On June 1, 2024, a collaborative group of scientists successfully deployed a 5,000-pound, sky-blue mooring approximately 60 miles north of Oahu, Hawaiʻi, from the NOAA Ship Oscar Elton Sette for the 20th consecutive year. This annual deployment is a key activity for the Woods Hole Oceanographic Institution (WHOI) Hawaiʻi Ocean Timeseries (HOT) Site (WHOTS) project. The mooring, aptly named WHOTS-20, joins a cohort of the nineteen moorings that have come before it, each collecting a year’s worth of high-resolution oceanic and atmospheric data before being swapped out for its successor. Twenty years since the initial deployment, the WHOTS moorings have revealed changes in ocean carbon storage and the importance of sustained observing systems for understanding ocean processes.
Translating ocean-atmosphere communications using data.
Each year, the WHOTS mooring is deployed within a 6-mile radius known as Station ALOHA, a collaborative research station within the central subtropical gyre of the North Pacific. Ocean conditions here influence storm events that impact the Hawaiian Islands, and the sustained monitoring efforts from WHOTS help improve models used in weather forecasts for Hawaiʻi, the Pacific, and the continental United States. Station ALOHA is also the oceanographic neighbor to the Mauna Loa Observatory where measurements that produce the Keeling Curve of atmospheric CO2 concentrations are collected. In fact, the WHOTS mooring was designed to be Mauna Loa’s marine counterpart to help understand how the increasing concentrations of atmospheric CO2 affect the ocean.
The atmosphere and the ocean “communicate” with each other through the exchange of heat and gases. In order to calculate the magnitude of this exchange, continuous observations of both “speakers” are needed. The WHOTS mooring bobs at the interface between the ocean and the air, and is outfitted with sensors above and below the water that record temperature, momentum and chemistry, among other variables, every minute. These measurements are then transmitted almost immediately via Iridium telemetry (check out real-time data from WHOTS-20 here – it’s updated hourly!). After twenty years, WHOTS has generated millions of oceanic and atmospheric observations.
So, what have two decades of ocean observations revealed?
Sustained data from WHOTS confirm that the ocean is becoming warmer and more acidic. A recent study that used several years of data from WHOTS and other instruments at Station ALOHA found that the ocean is continuing to absorb more carbon than it is releasing. Under these conditions, the ocean chemistry shifts to become more acidic, which can have major impacts on marine ecosystems, including coral reefs and marine mammals around the Hawaiian Islands.
Additionally, observations made by WHOTS highlight that some ocean-atmosphere processes are poorly captured by climate models. “The high-accuracy WHOTS observations show significant biases in both long-term averages and short-term variability of model-based surface fluxes,” according to Al Plueddemann, a senior scientist at WHOI. Identifying these biases is important because they can lead to inaccurate weather forecasts and projections of different modes of climate variability, like El Niño. Such biases would be difficult, or impossible, to identify from a dataset of just a few years, but the duration of measurements from the WHOTS moorings allows researchers to pull out long-term trends. According to WHOTS scientist Robert Weller, in order to improve reliability of weather forecasts and predictions of climate change, “sustained, accurate observations of air-sea fluxes, upper ocean properties and deep ocean variability are essential.”
Looking ahead to 20 more years.
Maintaining the WHOTS program not only means sustained scientific operations and knowledge production, but it also represents a substantial investment in local economies, workforces, and training for the next generation of ocean scientists. Much like the marine ecosystem in the subtropical Pacific, the WHOTS program consists of a diverse array of participants that create a singular, collaborative team. All research activities conducted at Station ALOHA, including WHOTS, could not function without research vessels (provided by the University of Hawaiʻi (UH) and NOAA) and the specialized workforce who operate them. “It requires people who are willing and able to work at sea, and who also understand the complex science and instrumentation needed,” says Jim Potemra, a professor at the UH at Mānoa and a principal investigator of the Hawaiʻi Ocean Timeseries program. “They are the real heroes of the operation.” WHOTS also shares shiptime with UH, which provides opportunities to monitor the local ecosystem (by deploying shark tracking devices or conducting coral adaptation experiments, for example!) and for UH students to gain experience working at sea.
As for the science, the researchers who have long been involved with WHOTS say that after 20 years of observations, the story is only just emerging. Regarding modes of climate variability that impact the subtropical Pacific, Potemra says “One might think ten years would resolve enough, but these sustained observations show that at least 30 years is needed.” WHOTS is one piece of the larger global ocean observing system supported by NOAA’s Global Ocean Monitoring and Observing program (GOMO), and the findings from WHOTS underscore the need to continue supporting such long-term monitoring efforts. Through sustained collaborations, outreach efforts, and scientific curiosity, WHOTS can continue to decipher the language spoken by the ocean and atmosphere. Congratulations to the WHOTS team on two successful decades, and here’s to another 20 years of high-resolution data!
The WHOTS program is supported by NOAA’s Global Ocean Monitoring and Observing program. The Hawaiʻi Ocean Timeseries program is funded by the U.S. National Science Foundation.