ARGO FLOAT TECHNOLOGY: ACHIEVEMENTS AND CHALLENGES Stephen C. Riser University of Washington

A short history of float technology: Drift bottles…ancient Swallow floats...the mid-1950s (Aires expedition) SOFAR (Rossby and Webb, 1970s) (MODE/POLYMODE) RAFOS (Rossby, 1980s) ALACE (Davis and Webb, 1990s; WOCE); Marvor (MARTEC, 1990s) (vi) PALACE (Davis, Webb, others, 1990s) (vii) APEX, SOLO, Provor, Ninja,…. (Argo, etc.)

Argo has defined a new set of float specifications: Long lifetime (~ 4-5 years; ~ 150-200 profiles) Acceptable losses (  10%) (ii) CTD stability (unattended over 4-5 years) Temperature ~ 0.001 C Pressure ~ 1 decibar Salinity ~ 0.01 (PSS-78) (iii) Diverse deployment methods (research vessels, VOS, aircraft) (iv) Profiles to  1000 m, preferably to 2000 m These requirements are unprecedented.

Float lifetime: 4-5 years, 180-200 profiles for Argo Argo deployments began in 2000, so there are no Argo floats that have yet met this requirement. Profiling floats deployed as part of non-Argo programs have shown that this specification is achievable. Example: floats deployed in the Japan/East Sea

 8/8/99  11/7/03 UW Float 230, deployed 2 August 1999 201 profiles as of 11/7/03 t = 7 days Parking depth 800 m Duration now > 4 years Russia  8/8/99  11/7/03 Japan/East Sea Japan [several others similar]

CTD sensor stability: UW Float 63 (SBE-41 CTD)  Deployed 8/24/97 Recovered 8/31/00 (91 profiles) Recalibrated at SBE Sensor stability over 3 years: T ~ 0.001 C p ~ 0.5 dbar S ~ 0.003 (PSS-78) [Other floats show similar results]

CTD sensor stability: UW Float 230 (SBE-41 CTD) Historical data The T/S properties of the Japan/East Sea are remarkably constant below 300 m, and this is reflected in the stability of the sensors on Float 230.

Float deployment methods: Research vessels Aircraft VOS

Argo floats have been deployed using all 3 methods:  Air RV VOS

Float parking/profiling depths: There are tradeoffs between deeper operation and energy consumption. Deeper operation allows for better sensor calibration; shallower operation uses less energy in pumping (E ~ p2). Presently all Argo floats can operate to 1000 m. The Argo Science Team has suggested that whenever feasible, floats should operate to a depth of 2000 m.

Float parking depths: Increasing the operating depth range means increasing the relative volume displacement. Displacement Equation V (ml) required for 0-2000 m operation for Webb APEX; ~ 1% of V0 .

Presently Argo floats are capable of 2000 m operation over more than 90% of the world ocean.

How reliable is the present generation of floats?  Float reliability (present UW statistics for Argo) [Note: presently the maximum duration is 3 years]

Conclusion: the specifications put forth in the early plans for Argo are generally being met. SOLO (cutaway) APEX

Technical advances: improvements to float technology Communications New sensors

  Improved float communcations: Iridium Iridium/GPS antenna 11/8/03   1/15/03 Track of Iridium APEX drifter

Iridium data transmission characteristics: Note: upload data rate ~ 220 bps (1760 baud); compare to Argos (~0.1 baud). A 20 kb file requires ~ 5 minutes to transmit (presently  12 hours are required for a < 1 kb file using Argos). Cost is $1 per minute. 2-way communication works well.

New sensors: Optical sensors (fluorometers; backscatter; etc.) See Bishop et al. (Nature, 2002) for an example. (ii) Dissolved oxygen (at least 2 types) (iii) Acoustic sensors (wind, rainfall, etc.)

New sensors: dissolved oxygen In a SBE/UW collaboration, a dissolved oxygen sensor for use on profiling floats has been developed and tested. The sensor is a Clark polarographic-type sensor similar to the present generation of SBE oxygen sensors on shipboard systems.

Dissolved oxygen:

UW Float 894 Operating at the HOTS site near Hawaii since 8/2002. Now in operation for over 1 year, 41 profiles. Dissolved O2 measured using SBE-43 sensor in conjunction with a SBE-41 CTD unit (Clark cell). Drift over one year is minimal (~0.03 ml/l). Results are promising; some other examples are not so good. Float 894 8/27/02 – 11/7/03 41 profiles [see N. Larson poster for details] [other O2 sensors are also under development]

Acoustic sensors on profiling floats: FLOAT ENDCAP AND CTD WITH BROADBAND HYDROPHONE Dr. Jeffrey Nystuen’s group at the UW Applied Physics Laboratory has designed and constructed an Acoustic Rain Gauge suitable for use on profilng floats. First deployment: Eastern N. Pacific, along the N. American coast, from the TG Thompson in November 2003.

The acoustic rain gauge: Rainfall and wind speed can be measured acoustically using the spectrum of ambient acoustic noise in the ocean. The results at right are from a TAO mooring in the equatorial Pacific (Nystuen, 1999). At moderate wind speeds there is good agreement between wind measured from an anemometer and winds inferred acoustically. [a float can make similar measurements while submerged]

Other new sensors: Addition of RAFOS capability for continuous tracking Biological sensors (nutrients, etc.) ??? The main limitations to adding new sensors are weight and power requirements. Floats provide an excellent platform for measuring many variables.

The future of Argo: What will the next generation of Argo look like? How and when will Argo become “operational”? Floats are complex devices that have great potential for revlutionizing our understanding of ocean circulation. This complexity will require the attention of the research community for many years to come.

A profiling float: SOLO schematic SBE-41 schematic

An XBT:

Conclusion: the evolution of Argo  Argo floats are relatively complex systems; their present reliability is reasonably good. Improving reliability is not simple since floats are generally not recovered.  Argo floats and profiling floats in general will become excellent platforms for making high quality measurements of a number of important oceanographic variables beyond the standard physical parameters.  Many groups in universities and public and private laboratories are now working to expand the capabilities of profiling floats. The float technologies and sensors available 5 years from now will possibly be considerably different from what we have now.  The path to operational capability is likely to be considerably different for Argo than for previous programs (ie, XBT ships of opportunity) due to the complexity of the instrument and the continuous research and development. There will be a need to keep the research community involved in Argo for the foreseeable future.