The Tropical Pacific Observing System (TPOS) centers in the tropical band of the Pacific Ocean from approximate latitudes of 10°S to 10°N. It takes advantage of the remarkable developments in both the in situ and satellite components of the observing system, including various satellite technologies (e.g., scatterometers and altimeters), autonomous profiling floats (i.e., Argo), and advances in moorings and other in situ technologies.
The observation network provides a better understanding of ocean climate and ecosystems, as well as human impacts and vulnerabilities. This requires the coordination of sustained ocean observations and the incorporation of new ocean observing technology to provide accurate weather and climate predictions. For example, TPOS in situ observations are important for initializing and constraining the ocean and atmospheric conditions used in forecast models and reanalyses and for validating satellite observations (e.g., SST, winds). At the heart of TPOS is observation and prediction of the El Niño-Southern Oscillation (ENSO), which represents the strongest year-to-year climate fluctuation on the planet. While ENSO was an early motivator, other phenomena and ocean processes are now observed and predicted using TPOS measurements.
In Situ Observations
Platforms for Sustained Observations
Moorings
Buoys and Moorings provide long-term, co-located, and subsurface measurements of sea surface temperature (SST) and surface vector winds, sea level pressure (SLP), position-change-based current, and air-sea flux variables. TPOS deploys surface drifting buoys and the Tropical Moored Buoy Array (TMA) to provide high temporal resolution and subsurface information in the region. Moorings are presently the only technology that provides long-term collocated ocean and atmosphere observations, sampling the full suite of variables to estimate air-sea fluxes.
Argo
Argo autonomous profiling floats measure the temperature and salinity of the upper 2,000 meters of the ocean providing coverage on larger spatial scales. TPOS deploys core Argo floats that measure temperature and salinity, and BGC-Argo floats, which are equipped with sensors to also measure nitrate, dissolved oxygen, pH, chlorophyll fluorescence, particulate backscatter and downwelling irradiance.*Photo Credit: NOAA/PMEL
Ships
Ships are important components of the TPOS Backbone and fill gaps by providing in situ observations that contribute to a broader spatial context and temporal resolution in the observing system. Both the moored and equatorial measurements and transects along mooring lines are considered essential for TPOS. In situ samples from mooring cruises, GO-SHIP lines, and other ships of opportunity would permit validation of satellite-based products (described below), maintain the pCO2 climate record, and enhance biogeochemical sampling in the tropical Pacific.
Pilot Platforms
Ocean Gliders
Ocean Gliders fill important roles in TPOS, especially in boundary currents and near-shore regions, by providing temperature and salinity profiles. Ocean Gliders are typically equipped with a CTD and have the capability to add various sensors like ADCP for velocity profiles, fluorometers for plankton biomass, and other biogeochemical samplers.
Uncrewed Surface Vehicle
Uncrewed Surface Vehicles are long duration autonomous surface vehicles equipped with various sensor packages to measure air temperature, humidity, diffuse and total irradiance, longwave radiation, barometric pressure, SST, SSS, air- and sea- pCO2, dissolved oxygen, chlorophyll, and an ADCP for measuring current profiles. This is a promising platform for providing important air-sea interaction observations without relying on shiptime.
*Photo Credit: NOAA/PMEL
Surface Gliders
Surface Gliders are uncrewed vehicles that demonstrate promise as long-term sampling platforms for surface and near-surface temperature and salinity sampling. Surface Gliders are equipped with a CTD (Conductivity, Temperature, and Depth) for profiling and sensors similar to those on the Uncrewed Surface Vehicles and are directed at long-term monitoring of surface fluxes in the west Pacific warm pool.
Remote Sensing
Satellites play an important role in assembling a complete and effective observing system for the tropical Pacific. While in situ observations such as Argo and the TMA offer high temporal resolution and subsurface information unattainable by satellites, space-borne platforms offer unmatched coverage and horizontal resolution, of particular importance over the ocean, where in situ measurement platforms are inevitably sparse.
Satellite data provide important context for observations made from ships, moorings, Argo floats and other autonomous platforms. For example, observations from the OCO-2 (atmospheric carbon) satellite, when combined with moored pCO2 data from the tropical Pacific, have improved our understanding of the timing of global changes in atmospheric CO2 linked to El Niño events (Chatterjee et al., 2017). As for SST, winds and altimetry, observations from other components of the observing system contribute important validation data for satellite ocean color and atmospheric CO2.