Drifting Buoys

For several years, oceanographers and meteorologists have deployed satellite-tracked drifting buoys in support of their research and operational programmes. These two bodies of users have, however, sought different capabilities from their drifters: the oceanographers have mainly looked for designs which accurately follow water parcels at a given depth, whereas the meteorologists have equipped their drifters with air-pressure sensors to collect real-time observations for weather forecasting. Despite efforts by both user groups to develop combined programmes, these two main requirements have been largely incompatible, particularly in respect to the size and above-surface exposure of the drifter. The success of the low-cost WOCE Surface Velocity Programme (SVP) oceanographic drifter, with its accurately quantified water following characteristics and proven longevity, prompted renewed interest in the development of a low cost met-ocean drifter capable of satisfying the needs of both user communities. The result is the SVP Barometer (SVP-B) drifter, whose design and use is described in the DBCP Report (Sybrandy et al. 2009). This design, refined over several years and after extensive testing, further develops the original SVP drifter by inclusion of a novel barometer port. This inexpensive but stable pressure sensor combined with a data filtering algorithm removes pressure spikes resulting from the repeated immersion of the drifter by waves.

Drifting buoys normally measure sea surface temperature (SST) and air pressure, and by tracking their positions the surface currents (resultant current arising from Ekman and geostrophic) can be determined. Some drifters also have sensors to measure wind, temperature profile and salinity. The buoys are battery powered and typically last for one to two years. The buoys are disposable and can be deployed at sea by regular ship crews. Measurements are normally made hourly and the data are transmitted by satellite. Most drifters use the ARGOS satellite system for data transmission and positioning, although new systems such as Iridium are currently being evaluated as a pilot programme. At present, users can access web pages at both ISDM (http://www.meds-sdmm.dfo-mpo.gc.ca/isdm-gdsi/drib-bder/ index-eng.htm) and AOML (http://www.aoml.noaa.gov/phod/dac/gdp.html), where products and data are available. Integrated Science Data Management (ISDM) in Canada became a Responsible National Oceanographic Data Centre (RNODC) for Drifting Buoy Data on behalf of JCOMMOPS. The present status of global drifters as on November 09, 2009 is shown in Fig. 3.3.

Drifting buoys, along with Voluntary Observing Ships, provide the primary source of air pressure data over the oceans that are needed to run global and regional weather forecasting models. The SST data provided by drifting buoys are important for climate data sets. The key application of surface drifter data is reduction of the bias error in satellite SST measurements, mapping large scale surface currents and identifying their role in heat transports and the generation of SST patterns and variability. They are invaluable as an independent validation tools for model and satellite derived currents (ekman+geostrophic), synthesized with altimetry and satellite winds to estimate absolute sea surface height (Niiler et al. 2003; Rio and Hernandez 2003) and used to understand the role of surface transport in the genesis of the El Niño (Picaut et al. 2002; Lagerloef et al. 2003; McPhaden 2004). Maximenko et al.

(2008) compared the drifter-sensemble averaged velocities and a sum of time averaged geostrophic and Ekman currents, and concluded that one drifter per 5 x 5° grid is not adequate to capture/resolve most of the surface current features. Dohan et al.

(2009) describes the data quality from drifters and the principal scientific insights during the last decade. There are numerous direct uses of sea surface velocity, such as for navigation and drift trajectories, advection calculations of ocean properties, spills, and Synthetic Aperture Radar (SAR) operations, etc. Also, drifting buoy data are used for many applications such as to study physical characteristics and climatology of sea ice within the Antarctic sea ice zone. These data are also used for many applications such as to trace the seasonal pathways of freshwater plumes (Sengupta et al. 2006), improving the surface current climatology (Shenoi et al. 1999), etc.

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