Long-term AOML Research Project Reaches Important Milestone

AOML oceanographers deploy a dropsonde at a monitoring site in the Straits of Florida

On October 15, 2015, the scientists, technicians, and engineers involved in the AOML Western Boundary Time Series (WBTS) project marked a milestone with the completion of the 100th successful dropsonde cruise in the Florida Current since the project’s inception in 2000.

NOAA research into the Florida Current at 27ºN dates back to the early 1980s, and many outstanding scientific results were obtained by the study throughout the 1980s and 1990s. In 1998, however, this Florida Current observing project, which had been jointly run by AOML and NOAA’s Pacific Marine Environmental Laboratory, came to an end. After significant outcry from the scientific community regarding the value of this long-time data series, a new team of AOML scientists restarted the Florida Current observations as part of the WBTS project in the year 2000. A cornerstone of the project is the continuous observations of the Florida Current volume transport via a submarine cable, which provided daily estimates from 1982 to 1998 in the earlier NOAA projects, and has produced similar daily estimates since the WBTS project started in 2000.

Another crucial part of the program has been regular ship observations of the Florida Current velocity using a freefalling float called a dropsonde at nine sites across the Straits of Florida at 27ºN. These ship sections are used to monitor the calibration of the cable measurements, and they have been critical tests that have helped researchers identify and fix a number of problems in the cable data over the years. The ocean instrument called a dropsonde (not to be confused with the atmospheric dropsonde) has been used since the 1960s, but the modern version was pioneered at AOML in the mid-1990s via the introduction of a Global Positioning System (GPS) receiver inside the dropsonde.

An earlier version of the dropsonde, developed at AOML in the mid 1990s and regularly improved by the engineering team until it was replaced in the mid-2000s by the modern generation of dropsonde.

Further improvements over the years have led to a very robust instrument that provides highly accurate estimates of depth-averaged ocean velocity. Deploying and collecting 100 dropsondes from cruises over the past 15 years in support of the WBTS project has been a collaborative effort of over 20 individuals, from the engineers who build the dropsondes, to the technicians who process the data, to the numerous oceanographers who go to sea to collect the data on small boats every year.

The datasets from the WBTS project have also contributed to a number of important publications including a recent study that compared data from three observing systems—free-falling floats, lowered acoustic Doppler current profilers, and a submarine cable—to assess how accurately they estimated the volume transport of the Florida Current at 27°N in the Straits of Florida. Through the careful comparisons completed in the study, AOML researchers Rigoberto Garcia and Dr. Christopher Meinen were able to demonstrate that the daily estimates of the Florida Current transport are accurate to within roughly 5% of the long-term mean, while annual averages of the Florida Current transport can be estimated to within about 1% of the long-term mean. The study concludes that observations from the submarine cable are capable of detecting small but significant climate changes in the volume transport of the Florida Current.

On average, the Florida Current carries approximately 32 million cubic meters of water northward through the Straits of Florida every second and represents a major component of the oceanic system for redistributing heat and freshwater globally.

To find out more about the Western Boundary Time Series, visit the project page.