GLOBAL DRIFTER PROGRAM

Peter Niiler (SIO)

Link to NOAA Strategic Plan: NOAA's Mission Goal 2: Understand Climate Variability and Change to Enhance Society's Ability to Plan and Respond

RESEARCH OBJECTIVES AND SPECIFIC PLAN TO ACHIEVE THEM

The “Global Drifter Program” (GDP) is the principal international component of the JCOMM “Global Surface Drifting Buoy Array.” It is a Scientific Project of the DBCP of WMO/IOC. It is a near-operational ocean-observing network of drifters that, through the ARGOS satellite system, returns data on ocean near-surface currents, SST and atmospheric pressure (and winds, subsurface T(z) and SSS) and provides a data processing system for scientific utilization of these data.

The scientific objectives of the GDP, and its operational and research partners, are to:

• Provide to GTS an operational, near-real-time data stream of SST, sea level pressure and surface velocity: 1250 drifters distributed globally is the presently desired array size.

• Observe the mixed layer velocity on a global basis with 0.5° resolution and, jointly with satellite altimeter data, produce new charts on the seasonal and interannual changing circulation of the world ocean at 0.5º resolution.

• Develop and introduce into the drifter construction technological advances in sensors, electronics, power, methods of assembly and deployment packaging, with special emphasis on sensors and aerial deployment methods for hurricane observations.

• Provide enhanced research quality data sets of ocean circulation that include drifter data from individual research programs, historical data from instruments different from the SVP Lagrangian Drifter and the corrected data sets for wind-produced slip of drifter velocity. The validation of OGCMs is the current major use of the global drifter velocity data sets.

RESEARCH ACCOMPLISHMENTS

Since September 2005 the fully implemented global drifter array has consisted of over 1250 drifters. This ‘required’ global drifter array size by JCOMM is based on the need to return instrumental observations of daily average SST (+/- 0.10C) over the global ocean at a 5º resolution, or the spatial scale of the error covariance function of operational NOAA satellite infrared SST sensors. Surface pressure sensors are also supported by international meteorological agencies based on regional needs. In 2006, the DBCP/North Pacific barometer array was completed with 50 desired elements in operation. In the period of July 2006 to July 2007, the weekly drifter count varied between 1350 and 1210. On July 23, 2007 there were 763 SVP drifters and 455 SVP-B (barometer) drifters, or a total of 1218, operational on GTS.

Drifter acquisitions in support of the “Global Surface Drifting Buoy Array”

In 2006-2007, 900 drifters with SST sensors were built and delivered by the GDP to AOML for deployment in the global ocean. This marks the 3rd consecutive year of level funding and acquisition of drifters to maintain a 1250 element global array with full cooperation of GDP national and international partners. Presently there is an array of 38 T-Chain Hurricane drifters and 24 SVP-W (wind) drifters in Keesler AFB waiting air-deployment into North Atlantic hurricanes in the August-October 2007 hurricane season. This marks the 4th consecutive year that GDP has provided air-deployed drifters for hurricane monitoring and research.

Global sea level maps derived from drifter and satellite data

A comparison of different methods that use the near surface momentum balance, wind, drifter, satellite altimeter and satellite gravity mission data to calculate sea level is shown on Figure 2. The differences are in how terms in the momentum equation are used to evaluate the sea level gradient and how a sea level gradient is converted to sea level. The important point is that both methods, using the same data, yield similar sea level maps that is equivalent to the uncertainty, or ‘noise level, of the combined data sets (to rms difference of 5 cm) and are thus relatively independent of the relatively subjective assumptions of how the statistical methodologies are configured.

Fig. The monthly mean age of the drifter array (upper panel) and the rate of at which drifters die (lower panel). These statistics show that a replacement rate of 95 drifters per month, or 1140 drifters per year, would be required to maintain a drifter population at 1250. Global drifter program provides 1000 drifters per year to this maintenance, the remainder coming from international partners and science research programs (courtesy of Dr. R. Lumpkin/AOML)

Fig. 2 The 1992-2002 time mean global sea level derived from momentum balance using drifter and satellite data by Maximenko and Niiler (2005) (left panel) and the difference between the time mean sea level derived by Rio and Hernandez (2004) (right panel) and the left panel center (predicted path is dotted green, actual solid).