2010

Publication type

Article

Authors

Marsh, R.; Megann, A.P. ORCID: https://orcid.org/0000-0003-0975-6317. 2002 Tracing water masses with particle trajectories in an isopycnic-coordinate model of the global ocean. Ocean Modelling, 4 (1). 27-53. 10.1016/S1463-5003(01)00011-7

Publication year

2002

Publication type

Article

Authors

Megann, A. ORCID: https://orcid.org/0000-0003-0975-6317; New, A. ORCID: https://orcid.org/0000-0002-3159-8872. 2001 The effects of resolution and viscosity in an isopycnic-coordinate model of the equatorial Pacific. Journal of Physical Oceanography, 31 (8). 1993-2018. 10.1175/1520-0485(2001)031<1993:TEORAV>2.0.CO;2

Publication year

2001

Publication type

Article

Authors

Marsh, R.; Nurser, A.J.G.; Megann, A.P. ORCID: https://orcid.org/0000-0003-0975-6317; New, A.L. ORCID: https://orcid.org/0000-0002-3159-8872. 2000 Water mass transformation in the Southern Ocean of a global isopycnal coordinate GCM. Journal of Physical Oceanography, 30 (5). 1013-1045. 10.1175/1520-0485(2000)030<1013:WMTITS>2.0.CO;2

Publication year

2000

Publication type

Article

Dr Alex Megann

Biography

Committee Memberships

Projects

Roles Provided

Publications

GOSI9: UK Global Ocean and Sea Ice configurations

Data supporting the North Atlantic Climate System Integrated Study (ACSIS) programme, including atmospheric composition; oceanographic and sea-ice observations (2016–2022); and output from ocean, atmosphere, land, and sea-ice models (1950–2050)

Ocean heat convergence and North Atlantic multidecadal heat content variability

Quantifying numerical mixing in a tidally forced global eddy-permitting ocean model

Assessment of the z ∼ time‐filtered arbitrary Lagrangian‐Eulerian coordinate in a global eddy‐permitting ocean model

Mechanisms for late 20th and early 21st century decadal AMOC variability

Exploring viscosity space in an eddy‐permitting global ocean model: Is viscosity a useful control for numerical mixing?

Evaluation of the z-tilde vertical coordinate in a 1/4° global NEMO

Importance of boundary processes for heat uptake in the Subpolar North Atlantic

The evaluation of the North Atlantic climate system in UKESM1 historical simulations for CMIP6

UK Global Ocean GO6 and GO7: a traceable hierarchy of model resolutions

Idealised modelling of ocean circulation driven by conductive and hydrothermal fluxes at the seabed

The Met Office Global Coupled Model 3.0 and 3.1 (GC3.0 and GC3.1) Configurations

Estimating the numerical diapycnal mixing in an eddy-permitting ocean model

Geothermal Heating in the Panama Basin: 1. Hydrography of the Basin

Geothermal heating in the Panama Basin. Part II: Abyssal water mass transformation

Model sensitivity of the Weddell and Ross seas, Antarctica, to vertical mixing and freshwater forcing

GO 5.0: The joint NERC-Met Office NEMO global ocean model for use in coupled and forced applications

Decadal-timescale changes of the Atlantic overturning circulation and climate in a coupled climate model with a hybrid-coordinate ocean component

High frequency variability of the Atlantic meridional overturning circulation

The sensitivity of a coupled climate model to its ocean component

Tracing water masses with particle trajectories in an isopycnic-coordinate model of the global ocean

The effects of resolution and viscosity in an isopycnic-coordinate model of the equatorial Pacific

Water mass transformation in the Southern Ocean of a global isopycnal coordinate GCM