TPOS Mission 7:  A new mission to the Tropical Pacific

TPOS-2024 (Mission 7) launched on 16 September 2024, with two Saildrone, Inc. uncrewed surface vehicles (USV) - SD1033 and SD1090 - departing from Alameda for the eastern/central (125°W/140°W) region of the Tropical Pacific Observing System (TPOS).  Pre-mission ADCP bottom track testing and an intercomparison with an NDBC Station (Site ID: 1801593, corresponding to SD1077) were conducted as the drones departed.  

The official mission began October 30, 2024 at 00:00 UTC when the drones arrived at their first waypoint near 10°N, 125°W.  The mission began with a meridional transect south to ~6°N 125°W, before sailing southwest to ~138°W, where a coordinated frontal study was performed with a University of Washington (UW) Seaglider, an underwater profiling glider deployed and operated from the MOTIVE (Mixing belOw Tropical Instability waVEs) cruise by Katie Kohlman (UW School of Oceanography).   An intercomparison with the TAO mooring at 0°N 140°W was also conducted before the drones were swept westward in the equatorial currents.  The drones ultimately sailed north to catch the eastward countercurrent and ended their mission around 5°N 140°W (having lost anemometer function).  Keeping a separation distance of ~5-20 km, while transecting thousands of kilometers, the drones provided both high resolution and widespread coverage of important air-sea interactions in this key region of the tropical Pacific.  With a hope that these missions can be repeated sufficiently frequently to capture the large-scale variations in the air-sea fluxes (and carbon dioxide uptake and outgassing in particular) along repeat lines, we refer to these as GO-USV transects.

Both drones carried the core Saildrone, Inc. suite of sensors (e.g. wind, air temperature, humidity, barometric pressure, PAR, 0.5m SST), the ASVCO2 suite of biological sensors, an ADCP, solar and longwave radiometers, and additional thermistors on the hull to better resolve the temperature profile in the top 2 meters of the water column.

In addition to the GO-USV transects, each mission has also tested additional capabilities of USVs integrated within the TPOS.  For this TPOS-2024 mission, we tested the ability to perform a coordinated frontal study using the two saildrones, wirewalkers, and the UW Seaglider.  Katie Kohlman, graduate student at the University of Washington's School of Oceanography, was the PI and pilot for the underwater glider.

For more details about this mission, please visit the OCS Saildrone TPOS 2024 Mission Blog.

Pre-mission plan:

The TPOS-2024 mission has a planned launch date of September 10, 2024, with two saildrones scheduled to depart Alameda for the first waypoint, which will be the official start of the mission, in the eastern tropical Pacific at 10°N 125°W.  The approximately 110-day mission will follow the GO-USV route, with a meridional transect from 10°N 125°W to the equator and a zonal transect along the equator to 0°N 140°W, where a coordinated frontal study with a Seaglider (PI & Pilot: Katie Kohlman, UW) will be performed. The saildrones will then head north to the ITCZ and transit eastward along the North Equatorial Counter Current.

Mission Summary (post-mission):

The TPOS 2024 mission was launched from Alameda ,CA on September 16, 2024 and officially began on October 30, 2024 when the drones reached 10°N, 125°W. The 98 day mission ended near 5°N, 140°W on February 4, 2025. Due to La Niña, Tropical Instability Waves (TIWs) were active throughout the mission and currents were very strong.

This year, the saildrones were each equipped with the standard suite of meteorological sensors: wind, air temperature, humidity, shortwave and longwave radiation, and atmospheric pressure. Both were also fitted with sensors on the hull or keel to measure upper ocean currents, sea surface temperature, salinity, conductivity, as well as carbon dioxide, chlorophyll and oxygen concentrations. For full sensor specifications, see the table in the blog post from 30 October.

The mission concluded early due to loss of wind sensors on January 15 (SD-1090) and January 18 (SD-1033). Despite the early termination of the mission due to sensor failures and challenges navigating the strong currents, the mission achieved several key objectives:

• Coordinated TIW study with the NSF-funded MOTIVE cruise, including a "UW APL wirewalkers sandwich" with the UW glider in close proximity
• Equatorial upwelling experiment at 0°, 140°W
• TAO mooring intercomparison at 0°, 140°W
• Long zonal and meridional transects with ASVCO2 sensors
• Observations of convective downdraft "cold pools"
• Frontal studies

Saildrone tracks for the entire mission with background Mercator SST for day one of the mission. Some key events marked which have associated blog posts.