Towards an Hybrid Antarctic Float Observation System (HAFOS)

Olaf Klatt, Olaf Boebel, Ismael Núñez-Riboni, and Eberhard Fahrbach

Alfred-Wegener-Institut
Bussestrasse 24
D-27570 Bremerhaven
Germany

The ARGO system is expected to become the backbone of a global ocean observing system. However, it can not be easily extended into polar oceans, since the floats have to return to the sea surface to be located and to transmit the measured data. Since location and data transmission under the ice is only possible by acoustic means, an observation system in the deep polar oceans requires the combination of different technologies. The envisioned system comprises ice avoiding profiling subsurface floats which are acoustically ranged by RAFOS technology. In a first step they transmit the recorded data only when they are able to reach open water in real-time, in a second phase, they provide delayed mode data transfer during the ice-free season. Acoustic real-time data transmission is planned as final step. Here we report on our efforts to develop an acoustically positioned, ice-sensing floats featuring delayed mode data transfer (iStore).

Since 1999 92 autonomous, profiling floats were deployed in the Weddell gyre area. Since 2001 these float have been registered with ARGO, forming a contribution of 61 floats in total. Since 2002, floats use the Ice Sensing Algorithm (ISA), which determines likeliness of ice coverage based on upper ocean temperatures. Of 18 ISA equipped floats 14 resurfaced after the first winter season while 11 of 13 resurfaced after the first and the second winter seasons. All of those surviving the first winter survived the second winter as well. A total of 20 floats are equipped with RAFOS receivers. Sound coverage for the RAFOS navigation system is provided by 3 sound sources installed since 2003 (2 refurbished in 2005) and 3 sources installed in 2005. Sound coverage varies according to season, location and sound source manufacturer, though a value of 400 km can be safely assumed for planning purposes.