News


There is much more supercooled ocean water on planet Earth than previously noticed

Dr. Alexander Haumann is an Associate Research Scholar at the program in Atmospheric and Oceanic Sciences at Princeton University, USA. He is interested in the interaction between the ocean, ice, and atmosphere in the Southern Ocean and how these processes affect our climate through their influence on the exchange of heat and carbon between the deep ocean and the atmosphere. In a new study, he analyzed MEOP, Argo and ship-based observations to provide first estimates of the horizontal and vertical extent of supercooled waters in the Southern Ocean and analyzed their underlying processes.    

Supercooled ocean water refers to seawater that has a temperature lower than the reference freezing point. In our analysis, we combine all available data collected by instrumented marine mammals, data from profiling floats, and ship-board observations in the Southern Ocean to study this process. While supercooling has been previously found in certain coastal locations due to melting of Antarctic glaciers, we here show that it not only occurs in most of the coastal ocean, but also far away from the coast under the sea ice that forms in winter. This latter process has not yet been observed in the Southern Ocean. The supercooled water sinks from the surface under the sea ice down to more than a hundred meters in the form of convective plumes. These sinking supercooled plumes might be an important process to exchange heat, carbon, oxygen and nutrients between the surface and the deeper layers of the ocean, but they are not yet represented in global climate models.

Southern elephant seals appear to preferentially dive in such freezing cold waters along the sea-ice edge or in polynya regions, which is why the signal is very prominent in the MEOP database. An interesting question is if they prefer these waters because of environmental conditions, such as feeding. 

Check out the paper: Haumann, F. A., Moorman, R., Riser, S. C., Smedsrud, L. H., Maksym, T., Wong, A. P. S., Wilson, E. A., Drucker, R., Talley, L. D., Johnson, K. S., Key, R. M., Sarmiento, J. L. (2020). Supercooled Southern Ocean waters. Geophysical Research Letters, 47, e2020GL090242. https://doi.org/10.1029/2020GL090242

The paper and the importance of data collected by Southern elephant seals were highlighted in Nature (21 October 2020, https://www.nature.com/articles/d41586-020-02956-4)    

Submesoscale flows described during the Southern Ocean winter for the first time

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Dr Louise Biddle is a researcher at the University of Gothenburg, Sweden. She is interested in the interactions between ice and ocean around Antarctica, with research focusing on both glacial meltwater and sea ice-ocean dynamics to understand how these processes affect the upper ocean. Using the MEOP database, she and co-author Dr Sebastiaan Swart described the variability in submesoscale processes under sea ice during the Southern Ocean winter for the first time, highlighting a significant submesoscale flux during the winter months of August and September (Biddle and Swart, 2020).

Submesoscale processes occur on small (1-10 km) and short (< day) scales, but can have a significant impact on vertical oceanic fluxes. Due to their submesoscale resolution, until recently they have been hard to observe – particularly so in the Southern Ocean. The oceanic uptake of heat and carbon in this region is of global significance and understanding any processes that can contribute to their vertical transport is of critical importance. However, with very few observations during the winter months, especially in the sea ice covered ocean, there was no knowledge of how the magnitude and impact of submesoscale processes may vary seasonally. To combat this challenge, we used the MEOP database, targeting Southern Elephant seals that like to transit through the sea ice zone during winter months. 

By focusing on a dataset tagged on Bouvet Island in 2008, we showed that the seal dives are able to resolve submesoscale features. To provide some statistical confidence in our results, we combined all 14 seals that dived under the sea ice to measure seasonal changes in mixed layer depth and horizontal buoyancy gradients (amongst other parameters), in order to estimate two submesoscale processes; Ekman Buoyancy Flux and Mixed Layer Eddy flux. Whilst Ekman Buoyancy Flux, which is a function of surface wind or ice stress, was moderated by the presence of sea ice, the deeper mixed layers formed during winter months in combination with increases in the horizontal buoyancy gradient resulted in a mid-winter maximum in Mixed Layer Eddy flux. This flux can result in stratification and shoaling of the mixed layer when it is greater than opposing wind or buoyancy fluxes, signifying a possible important driver in Southern Ocean vertical transport. 

We hope to extend this analysis to the circumpolar MEOP dataset, and make use of the new MEOP-SMS data to understand any regional differences in submesoscale processes – and any future high resolution observations under the sea ice can only enhance our current knowledge!


Reference: Biddle, L. C., & Swart, S. ( 2020). The observed seasonal cycle of submesoscale processes in the Antarctic marginal ice zone. Journal of Geophysical Research: Oceans, 125, e2019JC015587. https://doi.org/10.1029/2019JC015587

UPDATED: Aug 17, 2020. This research was highlighted in the New York Times. Link to the article “How 14 elephants seals assisted an Antarctic Ice Study” by Oliver Whang.

Pathways of warm water across the Antarctic slope

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Dr. Annie Foppert is Postdoctoral Fellow in the Centre for Southern Hemisphere Oceans Research at CSIRO Oceans and Atmosphere in Hobart, Tasmania. She is interested in the dynamics, circulation, and water-mass transformation of the Southern Ocean. Using the MEOP-CTD database, she and co-authors quantified eddy-driven transport of Circumpolar Deep Water across the East Antarctic slope and identified hotspots of this onshore transport.

The transport of relatively warm and salty water (known as Circumpolar Deep Water, CDW) from the open ocean to the Antarctic continental shelf has global significance. The amount of warm water that reaches the ice shelves regulates how quickly the ice melts which affects sea levels rise. At the same time, the amount of salt on the continental shelf regulates how much heat and carbon are stored in the deep ocean through dense water formation. Yet, the ways in which this circumpolar deep water makes it across the slope and onto the shelf, and the relative importance of the different processes that transport this water, are still unknown. 

Using the MEOP database of CTD profiles, we quantified the strength of one of these mechanisms for circumpolar deep water transport – eddy-driven transport – along the continental slope of East Antarctica. The data show that, in general, eddies transport this deep water across the continental slope to the shelf break – the point where the continental shelf transitions to the continental slope. Closer inspection of this eddy-driven transport mechanism revealed specific regions where eddies are much more efficient at carrying circumpolar deep water across the slope (while eddy-driven transport is negligible elsewhere). The waters at the shelf break are warmer and saltier in these hot spots of eddy-driven transport, and therefore more circumpolar deep water is available to move onto the continental shelf in these locations. What happens to these regional reservoirs of heat and salt once they move beyond the shelf break will ultimately determine which ice shelves are most vulnerable to ocean-driven melting and where the densest ocean waters are formed.

Check out her paper: Foppert, A., Rintoul, S. R., and England, M. H., 2019. Along-Slope Variability of Cross-Slope Eddy Transport in East Antarctica. Geophysical Research Letters, 46:8224-8233. doi: 10.1029/2019GL082999

MEOP-SMS: a new database for submesoscale-resolving CTD data

It is with great excitement that we are finally able to present you a brand new MEOP product, the MEOP-SMS database.

The MEOP-SMS database is a collection of CTD data gathered with CTD-SRDLs attached to elephant seals. The CTD-SRDLs were set in continuous recording mode, and could be recovered in the field, enabling retrieval of the entire CTD archive at the sampling frequency (0.5 Hz) for periods of several months. Instruments were deployed at the Kerguelen Islands and at Peninsula Valdes (Argentina) as part of the SO-MEMO program led by Christophe Guinet (CEBC/CNRS, France).

A set of 28 CTD-SRDL datasets is now available in the MEOP-SMS database, with a typical number of 50-100 profiles/day in the highly energetic oceanic regions around the Kerguelen Plateau and across the Argentine shelf break. With such a high sampling frequency, it becomes possible to observe directly the ocean submesoscale variabilibity for extended periods of time. 

A description of the dataset and the post-processing can be found in Siegelman et al., 2019. In this study, an improved post-processing method is presented, useful for both low-resolution satellite-transmitted and full-resolution archived CTD profiles. This post-processing procedure includes the correction of systematic biases, the correction of the thermal-mass effect on conductivity and temperature measurements and the removal of density inversion. A summary of this new procedure is provided HERE

Updated estimates of CTD data accuracy are also proposed, based on detailed comparisons between high- and low-resolution CTD data. For high-resolution data, accuracies are estimated to be of ±0.02°C for temperature and ±0.03 psu for salinity. For low-resolution data, transmitted data points have similar accuracies, however, reconstructed temperature profiles have a reduced accuracy associated with the vertical interpolation of ±0.04°C and a nearly unchanged salinity accuracy of ±0.03 psu (see Siegelman et al., 2019).

Check out the News Elephant Seals Enjoy Fine-Scale Fronts that describes the recent study of Lia Siegelman to find an example of how these unique data can be used to directly observe the submesoscale structure of the Southern Ocean.

Elephant seals enjoy fine-scale fronts

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Lia Siegelman is a graduate student at the University of Western Brittany, France, and currently a visiting student at Caltech/Jet Propulsion Laboratory in the US, studying fine ocean fronts (<50 km), known as submesoscale fronts. Here, she describes recent research that seeks to understand the distribution of these fronts in the under-sampled area of the Southern Ocean as well as their influence for higher trophic levels. 

Advances in theory and modeling of the last two decades suggest that submesoscale fronts are a conspicuous feature of the ocean. In particular, they are thought to play a key role in the transport of heat, carbon and nutrients between the atmosphere and the ocean. Yet, little is known about them in the real ocean as their short lifetime (< week) and small size (<50 km) make them hard to observe directly.

In a recently published study (Siegelman et al. 2019a), we used newly available CTD data collected by a single female southern elephant seal (Mirounga leonina) during its 3-month journey west of the Kerguelen Islands to study submesoscale fronts. The novelty of the dataset resides in its unprecedented high-resolution. Indeed, the tag was set in continuous recording mode, which means that it recorded every single dive realized by the seal – or in our case 6942 dives (see the newly released MEOP-SMS dataset and Siegelman et al. 2019b for more information), allowing us to study submesoscale fronts in this part of the ocean for the first time.  

These high-resolution observations provided evidence that submesoscale features are active in the Southern Ocean during summertime. Submeoscale fronts were principally observed on the edge of a standing meander of the Antarctic Circumpolar Current, accompanied by numerous temperature and salinity intrusions. The data also confirmed theoretical studies in showing that submesosale fronts are mainly generated by the background mesoscale strain field.

Strikingly, the seal increased its foraging activity at the standing meander site by up to 5 times. In particular, the seal spent significantly more time foraging in the vicinity of strong submesoscale features located on the edge of the meander where it also decreased its speed. Despite that most elephant seals target foraging grounds east of the Kerguelen Plateau, our findings suggest that excursions to the west are not accidental, and may be explained by the recurrently elevated physical and biological activity of the site. As such, other standing meanders of the ACC may also act as biological hotspots where trophic interactions are stimulated by submesoscale turbulence.

Reference:

  • Siegelman, L., O’Toole, M., Flexas, M., Rivière, P., & Klein, P. (2019). Submesoscale ocean fronts act as biological hotspot for southern elephant seal. Scientific reports9(1), 5588.
  • Siegelman, L., Roquet, F., Mensah, V., Rivière, P., Pauthenet, E., Picard, B., & Guinet, C. (2019). Correction and accuracy of high-and low-resolution ctd data from animal-borne instruments. Journal of Atmospheric and Oceanic Technology36(5), 745-760.

Find out more about in the NASA JPL news release

Ringed seals like meltwater plumes

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Alistair Everett is a Research Scientist at the Norwegian Polar Institute interested in ice-ocean interactions at marine-terminating glaciers. Here he describes recent research as part of the TIGRIF project, which seeks to understand the influence of tidewater glaciers on fjord dynamics and marine ecosystems, with particular focus on how these fjords may change in future when the glaciers retreat onto land.

In a recently published study (Everett et al. 2018), we investigated the water properties close to a glacier terminus in Kongsfjorden, northwestern Svalbard using a unique dataset collected by ringed seals (Pusa hispida) instrumented with GPS-CTD-SRDLs in 2012. The dataset had already been used to look at seal behaviour and their response to variations in sea ice (Hamilton et al., 2016). But in this new study the focus was instead on glaciology and the meltwater plume that could be observed very close to the glacier terminus; a place where it remains otherwise very challenging to collect data with boats and conventional CTD instruments.

The SRDLs used in this study (glued onto the hair of the seals) collected over one thousand CTD profiles within our study area during a period of around four months, providing us with a valuable time-series of oceanographic data. We were particularly interested in a number of profiles that showed ‘spikes’ of low temperature and salinity at depths of up to 60 metres. In our study, we showed that these ‘spikes’ were caused by the seals entering plumes of meltwater released from underneath the glacier. This was supported by running a clustering algorithm on the GPS locations returned by the SRDLs, which showed that the seals focused their foraging in plume locations, particularly during periods of high meltwater runoff from the glacier.

The data collected by the seals in Kongsfjorden provide important insights into a region where it has previously been very difficult to collect data. The results highlight the importance of plumes and marine-terminating glaciers for the ecosystems in these fjords. Further measurements within plumes can help us to constrain the volume of subglacial discharge, the associated melt rates and the influence on fjord circulation. In the future, as these glaciers retreat onto land, the plumes will eventually disappear, leading to the loss of this important foraging area for ringed seals and changes to the upwelling and circulation patterns within these fjords.


References:

Everett, A., Kohler, J., Sundfjord, A., Kovacs, K.M., Torsvik, T., Pramanik, A., Boehme, L., Lydersen, C., 2018. Subglacial discharge plume behaviour revealed by CTD-instrumented ringed seals. Sci. Rep. 8 (1), https://doi.org/10.1038/s41598-018-31875-8.

Hamilton, C. D., Lydersen, C., Ims, R. A. & Kovacs, K. M. 2016. Coastal habitat use by ringed seals Pusa hispida following a regional sea-ice collapse: importance of glacial refugia in a changing Arctic. Mar. Ecol. Prog. Ser. 545, 261–277, https://doi.org/10.3354/meps11598.    

Amundsen Sea data and Northern Seas data released in the public domain

A new release of the MEOP-CTD database is available publicly (version MEOP-CTD_2018-04-10) that now includes :

  • the deployment ct104 (M. Fedak, SMRU), obtained as part of the iSTAR A Programme (PI: K. Heywood, UEA)


  • A large number of deployments on hooded seals and harbour seals carried outover the previous decade by researchers at the Norwegian Polar Institute (PI: Kit Kovacs and Christian Lydersen).

Note also that the list of publication has been updated with more than 10 publications in 2017 and already 5 publications in 2018.

New release of the MEOP-CTD database

A new release of the MEOP-CTD database is now available. In this new release, about 80 CTD-SRDL tags have been added.

The data processing has been improved, with in particular two additional steps ensuring a better accuracy of salinity data:

  • A thermal cell effect correction has been applied on the entire database. Details of the method can be found in the following submitted manuscript:
    Mensah, V., Roquet, F., Picard, B., Pauthenet, E., Guinet, C., 2017.  A correction methodology for the thermal mass induced-errors of CTD tags mounted on marine mammals. In review in the Journal of Atmospheric and Oceanic Technologies. [PDF]
  • A density inversion removal algorithm is also applied, which seeks the minimum adjustment on the salinity profile to achieve neutral stability. The method is described in: Barker, P. M. and McDougall, T. J., 2017. Stabilizing Hydrographic Profiles with Minimal Change to the Water Masses. Journal of Atmospheric and Oceanic Technology, 34:1935-1945. doi: 10.1175/JTECH-D-16-0111.1

New data formats are also proposed, which hopefully will facilitate the use of our database by the largest number. In particular, a netCDF file version of data with profiles interpolated on a regular vertical grid is now available. Furthermore, the csv format is now proposed on a regular vertical grid as well.

If you want to learn more about the MEOP consortium, please read the recently published paper “Marine Mammals Exploring the Oceans Pole to Pole: A Review of the MEOP consortium” by Anne Treasure et al. published in Oceanography.

And don’t hesitate to send us feedbacks on your use of the MEOP-CTD database !!

Finally, you can follow the MEOP project on ResearchGate if you want to be the first to hear about the latest news.

Fabien and the MEOP consortium

The MEOP review is out in Oceanography

I am pleased to announce the publication of "Marine Mammals Exploring the Oceans Pole to Pole: A Review of the MEOP Consortium" by Anne Treasure et al. Marine Mammals Exploring the Oceans Pole to Pole: A Review o...

Another short note gives an overview of the current state of ocean observations using tagged animals: Ocean Observations Using Tagged Animals by Roquet et al.

These papers are part of the Special Issue on Autonomous and Lagrangian Platforms and Sensors (ALPS) in Oceanography, following the meeting ALPS (https://alps-ocean.us/) that took place in San Diego last February and was aimed at developing a broad vision for ALPS technology, capabilities, infrastructure, and user base in the next decade, and in the coming decades.

Note that this special issue also include the following papers on new exciting technologies of great potential for bio-logging:

      Do Southern Elephant Seals Behave Like Weather Buoys? by Cazau et al.

      A New Technology for Continuous Long-Range Tracking of Fish  by Rossby et al.

Antarctic predators unravel the mysteries of the Weddell Sea

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Sara Labrousse is Doctor in Marine and Antarctic Science interested in the foraging ecology of southern elephant seals (Mirounga leonina) in relation to physiography, hydrological factors, and sea ice condition. She tells us here about her recent experience in the Weddell Sea, where she deployed CTD-SRDL tags on Weddell seals as part of an oceanographic cruise led by Jean-Baptiste Sallée (CNRS researcher at LOCEAN in Paris) on board the R/V James Clark Ross.  [French link: http://phoques-antarctique-wapiti-2017.blogspot.se/]  


In January-March 2017, I was involved in an oceanographic cruise led by Jean-Baptiste Sallée (CNRS researcher at LOCEAN in Paris) in the Weddell Sea aboard the British Antarctic Survey vessel, the James Clark Ross. 

The Weddell Sea is a unique region responsible for 50 - 70% of the formation of deep water in Antarctica, motor of the global thermohaline circulation regulating the Earth's climate. The Weddell Sea also harbors rich ecosystems associated with the seasonal sea ice dynamics (krill, copepods, fish, birds and marine mammals). However, this sector remains under-sampled and little is known about the oceanographic and biological processes taking place in this region due to compact and extended sea ice making the access difficult even in summer. The cruise represented a unique opportunity to study the ecology of marine top predators by equipping them with satellite-relayed data loggers, in a region where little is known on their movements, foraging activity and their preferences in terms of oceanographic conditions.    

The cruise was part of the WAPITI project (Water-mass transformation and Pathways In The Weddell Sea: uncovering the dynamics of a global climate chokepoint from In-situ measurements) funded by the European Research Council (ERC). WAPITI aims to understand how and where water-masses are pre-conditioned and transported on the Antarctic shelf, before coming into contact with the ice shelves: an highly sensitive process ultimately controlling the rate of melt of the Earth’s largest ice reservoir. Associated with this cruise, we did opportunistic deployment from the R/V James Clark Ross of five Argos tags coupled with diving recorder and a CTD miniaturized sensor on one emblematic specie of pinnipeds, major consumer of the ecosystem of the Antarctic sea ice the Weddell seals (Leptonychotes weddellii). 

Such deployment coordinated within the framework of the physics-based WAPTITI will allow unique interdisciplinary research: (i) acquiring essential information on the feeding ecology of Antarctic seals, where only few studies are available, and (ii) acquiring a vast amount of information on the regional water-mass seasonal cycle and circulation acquired from instruments deployed as part of the WAPITI project, and valuable hydrological data under sea ice in late summer, autumn and winter from the CTD profiles recorded during dives of this predator. The study of the seal diving behavior under sea ice combined with simultaneously recorded hydrographic profiles will first highlight preferred feeding habitat in terms of oceanographic conditions. Secondly, this telemetry monitoring will help to quantify the role of hydrological and sea ice conditions on the abundance and availability of resources in the peri-Antarctic shelf, improving our still very patchy knowledge on ecosystem functioning under winter sea ice. 

Acknowledgements: Jean-Baptiste Sallée (ERC), Jean-Benoit Charrassin (CNES), Clive McMahon & Robert Harcourt (IMOS), and all the WAPITI TEAM;


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