Surface Mixed Layer at Submesoscales (SMILES)

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

Our current understanding of the Earth's climate is largely based on the predictions of numerical models that simulate the behaviour of, and interaction between, the atmosphere and the ocean. These models are crucially limited in their resolution, however, such that processes within the ocean that have horizontal scales of less than approximately 10 km cannot be explicitly represented and need to be parameterised for their effects to be included within the models. The purpose of this project, Surface Mixed Evolution at Submesoscales (SMILES), is to identify the potentially crucial role played by one variety of these unresolved processes, referred to as submesoscales, in influencing the structure and properties of the upper ocean, and thereby the transformation of surface water masses, within the Southern Ocean. Submesoscales are flows with spatial scales of 1-10 km that occur within the upper ocean where communication and exchange between the ocean and the atmosphere occurs. Previously considered unimportant to climate-scale studies due to their small scale and the presumed insignificance of their dynamics, recent evidence from high resolution regional models and observational studies is now emerging which suggests that submesoscales are actually widespread throughout the upper ocean and play a key role within climate dynamics due to their ability to rapidly restratify the upper ocean and reduce buoyancy loss from the ocean to the atmosphere. The impact of such a process is particularly important to the surface transformation of water masses such as Subantarctic Mode Water (SAMW), which is an important component of the Meridional Overturning Circulation (MOC) that redistributes heat, freshwater and tracers around the globe. Within the MOC, dense water masses such as SAMW are formed and transformed at high latitudes by surface processes before being subducted into the ocean interior. The properties of the subducted water masses and the tracers and dissolved gases such as carbon dioxide contained within them are vitally important to the global climate and geochemical cycles as these water masses remain out of contact with the surface over decennial to centennial timescales.

In the light of the recent discoveries concerning the ability of submesoscales to substantially influence the properties of the upper ocean, we will directly study the impacts of submesoscales on SAMW properties within the Scotia Sea. Using an integrated approach, we will both observe and simulate submesoscales within the upper ocean at a range of spatial and temporal scales, spanning from turbulence up to mode water formation. The principal goal of the study is the diagnosis of the role played by submesoscales in water mass transformation so that we can accurately incorporate these effects into climate-scale models which cannot explicitly resolve them. Our methods will entail a cruise approximately 200 miles south of the Falklands Islands at the Subantarctic Front (SAF), to the north of which SAMW is transformed, and a concurrent modelling study using a state-of-the-art global circulation model. During the cruise, we will use towed instruments to measure the length scales of variability in the temperature, salinity and related fields throughout the upper 300 m of the ocean. The data will enable us to identify the intensity and distribution of submesoscales within the vicinity of the SAF, and to ascertain the forcing mechanisms that generate them. In conjunction with the modelling component of the project, which will include both high resolution and coarse-scale simulations with the MITgcm and large eddy simulations (LES), we will assess how submesoscales ultimately impact on the properties of SAMW within the region and the ultimate effect this has on the formation of SAMW.

Planned Impact

The main beneficiaries of knowledge arising from this research are anticipated to be scientists working in related disciplines and policy-makers. The overall goal of the proposal is to understand how the unresolved processes studied within the project can impact on air-sea exchange and water mass transformation, and thereby influence climate. The scientists working in related disciplines for whom the results would be of interest fall into four groups: (i) observational and physical oceanographers interested in upper ocean processes, (ii) geophysical fluid dynamicists, (iii) marine biologists and (iv) ocean biogeochemists, particularly those concerned with air-sea gas exchange. The largest group of beneficiaries is therefore from the academic sector which includes the Intergovernmental Panel on Climate Change (IPCC) community, but additional identified users include:
- the Met Office (including the Hadley Centre), the European Centre for Medium-Range Weather Forecasting (ECMWF), the National Centre for Earth Observation (NCEO) the National Centre for Ocean Forecasting (NCOF), the UK OSMOSIS consortium and US 'LatMix' and CLIMODE teams (see Letters of Support).
These groups are tasked with developing accurate models to predict changes in air-sea exchange, ocean dynamics and thereby climate change. Ultimately the results from these models inform UK policy-makers.

The UK stands to benefit from this research as the initial recipients of our enhanced understanding of upper ocean physical processes will be UK institutionsnamed above. As a competetive area of research, the substantial improvements in model fidelity to be achieved from a more accurate representation of fundamentally important atmosphere-ocean interaction will provide a competitive advantage to the UK models. The projected timescale for realising these improvements is 3-5 years following initiation of the project.

Publications

10 25 50
 
Description To date the key findings funded by this grant have been largely technical and to do with designing the large-scale model of Drake Passage described elsewhere. In particular, the model derives it forcing from a best estimate of the Southern Ocean circulation and hydrography for 2005-2010. This best estimate has a drift, in part due to climate change, which had to be removed in designing the annual climatology we then used as surface forcing. In fact, this drift was the point behind designing the model in the first place; eliminating it allows for the production of a long-term (in excess of 10 years long) forcing series to drive higher resolution, but smaller in geographical area, nested models.

In generating the high resolution nested domains, we originally cloned; each of the 1/6 degree parent model's grid box to make the new nested model. For example, to form the 1/12 model, we copied each 1/6 grid box 4 times, but made them half the length on each side. This ensure that the same amount of, for example, momentum was provided by the wind. However, this created forcing fields full of small, homogenous regions, which resulted in an unacceptably noisy circulation. The solution has been to use an interpolation so that new forcing varies continuously and smoothly across the nested model grid.

In a similar vein, we also found it necessary to refine the nested models' bottom bathymetry via interpolation from the best available ocean sea bed map (which has grid boxes 1/60 degree across). This does two things; 1) eliminate another source of noise by not cloning each 1/6 degree box 4-16 times; 2) resolves small-scale ocean bed features to a higher degree and thus allows for the model's mesoscale eddy field to be stimulated by, and interact with, these smaller bathymetric features.

All three models were run to completion in the final six months of SMILES. Off-the-shelf eddy tracking algorithms were then applied to the output to begin comparing the different model resolutions. This analysis has not progressed to give definitive outcomes. However, the model output was shared with other members of the project to enable full use of the archive.
Exploitation Route The Drake Passage model designed by DRM will produce a large volume of mode output, in excess of 500Gb and possibly in excess of 1Tb. This could be used by other members of SMILES to investigate, for example, the formation of water masses and their pathways, as per the SMILES project description. In principle, the design of the model could be expanded to a full circumpolar domain and used to look at high resolution nested domain, and thus eddy formation/shedding process, in other parts of the Southern Ocean.
Sectors Environment

 
Description Prof Mike Meredith: leadership of Polar Regions chapter in IPCC Special Report on Oceans and Cryosphere
Geographic Reach Multiple continents/international 
Policy Influence Type Gave evidence to a government review
URL https://www.ipcc.ch/report/srocc/
 
Title SMILES Drake Passage Model 
Description A configuration of the MIT general circulation model (MITgcm) has been designed and developed to allow large-scale simulations of the SMILES observational area. This numerical model covers a 120 degree in longitude swath roughly centred upon Drake Passage. The initial configuration has a 1/6 grid spacing and its forcing is derived from the MITgcm-based Southern Ocean State Estimate (SOSE) of Mazloff et al. (2010, JPO, 40, 880-899, doi:10.1175/2009JPO4236.1) in order to give a realistic stratification and circulation. At present, this model configuration has been run to equilibrium at SOSE's native grid spacing of 1/6 degree. Higher resolution versions at 1/12 degree and 1/24 degree have been developed over a restricted spatial domain of roughly 1/4 the area of the 1/6 degree spinup. These nested domains are forced at their east/west/north/south boundaries by a 20-year, non-repeating series of temperature, salinity and velocity time-series derived from the initial 1/6 degree spinup. The nested domains have an improved representation and resolution of mesoscale eddy processes and bottom bathymetry. As such, they can be used to provide a large-scale, and potentially climatological view, of the eddy-shedding process observed in 2015 during the SMILES cruise. All three versions of the model were run to completion and eddy tracking algorithms applied in August 2016. This created a database of model output nearly 1Tb in size (the complete archive is twice this). This database was subsequently shared with other members of the project, Drs. Kate Adams and Scott Bachman, in order to facilitate further collaboration. 
Type Of Material Computer model/algorithm 
Provided To Others? No  
Impact The 1/6 degree spinup, and the associated 20-year forcing series for the 1/12 and 1/24 degree nested models, was completed in August 2016. No notable impacts are currently available to report. 
 
Description SMILES collaboration 
Organisation University of Cambridge
Department Department of Applied Mathematics and Theoretical Physics (DAMTP)
Country United Kingdom 
Sector Academic/University 
PI Contribution Dr. David Munday has been leading the development of a large-scale numerical model of the region of the Southern Ocean near Drake Passage. This model will provide a large-scale context to the rest of the SMILES project.
Collaborator Contribution Drs. John Taylor and Scott Bachman of the University of Cambridge have provided input on the design of the model and helped ensure that it fits within the aims of SMILES. Drs. Phil Hosegood and Kate Snow of the University of Plymouth have helped ensure that the forthcoming analysis of the numerical model will complement the observations collected in 2015.
Impact This collaboration has provided a large-scale model of Drake Passage, described as a separate outcome.
Start Year 2015
 
Description SMILES collaboration 
Organisation University of Plymouth
Country United Kingdom 
Sector Academic/University 
PI Contribution Dr. David Munday has been leading the development of a large-scale numerical model of the region of the Southern Ocean near Drake Passage. This model will provide a large-scale context to the rest of the SMILES project.
Collaborator Contribution Drs. John Taylor and Scott Bachman of the University of Cambridge have provided input on the design of the model and helped ensure that it fits within the aims of SMILES. Drs. Phil Hosegood and Kate Snow of the University of Plymouth have helped ensure that the forthcoming analysis of the numerical model will complement the observations collected in 2015.
Impact This collaboration has provided a large-scale model of Drake Passage, described as a separate outcome.
Start Year 2015
 
Description Academic Consultancy to BBC for series "Frozen Planet 2", Mike Meredith 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact This series is the successor to BBC's hugely successful Frozen Planet, which reached an audience of 500 million people globally. Prof Mike Meredith has been contracted by the BBC to serve as Academic Consultant for the production of its sequel. This work is underway; the series is due for broadcast in 2021, and will reach an audience similar to its predecessor.
Year(s) Of Engagement Activity 2019
 
Description Challenger Society Conference School Outreach Event 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Dr. David Munday took part in the school outreach event, organised by Emma Smith, as part of the Challenger Society Conference 2016. Over 20 students, from a number of schools, took part and were able to interact directly with around half-a-dozen conference delegates. A short poster presentation was made to a sub-group of the students and short bullet point summaries given to each student. In addition, a small A4 version of each delegate's poster was also prepared with a slightly simplified version of the material on the main poster. The event was very successful and David was asked to consider taking part at the next Challenger Society Conference.
Year(s) Of Engagement Activity 2016
URL https://www.liverpool.ac.uk/challenger-conference-2016/
 
Description Challenger conference talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A talk by Dr. Kate Adams presenting results form the SMILES cruise.
Year(s) Of Engagement Activity 2016
 
Description Presentation at Liege colloquia 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A talk by Dr. Kate Adams presenting results form the SMILES cruise.
Year(s) Of Engagement Activity 2016
 
Description Scripps CASPO seminar 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact A talk by Dr. Kate Adams presenting results form the SMILES cruise.
Year(s) Of Engagement Activity 2016
 
Description Scripps SOCCOM seminar 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact A talk by Dr. Kate Adams presenting results form the SMILES cruise.
Year(s) Of Engagement Activity 2016
 
Description Talk at Oregon State POA 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact A talk by Dr. Kate Adams presenting results form the SMILES cruise.
Year(s) Of Engagement Activity 2016
 
Description Talk at UoP MPRG seminar 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact A talk by Dr. Kate Adams presenting results form the SMILES cruise.
Year(s) Of Engagement Activity 2016