Marine Mammals Exploring the Oceans Pole to Pole

Yixi Zheng is a PhD student working on physical oceanography at the Centre for Ocean and Atmospheric Sciences, University of East Anglia, UK. She is interested in the interactions between sea ice, ice shelves and ocean in Antarctic continental shelf seas, and how they affect ice-shelf melting and our future climate through upper-ocean processes. In a recent Nature communication paper, she used MEOP data, combined with ship-based observations to reveal the wintertime meltwater distribution in front of Pine Island Glacier for the very first time. They show that the wintertime meltwater surfaces and provides near-surface heat that helps to maintain polynyas close to ice shelves.

Pine Island Glacier is melting rapidly and exporting glacial meltwater into the ocean. The glacial meltwater is thought to play a role in hydrography and sea ice distribution but it is poorly observed and its pathways poorly known due to the sparse observations, especially in winter. In this study we turn to MEOP observations to determine the seasonal distribution of near-surface meltwater exiting from beneath Pine Island Glacier in winter. In winter, the basal meltwater is much warmer (up to about 1 ºC above freezing) than other upper-ocean water masses which have temperatures near the freezing point (about -1.9 ºC). In this way we unambiguously distinguish ice shelf basal meltwater from the ambient water masses. 

Here we present, for the very first time, a set of wintertime full-depth measurements of 625 salinity and temperature profiles collected by sensors attached onto three seals. Our wintertime observations reveal clear signals of meltwater both at depth and near surface, connected by distinct meltwater-rich columns, while much of water above 450 m remains meltwater-poor. This spatial heterogeneity is in contrast to the relatively high and horizontally-uniform upper-ocean meltwater content indicated by summertime measurements. 

We argue that in winter, compared with the cold and dense ambient water, the meltwater-rich water has sufficient buoyancy to rise to near surface without undergoing intense lateral mixing. The winter processes revealed by our study are likely important for bringing nutrients to the near surface layer prior to the spring bloom, and for bringing heat to the surface to prevent sea ice from forming and thus maintaining the polynyas in front of the ice shelves.

Map of the study region. Positions of the seal-tag hydrographic profiles collected in winter (July to September) 2014 are indicated by solid dots and diamonds coloured by the conservative temperature, , above freezing at 2 dbar (upper colour scale in red and blue).

Fig. 2 Schematic representation of the meltwater pathways in winter and summer. a, Wintertime meltwater either spreads along pycnocline or rises through uniform water layers without undergoing intense lateral mixing. Wintertime meltwater rising to near surface melts sea ice and forms polynyas. b, Summertime meltwater spreads along pycnocline as well. However, in contrast to wintertime meltwater, summertime rising meltwater penetrating through stratified water mixes with ambient water intensively and spreads widely.

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.

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.

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.