Global Observations at the Air-Sea Interface from the Sofar Spotter Network

Historically, in situ open ocean observations of the air-sea interface have been exceedingly sparse - in turn limiting predictive skill across both domains given the critical nature of the air-sea interface in both atmospheric and oceanic predictions. The recent push towards compact buoy’s (e.g. the Spotter buoy, Figure 1; Raghukumar, 2019) has enabled a revolutionary change in monitoring at the air-sea interface by enabling the deployment of a global in situ network with comprehensive coverage of the open ocean providing real-time, hourly observations of surface ocean waves (including both bulk parameters and directional wave spectra), sea surface temperature, barometric pressure, inferred 10-meter wind, and surface drift. This novel observing network of over 500 free drifting buoys (Figure 2) is critical to, among many uses, improving wave forecast skill via data assimilation, advancing proxy methods for additional observation methods at the air-sea interface (e.g. inferred winds, currents), and providing key observations for validation and improvement of hurricane impact predictions. In this talk, we give a broad overview of the global Spotter network as well as its utilization at Sofar, within the research community, and with government partners.

Spotter Buoy

The Spotter buoy itself is a 42 cm diameter near-spherical free-drifting buoy equipped with solar panels that provide sufficient power for continuous, long-dwell deployment and Iridium satellite connection to transmit observations every hour. Commencing in late 2019, Sofar began conducting repeated deployments in order to develop and maintain a persistent network of approximately 500 globally distributed observing platforms. This has enabled a first of its kind global dataset of open ocean in situ observations of the air-sea interface as well as an ongoing data stream of real-time observations valuable for improving representation of the ocean surface and the tightly coupled atmospheric and oceanic domains.

Data Assimilation

With a global network of real-time observations, meaningful skill improvement for global wave forecasts becomes feasible, despite the historic underutilization of wave data assimilation given the scarcity of in situ observations. While satellite altimeters do provide valuable global coverage, the observations are limited to the total significant wave height (rather than spectra) and are often subject to low accuracy in energetic conditions (e.g. storms where observations are of particular value). With the availability of continuous, high quality Spotter observations of wave spectra, we have found the implementation of unique data assimilation strategies leveraging these observations to be highly impactful to forecast skill, particularly out to three days (Smit et al. 2021, Houghton et al., 2022, Houghton et al., 2023; Figure 3). Assimilation of Spotter data is also under development across a coupled modeling framework, with the understanding that the dynamics across the atmosphere-wave-ocean system are tightly connected and therefore observations across each domain should provide improved skill to all domains.

Rapid-response for Extreme Events

In addition to the persistent global network largely deployed by ships of opportunity, additional high density targeted observations are enabled by the agile deployment capacity of the Spotter buoy (e.g. via open-door airplane). As part of the National Oceanographic Partnership Program Hurricane Coastal Impacts project (nopphurricane.sofarocean.com), Sofar has deployed from a Navy P-3 aircraft a targeted array of Spotters for the two most recent landfalling hurricanes (Hurricane Ian 2022, and Hurricane Idalia 2023; Figure 2) and will continue to do so into the future. The observations collected from these high density Spotter arrays provide invaluable data at the air-sea interface during extreme events - a critical aspect to properly predicting the evolution of the tightly coupled atmosphere and ocean during hurricanes.

Government Collaboration

In addition to the research efforts that Sofar directly participates in, the Spotter network data is also utilized through a variety of other collaborations. Through partnership with the National Mesonet Program (NOAA/NWS), Sofar serves real-time data from more than 250 buoys to support regional forecasting offices. A number of Cooperative Research and Development Agreements (CRADAs) are also ongoing with the National Data Buoy Center (NDBC), U.S. Naval Oceanography and U.S. National Ice Center to explore network augmentation, data assimilation and Arctic research. Further, Sofar supports many regional IOOS associations for localized ocean data in the coastal zone as well as Backyard Buoys - a collaboration with several IOOS regions, NSF, and indigenous groups for Sofar buoys to support coastal communities.

The Sofar Spotter network represents a first of its kind observing network of the air-sea interface that is driving innovation in data assimilation and forecasting of extreme events over the open ocean. These long-dwell in situ buoys provide valuable observations that are complementary to existing observing networks to drive forward our capacity to understand and predict conditions at the ocean surface.