Dr Alexis Megann

Group: 
Marine Systems Modelling
Email: 
apm@noc.ac.uk
Location: 
Southampton
Telephone: 
+44 (0)23 8059 6645

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 Ocean Modelling Project (JOMP), which is developing, jointly between NERC and the Met Office, the ocean component of the next-generation Earth System Model for the IPCC Sixth Assessment Report (AR6) and the associated Coupled Model intercomparison Project (CMIP6). 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, and this is used in the current GC1 and GC2 climate models. The most recent deliverable is GO6.0, aimed at AR6: this 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 GO5.0 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 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 am leading the modelling component of OSCAR, a large interdisciplinary project investigating the effect of geothermal heat input on ocean circulation. This is focused on the nearly enclosed Panama Basin in the eastern tropical Pacific, which is a region with a high level of hydrothermal activity, but will also illuminate the influence of geothermal heating on global ocean circulation. OSCAR involves 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 am a modelling advisor to the Poles Apart project (involving NOC and the British Antarctic Survey), whose aim is firstly to understand firstly why sea ice cover in the Southern Ocean has increased in recent years, in contrast to the rapid decline in Arctic sea ice, and secondly to understand why the coupled climate models contributing to the IPCC Fifth Assessment Report fail to realistically reproduce this increase.

I am a collaborator in the PIRATE project, a study funded through CNES/EuMetSat and based at LGGE in Grenoble. It will use high-resolution (10km) ocean models, along with an ensemble of medium-resolution models, to help to understand the evolution of the chaotic eddying regime detected by satellite alimetry and its effect on ocean uncertainty.

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 am 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.

Other research interests are the variability and stability of the Atlantic Meridional Overturning Circulation (AMOC), oceanic heat uptake under global climate change and equatorial ocean dynamics.

Links to further information on specialist area or projects:

OSCAR

Poles Apart

JOMP

 

Publications

Article

Kjellsson, Joakim; Holland, Paul R.; Marshall, Gareth J.; Mathiot, Pierre; Aksenov, Yevgeny; Coward, Andrew C.; Bacon, Sheldon; Megann, Alex P.; 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
Megann, A.P.; Storkey, D.; Aksenov, Y.; Alderson, S.; Calvert, D.; Graham, T.; Hyder, P.; Siddorn, J.; 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
Persechino, A.; Marsh, R.; Sinha, B.; Megann, A.P.; Blaker, A.T.; New, A.L.. 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
Balan Sarojini, B.; Gregory, J.M.; Tailleux, R.; Bigg, G.R.; Blaker, A.T.; Cameron, D.R.; Edwards, N.R.; Megann, A.P.; 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
Megann, A.P.; New, A.L.; Blaker, A.T.; 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
Marsh, R.; Nurser, A.J.G.; Megann, A.P.; New, A.L.. 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

Electronic output

Megann, Alexis; Sinha, Bablu. Global Ocean Configuation 5.0. [Output (Electronic)]