Circumpolar EKE/tides from available mooring records (Deliverable D1.2)

Ocean Cryosphere Exchanges in Antarctica: Impacts on Climate and the Earth system Circumpolar EKE/tides from available mooring records Deliverable D1.2 UK Partners are funded by UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding Guarantee. https://ocean-ice.eu/ OCEAN ICE – GA 101060452 Deliverable D1.2 2 About this document Deliverable: D1.2 Circumpolar EKE/tides from available mooring records. Work Package: WP1 Subpolar circulation, heat delivery and water mass Export, Task 1.1: Dynamic Processes Type of document: Report Lead beneficiary and author: PP13-United Kingdom Research and Innovation - British Antarctic Survey (UKRI-BAS): Pierre Dutrieux Contributors (Researchers involved in the deliverable): PP2-Norwegian Research Centre (NORCE): Svein Østerhus PP3-Alfred-Wegener-Institut Helmholtz-Zentrum Fur PolarUnd Meeresforschung (AWI): Markus Janout PP10University of Gothenburg (UGOT): Anna Wåhlin PP11Norwegian Polar Institute (NPI): Tore Hattermann PP13-United Kingdom Research and Innovation - British Antarctic Survey (UKRI-BAS): Pierre Dutrieux, Shenjie Zhou, Povl Abrahamsen, Mike Meredith, Peter Davis, Keith Nicholls PP15-University of Southampton (UoS): Alessandro Silvano, Kathryn Gunn Review: PP1: Danish Meteorological Institute (DMI): Ruth Mottram (ru[email protected]), Chiara Bearzotti ([email protected]) Disclaimer: Funded by the European Union and UK Research and Innovation. Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or UK Research and Innovation. Neither the European Union nor UK Research and Innovation can be held responsible for them. OCEAN ICE – GA 101060452 Deliverable D1.2 3 Table of contents 1.Publishable summary 4 2. Work performed and main achievements 4 2.1 Standardisation of the mooring timeseries 5 2.2 Tidal fit and data filtering method 5 3. Results 6 3.1 Spectral characteristics 6 3.2 Spatial distributions 6 3.2 Direction for future work 9 References 9 4. Impacts 10 OCEAN ICE – GA 101060452 Deliverable D1.2 4 1.Publishable summary In this deliverable, we present the first version of a series of historical moored timeseries that have been deployed over the past 50 years over Antarctic continental shelves and slopes and provide an overview of energetic characteristics of ocean circulation on and off the continental shelves. The first standardized data collection of moored hydrography and current velocities records we present is restricted to the region south of 60°S (Figure 1) and is freely accessible from SEANOE (https://www.seanoe.org/data/00887/99922/), and we hope to complement it with additional records and maintain in the future. We refer to this data compilation as the OCEAN ICE mooring compilation herein. This dataset provides an opportunity for a systematic study on the pan-Antarctic water mass transport and shelf connectivity, a task made challenging by the typically multi-source and multiformat nature of these records. Perhaps unsurprisingly, spectral analysis of the compiled current velocity timeseries demonstrates the dominating presence of tidal variability within most records. This component of the variability is fitted using multi-linear regression to tidal frequencies, and the tidal fit is removed from the original timeseries to leave tide-filtered variability. Keeping in mind that records are limited to months to years in duration, the latter is predominantly composed of synoptic (3-10 days period), intraseasonal (10-80 days) and seasonal (~6 months-1 year) variability. The spatial distribution of the kinetic energy integrated within each frequency band (tidal and non-tidal) is presented and discussed. Fig. 1: A comparison between OCEAN ICE moored timeseries compilation and SOOS map metadata information. Each square in the map stands for a record in either SOOS map or OCEAN ICE compilation. The red-filled squares are those contained in OCEAN ICE compilation. Squares with orange crosses are valid sites in SOOS map (with status of recovered or deployed). The overlapping sites across OCEAN ICE compilation and SOOS map are then denoted as a red square with orange cross on top. The blue cross are those sites in SOOS map that are shown as either planned, failed or unknown status. 2. Work performed and main achievements The purpose of this deliverable is to assemble the pan-Antarctic mooring records and retrieve an overview on the Eddy Kinetic Energy (hereafter defined as ocean current energy with annual of shorter periodicity) and tidal energy distribution directly from the in-situ observation to help us understand OCEAN ICE – GA 101060452 Deliverable D1.2 5 the relative role of the dynamic processes contributed to the kinetic energy in the circulation on and off the shelf. 2.1 Standardisation of the mooring timeseries In this section, we describe the standardisation performed on mooring timeseries that we assembled from various data centres and individual data owner. In the OCEAN ICE mooring compilation, we collected over 500 mooring timeseries (including temperature, pressure, salinity, velocity) including those being turned over on a regular basis on the same mooring sites (Figure 1). The comparison with SOOS mooring map shows that our compilation has overall a better coverage of mooring records in front of the Ross Ice Shelf and through different ice shelves boreholes in Ross Ice Shelf and Amery Ice Shelf. To these, records from Antarctic Slope Current and Antarctic Bottom Water transport over the slope current from places such as southeastern Weddell Sea (Heywood et al. 2012), Princess Elizabeth Trough (Heywood et al. 1996) and Australian-Antarctic Basin (Peña-Molino et al. 2016), not yet present within SOOS map, were added. The mooring timeseries are acquired from various sources. Some of them are archived in public database such as Pangaea Data repository (Germany), British Oceanographic Data Centre (UK), Polar Data Centre (UK), US Antarctic Program Data Centre (USA), Australian Antarctic Data Centre (AU) and Korean Polar Data Centre (Korean). Others are sporadically stored in places that are less commonly considered as Antarctic mooring data centres such as NCEI/NOAA, or local database developed by individual institute such as Lamont-Doherty Earth Observatory or Oregon State University. We retained the original temporal resolution of all the mooring records – meaning the highest temporal resolution possible, and renamed variables following a consistent variable name structure throughout the database. Some rudimentary data clean up was applied on velocity timeseries, replacing bad data points with NaN. The brushed timeseries are then grouped by mooring sites – if one mooring string contains multiple instruments and sensors, the recorded timeseries regardless of variables, are stored in single NetCDF file, including all hydrographic (temperature, salinity, pressure) and current velocity measurements wherever available. The resulting timeseries are available in SEANOE (https://www.seanoe.org/data/00887/99922/). We note that the database contains current velocity records obtained from point current meters and moored Acoustic Doppler Current Profilers (ADCP). In the remainder, we average ADCP timeseries with depth, providing us with one single timeseries per instrument so it can be more readily compared to the more prevalent point observations. Finally, it is interesting to note that most of the instrumented current meters are situated near the seabed to avoid iceberg collisions. 2.2 Tidal fit and data filtering method In the following, we proceed with analysing and discussing the frequency content and spatial distribution of the kinetic energy contained within the compiled records. We first isolate tidal components of the variability from individual records. We use UTide (reference here) to fit all tidal components in the variability in the original records, providing us with one tidal harmonics fit for each record. That fit is then removed from the original, giving us a third, ‘filtered’ timeseries for each record. OCEAN ICE – GA 101060452 Deliverable D1.2 6 3. Results 3.1 Spectral characteristics Fig. 2: The spectral PDF of a original velocity timeseries, b tide-free velocity timeseries and c the tide harmonics. We now have an overall estimation of the tidal and eddy kinetic energy across the Antarctic continental shelves from the mooring records. Figure 2 shows the probabilistic distribution function (PDF) plot of the current speed spectra from all the mooring sites where current meters or current profilers were instrumented. Figure 2a shows the original spectra PDF. The latter is predominantly characterized by a classic red spectrum, with pronounced tidal energy in the semi-diurnal, diurnal and fortnight frequency bands, but also showing peaks for higher and lower frequency tidal harmonics. The spectra pdf of the tidal harmonics fit is shown in figure 2c and indeed highlights the presence of the numerous tidal harmonics and their elevated energy levels. Tide-free spectra (figure 2b) show a smoother red form, with an overlay of relatively elevated energy peaks around the semi-diurnal frequency range and a smaller energy bump around synoptic timescales (3-10 days periodicity). 3.2 Spatial distributions We further estimate the spatial distribution of EKE by integrating spectra all three sets of timeseries (original, tidal harmonics, filtered) for each record over a set of frequency ranges, namely the semidiurnal (0.5+/-0.1 day period), diurnal (1+/-0.1 day), fortnightly, synoptic (3 to 10 days), intraseasonal (10-80 days) and seasonal (80 days-1.2 tears) timescales. OCEAN ICE – GA 101060452 Deliverable D1.2 7 An example of the result is shown for the diurnal range in figure 3. Original timeseries (Figure 3a) in fact contain a range of EKE with close to diurnal periodicity. Most of this energy cleanly corresponds to the exact tidal harmonics (Figure 3c). In general, most of the EKE with diurnal (Figure 3 b-c), semidiurnal (Figure 4 a-b) and fortnight (Figure 4 c-d) periodicity is indeed driven by the tide, and the filtered energy on these three frequency bands is much lower than the original and tidal harmonic estimations. Fig. 3: Diurnal frequency energy estimated from a original timeseries, b tide-free timeseries and c tide-only timeseries. However, we note that some of the filtered records still retain high energy levels within the diurnal and semi-diurnal range. This property is particularly pronounced in regions of dense shelf water outflows out of the Ross Sea, the Cosmonaut Sea (off Amery ice shelf) and the Terre Adélie sea. The ice front of the Ross and Filchner Ronne ice shelves and the entrance of the Filchner trough also shows elevated levels of diurnal and/or semi-diurnal variability in filtered timeseries. The presence of diurnal variability within the filtered records may results from other sources of variability, e.g. within the inertial range, and/or dispersion of tidal energy around the exact tidal harmonic via mixing processes or spectral diffusion. The fact that there is little EKE in the filtered records at fortnightly periodicity may indicate a generally low level of spectral diffusion, but this factor is frequency dependent. OCEAN ICE – GA 101060452 Deliverable D1.2 8 Fig.4: Tide-free (filtered) and Tide-only (tide harmonics) energy distribution on semi-diurnal and fortnightly frequency bands. At lower frequency, EKE tends to be contained within three distinct bands (Figure 5). The synoptic (310 days) band shows elevated energy levels along most of the Antarctic continental shelf break (Figure 5a). Inshore, and along glacier fronts, an notable energy distribution pattern emerges, where regions corresponding to relatively high glacial melt, namely the Amundsen and Bellingshausen seas and the Totten and Denman glacier fronts all show relatively low energy level, contrasting with higher energy levels of cold regimes characterizing the Ross, Filchner-Ronne and Amery ice shelves. One reason for this difference may be the heightened sensitivity of cold regions to synoptic atmospheric variability and the associated response of the ocean surface to sea ice processes, whilst warmer regimes tend to be relatively more stratified and the lower part of the water column more insulated from surface variability. More work is needed to confirm this hypothesis. Another source of synoptic timescale energy are short coastal waves, excited by the atmosphere or resulting from local flow instability (Chavane et al 2010). A similar regional pattern of variability tends to hold for longer, intraseasonal periodicity (Figure 5b). Cross correlations with atmospheric variables is out of scope for this brief presentation, but may help elucidate the processes driving EKE within this frequency bands. Finally, we present EKE within the seasonal range (Figure 5c). The same spatial distribution pattern, with enhanced EKE levels in cold regimes shelf seas, holds. This perhaps further highlights the influence of sea ice production and associated variability on shelf sea dynamics, as we expect cold regimes to host deep convection events, whilst convection is mostly suppressed in warm regimes. OCEAN ICE – GA 101060452 Deliverable D1.2 9 Fig. 5: Tide-free (filtered) kinetic energy over a synoptic, b intra-seasonal and c seasonal timescales as an indication of energy distribution over the typical eddy timescales. 3.2 Direction for future work Our brief analysis suggest that current energetics around Antarctica are mostly dominated by the tidal component, especially in continental shelf seas. A more detailed comparison of our tidal harmonics with tidal models predictions such as those of CAT2008 (Padman 2002) could be useful to extract additional information on the either the model validity or the source of the variability itself. We find an interesting distinction between warm and cold ice shelf regimes in terms of EKE. This ought to be the subject to a more detailed analysis in the future to elucidate the reasons behind such a distinction, and the repercussions on local circulation and mixing. References Chavanne, C.P., K.J. Heywood, K.W. Nicholls, I. Fer (2010), Observations of the Antarctic Slope Undercurrent in the southeastern Weddell Sea, Geophys. Res. Lett., 37, L13601. Heywood, K.J., M. Sparrow, J. Brown and R.R. Dickson (1996), Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough, Antarctica, Deep-Sea Res. Pt. I, 46, 1181-1200. Heywood, K.J., R. Muench and G. Williams (2012), An Overview of the Synoptic Antarctic Shelf-Slope Interactions (SASSI) project for the International Polar Year, Ocean Sci., 8, 1111-1116, 2012. Padman, L., H. A. Fricker, R. Coleman, S. Howard, and S. Erofeeva (2002), A new tidal model for the Antarctic ice shelves and seas, Ann. Glaciol., 34, 247-254. Peña-Molino, B., M.S. McCartney, S.R. Rintoul (2016), Direct observations of the Antarctic Slope Current transport at 113°E, J. Geophys. Res. Oceans, 121, 7390-7407 Zhou et al. The OCEAN ICE mooring compilation: a standardised, pan-Antarctic database of ocean hydrography and current time series, Earth Syst. Sci. Data, 28 Oct 2025, DOI: https://doi.org/10.5194/essd-17-5693-2025