HAFOS - The Hybrid Antarctic Float Observing System

Motivation and Background

Figure 1 Schematic of physical processes at work in the Southern Ocean (modified from Williams et al. 2015)

The oceans are key elements of the global climate system, not at last due to their ability to extract and store heat and CO₂ from the atmosphere. Polar oceans are of particular relevance, as it is here where dense waters are formed by cooling and salinification (through formation of sea-ice). The dense water formed in the Weddell Sea makes a major contribution to the Antarctic Bottom Water (AABW), which fills and ventilates most of the deep basins of the world ocean. Recognizing and understanding the underlying mechanisms and potential changes of these processes requires detailed oceanographic measurements at highest precision over long periods (decades) and at high temporal and spatial resolution (hourly at best).

Team

Dr. Olaf Boebel, Scientist
Dr. Mario Hoppema, Scientist
Matthias Monsees, Technician
Dr. Ilse van Opzeeland, Scientist
Stefanie Spiesecke, Technician

Results

Repeat CTD sections

Separated in time by usually years, repeat CTD sections provide spatially highly resolved snapshots of water properties along a given transect through the ocean. This data serve to, e.g., establish the volume of water belonging to a certain water mass and to estimate the amount of heat or gases and dissolved substances such as CO₂ and nutrients stored therein. In Figure 7for example the volume of Antarctic Bottom Water (delineated by a lower black line) steadily decreases while this water mass slowly gets warmer, i.e. takes up heat.

Repeat sections employing high-precision CTDs (with absolute accuracies better than 0.001°C in temperature upheld for now approaching three decades) allow capturing the ever so slightly increase of ocean bottom temperatures as depicted in Figure 8 by one of the longest oceanic time series at the 61°S 00°E, in the northeastern-most extent of the Weddell Gyre, where at least some of the AABW is believed to escape to the North.

Along with (electronic) CTD measurements, water samples are collected throughout the water column to be retrospectively analyzed on board or back in the laboratory for concentrations of various trace gases and carbon constituents. Measurements of Total CO₂ (also known as DIC) in the surface and bottom waters of the Weddell Sea, reveal a steady CO₂ increase of anthropogenic origin, as surface water charged with anthropogenic CO₂ is a major component in the formation of WSBW (Van Heuven et al. 2011). The findings reflect the deep-sea sequestration of anthropogenic CO₂, thus contributing to diminishing the burden of excess CO₂ in the atmosphere. The largest increase of CO₂ is found in the surface layer (Van Heuven et al. 2014). Here, uptake of excess CO₂, i.e. anthropogenic CO₂, from the atmosphere has obviously occurred. Somewhat surprisingly, steady state tracer oxygen shows a decreasing trend in the WSBW. This confirms that the Weddell Gyre circulation is not in steady state, as also detected in potential temperature and salinity data collected at AWI (Fahrbach et al. 2011). A decrease of oxygen in the WSBW may be caused by a changing composition of the water mass, where the contribution of the one component surface water is decreasing compared to that of the other component (Modified) Warm Deep Water.

Fig. 7: Ocean temperatures for multiple years along the Greenwich meridian from North (left) to South (right). Vertical extent from the sea surface (top) to the sea floor (white).

Fig. 8: Ocean temperatures near the ocean bottom collected by repeat CTD a surveys, comprising 14 visits over a period of more than two decades.

Figure 9: Time series (1973-2010) of normalized Total CO2 (NCT) and Oxygen within the Weddell Sea at the Prime Meridian. Displayed are linear trends in the Antarctic Surface Water (top) and Weddell Sea Bottom Water (WSBW, bottom). Statistically significa

Mooring-borne hydrographic recorders

While the repeat CTD sections lack appropriate temporal resolution to understand daily or seasonal fluctuations in the data (and hence bear the risk of a single realization representing a complete outlier), mooring borne hydrographic-long term recorders collect hourly measurements, albeit at only few selected locations. Figure 10 shows the temperature record for one such recorder at 59°S 0°E, revealing a significant change in the coldest AABW temperatures between 2008 and 2011.

Fig. 10: Near bottom ocean temperatures at recorded at 59°S 0°E.

Argo Floats

With moorings and repeat CTD sections being constrained to a limited number of locations, one wonders how representative the observations made there are for the remainder of the sea. Argo floats are aptly suited to resolve this problem, as, once released, they drift freely through the ocean with its deep currents. In the Weddell Sea, a steady clockwise circulation, the Weddell Gyre, is particularly helpful as any float deployed in the East, will travel with time into the inner Weddell Sea to emerge – years later – from under the ice near the Antarctic Peninsula (Figure 11)

Along their tracks they collect profiles of temperature and salinity between 2000m depth and the sea surface, revealing the annual cycle of water property modifications through freezing and melting in the upper 200m of the water column (Figure 12).

Figure 12 Time series of upper ocean temperatures recorded along the trajectory of a single float. Note the near-surface warming event in austral summer 2016/17 (Graphic: Alfred-Wegener-Institut)

Mapping the temperature and salinity profiles of many floats to specific surfaces (e.g. along 1000m depth, or, as in Figure 13, along the deep vertical temperature maximum, reveals the overall circulation patterns at depth, e.g. the inflow of warmer water from the East (the yellow turning to green/cyan core south of 65°S) into the Weddell Gyre and northerly gyre core featuring the coldest waters.

Figure 13 Conservative temperature (◦C) at the sub-surface temperature maximum for the entire time period from objective mapping of float data. (From Reeve et al., 2016) (Graphic: Alfred-Wegener-Institut)

More advanced analysis techniques even allow quantities estimates the transports associated with this circulation pattern (Figure 14) and confirm the double gyre structure as proposed by numerical models of the ocean circulation.

Figure 14 (Graphic: Alfred-Wegener-Institut)

Passive acoustic recorders

With the mooring infrastructure in place for oceanographic studies, addition of passive acoustic recorders to the mooring set up allows monitoring the ocean for abiotic and biotic sound.

Acoustic recordings reveal both impacts of biotic and abiotic sources on the polar acoustic environment. Figure 15 shows how frequently certain sound levels (y-axis) are attained for a given frequency. The observed bi-model structure (two yellow/red bands) result from different acoustic conditions in summer (open ocean) and winter time (ice covered ocean). Peaks result from choruses of baleen whale.

Figure 15 Spectral statistics at 66° S. Colour shows empirical spectral probability density as a function of frequency and spectral levels, the black solid lines the 5th, 50th and 95th percentile spectral levels and the white lines the average spectra d (Graphic: Alfred-Wegener-Institut)

Figure 16, showing the intensity of sound at a given frequency and day, reveals the impact of marine mammal choruses even more clearly. The acoustic energy from fin, Antarctic minke and Antarctic Blue whale calls as well as leopard seal clearly stand out against the acoustic background.