Surface Mixed Layer Evolution at Submesoscales (SMILES)

Lead Research Organisation: Plymouth Marine Laboratory
Department Name: Plymouth Marine Lab

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 Submesoscale dynamics can have a leading role in the exchange of materials and substances between the deep ocean and the surface layer in addition to promoting an active link between the sea surface and the atmosphere. The important result demonstrated in this work is that the concentration of nutrients does not drop as fast as it should given the observed primary production, suggesting an additional source of nutrients coming from somewhere. We demonstrated that these weren't coming from turbulent entrainment across the mixed layer base, nor from Ekman pumping caused by divergent winds. Instead, the periphery of the filaments were susceptible to submesoscale instabilities, direct evidence for which was obtained from drifting ADCPs and tracer release experiments. Rapid changes in temperature and salinity observed in the Antarctic circumpolar current suggest that modification of mode water may occur during the formation of mesoscale eddies on time scales ~ 1 day. Eddies in the Southern Ocean are important for transporting heat, nutrients and also energy to the north and south of the ACC. These mechanisms, often absent in climate models can have key implications in our ability to accurately forecast future climate scenarios.
Exploitation Route The SMILES numerical modelling group has developed a submesoscale parameterization which focuses on a special type of fluid instability for which no parameterization has previously been developed: symmetric instability (SI). This parameterization is dependent on external forcing by either surface buoyancy loss (i.e. atmospheric cooling or precipitation) or down-front winds (which blow parallel to regions where the fluid density changes in the horizontal), which lead to conditions favorable for SI. Previously-developed theory of SI has been leveraged to estimate the rates of momentum, buoyancy, and passive tracer mixing appropriate for this type of turbulence. With this parameterization the mixing rates of these quantities in GCM's are then adjusted to reflect this "missing" turbulence. This can be used in climate models.
Sectors Aerospace, Defence and Marine,Environment

URL http://www.smiles-project.org/
 
Title Flowcytometry samples analysis during SMILES cruise (April-May 2015) 
Description Surface and depth resolving analysis of water samples collected during SMILES cruise in the Southern Atlantic (Scotia Ridge Sea). The analysis of the samples is on-going. About 20% of the samples have been analysed (from a total of ~900). 
Type Of Material Database/Collection of data 
Provided To Others? No  
Impact The datasets allowed the biogeochemical characterisation of surface waters in the vicinity of the Antarctic Circumpolar Current and is contributing to the understanding of Air-Sea gas exchange processes as well as vertical processes associated with submesoscale dynamics. 
URL http://www.smiles-project.org/index.html
 
Title Nutrient samples analysis during SMILES cruise (April-May 2015) 
Description Analysis of ~200 water samples of surface waters collected during SMILES cruise in 2015. The samples were analysed for Nitrate, Nitrite, Silicate and Phosphate. 
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
Impact The datasets allowed the biogeochemical characterisation of surface waters in the vicinity of the Antarctic Circumpolar Current and is contributing to the understanding of Air-Sea gas exchange processes as well as vertical processes associated with submesoscale dynamics. 
URL http://www.smiles-project.org/
 
Title processed ADCP dataset collected during the SMILES cruise in April-May 2015 
Description Processed ADCP dataset from SMILES cruise JR311 
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
Impact The data have been used in the preparation of several manuscript at the University of Plymouth and Cambridge University 
 
Description Poster presentation at Ocean Sciences international conference (New Orleans, 2016) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Presentation of a subset of the results arising from SMILES to a wide and large international audience. We presented a description of the evolution of a cyclonic eddy in the Southern Atlantic region from birth to its demise and discussed the role of submesoscale dynamics to its evolution. The work was presented in the Past and Present Climate in the Southern Ocean session.
Year(s) Of Engagement Activity 2016
 
Description Presentation on ERSEM ecosystem model relevance to Arctic environments 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Workshop attended by mixed of funders, users and scientists for discussing proposal submission to look at relevance of sea-ice algae in Arctic environments to evaluate potential impacts from offshore oil extraction. Resulting form this workshop, a proposal was submitted to Norway's STATOIL company.
Year(s) Of Engagement Activity 2015